Neurotransmitters Explained

1 - Introduction

Theres this idea thats been sitting in everyones head since like the nineties: the brain as a chemical mixing board, with too much dopamine here & too little serotonin there, & psychiatric illness being basically the result of poorly tuned dials. its a comfortable picture, its visual, & its more or less wrong. it survives anyway because it works so smoothly in forums, in talk shows, in patient education, you dont have to explain plasticity or networks or glutamate homeostasis or receptor diversity or developmental windows, you just say “serotonin imbalance” & everyone nods. so let me try & clean this up a bit, without turning it into a 100 page reader. what neurotransmitters actually are, what they actually do, & why the popular explanations that have been floating around the internet for thirty years are mostly either outdated, oversimplified or shaped by pharma marketing. & equally importantly, what does this have to do with psychology, with attachment, with stress, with addiction, with therapy, with the whole field beyond pure biochemistry. because psychiatry is not half pharmacology & half “the rest”, its a discipline where biology & psychology generate each other & dont really function without each other at all. so lets walk thru this.

2 - What Even Is a Neurotransmitter?

a neurotransmitter in the classical textbook sense is a messenger that gets released from one neuron into the synaptic cleft, binds to receptors on the receiving cell & there either triggers an electrical signal or modulates one. thats the mechanics: vesicles, calcium influx, exocytosis, binding, effect, reuptake or enzymatic degradation. so far so 1980s. where it gets interesting: the line between neurotransmitter, neuromodulator & hormone has gotten pretty soft in modern thinking. acetylcholine acts as a lightning fast point to point transmitter at the motor endplate but as a slowly diffusing modulator in the cortex. oxytocin & vasopressin are central neurotransmitters & at the same time classical hormones of the posterior pituitary, the same molecule doing both jobs depending on location & receptor. this isnt a special case anymore, its the rule. second important thing: ionotropic vs metabotropic. ionotropic receptors are ion channels that open & close within milliseconds & generate fast electrical signals (classic: glutamate AMPA, GABA-A, nicotinic ACh). metabotropic receptors are G-protein coupled, they take hundreds of milliseconds to minutes, & they mainly do modulation, ie how loud or quiet a neuron reacts to other signals. almost all monoamine receptors are metabotropic. that means dopamine & serotonin dont generate fast signals, they change how loudly other signals get heard. third point, maybe the most important one for understanding all the rest: the same transmitter can do the exact opposite depending on which receptor it hits. GABA-A inhibits fast, GABA-B inhibits slowly & via different mechanisms. glutamate at AMPA receptors excites acutely, at mGluR receptors it modulates synapse strength long term. serotonin has more than fourteen receptor subtypes, some calm anxiety, others trigger it. “more serotonin” is therefore meaningless without knowing at which receptor in which brain region. & finally the concept thats become central to modern psychiatry: the E/I balance, ie the balance between excitation (glutamate) & inhibition (GABA). most psychiatric symptoms are today better understood as disturbances of this dynamic balance at the circuit level, rather than too much or too little of any single substance

3 - The Neurotransmitters

3.1 - The Workhorses: Glutamate, GABA, Glycine

glutamate is the primary excitatory transmitter in the brain. roughly 80 to 90 percent of cortical synapses are glutamatergic. if learning & memory happen molecularly anywhere, its here, at NMDA & AMPA receptors, via the famous long-term potentiation. glutamate is quantitatively the language the brain speaks. all monoamines together, ie dopamine, serotonin, noradrenaline, histamine, make up a tiny fraction of synapses, which gets overlooked in lay coverage all the time. GABA is the exact opposite, the principal inhibitory transmitter. without GABAergic inhibition the system goes into seizure, & thats not metaphorical, thats the pharmacology of epilepsy. GABA-A receptors are chloride channels & at the same time the binding site of basically all clinically important sedatives & anxiolytics: benzos, barbiturates, neurosteroids, alcohol also has a modulatory action there. GABA-B is metabotropic, slower, its eg the target of baclofen. glycine is the dominant inhibitory transmitter in spinal cord & brainstem, but it has a beautiful second job: its an obligatory co-agonist at the NMDA receptor. without glycine or D-serine the NMDA receptor wont open, even with glutamate present. same molecule, two completely different jobs depending on where it shows up. exactly the principle from the previous section, same transmitter, totally different effect depending on receptor & location. take home: when you see “substance X increases serotonin” in a forum thread, thats about as informative as “substance X changes mood somewhere in the brain”. the actual work is being done by glutamate & GABA, quietly in the background, & modulating that work is the job of the more famous monoamines.

