Neurotransmitter Framework, Signaling Systems, and Receptor Dynamics

A comprehensive collection of 100 vocabulary flashcards covering the neurotransmitter framework, catecholamines, serotonin, amino acids, acetylcholine, receptor structures, and metabotropic signaling pathways based on Lecture 6.

Neurotransmitter Criterion 1: Synthesis and Inactivation

The presynaptic neuron must synthesize and store the chemical messenger, and a specific mechanism (enzymatic degradation or cellular reuptake) must exist to terminate the signal by clearing it from the synapse.

Neurotransmitter Criterion 2: Stimulated Release

Experimental stimulation of the presynaptic neuron must produce measurable, $\text{Ca}^{2+}$-dependent release of the chemical from the axon terminal.

Neurotransmitter Criterion 3: Receptor Availability

The postsynaptic target cell must express specific receptors designed to detect and bind the released chemical messenger.

Neurotransmitter Criterion 4: Functional Mimicry

Direct experimental application of the exogenous chemical onto the postsynaptic cell must produce a physiological response identical to that evoked by natural presynaptic release.

Neurotransmitter Criterion 5: Antagonist Blockade

The actions of the chemical messenger must be prevented by the application of a receptor-specific antagonist, confirming that its effects are mediated by specific receptor pathways.

Functional Mimicry Electrophysiological Testing

An experimental approach where a presumptive neurotransmitter is applied via a drug-containing micropipette while a microelectrode measures membrane potential ($V_m$) changes to compare them against presynaptic stimulation.

Agonist

A pharmacological agent or endogenous ligand that binds to a receptor and activates it, triggering the receptor's characteristic biological response.

Antagonist

A molecule that binds to a receptor without activating it, thereby blocking or inhibiting the actions of endogenous ligands or agonists.

Ketamine

A dissociative anesthetic that acts as an antagonist of the NMDA subtype of glutamate receptor and is used in clinical treatments for treatment-resistant depression.

Nicotine

A plant alkaloid derived from tobacco that functions as an agonist at ionotropic nicotinic acetylcholine receptors.

"-ergic" Suffix

A neuroscientific naming convention appended to neurotransmitter names to describe the entire signaling system, neurons, or pathways utilizing that specific transmitter (e.g., dopaminergic, cholinergic).

Monoamines (Receptor Profiles)

A transmitter class comprising dopamine, norepinephrine, epinephrine, and serotonin that signals primarily through metabotropic (GPCR) receptors, with few ionotropic exceptions such as the $5\text{-HT}_3$ receptor.

Amino Acid Transmitters (Receptor Profiles)

A ubiquitous transmitter class comprising glutamate, GABA, and glycine that signals through a combination of both fast ionotropic channels and modulatory metabotropic receptors.

Acetylcholine Class (Receptor Profiles)

A neurotransmitter family consisting of acetylcholine (ACh) that utilizes a combination of fast ionotropic (nicotinic) and slower metabotropic (muscarinic) receptors.

Rate-Limiting Step

The slowest enzymatic conversion in a metabolic pathway that determines and restricts the overall rate of neurotransmitter synthesis.

Tyrosine

The precursor amino acid that initiates the catecholamine biosynthetic pathway.

Tyrosine Hydroxylase (TH)

The rate-limiting cytosolic enzyme in catecholamine biosynthesis that catalyzes the conversion of tyrosine to L-Dopa.

L-DOPA (L-Dihydroxy-phenylalanine)

The intermediate catecholamine metabolite synthesized from tyrosine by tyrosine hydroxylase and decarboxylated into dopamine.

Dopa Decarboxylase

The cytosolic enzyme that converts L-DOPA into dopamine during catecholamine synthesis.

Dopamine (DA)

A catecholamine neurotransmitter produced by the decarboxylation of L-DOPA, acting both as an independent signaling molecule and as the metabolic precursor to norepinephrine.

Dopamine-beta-hydroxylase (DBH)

The enzyme localized exclusively inside synaptic vesicles that converts dopamine into norepinephrine in noradrenergic neurons.

