Neurotransmitters, Synaptic Transmission, and Neural Integration

Neurotransmitters: Structure, Synthesis, and Degradation

Overview of Neurotransmitter Classes

Neurotransmitters are chemical messengers that transmit signals across synapses in the nervous system. They are classified into several major groups based on their chemical structure and function.

Choline derivative	Biogenic amines	Amino acids	Purines	Neuropeptides	Unique molecules
Acetylcholine	Catecholamines (Dopamine, Epinephrine, Norepinephrine), Serotonin, Histamine	Glutamate, Aspartate, Glycine, GABA	ATP, ADP, Adenosine	TRH, Vasopressin, Oxytocin, Substance P, Cholecystokinin, Endogenous opioids (Enkephalins, Endorphins), Orexin	Nitric oxide, Endocannabinoids

Acetylcholine (ACh)

Synthesis, Release, and Degradation

Acetylcholine is the most abundant neurotransmitter in the peripheral nervous system and is found in both the somatic and autonomic branches. It is synthesized in the axon terminal cytosol from acetyl CoA and choline, catalyzed by choline acetyltransferase (CAT):

• Synthesis: Acetyl CoA + Choline (via CAT) → Acetylcholine + CoA

• Storage: ACh is stored in synaptic vesicles until released by exocytosis, triggered by an action potential.

• Degradation: Acetylcholinesterase (AChE) breaks down ACh into acetate and choline in the synaptic cleft. Choline is recycled by active transport into the presynaptic neuron.

Cholinergic Receptors

• Nicotinic receptors: Ionotropic, ligand-gated channels that allow Na+ and K+ movement, producing a fast excitatory postsynaptic potential (EPSP). Found at neuromuscular junctions and some CNS regions.

• Muscarinic receptors: Metabotropic, G protein-coupled receptors that can open/close ion channels or activate second messenger systems, producing slower, diverse effects. Found in effector organs of the autonomic nervous system and CNS.

Key Concept: The effect of a neurotransmitter depends on the receptor and its signal transduction mechanism, not the transmitter itself.

Biogenic Amines

Catecholamines and Other Amines

Biogenic amines are derived from amino acids and include catecholamines (dopamine, norepinephrine, epinephrine), serotonin, and histamine. Catecholamines are synthesized from tyrosine and act via G protein-coupled receptors (adrenergic and dopaminergic receptors).

• Adrenergic receptors: Alpha (α) and beta (β) subtypes, with different affinities for norepinephrine and epinephrine.

• Degradation: Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) degrade catecholamines in the synaptic cleft and presynaptic terminals.

• Serotonin: Regulates mood and sleep; targeted by antidepressants such as SSRIs.

• Histamine: Functions as a neurotransmitter in the CNS, especially the hypothalamus.

Clinical Connection: Treating Depression

• Monoamine oxidase inhibitors (MAOIs): Inhibit breakdown of norepinephrine and serotonin, increasing their synaptic concentration.

• Selective serotonin reuptake inhibitors (SSRIs): Block serotonin reuptake, increasing its synaptic availability.

• Tricyclic antidepressants: Affect adrenergic and serotonergic synapses, though mechanisms are less understood.

Amino Acid Neurotransmitters

Excitatory and Inhibitory Amino Acids

Amino acid neurotransmitters are the most abundant in the CNS.

• Excitatory: Glutamate and aspartate (bind to AMPA, NMDA, and kainate receptors; glutamate is the most common excitatory neurotransmitter).

• Inhibitory: GABA and glycine (GABA binds to GABAA, GABAB, and GABAC receptors; glycine acts mainly in the spinal cord).

Clinical Connection: GABAergic Agents

• Benzodiazepines (e.g., Valium): Enhance GABAA receptor activity, reducing anxiety and CNS activity.

• Alcohol: Also enhances GABAA receptor activity, leading to CNS depression.

• Sleep aids (e.g., zolpidem): Bind to specific GABAA receptor subtypes, promoting sedation.

