Dopamine

Dopamine (DA), an abbreviation for 3,4-dihydroxyphenethylamine, functions as a neuromodulatory molecule with diverse critical cellular roles. This compound is classified as an organic chemical belonging to both the catecholamine and phenethylamine families. As an amine, it is synthesized through the decarboxylation of its precursor, L-DOPA, a substance produced within the brain and kidneys. Furthermore, dopamine synthesis occurs in plants and the majority of animal species. Within the brain, dopamine operates as a neurotransmitter, facilitating signal transmission between neurons (nerve cells). The brain contains multiple distinct dopaminergic pathways, with one notably contributing to the motivational aspects of reward-driven behaviors. Anticipation of various rewards elevates cerebral dopamine levels, while numerous addictive substances either enhance dopamine release or inhibit its neuronal reuptake post-secretion. Additional dopaminergic pathways in the brain are implicated in motor control and the regulation of diverse hormone secretions. Collectively, these pathways and associated cell groups constitute a neuromodulatory dopamine system.

While popular culture frequently depicts dopamine as the primary neurochemical of pleasure, contemporary pharmacological understanding posits that dopamine primarily confers motivational salience. This implies that dopamine signals the perceived motivational significance (i.e., the desirability or aversiveness) of a particular outcome, thereby influencing an organism's behavioral trajectory towards or away from that outcome.

Beyond the central nervous system, dopamine predominantly acts as a local paracrine messenger. Specifically, in blood vessels, it inhibits norepinephrine release and functions as a vasodilator. In the kidneys, it enhances sodium excretion and diuresis. Within the pancreas, it diminishes insulin production. In the digestive system, it decreases gastrointestinal motility and safeguards the intestinal mucosa. Furthermore, in the immune system, it attenuates lymphocyte activity. Except for its role in blood vessels, dopamine within these peripheral systems is locally synthesized and exerts its effects in close proximity to its secreting cells.

Numerous significant neurological disorders are linked to dysfunctions within the dopamine system, and several primary therapeutic agents for these conditions operate by modulating dopaminergic effects. Parkinson's disease, a degenerative disorder characterized by tremor and motor impairment, results from the degeneration of dopamine-secreting neurons in the substantia nigra, a midbrain region. Its metabolic precursor, L-DOPA, can be synthesized; Levodopa, a purified form of L-DOPA, constitutes the most prevalent treatment for Parkinson's disease. Evidence suggests that schizophrenia involves altered dopamine activity levels, and the majority of antipsychotic medications employed for its treatment are dopamine antagonists, which diminish dopaminergic activity. Analogous dopamine antagonist drugs are also recognized as highly effective anti-nausea agents. Restless legs syndrome and attention deficit hyperactivity disorder (ADHD) are both correlated with reduced dopamine activity. While dopaminergic stimulants can be addictive at high dosages, certain formulations are utilized at lower doses for ADHD management. Dopamine itself is available as a pharmaceutical preparation for intravenous administration. It proves beneficial in managing severe heart failure or cardiogenic shock. In neonates, it may be administered for hypotension and septic shock.

Structure

A dopamine molecule is characterized by a catechol structure—a benzene ring substituted with two hydroxyl groups—to which an amine group is attached via an ethyl chain. Consequently, dopamine represents the simplest catecholamine, a family that also encompasses the neurotransmitters norepinephrine and epinephrine. The presence of a benzene ring with this specific amine attachment classifies it as a substituted phenethylamine, a chemical class containing numerous psychoactive compounds.

Dopamine, an organic base like most amines, undergoes protonation in acidic conditions through an acid-base reaction. The resulting protonated form exhibits high water solubility and considerable stability, though it remains susceptible to oxidation upon exposure to oxygen or other oxidizing agents. Conversely, in basic environments, dopamine exists in its unprotonated, free base form, which is characterized by lower water solubility and enhanced reactivity. Given the superior stability and aqueous solubility of its protonated state, dopamine is typically provided for chemical and pharmaceutical applications as dopamine hydrochloride—a salt formed by the reaction of dopamine with hydrochloric acid. When dry, dopamine hydrochloride presents as a fine powder, ranging in color from white to yellow.

Biochemistry

Synthesis

Dopamine synthesis occurs within a limited range of cell types, predominantly neurons and cells located in the adrenal medulla. The following outlines the primary and minor metabolic pathways:

Primary: L-Phenylalanine → L-Tyrosine → L-DOPA → Dopamine

Minor: L-Phenylalanine → L-Tyrosine → p-Tyramine → Dopamine

Minor: L-Phenylalanine → m-Tyrosine → m-Tyramine → Dopamine

The immediate precursor to dopamine, L-DOPA, can be generated either indirectly from the essential amino acid phenylalanine or directly from the non-essential amino acid tyrosine. These amino acids are ubiquitous in proteins, making them widely accessible through dietary intake, with tyrosine being particularly prevalent. Despite its presence in various food sources, dopamine cannot traverse the blood–brain barrier, which encapsulates and safeguards the brain. Consequently, its synthesis within the brain is imperative for its neuronal functions.

The conversion of L-phenylalanine to L-tyrosine is catalyzed by phenylalanine hydroxylase, utilizing molecular oxygen (O§45§) and tetrahydrobiopterin as cofactors. Subsequently, L-tyrosine is transformed into L-DOPA through the action of tyrosine hydroxylase, requiring tetrahydrobiopterin, O§1011§, and iron (Fe²⁺) as cofactors. Finally, L-DOPA is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase (also known as DOPA decarboxylase), with pyridoxal phosphate serving as the cofactor.

Dopamine itself serves as a precursor in the biosynthesis of the neurotransmitters norepinephrine and epinephrine. Dopamine β-hydroxylase catalyzes the conversion of dopamine to norepinephrine, with O₂ and L-ascorbic acid acting as cofactors. Subsequently, phenylethanolamine N-methyltransferase facilitates the transformation of norepinephrine into epinephrine, using S-adenosyl-L-methionine as a cofactor.

Certain cofactors necessitate their own synthetic pathways. A deficiency in any essential amino acid or cofactor can compromise the synthesis of dopamine, norepinephrine, and epinephrine.