Norepinephrine (NE)

A catecholamine neurotransmitter synthesized from dopamine by DBH, serving as the primary transmitter of the sympathetic nervous system and the locus coeruleus.

Phenylethanolamine N-methyltransferase (PNMT)

The cytosolic enzyme found in adrenergic neurons and adrenal chromaffin cells that converts norepinephrine into epinephrine.

DBH Vesicular Compartmentalization

The unique anatomical restriction of dopamine-beta-hydroxylase to the interior of synaptic vesicles, requiring cytosolic dopamine to be imported before it can be converted to norepinephrine.

Cytosolic Catecholamine Enzymes

The three catecholamine synthesizing enzymes located in the cytoplasm of the axon terminal: tyrosine hydroxylase (TH), Dopa decarboxylase, and PNMT.

Calcium Regulation of Tyrosine Hydroxylase

An activity-dependent regulatory mechanism whereby elevated intracellular $\text{Ca}^{2+}$ levels resulting from high synaptic firing stimulate and enhance TH catalytic activity.

Morphine Regulation of Tyrosine Hydroxylase

An adaptive transcriptional regulation where chronic morphine exposure significantly upregulates TH expression at the RNA level in the locus coeruleus.

Dopaminergic Midbrain Hubs (SNr and VTA)

Midbrain nuclei (Substantia Nigra and Ventral Tegmental Area) that express TH and Dopa decarboxylase to produce dopamine, but lack DBH.

Noradrenergic Hub (Locus Coeruleus)

A primary noradrenergic brainstem nucleus expressing TH, Dopa decarboxylase, and DBH, converting all synthesized dopamine into norepinephrine.

Vesicular Monoamine Transporters (VMAT1 and VMAT2)

Transporter proteins that sequester cytosolic monoamines into synaptic vesicles against a steep concentration gradient.

VMAT Stoichiometry and Mechanism

An active antiporter transport mechanism in which VMAT moves one monoamine molecule into the vesicle in exchange for two $\text{H}^+$ ions moving down their gradient into the cytosol.

Dopamine Transporter (DAT)

A plasma membrane $\text{Na}^+$-dependent symporter that uses the electrochemical gradient of sodium to clear dopamine from the synaptic cleft back into the terminal.

Norepinephrine Transporter (NET)

A plasma membrane $\text{Na}^+$-dependent symporter that terminates noradrenergic signaling by transporting synaptic norepinephrine back into the presynaptic terminal.

Monoamine Oxidase (MAO)

The primary intracellular degradative enzyme localized to the outer mitochondrial membrane that metabolizes unvesiculated monoamines in the presynaptic terminal.

Nigrostriatal Pathway

A dopaminergic projection originating in the Substantia Nigra and terminating in the dorsal striatum, essential for voluntary motor control.

Mesolimbic Pathway

A dopaminergic pathway projecting from the Ventral Tegmental Area (VTA) to the Nucleus Accumbens, playing a central role in reward, motivation, and addiction.

Mesocortical Pathway

A dopaminergic projection from the VTA to the prefrontal cortex involved in executive function, decision-making, and cognitive control.

Tuberoinfundibular Pathway

A dopaminergic circuit projecting from the hypothalamus to the pituitary gland to regulate neuroendocrine secretion.

Locus Coeruleus Projections

Extensive noradrenergic projections innervating the prefrontal cortex to manage arousal and the amygdala/hippocampus to modulate fear conditioning and escape behaviors.

Signal-to-Noise Ratio (Norepinephrine System)

A functional modulation whereby norepinephrine suppresses background spontaneous firing while enhancing responses to strong sensory stimuli, improving input clarity.

Autonomic Role of Norepinephrine

Functions as the principal chemical transmitter released by postganglionic sympathetic fibers of the autonomic nervous system to drive the "fight-or-flight" response.

Epinephrine Distribution (Peripheral vs Central)

Found centrally in only a small subset of brainstem neurons; primarily synthesized peripherally by chromaffin cells in the adrenal gland to regulate cardiac and respiratory function during stress.

