Neuropharmacology: Neurotransmitter Systems (Acetylcholine, Catecholamines, Glutamate, GABA, and Serotonin)

Comprehensive vocabulary flashcards covering acetylcholine, catecholamines (dopamine and norepinephrine), glutamate, GABA, and serotonin synthesis, storage, receptors, pathways, and pharmacology.

Choline Acetyltransferase (ChAT)

The specific enzyme found exclusively in cholinergic neurons that synthesizes acetylcholine in a single step from choline and acetyl coenzyme A ($\text{acetyl CoA}$). Its rate of synthesis depends on precursor availability and cell firing rate, and it has no known pharmacological inhibitors.

Vesicular ACh Transporter (VAChT)

The protein responsible for loading acetylcholine into synaptic vesicles in the presynaptic terminal, which can be blocked by the experimental drug vesamicol to decrease acetylcholine release upon nerve firing.

Vesamicol

An experimental drug that selectively blocks the vesicular acetylcholine transporter ($\text{VAChT}$), thereby preventing acetylcholine from entering synaptic vesicles and reducing the amount released during neurotransmission.

Botulinum Toxin

A bacterial toxin produced by the anaerobic bacterium Clostridium botulinum (types A through G) that blocks acetylcholine release at peripheral cholinergic terminals, including the neuromuscular junction, by preventing synaptic vesicle fusion with the terminal membrane, resulting in muscle paralysis.

Black Widow Spider Venom

A toxin that induces massive, uncontrolled release of acetylcholine in the peripheral nervous system.

Forms of Acetylcholinesterase (AChE)

Three molecular arrangements of the enzyme that hydrolyzes acetylcholine into choline and acetic acid: soluble $G_4$ (free tetramers inside the cell), membrane-bound $G_4$ (tetramers tethered to the external membrane), and the $A_{12}$ asymmetric form (12 catalytic subunits anchored by a collagen-like tail at the neuromuscular junction for ultra-rapid inactivation).

Hemicholinium-3 (HC-3)

A pharmacological compound that selectively blocks high-affinity choline reuptake transporters on presynaptic cholinergic nerve terminals, reducing intracellular choline availability and declining acetylcholine synthesis.

Physostigmine (Eserine)

A reversible acetylcholinesterase inhibitor naturally derived from plants that readily crosses the blood–brain barrier to exert central cholinergic effects; used topically to treat glaucoma but produces systemic toxicity.

Organophosphorus Compounds

Irreversible acetylcholinesterase inhibitors used as agricultural insecticides and chemical warfare nerve agents (e.g., sarin and soman) that permanently phosphorylate the enzyme, causing lethal hyperstimulation of cholinergic transmission.

Pyridostigmine (Mestinon)

A synthetic reversible acetylcholinesterase inhibitor that does not cross the blood–brain barrier; used therapeutically for myasthenia gravis and deployed pre-treatment as an antidote to temporarily shield the enzyme from irreversible nerve agents.

Myasthenia Gravis

An autoimmune disorder characterized by severe muscle weakness and fatigue caused by circulating autoantibodies directed against nicotinic acetylcholine receptors at the neuromuscular junction, resulting in receptor loss and impaired muscle contraction.

Autonomic Cholinergic Synapses

The structural arrangement wherein all preganglionic autonomic neurons release acetylcholine onto ganglionic nicotinic receptors, parasympathetic ganglionic neurons release acetylcholine onto target organ muscarinic receptors, and sympathetic ganglionic neurons release norepinephrine.

Basal Forebrain Cholinergic System (BFCS)

A major subcortical network comprising cholinergic projection neurons in the medial septum, diagonal band nuclei, substantia innominata, and nucleus basalis that innervate the neocortex, hippocampus, and limbic structures to mediate declarative memory and sustained attention.

Laterodorsal and Pedunculopontine Tegmental Nuclei (LDTg and PPTg)

Cholinergic nuclei located in the dorsolateral pons that project to the thalamus, basal ganglia, lower brainstem, and midbrain dopamine neurons, playing essential roles in sleep architecture (initiation of REM sleep) and reward signaling.

Nicotinic Acetylcholine Receptor (nAChR)

A pentameric ligand-gated ionotropic receptor family consisting of combinations of $\alpha$, $\beta$, $\gamma$, $\delta$, and $\varepsilon$ subunits that mediates fast excitatory transmission by opening an intrinsic channel permeable to $\text{Na}^+$ and $\text{Ca}^{2+}$.