Degradation

Dopamine undergoes degradation into inactive metabolites through the sequential action of several enzymes: monoamine oxidase (MAO), catechol-O-methyltransferase (COMT), and aldehyde dehydrogenase (ALDH). Both MAO-A and MAO-B isoforms of monoamine oxidase efficiently metabolize dopamine. While multiple degradation pathways exist, the principal end-product is homovanillic acid (HVA), which lacks known biological activity. Homovanillic acid is filtered from the bloodstream by the kidneys and subsequently excreted in the urine. The two primary metabolic routes converting dopamine to HVA are:

• Dopamine → DOPAL → DOPAC → HVA – catalyzed by MAO, ALDH, and COMT respectively
• Dopamine → 3-Methoxytyramine → HVA – catalyzed by COMT and MAO+ALDH respectively

In clinical investigations concerning schizophrenia, plasma homovanillic acid measurements have served as an indicator for estimating cerebral dopamine activity. However, a significant challenge with this methodology lies in distinguishing the substantial plasma homovanillic acid levels derived from norepinephrine metabolism.

While dopamine is typically catabolized by oxidoreductase enzymes, it is also vulnerable to direct oxidation by molecular oxygen, resulting in the formation of quinones and various free radicals. The oxidation rate can be augmented by factors such as the presence of ferric iron. Quinones and free radicals generated through dopamine's autoxidation possess cytotoxic properties, and evidence suggests this mechanism may contribute to the neuronal degeneration observed in Parkinson's disease and other pathological states.

Functions

Cellular Effects

Dopamine mediates its physiological effects through the activation of cell surface receptors. In humans, dopamine exhibits high binding affinity for both dopamine receptors and human trace amine-associated receptor 1 (hTAAR1). Mammalian systems feature five distinct dopamine receptor subtypes, designated D₁ through D₅. These receptors are all metabotropic G protein-coupled receptors, mediating their cellular responses through intricate second messenger pathways. They are categorized into two primary families: D§45§-like and D§6_{7§-like. Within the nervous system, neuronal D§8}9§-like receptor activation (encompassing D§1011§ and D§1213§) can induce either excitation (through sodium channel opening) or inhibition (through potassium channel opening). Conversely, D§14_{15§-like receptor activation (including D§1617§, D§1819§, and D§2021§) typically results in the inhibition of the target neuron. Therefore, classifying dopamine inherently as either excitatory or inhibitory is inaccurate; its impact on a target neuron is contingent upon the specific receptor subtypes expressed on the neuronal membrane and the neuron's intracellular responses to the second messenger cAMP. In the human nervous system, D§2223§ receptors are the most abundant, followed by D§2425§ receptors, while D§2627§, D§2829§, and D§3031§ receptors are expressed at considerably lower concentrations.}

Storage, Release, and Reuptake

Within the brain, dopamine acts as both a neurotransmitter and a neuromodulator, with its activity regulated by mechanisms characteristic of all monoamine neurotransmitters. Following its synthesis, dopamine is translocated from the cytosol into secretory vesicles, such as synaptic vesicles and both small and large dense core vesicles, via the vesicular monoamine transporter (VMAT2), a solute carrier. It remains stored within these vesicles until its release into the synaptic cleft. Dopamine release primarily occurs through action potential-induced exocytosis, though it can also be triggered by the activity of the intracellular trace amine-associated receptor 1 (TAAR1). TAAR1, a high-affinity receptor for dopamine, trace amines, and specific substituted amphetamines, is situated on membranes within the presynaptic cell's intracellular environment. Its activation can modulate dopamine signaling by promoting dopamine reuptake inhibition and efflux, and by suppressing neuronal firing through various mechanisms.

Upon entering the synaptic cleft, dopamine binds to and activates dopamine receptors. These receptors include postsynaptic dopamine receptors situated on the dendrites of the postsynaptic neuron, and presynaptic autoreceptors (e.g., D₂sh and presynaptic D₃ receptors) located on the axon terminal membrane of the presynaptic neuron. Following the generation of an action potential by the postsynaptic neuron, dopamine molecules rapidly dissociate from their receptors. Subsequently, they are reabsorbed into the presynaptic cell through reuptake, a process facilitated by either the dopamine transporter or the plasma membrane monoamine transporter. Within the cytosol, dopamine can either undergo degradation by monoamine oxidase or be repackaged into vesicles by VMAT2, thereby becoming available for subsequent release.

Extracellular dopamine levels in the brain are modulated by two distinct mechanisms: phasic and tonic transmission. Phasic dopamine release, akin to most neurotransmitter releases within the nervous system, is directly instigated by action potentials originating in dopaminergic neurons. Tonic dopamine transmission involves the sustained release of small dopamine quantities, independent of preceding presynaptic action potentials. This tonic transmission is influenced by various factors, such as the activity of other neurons and the rate of neurotransmitter reuptake.

Central Nervous System

Within the brain, dopamine is crucial for executive functions, motor control, motivation, arousal, reinforcement, and reward, alongside contributing to lower-level processes such as lactation, sexual gratification, and nausea. The collective dopaminergic cell groups and pathways constitute the neuromodulatory dopamine system.

Dopaminergic neurons, which are dopamine-producing nerve cells, are relatively scarce, numbering approximately 400,000 in the human brain. Their cell bodies are localized in distinct clusters within a few relatively circumscribed brain regions. Nevertheless, their axonal projections extend to numerous other cerebral regions, profoundly influencing their target structures. These dopaminergic cell groups were initially delineated in 1964 by Annica Dahlström and Kjell Fuxe, who designated them with "A" labels, signifying their aminergic nature. Within their classification system, regions A1 through A7 encompass norepinephrine-containing neurons, while A8 through A14 comprise dopaminergic neurons. The identified dopaminergic regions include the substantia nigra (groups A8 and A9), the ventral tegmental area (group A10), the posterior hypothalamus (group A11), the arcuate nucleus (group A12), the zona incerta (group A13), and the periventricular nucleus (group A14).

The substantia nigra is a compact mesencephalic region integral to the basal ganglia system. It comprises two distinct subdivisions: the pars reticulata, serving as an input zone, and the pars compacta, functioning as an output zone. Dopaminergic neurons are predominantly situated within the pars compacta (cell group A8) and adjacent areas (group A9). In human neuroanatomy, the projection of dopaminergic neurons originating from the substantia nigra pars compacta to the dorsal striatum, designated as the nigrostriatal pathway, is critically involved in regulating motor function and facilitating the acquisition of novel motor skills. These specific neurons exhibit heightened susceptibility to damage; extensive neuronal loss in this population leads to the manifestation of a parkinsonian syndrome.