Parkinson’s Disease

A neurodegenerative pathology marked by the progressive degeneration of dopaminergic neurons in the nigrostriatal pathway, resulting in striatal dopamine depletion, bradykinesia, resting tremors, and rigidity.

L-DOPA Prodrug Therapy

A pharmacological strategy for Parkinson's disease utilizing L-DOPA as a metabolic precursor that can cross the blood-brain barrier (which dopamine cannot) to replenish striatal dopamine.

Serotonin (5-Hydroxytryptamine / 5-HT)

An indoleamine monoamine neurotransmitter synthesized from tryptophan that stabilizes emotional tone, mood, and homeostatic physiological states.

Tryptophan

An essential dietary amino acid (found in salmon, eggs, and turkey) that serves as the metabolic precursor for serotonin biosynthesis.

Tryptophan Hydroxylase (TPH)

The rate-limiting enzyme in CNS serotonin synthesis that converts L-tryptophan to 5-hydroxytryptophan (5-HTP), serving as a definitive molecular marker for serotonergic neurons.

TPH Phosphorylation Regulation

The dynamic regulation of central tryptophan hydroxylase activity through phosphorylation by protein kinases in response to intracellular signaling cascades.

5-HTP and 5-HTP Decarboxylase

The intermediate metabolite 5-hydroxytryptophan (5-HTP) and the enzyme that rapidly decarboxylates it into serotonin (5-HT).

Gut-Brain Serotonin Compartmentalization

The physiological distribution wherein $90\text{--}95\%$ of the body's serotonin is synthesized in the gastrointestinal tract and cannot cross the blood-brain barrier, making CNS levels entirely dependent on local synthesis from tryptophan.

Serotonin Transporter (SERT)

The plasma membrane transporter responsible for the primary reuptake and clearance of synaptic serotonin back into the presynaptic terminal.

5-HT3 Receptor

The only known ionotropic serotonin receptor subtype, functioning as a fast, ligand-gated cation channel rather than a G-protein-coupled receptor.

Raphe Nuclei

Midline brainstem clusters where serotonergic cell bodies are concentrated, giving rise to expansive ascending and descending projections across the neuroaxis.

Raphe Nuclei Sleep-Wake Firing Patterns

The state-dependent firing profile of serotonergic neurons, showing maximal activity during waking, reduced firing during slow-wave sleep (SWS), and complete quiescence during REM sleep.

Cortical Area 17 Serotonergic Modulation

The modulatory control exerted by serotonin over the firing properties and responsiveness of primary visual cortex neurons to visual sensory stimuli.

Selective Serotonin Reuptake Inhibitors (SSRIs)

Antidepressant and anxiolytic drugs (e.g., fluoxetine, sertraline) that selectively inhibit SERT to prolong the synaptic residence time of serotonin.

Excitatory-Inhibitory Balance

The essential neuroprotective homeostatic equilibrium between excitatory glutamate and inhibitory GABA/glycine signaling necessary to maintain neural function and prevent excitotoxicity.

Excitotoxicity

A pathological cascade of excessive neuronal overstimulation and death triggered by impaired glutamate clearance, frequently occurring in stroke and ischemia.

Glutamic Acid Decarboxylase (GAD)

The synthesizing enzyme that converts glutamate to GABA; serves as a definitive marker for inhibitory neurons because it is absent from glutamatergic neurons and glia.

VGLUT1-3

Vesicular glutamate transporter isoforms that sequester glutamate into synaptic vesicles using an antiporter mechanism driven by a proton ($\text{H}^+$) gradient.

VIAAT (Vesicular Inhibitory Amino Acid Transporter)

A vesicular antiporter that uses a proton ($\text{H}^+$) electrochemical gradient to load both GABA and glycine into synaptic vesicles.

Excitatory Amino Acid Transporters (EAATs)

Plasma membrane cotransporters on neurons and astrocytes that clear extracellular glutamate by coupling one glutamate molecule to the inward movement of two $\text{Na}^+$ ions.