3.2 - The Modulators: Monoamines, Dopamine, Serotonin, Noradrenaline, Histamine

These four are what shows up in popular discourse all the time, even tho theyre quantitatively small systems. small here is not derogatory, their function is huge. but they work differently from glutamate or GABA, just a few small nuclei in the brainstem & midbrain send out widely branching axon trees into nearly the whole brain & set the tone there: how loud, how awake, how motivated, how anxious, how attentive a region currently runs. modulation, not the main signal. dopamine has four classical pathways: nigrostriatal (movement, dies in parkinsons), mesolimbic (motivation, reinforcement, cue salience), mesocortical (executive function, working memory), tuberoinfundibular (prolactin inhibition, which is why you get hyperprolactinemia under antipsychotics). what dopamine isnt: the happiness molecule. more on that in the myths section. serotonin is probably the single most oversimplified transmitter in existence. more than fourteen receptor subtypes, with partly opposite effects. 5-HT1A is associated with anxiolysis & antidepressant effect, 5-HT2A is the plasticity & psychedelic receptor, 5-HT2C regulates appetite & mood, 5-HT3 is the only ionotropic serotonin receptor & the target of ondansetron for nausea. more than 90 percent of body serotonin actually doesnt sit in the brain at all, its in the gut, in enterochromaffin cells, where it controls motility & secretion & signals to the brain via vagal afferents. the gut-brain axis is exactly that loop.

3.3 - Acetylcholine, the Bridge Transmitter

acetylcholine is the transmitter that bridges three worlds which textbooks usually keep neatly separate: central nervous system, autonomic nervous system, & the neuromuscular endplate. in the brain it projects mainly from the nucleus basalis of Meynert into cortex & controls attention & memory encoding. exactly this nucleus degenerates early in alzheimers, which is why acetylcholinesterase inhibitors like donepezil or rivastigmine are used as symptomatic treatment, they cant stop the disease but they push the cholinergic signal back up for a while. in the autonomic system acetylcholine is the transmitter at all preganglionic neurons & all postganglionic parasympathetic fibers. thats the rest-and-digest side, bradycardia, bronchoconstriction, salivation & gastric secretion, increased GI motility. & at the motor endplate finally its the transmitter that triggers muscle contraction, here via nicotinic receptors. precisely because ACh works at so many sites the toxicology here is didactically rich. acetylcholinesterase inhibitors, ie organophosphates from agriculture or nerve agents like sarin, prevent ACh breakdown & flood every cholinergic synapse simultaneously. clinically that means SLUDGE: salivation, lacrimation, urination, defecation, GI cramping, emesis, plus fasciculations, peripheral paralysis & central seizures. a cholinergic crisis is life threatening without antidote. the antidote is atropine, a competitive muscarinic antagonist, plus pralidoxime which regenerates the esterase. botulinum toxin works on the same system but in the exact opposite direction: it cleaves SNARE proteins & blocks ACh release. result, flaccid paralysis. clinically diluted thats the botulinum toxin therapy used for dystonia, spasticity, migraine & in cosmetics. curare & alpha-bungarotoxin block postsynaptic nicotinic receptors at the endplate & are historically the tools thru which neuromuscular transmission was actually understood. nice point for forum readers: donepezil & sarin are chemical cousins. both inhibit acetylcholinesterase. the difference is selectivity, reversibility & dose. pharmacology is always dose & context.