The ventral tegmental area (VTA) is a distinct mesencephalic region. The primary contingent of VTA dopaminergic neurons projects to the prefrontal cortex through the mesocortical pathway, while a secondary, smaller group extends to the nucleus accumbens via the mesolimbic pathway. Collectively, these two pathways constitute the mesocorticolimbic projection. Furthermore, the VTA dispatches dopaminergic projections to the amygdala, cingulate gyrus, hippocampus, and olfactory bulb. Mesocorticolimbic neurons are fundamental to reward processing and various facets of motivation. A growing body of research indicates that dopamine is also critically involved in aversive learning, mediated by its influence on multiple cerebral regions.

Dopaminergic neurons within the posterior hypothalamus project to the spinal cord; however, their precise functional role remains largely undefined. Preliminary evidence suggests that pathological alterations in this region may contribute to restless legs syndrome, a neurological disorder characterized by an irresistible urge to move body parts, particularly the legs, leading to significant sleep disturbances.

Dopaminergic neurons originating from the arcuate and periventricular nuclei of the hypothalamus establish a critical projection, the tuberoinfundibular pathway, which extends to the pituitary gland and modulates prolactin hormone secretion. Dopamine functions as the principal neuroendocrine inhibitor of prolactin secretion from the anterior pituitary gland. Dopamine synthesized by arcuate nucleus neurons is released into the hypophyseal portal system of the median eminence, subsequently reaching the pituitary gland. Lactotrophs, the prolactin-producing cells, continuously secrete prolactin in the absence of dopamine; conversely, dopamine exerts an inhibitory effect on this secretion.

The zona incerta, anatomically positioned between the arcuate and periventricular nuclei, projects to various hypothalamic regions and contributes to the regulation of gonadotropin-releasing hormone. This hormone is essential for initiating the post-pubertal development of both male and female reproductive systems.

A distinct population of dopamine-secreting neurons is located within the ocular retina. These neurons are classified as amacrine cells, characterized by the absence of axons. They release dopamine into the extracellular milieu and exhibit activity predominantly during photopic (daylight) conditions, becoming quiescent during scotopic (nighttime) periods. This retinal dopamine modulates photoreceptor function by enhancing cone cell activity while simultaneously suppressing rod cells. Consequently, this action augments sensitivity to color and contrast under bright illumination, albeit at the expense of diminished sensitivity in low-light environments.

Basal ganglia

Within the vertebrate brain, the substantia nigra and the ventral tegmental area constitute the primary and most significant sources of dopamine. These two midbrain structures exhibit close anatomical relationships and share numerous functional similarities. The striatum represents the most substantial constituent of the basal ganglia. Specifically, the substantia nigra projects dopaminergic fibers to the dorsal striatum, whereas the ventral tegmental area directs a comparable dopaminergic projection to the ventral striatum.

Elucidating the precise functions of the basal ganglia has proven to be a protracted endeavor. The predominant hypotheses generally posit that the basal ganglia are instrumental in the process of action selection. In its fundamental formulation, the action selection theory suggests that in scenarios presenting multiple behavioral options, basal ganglia activity dictates the selection of a specific action by disinhibiting the chosen response while concurrently suppressing other motor systems that could produce competing behaviors. Consequently, within this conceptual framework, the basal ganglia are implicated in initiating behaviors rather than specifying the intricate details of their execution. Essentially, they function as a critical component of a decision-making system.

The basal ganglia are compartmentalized into distinct sectors, each responsible for regulating specific categories of actions. The ventral sector, encompassing the ventral striatum which receives projections from the ventral tegmental area, occupies the apex of this hierarchical organization, orchestrating actions at the organismal level. Conversely, the dorsal sectors, which include the dorsal striatum receiving input from the substantia nigra, function at subordinate levels, determining the precise musculature and movements required to execute a particular behavioral pattern.

Dopamine contributes to the action selection process through at least two significant mechanisms. Firstly, it modulates the threshold for initiating actions. Elevated dopamine activity correlates with a reduced impetus necessary to elicit a specific behavior. Consequently, high dopamine concentrations are associated with heightened motor activity and impulsive conduct, whereas diminished dopamine levels result in lethargy and decelerated responses. Parkinson's disease, characterized by a substantial reduction in dopamine levels within the substantia nigra circuit, manifests as rigidity and impaired movement initiation; however, individuals afflicted with the disease can exhibit reactions as robust as those of healthy individuals when exposed to potent stimuli, such as a severe threat. Conversely, pharmacological agents that augment dopamine release, including cocaine and amphetamine, can induce elevated activity levels, potentially culminating in psychomotor agitation and stereotyped movements in extreme cases.

The second crucial function of dopamine involves its role as a "teaching" signal. Following an action, an increase in dopaminergic activity modifies the basal ganglia circuit, thereby facilitating the subsequent evocation of that same response in analogous future contexts. This mechanism exemplifies a form of operant conditioning, wherein dopamine functions as a reward signal.

Reward

In the context of the reward system, reward is defined as the appealing and motivating characteristic of a stimulus that elicits both appetitive (or approach) behavior and consummatory behavior. A stimulus is considered rewarding if it prompts an organism to seek it out and engage in its consumption. The primary functions of reward encompass pleasure, learning (such as classical and operant conditioning), and approach behavior. While pleasure serves as a defining element of reward, it is important to note that although all pleasurable stimuli are inherently rewarding, not all rewarding stimuli necessarily induce pleasure (e.g., extrinsic motivators like monetary compensation). The motivational or desirable dimension of rewarding stimuli is manifested through the approach behaviors they provoke, whereas the pleasure derived from intrinsic rewards arises from their consumption subsequent to acquisition. The incentive salience model is a neuropsychological framework that differentiates these two constituents of an intrinsically rewarding stimulus: "wanting" or desire (sometimes termed "seeking") aligns with appetitive or approach behavior, while "liking" or pleasure corresponds to consummatory behavior. In individuals with drug addiction, "wanting" becomes decoupled from "liking," meaning the craving for an addictive substance intensifies even as the pleasure derived from its consumption diminishes due to the development of drug tolerance.