Purines

ATP and Related Compounds

ATP, ADP, and adenosine act as neurotransmitters, especially in the enteric nervous system. Purinergic receptors include:

• P2X: Ionotropic, cation channels (excitatory).

• P2Y: Metabotropic, G protein-coupled (modulatory effects).

• Adenosine receptors: Bind adenosine specifically.

Purines are degraded by nucleotidases and adenosine deaminase.

Neuropeptides

Synthesis and Function

Neuropeptides are short chains of amino acids synthesized in the cell body, packaged in dense core vesicles, and transported to axon terminals. They often act as neuromodulators and are co-released with small neurotransmitters.

• Examples: TRH, vasopressin, oxytocin, substance P, cholecystokinin, endogenous opioids (enkephalins, endorphins), orexin.

• Orexin: Regulates sleep-wake cycles; potential target for narcolepsy treatment.

Unique Neurotransmitters

Nitric Oxide and Endocannabinoids

• Nitric oxide (NO): A gaseous neurotransmitter synthesized on demand and diffuses to target cells, altering protein activity without binding to surface receptors. It is rapidly degraded.

• Endocannabinoids: Lipid-derived messengers (e.g., anandamide, 2-AG) that act on CB1 receptors in the CNS. They modulate neurotransmission and are targets of THC (cannabis).

Synaptic Transmission

Electrical Synapses

Electrical synapses connect neurons via gap junctions, allowing direct ion flow and rapid, bidirectional communication. Found in the retina, cortex, and brainstem for synchronizing activity.

Chemical Synapses

Most synapses are chemical, involving neurotransmitter release from a presynaptic neuron to a postsynaptic cell. Types of synapses include axodendritic, axosomatic, and axoaxonic.

Mechanism of Neurotransmitter Release

• Action potential arrives at axon terminal, opening voltage-gated Ca2+ channels.

• Ca2+ influx triggers exocytosis of synaptic vesicles, releasing neurotransmitter into the synaptic cleft.

• Neurotransmitter binds to postsynaptic receptors, inducing a response.

• Termination: Neurotransmitter is degraded, reuptaken, or diffuses away.

Signal Transduction at Chemical Synapses

Ionotropic vs. Metabotropic Receptors

• Ionotropic (channel-linked): Fast, direct opening of ion channels (e.g., nicotinic ACh receptors).

• Metabotropic (G protein-coupled): Slow, indirect effects via G proteins and second messengers (e.g., muscarinic ACh receptors).

Excitatory and Inhibitory Synapses

• Excitatory synapses: Cause depolarization (EPSP), often via Na+ influx.

• Inhibitory synapses: Cause hyperpolarization (IPSP), often via K+ efflux or Cl- influx.

Neural Integration

Divergence and Convergence

• Divergence: One neuron communicates with multiple postsynaptic neurons.

• Convergence: One neuron receives input from multiple presynaptic neurons.

Summation of Postsynaptic Potentials

• Temporal summation: Multiple signals from one synapse in rapid succession sum together.

• Spatial summation: Signals from different synapses occurring simultaneously sum together.

Presynaptic Modulation

• Presynaptic facilitation: A modulatory neuron increases neurotransmitter release from another presynaptic neuron, enhancing the postsynaptic response.

• Presynaptic inhibition: A modulatory neuron decreases neurotransmitter release, reducing the postsynaptic response.

• Occurs at axoaxonic synapses and affects only the specific synapse involved.

Summary Table: Major Neurotransmitter Classes and Examples

Class	Examples	Main Functions
Choline derivative	Acetylcholine	Muscle activation, autonomic functions
Biogenic amines	Dopamine, Norepinephrine, Epinephrine, Serotonin, Histamine	Mood, arousal, autonomic regulation
Amino acids	Glutamate, Aspartate, GABA, Glycine	Excitation/inhibition in CNS
Purines	ATP, ADP, Adenosine	Neuromodulation, enteric nervous system
Neuropeptides	Substance P, Endorphins, Enkephalins, Orexin	Pain modulation, stress, arousal
Unique molecules	Nitric oxide, Endocannabinoids	Retrograde signaling, neuromodulation