3.4 - Peptides & Atypical Messengers

neuropeptides are the largest & most heterogeneous family, more than a hundred have been described. endogenous opioids, ie beta-endorphin, enkephalins, dynorphins, bind at mu, delta, kappa & mediate pain inhibition, social bonding reward & the affective component of pain. substance P is a key pain transmitter. oxytocin & vasopressin are the textbook example of the blurry transmitter-vs-hormone line, they act centrally on social cognition, attachment & stress buffering, peripherally on labor, milk ejection, water balance. CRH, NPY, orexin & some others are clinically directly relevant, orexin for example is the transmitter thats lost in narcolepsy. important to know: peptides are almost always co-released with classical transmitters from the same cell, ie the same synapse can release fast glutamate & slow peptide simultaneously, on completely different timescales. thats one of the things that finally killed the old “one neuron, one transmitter” picture. & then there are atypical messengers like endocannabinoids, anandamide & 2-AG, which act as retrograde lipid messengers, ie from the postsynaptic neuron backwards onto the presynaptic, to dampen its own input. plus gases like nitric oxide that diffuse freely thru membranes & deliver messages where there isnt a classical synaptic cleft at all. nerdy stuff, but it shows how much the concept of “transmitter” has been softened by now.

4 - Myths

4.1 - Myth One: Dopamine Equals Happiness

the classic. probably the most stubborn myth in popular neuroscience as a whole. “dopamine makes you happy, chocolate gives a dopamine kick, likes give dopamine.” these sentences arent entirely wrong but they are misleading in a way that actually matters. what dopamine actually does fits roughly into two sentences. first, dopamine signals a reward prediction error, ie the difference between expected & actual reward. when something turns out better than expected, the mesolimbic dopamine system fires. when it turns out exactly as expected, it doesnt fire, even tho the reward is right there. thats the learning substrate the brain uses to figure out which behaviors are worth doing. shown elegantly in monkey experiments back in the nineties & replicated to the point of nausea since.

second, dopamine assigns “incentive salience”, roughly meaning how much pull a stimulus has on you. the concept is from kent berridge & it splits “wanting” from “liking”. wanting is the motivational pull, the i-need-this-now feeling, & thats dopamine mediated. liking is the actual hedonic experience, ie does it actually feel good, & that gets generated in tiny anatomical hotspots in the nucleus accumbens & ventral pallidum, mediated not by dopamine but by mu-opioids & endocannabinoids. practical demonstration: in animals where you selectively destroy mesolimbic dopamine, the willingness to work for sugar disappears, but the orofacial liking reactions to sucrose stay intact. they dont want it anymore, but if you put it in their mouth they enjoy it just as much. conversely, mu-opioid stimulation in a hotspot increases liking without changing wanting. this explains a lot. compulsive scrolling, gambling, porn, cigarettes despite fading enjoyment, anhedonic depression with retained drive vs retained experience, all of it becomes way more legible once you separate wanting from liking. dopamine is closer to “this is important, turn towards it & learn from it” than to “this feels nice”. happiness, if the word is even neurobiologically meaningful, lives more on the opioid & endocannabinoid side.