GABA Transporters (GATs)

Plasma membrane symporters that terminate GABAergic signaling by transporting GABA into cells using the electrochemical energy of two cotransported $\text{Na}^+$ ions.

Acetylcholine (ACh)

The primary neurotransmitter of the parasympathetic "rest and digest" system and the obligate transmitter driving skeletal muscle contraction at the neuromuscular junction.

Choline and Acetyl-CoA

The two biochemical precursors of acetylcholine, consisting of extracellularly derived choline and mitochondrially generated Acetyl-CoA.

Choline Acetyltransferase (ChAT)

The enzyme that synthesizes acetylcholine in the presynaptic terminal by transferring an acetyl group from Acetyl-CoA to choline.

Rate-Limiting Step of ACh Synthesis

The $\text{Na}^+$-dependent high-affinity reuptake of choline from the extracellular fluid back into the presynaptic terminal.

Acetylcholinesterase (AChE)

A high-velocity enzyme situated in the synaptic cleft that terminates cholinergic signaling by cleaving acetylcholine into acetic acid and choline.

Basal Forebrain Cholinergic System

A major projection system originating in the basal forebrain and terminating in the neocortex and hippocampus to modulate attention, memory, and cognitive arousal.

Brainstem Cholinergic System

A cholinergic network projecting to the thalamus that regulates thalamocortical sensory gating, sustained attention, and the induction of REM sleep.

Neuromuscular Junction (NMJ) Nicotinic Signaling

Rapid excitatory cholinergic transmission where motor neurons release ACh onto ionotropic nicotinic receptors to cause fast skeletal muscle contraction.

Cardiac M2 Muscarinic Receptors

Metabotropic acetylcholine receptors in the heart coupled to $\text{G}\alpha_i$ that open $\text{K}^+$ channels, inducing membrane hyperpolarization and slowing heart rate.

Ionotropic Receptors

Fast, ligand-gated ion channels that open directly upon neurotransmitter binding to allow rapid ion flux for immediate sensory and motor signaling.

Metabotropic Receptors

G-protein-coupled receptors (GPCRs) that operate on a slower timescale to trigger intracellular enzymatic cascades, modulating long-term neuronal excitability and metabolic state.

Ionotropic Glutamate Receptor Subtypes

Three structurally distinct, excitatory ligand-gated channel families classified by their selective pharmacological agonists: AMPA, NMDA, and Kainate.

Tetrameric Assembly of Glutamate Receptors

The quaternary structure of ionotropic glutamate receptors, which require the assembly of four distinct or identical subunits to form a functional ion pore.

Glutamate Receptor Activation Stoichiometry

The ligand-binding requirement whereby all four subunits of the tetrameric glutamate receptor must bind a glutamate molecule to open the channel.

Transmembrane Topology of Glutamate Receptors

The structural arrangement of ionotropic glutamate receptor subunits, consisting of three complete transmembrane alpha helices and one partial pore-forming re-entrant loop ($\text{M}_2$).

Monovalent Cation Permeability (AMPA and Kainate)

The conductance property of AMPA and Kainate receptor channels that selectively permits the flow of monovalent cations, specifically $\text{Na}^+$ and $\text{K}^+$.

Calcium Permeability of NMDA Receptors

The unique biophysical property of NMDA receptors allowing significant divalent $\text{Ca}^{2+}$ influx in addition to monovalent $\text{Na}^+$ and $\text{K}^+$ flux.

NMDA Receptor as a Coincidence Detector

The functional specialization of the NMDA receptor requiring both presynaptic glutamate release and postsynaptic membrane depolarization to open.

Voltage-Dependent Magnesium ($\text{Mg}^{2+}$) Block

The physical plugging of the NMDA receptor pore by an $\text{Mg}^{2+}$ ion at resting membrane potentials (approximately $-65\,\text{mV}$), preventing ion flow even when glutamate is bound.