Depolarization Block

A loss of membrane resting potential that occurs during continuous agonist exposure when non-desensitized nicotinic receptors persistently depolarize the cell, rendering the membrane unable to generate subsequent action potentials until repolarization occurs.

D-Tubocurarine

The active, paralyzing alkaloid derived from curare that acts as a competitive antagonist selectively at muscle-type nicotinic acetylcholine receptors.

Muscarinic Acetylcholine Receptors

A family of five metabotropic receptor subtypes ($M_1$–$M_5$), where $M_1$, $M_3$, and $M_5$ couple to $G_q$ to stimulate phosphoinositide metabolism, and $M_2$ and $M_4$ couple to $G_i$ to inhibit adenylyl cyclase and open $\text{K}^+$ channels.

Tyrosine Hydroxylase (TH)

The initial, rate-limiting enzyme in catecholamine synthesis that converts L-tyrosine to L-DOPA. Its activity is subject to negative end-product feedback inhibition by DA and NE and activation via phosphorylation by protein kinases during cell firing.

$\alpha$-Methyl-para-tyrosine (AMPT)

A pharmacological inhibitor that selectively blocks tyrosine hydroxylase ($\text{TH}$), thereby halting the synthesis of dopamine, norepinephrine, and epinephrine.

Vesicular Monoamine Transporter (VMAT)

A transmembrane vesicular protein ($\text{VMAT2}$ in brain) that packages dopamine, norepinephrine, and serotonin into synaptic vesicles, protecting them from cytoplasmic enzymatic degradation.

Reserpine

An irreversible drug blocker of the vesicular monoamine transporter ($\text{VMAT}$) that depletes intracellular monoamine stores, causing marked sedation and depressive symptoms in animals and humans.

Varicosities Along Monoamine Axons

Repeated, bead-like axon swellings forming en passant ('in passing') synapses filled with vesicles along the length of dopamine and norepinephrine fibers, though only approximately $30\%$ possess specialized active zones.

Catecholamine Enzymatic Breakdown Pathways

The metabolic degradation mediated by monoamine oxidase ($\text{MAO}$) and catechol-O-methyltransferase ($\text{COMT}$), which yields homovanillic acid ($\text{HVA}$) from dopamine, and 3-methoxy-4-hydroxy-phenylglycol ($\text{MHPG}$) in the brain or vanillymandelic acid ($\text{VMA}$) in the PNS from norepinephrine.

Ascending Dopamine Pathways

Three major central projection systems: the nigrostriatal tract ($\text{A9}$ in substantia nigra to caudate-putamen/dorsal striatum), the mesolimbic pathway ($\text{A10}$ in VTA to nucleus accumbens and olfactory tubercle), and the mesocortical pathway ($\text{A10}$ in VTA to prefrontal cortex).

Signaling Mechanisms of $D_1$ and $D_2$ Receptors

The divergent intracellular cascades of dopamine receptors: $D_1$-like receptors couple to $G_s$ to stimulate adenylyl cyclase and elevate $\text{cAMP}$, whereas $D_2$-like receptors couple to $G_i$ to inhibit adenylyl cyclase, decrease $\text{cAMP}$ formation, and open $\text{K}^+$ channels.

Dopamine-Deficient (DD) Mouse

A genetically modified animal model in which the gene for tyrosine hydroxylase is knocked out while dopamine $\beta$-hydroxylase remains intact, producing severe aphagia, adipsia, and hypoactivity that can be reversed by L-DOPA.

Behavioral Supersensitivity

An exaggerated behavioral response to dopamine agonists that develops following chronic administration of $D_2$ receptor antagonists (such as haloperidol), caused by the up-regulation and increased density of postsynaptic $D_2$ receptors.

Locus Coeruleus Noradrenergic Pathways

The principal central noradrenergic nucleus ($\text{A6}$ group) located in the dorsal pons whose axons project broadly throughout the cortex, hippocampus, thalamus, cerebellum, and spinal cord to modulate arousal, vigilance, and sensory processing.