4.2 - Myth Two: Addiction Equals Dopamine

right after the first myth comes the second one, which usually gets told in the same breath: “addiction is a dopamine disorder, drugs hijack the reward system, thats why you cant stop.” its half true, & precisely because its half true it has stuck around for so long. true: practically all substances of abuse, from alcohol to nicotine to heroin, cocaine & methamphetamine, acutely raise mesolimbic dopamine release. they flood the system with a reward prediction signal of a magnitude that has nothing to do with the natural history of the brain. the consequence is a massive cue salience assignment, ie the cigarette pack, the slot machine sound, the smell in the regular bar hallway become high potency behavior triggers, long after the acute drug effect has worn off. thats the initial, dopaminergic chunk of addiction. what happens chronically tho is something else. chronic substance exposure changes glutamatergic plasticity at the connections between prefrontal cortex, nucleus accumbens & amygdala. the cystine-glutamate exchanger & the glial glutamate transporter GLT-1 get downregulated, tonic extrasynaptic glutamate drops, phasic synaptic glutamate during cue confrontation goes up disproportionately. dendritic spines in the accumbens get rebuilt, the top-down control of prefrontal cortex over striatal drug seeking gets weaker. thats what makes the chronic, repeatedly relapsing form of addiction. its a glutamatergic plasticity disorder, & its substantially harder to treat than the initial dopaminergic high. a short formula that should stick on a forum: dopamine initiates, glutamate consolidates. its also the reason why treatments that purely target dopamine, eg naltrexone in alcohol use disorder, often disappoint in practice, while substances that intervene in glutamate homeostasis, like N-acetylcysteine in some studies, or acamprosate, theoretically fit the mechanism better. clinically the problem is far from solved obviously, but the understanding has moved forward. for the forum discussion this means: someone with a manifest substance use disorder doesnt have “too much” or “too little” dopamine. they have a permanently rebuilt glutamatergic network that responds to certain cues the way it should respond to life threat. thats neurobiologically the more honest description, & it also takes a chunk of moral stigma out, because “cant just stop” is here to be taken literally.

4.3 - Myth Three: Serotonin Deficit Equals Depression

probably the psychiatrical most important myth, because its been used millions of times in patient education & advertising. the story: depression is caused by a serotonin deficit, SSRIs raise the serotonin level, patient gets better. nice story, clean, causal, easy to market. the problem, it isnt sufficiently supported & basically never was. in 2022 an umbrella review by joanna moncrieff & colleagues was published in molecular psychiatry, synthesizing the evidence from six domains: serotonin & 5-HIAA levels in body fluids, 5-HT1A receptor binding, SERT imaging & post-mortem findings, tryptophan depletion studies, SERT polymorphisms, & gene-environment interactions. conclusion: no consistent evidence that depression is caused by a lowered serotonin concentration or activity. that caused considerable noise in press releases, patient conversations & also in academic twitter. the reply came promptly, a 36 author response in the same journal criticized the methodology, particularly selective inclusion, lack of own data analysis & the handling of receptor pharmacology. both sides have a point. the honest reading i find tenable for a forum with a mixed audience: the simple “depression equals serotonin deficit” picture is not defensible, never was, & was to no small degree marketing of the nineties & 2000s. at the same time the serotonin system is still deeply involved in mood regulation, & SSRIs do work better than placebo, particularly in moderate to severe depression. so what do they actually do, if not topping up missing serotonin: the most likely current answer is they boost neuroplasticity. they upregulate BDNF & its receptor tyrosine kinase TrkB, they promote hippocampal neurogenesis, they even appear to reopen developmentally-similar critical periods in which the brain can be rebuilt. a beautiful paper from 2021 even showed that fluoxetine & imipramine bind directly to TrkB & allosterically enhance BDNF signaling, ie they do more than just flood the synapse with serotonin. at the network level functional imaging studies show that default mode network connectivity, which in depression is typically hyperconnected & ruminative, normalizes during a successful SSRI course. meaning, SSRIs let the brain relearn, theyre less a chemical compensation than a pharmacological permission for plasticity. what gets done with this permission depends on therapy, life circumstances & relationships, ie very largely on psychology & social context.