Depolarization Relief of Magnesium Block

The electrostatic expulsion of the blocking $\text{Mg}^{2+}$ ion from the NMDA receptor pore when the postsynaptic membrane is depolarized by nearby AMPA receptors to approximately $-30\,\text{mV}$.

Calcium-Mediated Plasticity Signaling

The intracellular signaling cascade triggered when $\text{Ca}^{2+}$ enters through unblocked NMDA receptors to act as a second messenger, activating downstream biochemical pathways.

Long-Term Potentiation (LTP)

A persistent, activity-dependent increase in synaptic efficacy triggered by high-frequency stimulation and mediated by NMDA receptor-dependent $\text{Ca}^{2+}$ entry.

Hebbian Principle

The neurobiological theory summarized as "cells that fire together, wire together," describing synaptic strengthening when presynaptic and postsynaptic neurons fire concurrently.

Pentameric Architecture of GABAA and Glycine Receptors

The structural assembly of inhibitory ionotropic $\text{GABA}_\text{A}$ and glycine receptors, which require five protein subunits surrounding a central ion pore.

GABAA Subunit Gene Diversity

The structural heterogeneity of $\text{GABA}_\text{A}$ receptors, whose pentamers are assembled from combinations of subunits encoded by 19 distinct genes ($\alpha$, $\beta$, $\delta$, $\gamma$, and $\epsilon$).

GABAA Channel Activation Stoichiometry

The biochemical requirement that exactly two GABA molecules must bind to the $\text{GABA}_\text{A}$ receptor complex to gate the ion channel open.

Chloride ($\text{Cl}^-$) Conductance and IPSPs

The influx of negative $\text{Cl}^-$ ions through opened $\text{GABA}_\text{A}$ and glycine receptor channels, driving membrane hyperpolarization and inhibitory postsynaptic potentials (IPSPs).

Strychnine

A potent alkaloid that acts as a specific competitive antagonist at ionotropic glycine receptors in the spinal cord and brainstem.

Benzodiazepines

Positive allosteric modulators of the $\text{GABA}_\text{A}$ receptor (e.g., Valium) that increase the frequency of channel openings to produce sedative and anxiolytic effects.

Barbiturates

Allosteric modulators of the $\text{GABA}_\text{A}$ receptor (e.g., Pentobarbital) that increase the duration of channel open time, generating stronger IPSPs and producing anesthetic states.

Alcohol (Ethanol) on GABAA Receptors

A pharmacological modulator that enhances overall $\text{GABA}_\text{A}$ receptor conductance, resulting in sedative-hypnotic and anticonvulsant states.

7-Transmembrane (7-TM) Motif

The canonical protein architecture of metabotropic receptors, consisting of a single polypeptide with seven membrane-spanning alpha helices whose extracellular loops form the ligand-binding pocket.

Heterotrimeric G-Protein Activation Cycle

A molecular signaling sequence where receptor activation triggers the exchange of GDP for GTP on the $\text{G}\alpha$ subunit, causing it to dissociate from the $\text{G}\beta/\gamma$ complex until intrinsic GTPase activity converts GTP back to GDP.

Stimulatory G-Protein Pathway ($\text{G}\alpha_s$)

A GPCR effector pathway that activates adenylyl cyclase, elevates cAMP, activates PKA, and leads to the phosphorylation and closure of $\text{K}^+$ channels, reducing $\text{K}^+$ efflux and increasing neuronal excitability.

Inhibitory G-Protein Pathway ($\text{G}\alpha_i$)

A GPCR effector pathway that inhibits adenylyl cyclase activity, thereby decreasing intracellular cAMP levels and reducing PKA-mediated protein phosphorylation.

Calcium-Mobilizing G-Protein Pathway ($\text{G}\alpha_q$)

A GPCR signaling pathway that activates downstream cascades to trigger the release of divalent $\text{Ca}^{2+}$ from intracellular stores.

GPCR Signal Amplification Cascade

A geometric biochemical progression where a single activated receptor stimulates multiple G-proteins, each activating enzymes to generate thousands of second messengers (e.g., cAMP), leading to sustained cellular responses persisting for minutes or longer.