In the cerebral context, dopamine serves, in part, as a comprehensive reward signal. An initial dopaminergic response to a rewarding stimulus encodes critical information regarding the reward's salience, intrinsic value, and situational context. Furthermore, within the domain of reward-related learning, dopamine operates as a reward prediction error signal, quantifying the extent to which a reward's value deviates from expectation. As posited by Montague, Dayan, and Sejnowski, this hypothesis suggests that anticipated rewards do not elicit a subsequent phasic dopamine response in specific dopaminergic neurons. Conversely, unexpected rewards, or those exceeding expectations, trigger a transient elevation in synaptic dopamine. The absence of an expected reward, however, results in a reduction of dopamine release below baseline levels. The "prediction error" hypothesis has garnered significant attention from computational neuroscientists, largely because temporal difference learning, a prominent computational-learning methodology, extensively utilizes a signal that encodes prediction error. This convergence of theoretical frameworks and empirical data has fostered productive collaborations between neuroscientists and computer scientists engaged in machine learning research.

Microelectrode recordings from animal brains provide evidence that dopamine neurons situated in the ventral tegmental area (VTA) and substantia nigra exhibit robust activation in response to diverse rewarding events. These dopamine neurons, responsive to reward and located in the VTA and substantia nigra, are indispensable for reward-related cognitive processes and constitute a core element of the reward system. The functional role of dopamine diverges across distinct axonal projections originating from the VTA and substantia nigra. For instance, the VTA–nucleus accumbens shell projection attributes incentive salience (i.e., "wanting") to rewarding stimuli and their associated cues. The VTA–prefrontal cortex projection is responsible for recalibrating the value of various goals based on their incentive salience. Furthermore, the VTA–amygdala and VTA–hippocampus projections facilitate the consolidation of memories pertinent to reward. Both the VTA–nucleus accumbens core and the substantia nigra–dorsal striatum pathways contribute to the acquisition of motor responses that aid in obtaining rewarding stimuli. Additionally, certain activity within the VTA dopaminergic projections appears to correlate with reward prediction.

Pleasure

While dopamine plays a pivotal role in generating "wanting," which is associated with appetitive or approach behaviors in response to rewarding stimuli, extensive research indicates that dopamine should not be simply equated with hedonic "liking" or pleasure, as manifested in consummatory behavioral responses. Dopamine neurotransmission contributes to certain aspects of pleasure-related cognition, but not all, given that pleasure centers have been identified both within the dopaminergic system (e.g., the nucleus accumbens shell) and independently of it (e.g., the ventral pallidum and parabrachial nucleus). For instance, direct electrical stimulation of dopamine pathways via implanted brain electrodes elicits pleasurable sensations, prompting many animal species to actively seek such stimulation. Conversely, antipsychotic medications, which reduce dopamine levels, frequently induce anhedonia, characterized by a diminished capacity to experience pleasure. Numerous pleasurable activities, including sexual intercourse, eating, and engaging in video games, are known to elevate dopamine release. All addictive substances, whether directly or indirectly, influence dopamine neurotransmission within the nucleus accumbens; repeated administration of high doses of these drugs intensifies drug "wanting," leading to compulsive drug use, presumably through the sensitization of incentive-salience. Psychostimulants, such as methamphetamine and cocaine, increase synaptic dopamine concentrations, thereby enhancing "wanting" behaviors without significantly altering expressions of pleasure or levels of satiation. In contrast, opiate drugs like heroin and morphine augment both "liking" and "wanting" behaviors. Furthermore, animals with an inactivated ventral tegmental dopamine system fail to seek food and will perish from starvation if left unattended; however, if food is directly provided, they will consume it and exhibit signs of pleasure.

A clinical investigation conducted in January 2019 examined the impact of a dopamine precursor (levodopa), a dopamine antagonist (risperidone), and a placebo on human reward responses to music. This assessment included quantifying the pleasure experienced during musical chills, utilizing both electrodermal activity measurements and subjective participant ratings. The study concluded that manipulating dopamine neurotransmission bidirectionally modulates pleasure cognition, specifically the hedonic impact of music, in human subjects. This research further established that elevated dopamine neurotransmission constitutes a sine qua non condition for experiencing pleasurable hedonic reactions to music in humans.

A seminal study published in Nature in 1998 provided evidence that engaging in video games triggers dopamine release within the human striatum. This dopamine release is implicated in various cognitive processes, including learning, behavioral reinforcement, attention, and sensorimotor integration. Utilizing positron emission tomography (PET) scans and ¹¹C-labelled raclopride, researchers monitored cerebral dopamine levels during goal-directed motor tasks. Their findings indicated a positive correlation between dopamine release and task performance, with the highest concentrations observed in the ventral striatum. This investigation marked the first demonstration of the specific behavioral conditions under which dopamine is released in human subjects, underscoring the utility of PET in detecting neurotransmitter fluctuations during behavioral changes. Subsequent research suggests that potentially problematic video game engagement is associated with certain personality characteristics, such as diminished self-esteem and self-efficacy, heightened anxiety, increased aggression, and clinical manifestations of depression and anxiety disorders. Furthermore, individual motivations for playing video games are diverse, encompassing coping mechanisms, social interaction, and personal gratification. The DSM-5 classifies Internet Gaming Disorder as a mental disorder, noting its close relationship to Gambling Disorder, a categorization that has garnered support from some researchers while simultaneously generating considerable debate.

Outside the central nervous system

Given that dopamine cannot traverse the blood-brain barrier, its synthesis and functional roles in peripheral regions largely operate independently of its cerebral counterparts. Although a significant quantity of dopamine circulates within the bloodstream, its precise functions in this context remain largely undefined. While dopamine concentrations in blood plasma are comparable to those of epinephrine, more than 95% of plasma dopamine in humans exists as dopamine sulfate, a conjugate formed by the action of the enzyme sulfotransferase 1A3/1A4 on free dopamine. The majority of this dopamine sulfate originates in the mesenteric organs. This production of dopamine sulfate is hypothesized to serve as a detoxification mechanism for dopamine consumed through diet or generated during digestion, as plasma levels commonly increase over fifty-fold following a meal. Dopamine sulfate possesses no identified biological functions and is subsequently eliminated via urine.