Adrenergic Receptor Families

Metabotropic receptors comprising $\beta_1$ and $\beta_2$ (which couple to $G_s$ to stimulate adenylyl cyclase and increase $\text{cAMP}$), $\alpha_2$ (which couples to $G_i$ to reduce $\text{cAMP}$ and increase $\text{K}^+$ conductance), and $\alpha_1$ (which stimulates the phosphoinositide system to elevate intracellular $\text{Ca}^{2+}$).

Clonidine

An $\alpha_2$-adrenergic agonist that stimulates autoreceptors to inhibit noradrenergic cell firing, used to treat hypertension and suppress autonomic hyperactivity during opioid withdrawal.

Yohimbine

An $\alpha_2$-adrenergic antagonist that blocks autoreceptors, provoking noradrenergic cell firing, elevating norepinephrine release, and inducing feelings of anxiety and withdrawal symptoms.

Guanfacine

A selective $\alpha_2$-adrenoceptor agonist that enhances cognitive operations and working memory in the prefrontal cortex by optimizing post-synaptic noradrenergic tone without the adverse activation of lower-affinity $\alpha_1$ receptors.

Propranolol (Inderal)

A nonselective $\beta$-adrenergic antagonist that lowers blood pressure by reducing heart contractile force, and is additionally prescribed to treat somatic anxiety and generalized anxiety disorder.

Glutamate–Glutamine Cycle

The metabolic cooperation between astrocytes and glutamatergic neurons, where astrocytic EAATs clear synaptically released glutamate, glutamine synthetase converts it to glutamine, and neuronal glutaminase converts transported glutamine back into glutamate.

Excitatory Amino Acid Transporter 2 (EAAT2)

An astrocytic glutamate transporter responsible for approximately $90\%$ of total glutamate clearance from the extracellular space in the brain; its genetic deletion leads to fatal spontaneous seizures and shortened life spans.

Ion Permeabilities of Ionotropic Glutamate Receptors

The channel conductance profiles of fast excitatory receptors: AMPA and kainate receptors primarily admit $\text{Na}^+$, whereas NMDA receptors are uniquely permeable to both $\text{Na}^+$ and substantial amounts of $\text{Ca}^{2+}$.

NMDA Receptor Properties and Binding Sites

A specialized heterotetrameric channel requiring simultaneous agonist (glutamate) and co-agonist (glycine or D-serine) binding, relief of resting $\text{Mg}^{2+}$ channel blockade by membrane depolarization, and containing an uncompetitive antagonist site that binds PCP, ketamine, and MK-801.

NBQX

A selective competitive antagonist at AMPA and kainate ionotropic glutamate receptors that does not inhibit NMDA receptors, clinically causing sedation, motor ataxia, and anticonvulsant protection in animal tests.

Hippocampal Trisynaptic Circuit of LTP

The hippocampal pathways widely used to investigate long-term potentiation: perforant path fibers from entorhinal cortex project to dentate gyrus granule cells, mossy fibers synapse onto CA3 pyramidal neurons, and Schaffer collaterals project from CA3 to CA1 pyramidal cells.

Silent Synapse

A glutamatergic synapse expressing functional NMDA receptors but lacking surface AMPA receptors; converted into a functional transmitting synapse during LTP through $\text{Ca}^{2+}$/CaMKII-mediated AMPA receptor insertion.

Excitotoxicity Hypothesis

The principle that excessive, prolonged exposure to extracellular glutamate triggers excessive postsynaptic depolarization and massive $\text{Ca}^{2+}$ influx, leading to cellular lysis (necrosis) or programmed cell death (apoptosis).

Glutamic Acid Decarboxylase (GAD)

The biosynthetic enzyme found exclusively in GABAergic neurons that synthesizes $\gamma$-aminobutyric acid ($\text{GABA}$) in one step from glutamate.

Vesicular GABA Transporter (VGAT)

The membrane transport protein that packages both GABA and glycine into synaptic vesicles for vesicular exocytosis.

Tiagabine (Gabitril)

A prescription antiepileptic drug that selectively blocks the high-affinity plasma membrane transporter GAT-1, elevating extracellular GABA levels to enhance inhibitory neurotransmission.

Cycling of GABA between Neurons and Astrocytes

The metabolic fate of synaptically released GABA: clearance via GAT-1/2/3, breakdown by GABA-T into glutamate and succinate, conversion to glutamine by astrocytic glutamine synthetase, and transport back into neurons for GAD-mediated resynthesis.