4.4 - Myth Four: Endorphins Make the Runners High

everyone knows the line, run long enough & the endorphins kick in, thats the famous runners high. sounds good af, gets repeated in every other sports article, & by current evidence its probably wrong on the point that matters most to people. two problems. first: beta-endorphin is a 31 amino acid peptide & does not cross the blood brain barrier in any meaningful amount. so when endorphin in plasma rises during running, which it does, the stuff doesnt actually arrive centrally, at least not in concentrations that would explain the euphoric mood change. second, in 2015 fuss & colleagues showed cleanly in mice that the anxiolytic & analgesic effects of running depend on CB1 cannabinoid receptors on forebrain GABAergic neurons, not on opioid receptors. pharmacological blockade of the cannabinoid system abolished the effect, opioid blockade did not. anandamide levels in plasma rise substantially under exertion, & anandamide is lipid soluble, so it gets centrally without trouble. a double blind placebo controlled treadmill study in humans from 2021 by the same group supports this, with the honest comment that the purely euphoric component is harder to pin down than the anxiolytic one. recent reviews summarize the current state along roughly the same lines: the runners high is primarily endocannabinoid mediated, endorphins probably play a smaller secondary role, & monoamines like dopamine & noradrenaline certainly contribute as well. for the forum discussion: endorphins arent wrong, theyre just not the central driver. so when someone says “exercise makes you happy because of endorphins” thats culturally well established but biologically probably outdated. endocannabinoids are the more honest answer, & whats kinda funny is that the endogenous cannabinoid system gets a function here thats almost the opposite of what most people associate with the word cannabis, ie not sedation but adaptive regulation under stress.

5 - Where We Actually Are, Modern Psychiatry

psychiatry has shifted pretty fundamentally over the last two decades, even if that hasnt really landed in popular perception yet. individual transmitters & their concentrations are not the central language anymore. instead three concepts that together make up the more current picture. first, networks. psychiatric symptoms map better onto altered connectivity of large networks than onto single substances. the default mode network, ie the regions active during inward directed thought, is typically hyperconnected in depression, which fits the phenomenology of rumination. the salience network, which switches between inner & outer focus, is dysregulated in schizophrenia. the executive network, prefrontal-parietal, has weaker control in ADHD. successful treatments, whether SSRI, ketamine, psilocybin in trials, TMS or ECT, reorganize these networks. second, plasticity. its the red thread running thru everything weve just talked about. antidepressants as plasticity promoters, learning as a glutamatergic phenomenon, addiction as a plasticity disorder, trauma as pathologically consolidated plasticity, therapy as targeted re-plasticity induction. BDNF-TrkB signaling, dendritic spine remodeling, hippocampal neurogenesis, reopening of critical periods. theres a beautiful framing that antidepressants dont fix the problem, they put the brain into the mode where it can be rebuilt by experience again. what then happens depends on the experience. third, ketamine & psychedelics as a paradigm break. ketamine in subanesthetic dose works antidepressively within hours, via NMDA antagonism, glutamate surge, AMPA receptor activation, BDNF release, mTORC1, synaptogenesis. that broke the pure monoamine paradigm in 2010 once & for all. psychedelics of the 5-HT2A class, psilocybin, LSD, DMT, are described as so-called psychoplastogens, a single dose produces lasting dendritic spine & synaptogenesis effects on cortical pyramidal neurons. clinical trials are promising but methodologically tricky, because blinding doesnt really work given the obvious effects, & expectancy effects are huge. no hype please, but also no reflexive rejection, the field is moving. & in classification: ICD-11, in force since 2022, frames psychiatric disorders explicitly biopsychosocioculturally & developmentally, with dimensional elements particularly in personality disorders & a separate group for stress associated disorders, including complex PTSD as its own entity. thats not just bureaucracy, thats an attitude shift.

6 - Closing

if i had to condense all of this into one sentence to leave on the forum, it might be this one: brain chemistry is real & important, but brain chemistry is never a single dial. its a dynamic network of excitation & inhibition, fast & slow signals, local & diffusing messages, which together are the substrate for what we call experience, learning, feeling, wanting & remembering. so the next time you read that dopamine makes you happy, that addiction is just dopamine, that depression is a serotonin deficit or that endorphins make exercise nice, you now know: each contains a grain of truth, but as explanations theyre about as useful as saying a car drives because theres petrol inside. true, but a tiny slice. & this tiny slice understanding is precisely what makes mental illness so hard to really understand, destigmatize & treat well. the good news: the field is much further along than the popular slogans suggest. plasticity, networks, E/I balance, psychoplastogens, integrative biopsychosocial models, those are the concepts modern psychiatry actually works with. & how we live, love, learn & care for each other is part of this biology, not its opposite.