The comparatively small amount of unconjugated dopamine present in the bloodstream may originate from the sympathetic nervous system, the digestive system, or potentially other organs. This unconjugated dopamine can either interact with dopamine receptors in peripheral tissues, undergo metabolic breakdown, or be converted into norepinephrine by the enzyme dopamine beta hydroxylase, which is secreted into the bloodstream by the adrenal medulla. Dopamine receptors are also situated within arterial walls, where they mediate vasodilation and inhibit norepinephrine release from postganglionic sympathetic nerve terminals. This inhibitory effect on norepinephrine release can occur through dopamine's action on both presynaptic dopamine receptors and presynaptic α-1 receptors, similar to norepinephrine itself. While these responses may be triggered by dopamine released from the carotid body during hypoxic conditions, the extent to which arterial dopamine receptors fulfill other biologically significant roles remains undetermined.

In addition to its involvement in blood flow regulation, dopamine operates within several peripheral systems where it circulates locally, exerting either exocrine or paracrine effects. Key peripheral systems where dopamine plays a significant role encompass the immune system, the kidneys, and the pancreas.

Immune system

Within the immune system, dopamine exerts its effects by binding to receptors located on immune cells, particularly lymphocytes. Furthermore, dopamine influences immune cells found in the spleen, bone marrow, and throughout the circulatory system. Notably, immune cells themselves are capable of synthesizing and releasing dopamine. The primary impact of dopamine on lymphocytes involves a reduction in their activation state. While the precise functional significance of this intricate system remains to be fully elucidated, it presents a potential pathway for interactions between the nervous and immune systems and may hold relevance for certain autoimmune disorders.

Kidneys

The renal dopaminergic system is situated within the nephron cells of the kidney, where all known subtypes of dopamine receptors are expressed. Dopamine is additionally synthesized by tubule cells within the nephron and subsequently released into the tubular fluid. Its physiological effects encompass enhancing renal blood supply, elevating the glomerular filtration rate, and promoting the urinary excretion of sodium. Consequently, impairments in renal dopamine function can lead to diminished sodium excretion, thereby contributing to the development of hypertension. Substantial evidence indicates that deficiencies in dopamine production or receptor function can precipitate various pathologies, such as oxidative stress, edema, and both genetic and essential hypertension. Notably, oxidative stress itself is implicated as a causative factor in hypertension. Furthermore, systemic defects can arise from genetic predispositions or pre-existing hypertension.

Pancreas

The role of dopamine within the pancreas is notably intricate. The pancreas comprises two distinct components: an exocrine and an endocrine portion. The exocrine component is responsible for synthesizing and secreting digestive enzymes and other substances, including dopamine, directly into the small intestine. The precise function of this secreted dopamine upon its entry into the small intestine remains to be definitively established; however, proposed roles include safeguarding the intestinal mucosa from injury and attenuating gastrointestinal motility, which refers to the rate of content transit through the digestive system.

Comprising the endocrine component of the pancreas, pancreatic islets are responsible for the synthesis and secretion of hormones, such as insulin, directly into the bloodstream. Research indicates that the beta cells within these islets, which produce insulin, possess dopamine receptors. Dopamine's action on these receptors appears to diminish insulin release. The precise origin of this dopamine input remains undefined; potential sources include circulating dopamine derived from the sympathetic nervous system or local synthesis by other pancreatic cell types.

Medical Applications

As a pharmaceutical agent, dopamine is marketed under various trade names, including Intropin, Dopastat, and Revimine. It is recognized on the World Health Organization's List of Essential Medicines. Its primary therapeutic application involves its use as a stimulant in managing severe hypotension, bradycardia, and cardiac arrest, particularly critical in the treatment of newborn infants. Administered intravenously, dopamine exhibits a very short plasma half-life—approximately one minute in adults, two minutes in newborn infants, and up to five minutes in preterm infants. Consequently, it is typically delivered via a continuous intravenous infusion rather than a bolus injection.

The physiological effects of dopamine are dose-dependent, encompassing enhanced renal sodium excretion, increased urine production, elevated heart rate, and augmented blood pressure. At lower dosages, dopamine engages the sympathetic nervous system, leading to an intensification of myocardial contractility and heart rate, which subsequently boosts cardiac output and systemic blood pressure. Conversely, higher dosages induce vasoconstriction, further contributing to blood pressure elevation. Although earlier publications suggested that very low doses could improve renal function without adverse effects, contemporary reviews indicate that such minimal dosages are ineffective and potentially detrimental. While certain outcomes arise from dopamine receptor stimulation, its notable cardiovascular effects are primarily attributed to its action as a weak agonist at α₁, β§23§, and β§4_{5§ adrenergic receptors.}

Adverse effects associated with dopamine administration include impaired renal function and cardiac arrhythmias. The LD₅₀, defined as the lethal dose anticipated to cause mortality in 50% of a population, has been determined as follows: 59 mg/kg for mice (intravenous administration); 95 mg/kg for mice (intraperitoneal administration); 163 mg/kg for rats (intraperitoneal administration); and 79 mg/kg for dogs (intravenous administration).

Disease, Disorders, and Pharmacology

The dopaminergic system is fundamentally implicated in the pathophysiology of several prominent medical conditions, such as Parkinson's disease, attention deficit hyperactivity disorder, Tourette syndrome, schizophrenia, bipolar disorder, and various forms of addiction. Beyond dopamine itself, numerous other pharmacological agents exert their effects on dopamine systems throughout the brain and body. While some of these compounds are utilized for therapeutic or recreational purposes, neurochemists have also synthesized a diverse array of research compounds. These include agents that exhibit high-affinity binding to specific dopamine receptor subtypes, functioning as either agonists or antagonists, as well as compounds that modulate other facets of dopamine physiology, such as dopamine transporter inhibitors, VMAT inhibitors, and enzyme inhibitors.

Aging Brain

Multiple investigations have documented an age-associated reduction in both dopamine synthesis and the density of dopamine receptors within the brain. This decline is observed in both striatal and extrastriatal regions. Specifically, reductions in D₁, D₂, and D§45§ receptors are extensively substantiated. The age-related diminution of dopamine is hypothesized to underlie numerous neurological symptoms that become more prevalent with advancing age, including diminished arm swing and heightened rigidity. Furthermore, alterations in dopamine concentrations may contribute to age-dependent shifts in cognitive flexibility.