$\text{GABA}_A$ Receptor Subunits and Modulatory Binding Sites

A pentameric ligand-gated chloride channel (typically $2\alpha, 2\beta, 1\gamma$) containing an intrinsic ion pore and discrete allosteric binding domains for neurosteroids, benzodiazepines, barbiturates, ethanol, and picrotoxin.

Bicuculline

A competitive antagonist at the GABA-binding site of the $\text{GABA}_A$ receptor that blocks chloride conductance, inducing severe systemic epileptic convulsions.

$\text{GABA}_B$ Receptor

A metabotropic inhibitory receptor that suppresses neuronal firing postsynaptically by opening $\text{K}^+$ channels and inhibits neurotransmitter release presynaptically by closing $\text{Ca}^{2+}$ channels and inhibiting adenylyl cyclase.

Baclofen (Lioresal)

A selective agonist at metabotropic $\text{GABA}_B$ receptors used clinically as a central muscle relaxant.

Serotonin Synthesis Pathway

The two-step enzymatic reaction wherein dietary L-tryptophan is hydroxylated by tryptophan hydroxylase ($\text{TPH}$) into 5-hydroxytryptophan ($\text{5-HTP}$), which is then decarboxylated by aromatic L-amino acid decarboxylase ($\text{AADC}$) into 5-hydroxytryptamine ($\text{5-HT}$).

TPH1 and TPH2

The two distinct isoforms of tryptophan hydroxylase: $\text{TPH2}$ is expressed exclusively in serotonergic neurons of the central nervous system, whereas $\text{TPH1}$ is expressed in nonneuronal peripheral cells such as gut enterochromaffin cells and the pineal gland.

Large Neutral Amino Acids (LNAA)

Amino acids that compete with tryptophan for the shared transporter across the blood–brain barrier; meals high in carbohydrates and low in protein stimulate insulin secretion to drive competing LNAAs into muscle, elevating the tryptophan/LNAA ratio and enhancing brain 5-HT synthesis.

Para-chlorophenylalanine (PCPA)

A classic pharmacological tool that selectively and irreversibly inhibits tryptophan hydroxylase, depleting endogenous serotonin levels in experimental animals.

Serotonin Transporter (SERT)

The high-affinity plasma membrane carrier on presynaptic serotonergic neurons that removes 5-HT from the synaptic cleft into the cytoplasm, representing the primary molecular target of selective serotonin reuptake inhibitors (SSRIs).

Serotonergic Pathways from Raphe Nuclei

The anatomical organization of central 5-HT neurons distributed along the brainstem midline ($\text{B1}$–$\text{B9}$), with the dorsal raphe nucleus ($\text{DRN/B7}$) and median raphe nucleus ($\text{MRN/B8}$) providing nearly all serotonergic ascending projections to the forebrain.

$5\text{-HT}_{1A}$ Receptor

A metabotropic inhibitory receptor subtype concentrated in the hippocampus, amygdala, and raphe nuclei that inhibits adenylyl cyclase and opens $\text{K}^+$ channels, acting as both a somatodendritic autoreceptor and a postsynaptic receptor that reduces anxiety.

$5\text{-HT}_{2A}$ Receptor

A metabotropic receptor expressed abundantly in neocortex that stimulates the phosphoinositide second-messenger system, increasing intracellular $\text{Ca}^{2+}$ and activating protein kinase C; it serves as the target mediating the hallucinogenic actions of LSD and DOI.

Serotonin Deficiency Hypothesis of Aggression

The neurochemical theory positing that low central serotonergic activity is directly linked to increased impulsive or reactive aggression, supported by TPH2-knockout mouse models displaying hyperaggressiveness that is reversible by 5-HTP.

Hypothalamic Circuitry of Serotonergic Appetite Regulation

The neural mechanism in the arcuate nucleus wherein 5-HT inputs excite POMC neurons via $5\text{-HT}_{2C}$ receptors to release $\alpha\text{-MSH}$ onto anorexigenic $MC_{3/4}$ receptors, while simultaneously inhibiting orexigenic NPY/AgRP neurons via $5\text{-HT}_{1B}$ receptors to reduce food intake.

Enterochromaffin (EC) Cells

Specialized mucosal cells of the enteric nervous system in the gut that synthesize the vast majority of the body's serotonin via TPH1 and release it in response to food stimulation to stimulate gut peristalsis.