Multiple Sclerosis

Research indicates that an imbalance in dopamine levels contributes to fatigue experienced by individuals with multiple sclerosis. Specifically, in patients afflicted with multiple sclerosis, dopamine has been shown to suppress the production of interleukin-17 (IL-17) and interferon-gamma (IFN-γ) by peripheral blood mononuclear cells.

Parkinson's Disease

Parkinson's disease, an age-related neurological condition, is characterized by motor impairments such as muscular rigidity, bradykinesia, and resting tremor. In its advanced stages, the disease progresses to dementia and ultimately results in mortality. The cardinal symptoms are attributed to the degeneration of dopamine-secreting neurons within the substantia nigra. These dopaminergic cells are particularly vulnerable to various insults, including encephalitis (as depicted in the book and movie Awakenings), repeated sports-related concussions, and certain chemical poisonings like MPTP, which can lead to substantial cellular loss. Such damage can precipitate a parkinsonian syndrome, exhibiting core features similar to Parkinson's disease. However, most instances of Parkinson's disease are idiopathic, meaning the underlying cause of cell death remains unidentified.

The predominant therapeutic approach for parkinsonism involves the administration of L-DOPA, a metabolic precursor to dopamine. L-DOPA undergoes conversion into dopamine within both the brain and peripheral tissues through the action of the enzyme DOPA decarboxylase. Its preference over direct dopamine administration stems from its capacity to traverse the blood–brain barrier, a property dopamine lacks. To optimize cerebral L-DOPA delivery, it is frequently co-administered with a peripheral decarboxylase inhibitor, such as carbidopa or benserazide, which curtails its conversion to dopamine outside the brain. While prolonged and regular L-DOPA therapy often precipitates adverse effects like dyskinesia, it remains widely regarded as the most effective long-term treatment modality for the majority of Parkinson's disease cases.

While L-DOPA therapy does not regenerate lost dopaminergic neurons, it stimulates the surviving cells to augment dopamine production, thus partially compensating for neuronal depletion. In advanced disease stages, the efficacy of this treatment diminishes as severe cellular loss renders the remaining neurons incapable of producing sufficient dopamine, irrespective of L-DOPA concentrations. Alternative pharmacological agents that enhance dopamine function, including bromocriptine and pergolide, are occasionally employed in parkinsonism management; however, L-DOPA generally offers the most favorable balance between therapeutic benefits and adverse reactions.

Dopaminergic medications prescribed for Parkinson's disease are occasionally linked to the emergence of dopamine dysregulation syndrome. This syndrome is characterized by the excessive use of dopaminergic agents and a medication-induced compulsive pursuit of natural rewards, such as gambling and sexual activity. These latter behaviors bear resemblance to those observed in individuals diagnosed with behavioral addictions.

Pharmacological Addiction and Psychostimulants

Cocaine, substituted amphetamines (including methamphetamine), Adderall, methylphenidate (marketed as Ritalin or Concerta), and other psychostimulant compounds primarily or partially exert their pharmacological effects by elevating cerebral dopamine levels through diverse mechanisms. Specifically, cocaine and methylphenidate function as dopamine transporter blockers or reuptake inhibitors, non-competitively impeding dopamine reuptake and consequently increasing dopamine concentrations within the synaptic cleft. Similarly, substituted amphetamines and amphetamine also augment synaptic dopamine levels, albeit via distinct mechanisms compared to cocaine.

Psychostimulants elicit various physiological and psychological effects, including elevated heart rate, increased body temperature, and enhanced perspiration. Cognitively, they improve alertness, attention, and endurance, while also intensifying pleasure derived from rewarding stimuli. Conversely, at higher dosages, these substances can induce agitation, anxiety, or even a detachment from reality. The high addictive potential of psychostimulants stems from their activating influence on the brain's dopamine-mediated reward system. Nevertheless, at lower therapeutic doses, certain psychostimulants are efficacious in managing attention deficit hyperactivity disorder (ADHD) and narcolepsy. A critical distinction among these compounds lies in their onset and duration of action. For instance, cocaine, when injected or inhaled in its free base form, can manifest effects within seconds, which typically persist for 5 to 90 minutes. This rapid onset and brief duration contribute to its easily perceived effects and, consequently, its high addiction liability. In contrast, oral methylphenidate may require up to two hours to achieve peak plasma concentrations, with effects lasting up to 12 hours depending on the specific formulation. Such extended-release formulations offer advantages by mitigating abuse potential and enhancing treatment adherence through more convenient dosing schedules.

Numerous addictive substances elevate dopamine activity within the brain's reward pathways. Stimulants, including nicotine, cocaine, and methamphetamine, significantly increase dopamine levels, a mechanism widely considered central to their addictive properties. However, for other addictive drugs, such as the opioid heroin, the contribution of increased dopamine in the reward system to addiction may be less pronounced. Individuals undergoing withdrawal from stimulants typically do not experience the severe physical distress associated with alcohol or opiate withdrawal. Instead, their withdrawal syndrome is characterized by intense craving—a profound desire for the drug—accompanied by irritability, restlessness, and other symptoms of psychological arousal, indicative of psychological dependence.

The dopamine system is pivotal in multiple facets of addiction. Initially, genetic variations influencing the expression of dopamine receptors in the brain can predispose individuals to perceive stimulants as either appealing or aversive. Acute stimulant consumption leads to elevated brain dopamine levels, persisting from minutes to hours. Ultimately, the sustained increase in dopamine resulting from chronic, high-dose stimulant use instigates extensive structural alterations within the brain, which underlie the characteristic behavioral pathologies of addiction. Treating stimulant addiction presents significant challenges because, even after cessation of use, the psychological craving often persists. Furthermore, even when craving appears to have subsided, it can readily re-emerge upon exposure to drug-associated cues, such as specific individuals, environments, or circumstances, reflecting the highly interconnected nature of neural association networks.

Psychosis and Antipsychotic Pharmacotherapy

In the early 1950s, psychiatrists identified that a category of medications, termed typical antipsychotics (also known as major tranquilizers), frequently alleviated the psychotic manifestations of schizophrenia. The widespread adoption of the inaugural antipsychotic, chlorpromazine (Thorazine), during the 1950s, facilitated the discharge of numerous schizophrenia patients from institutional care in subsequent years. By the 1970s, researchers elucidated that these typical antipsychotics functioned as antagonists at D₂ receptors. This understanding gave rise to the dopamine hypothesis of schizophrenia, which posits that the disorder primarily results from hyperactive dopamine systems in the brain. Further corroboration for this hypothesis emerged from observations that dopamine-enhancing stimulants, such as methamphetamine, often exacerbated psychotic symptoms and could induce psychosis in healthy individuals when administered in sufficient doses. Over the ensuing decades, novel atypical antipsychotics were developed, characterized by a reduced incidence of severe side effects. Many of these newer agents do not directly target dopamine receptors but rather modulate dopamine activity through indirect mechanisms. These drugs also found application in treating other psychotic disorders. Generally, antipsychotic medications exert a broad suppressive effect on most forms of active behavior, notably diminishing the delusional and agitated conduct typical of overt psychosis.

Subsequent research has diminished the prominence of the dopamine hypothesis, particularly its initial simplistic formulation. A key reason is that individuals with schizophrenia generally do not exhibit demonstrably elevated brain dopamine activity. Nevertheless, a significant number of psychiatrists and neuroscientists maintain that schizophrenia is associated with some form of dopamine system dysfunction. Over time, the "dopamine hypothesis" has evolved, proposing increasingly nuanced and intricate dysfunctions.

In a 2018 review, psychopharmacologist Stephen M. Stahl proposed that in numerous instances of psychosis, including schizophrenia, an overexcitation of dopamine D₂ receptors in the ventral striatum is attributable to the interplay of three interconnected neurotransmitter networks: dopamine, serotonin, and glutamate, either individually or in various combinations.

Attention Deficit Hyperactivity Disorder

Dysregulated dopamine neurotransmission is implicated in attention deficit hyperactivity disorder (ADHD), a condition characterized by compromised cognitive control. This impairment manifests as difficulties in attentional regulation, behavioral inhibition, and working memory deficits, among other issues. Genetic associations have been identified between ADHD and dopamine receptors, the dopamine transporter, as well as other neurotransmitter receptors and transporters. A critical aspect of the relationship between dopamine and ADHD pertains to pharmacological treatments. Highly effective therapeutic agents for ADHD include psychostimulants like methylphenidate (e.g., Ritalin, Concerta) and amphetamine (e.g., Evekeo, Adderall, Dexedrine), which elevate both dopamine and norepinephrine concentrations in the brain. The clinical efficacy of these psychostimulants in managing ADHD is achieved via the indirect activation of dopamine receptor D₁ and adrenoceptor α₂ within the prefrontal cortex.

Pain

Dopamine contributes to pain processing across various levels of the central nervous system, encompassing the spinal cord, periaqueductal gray, thalamus, basal ganglia, and cingulate cortex. Reduced dopamine concentrations have been linked to the painful symptoms commonly observed in Parkinson's disease. Furthermore, dysfunctions in dopaminergic neurotransmission are evident in several painful clinical disorders, such as burning mouth syndrome, fibromyalgia, and restless legs syndrome.

Nausea

The phenomena of nausea and vomiting are primarily regulated by activity within the area postrema, located in the medulla of the brainstem, specifically within the chemoreceptor trigger zone. This region is characterized by a substantial density of type D₂ dopamine receptors. Consequently, pharmacological agents that activate D₂ receptors possess a significant propensity to induce nausea. This category includes certain medications prescribed for Parkinson's disease, alongside other dopamine agonists such as apomorphine. Conversely, D§4_{5§-receptor antagonists, such as metoclopramide, are sometimes employed as effective antiemetic agents.}

Fear and Anxiety

Concurrent positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies have demonstrated that the magnitude of dopamine release correlates with the intensity of conditioned fear responses and exhibits a linear relationship with learning-induced activity in the amygdala. While dopamine is commonly associated with reward learning, it also performs a crucial function in the acquisition and extinction of fear, facilitating the formation, storage, and modification of fear memories through its interactions with other cerebral areas, including the amygdala, ventromedial prefrontal cortex, and striatum.

Comparative Biology and Evolution

Microorganisms

While dopamine has not been reported in archaea, its presence has been identified in certain bacterial species and in the protozoan Tetrahymena. Significantly, some bacteria possess homologs of all the enzymes utilized by animals for dopamine synthesis. A hypothesis suggests that animals acquired their dopamine-synthesizing apparatus from bacteria through horizontal gene transfer, potentially occurring relatively late in evolutionary history, possibly stemming from the symbiotic integration of bacteria into eukaryotic cells that subsequently evolved into mitochondria.

Animals

Dopamine functions as a neurotransmitter across the majority of multicellular animal species. While a solitary report notes dopamine's presence in sponges, its functional role remains undefined. Conversely, dopamine has been identified within the nervous systems of numerous other radially symmetric organisms, such as cnidarian jellyfish, hydra, and various corals. This suggests that dopamine's role as a neurotransmitter originated concurrently with the earliest nervous systems, more than 500 million years ago during the Cambrian Period. Furthermore, dopamine serves as a neurotransmitter in vertebrates, echinoderms, arthropods, mollusks, and various worm species.

Across all animal taxa investigated, dopamine consistently modulates motor behavior. For instance, in the model nematode Caenorhabditis elegans, dopamine diminishes locomotion while enhancing food-seeking behaviors. In flatworms, it induces distinctive "screw-like" movements. In leeches, dopamine suppresses swimming and encourages crawling. Throughout diverse vertebrate species, dopamine exerts an "activating" influence on behavioral transitions and response selection, a function analogous to its role in mammals.

Furthermore, dopamine has been consistently implicated in reward learning across all animal phyla. Similar to vertebrates, various invertebrates, including roundworms, flatworms, mollusks, and common fruit flies, can be conditioned to reiterate an action when it is reliably succeeded by elevated dopamine levels. Within fruit flies, specific components of reward learning indicate a modular organization of the insect's reward processing system, which exhibits broad parallels with its mammalian counterpart. For instance, dopamine modulates both short-term and long-term learning in monkeys. Correspondingly, in fruit flies, distinct populations of dopaminergic neurons mediate reward signals for short-term and long-term memories.

Historically, arthropods were considered an exception, with dopamine perceived as having an adverse effect. Reward processing in these organisms was thought to be mediated by octopamine, a neurotransmitter structurally similar to norepinephrine. Nevertheless, more recent investigations have demonstrated dopamine's involvement in reward learning within fruit flies. Furthermore, the rewarding properties of octopamine have been attributed to its activation of specific dopaminergic neurons that were not previously identified in earlier research. Additionally, dopamine is present in cephalopod ink.

Plants

Numerous plant species, including various food crops, synthesize dopamine in differing quantities. The most elevated concentrations have been detected in bananas, where the fruit pulp of both red and yellow varieties contains dopamine at levels ranging from 40 to 50 parts per million by weight. Potatoes, avocados, broccoli, and Brussels sprouts can also contain dopamine at concentrations of 1 part per million or higher. Conversely, oranges, tomatoes, spinach, beans, and other botanical species exhibit measurable concentrations below 1 part per million. Plant dopamine is synthesized from the amino acid tyrosine through biochemical pathways analogous to those employed by animals. This dopamine can undergo diverse metabolic processes, yielding melanin and various alkaloids as byproducts. While the precise functions of plant catecholamines remain to be definitively established, evidence suggests their involvement in stress responses, such as bacterial infection, their capacity to act as growth-promoting agents under certain conditions, and their influence on sugar metabolism. The specific receptors mediating these actions, along with the intracellular mechanisms they activate, have not yet been identified.

Dietary dopamine cannot exert effects on the brain due to its inability to traverse the blood–brain barrier. Nevertheless, numerous plant species contain L-DOPA, which serves as a metabolic precursor to dopamine. The most significant concentrations are present in the leaves and bean pods of plants belonging to the genus Mucuna, particularly Mucuna pruriens (velvet beans), which have historically been utilized as a pharmaceutical source of L-DOPA. Another botanical source rich in L-DOPA is Vicia faba, the plant responsible for producing fava beans (also known as "broad beans"). However, the L-DOPA content within the beans themselves is considerably lower than that found in the pod shells and other plant components. Furthermore, the seeds of Cassia and Bauhinia trees also contain notable quantities of L-DOPA.

Dopamine is found in exceptionally high concentrations, approximately 4.4% of dry weight, within the marine green algae species Ulvaria obscura, which constitutes a significant component of certain algal blooms. Evidence suggests that this dopamine acts as an anti-herbivore defense mechanism, diminishing consumption by snails and isopods.

Role as a Melanin Precursor

Melanins represent a diverse group of dark-pigmented compounds prevalent across numerous organisms. These substances exhibit a close chemical relationship with dopamine, and a specific melanin variant, termed dopamine-melanin, can be synthesized through the enzymatic oxidation of dopamine by tyrosinase. In contrast, the melanin responsible for human skin pigmentation is produced via a distinct pathway utilizing L-DOPA as a precursor, rather than dopamine. Nevertheless, considerable evidence indicates that the neuromelanin imparting a dark hue to the substantia nigra in the brain is, at least partially, composed of dopamine-melanin.

Dopamine-derived melanin is also hypothesized to occur in various other biological systems. A portion of plant-based dopamine is likely utilized as a precursor for dopamine-melanin synthesis. Furthermore, the intricate patterns observed on butterfly wings and the distinct black-and-white striations on insect larvae bodies are believed to result from spatially organized accumulations of dopamine-melanin.

Historical Context and Discovery

Dopamine was initially synthesized in 1910 by George Barger and James Ewens at Wellcome Laboratories in London, England. Its presence in the human brain was subsequently identified by Katharine Montagu in 1957. The compound received its name, dopamine, due to its classification as a monoamine and its precursor in the Barger-Ewens synthesis, 3,4-dihydroxyphenylalanine (also known as levodopa or L-DOPA). The neurotransmitter function of dopamine was first elucidated in 1958 by Arvid Carlsson and Nils-Åke Hillarp at the Laboratory for Chemical Pharmacology, National Heart Institute of Sweden. Carlsson later received the 2000 Nobel Prize in Physiology or Medicine for demonstrating that dopamine serves not only as a precursor for norepinephrine (noradrenaline) and epinephrine (adrenaline) but also functions independently as a neurotransmitter.

Polydopamine

In 2007, investigations into the adhesive polyphenolic proteins found in mussels led to the discovery that diverse materials, when immersed in a slightly basic dopamine solution, spontaneously acquire a coating of polymerized dopamine, commonly termed polydopamine. This polymerized form of dopamine arises from a spontaneous oxidation reaction and is formally classified as a type of melanin. Moreover, the self-polymerization of dopamine offers a mechanism to modulate the mechanical characteristics of peptide-based gels. Typically, polydopamine synthesis involves the reaction of dopamine hydrochloride with Tris, acting as a base, in an aqueous medium. The precise structural composition of polydopamine remains undetermined.

Polydopamine coatings are capable of forming on objects spanning a wide size range, from nanoparticles to extensive surfaces. The chemical attributes of polydopamine layers present considerable utility, prompting extensive research into their potential applications. Fundamentally, these coatings can provide protection against photodamage or serve as encapsulants for drug delivery systems. At a more advanced level, their inherent adhesive properties suggest their utility as substrates for biosensors or other biologically active macromolecules.

Dopamine fasting

• Dopamine fasting
• Breastfeeding and fertility
• Endorphin
• Serotonin
• Oxytocin
• Enkephalin

References

Szalavitz M (13 September 2024). "A 'Dopamine Fast' Will Not Save You From Addiction." The New York Times.

• Szalavitz M (13 September 2024). "A 'Dopamine Fast' Will Not Save You From Addiction". The New York Times.Smith DG (30 June 2023). "We Have a Dopamine Problem." The New York Times.Metz C (29 September 2021). "One Man's Endless Hunt for a Dopamine Rush in Virtual Reality." The New York Times.Lembke A (2021). Dopamine Nation. Headline. ISBN 9781472294128.
  • The dictionary definition of Dopamine at Wiktionary
  • Media related to Dopamine at Wikimedia Commons