Neurotransmitters

Neurotransmitters- Classification and Transmission

Neurotransmitters are intercellular messengers categorized into conventional and unconventional types based on their storage and release mechanisms.

Classification of Neurotransmitters

Conventional Neurotransmitters: These are stored in synaptic vesicles (SVs) and released by Ca 2+ -dependent exocytosis. They include:
- Small-Molecule Transmitters: Acetylcholine (ACh), purines like ATP, and amino acids such as glutamate (the main excitatory transmitter in the brain), GABA, and glycine (inhibitory).
- Biogenic Monoamines: These include catecholamines (dopamine, norepinephrine, and epinephrine) derived from tyrosine, serotonin (derived from tryptophan), and histamine.
- Neuropeptides: Large molecules ranging from 3 to 100 amino acids, such as substance P, enkephalins, and cholecystokinin (CCK), often stored in large dense-core vesicles.
Unconventional Neurotransmitters: These are not stored in vesicles but are synthesized and released "on demand". They include:
- Gases: Nitric oxide (NO) and carbon monoxide (CO), which are highly membrane-permeant and diffuse directly to target cells.
- Lipids: Endocannabinoids like 2-AG and anandamide, which often function as retrograde messengers by traveling from the postsynaptic neuron back to the presynaptic terminal.

Mechanisms of Chemical Transmission

Chemical synaptic transmission is a unidirectional, polarized process occurring in seven discrete steps:

Synthesis and Storage: Neurotransmitters are synthesized in the nerve terminal or cell body and packaged into SVs. This packaging often uses a V-ATPase to create an H+ gradient that powers secondary active transporters.
Action Potential Arrival: An action potential travels down the axon and depolarizes the presynaptic terminal.
Ca 2+ Influx: Depolarization opens voltage-gated Ca 2+ channels, allowing Ca 2+ to flow into the terminal down its electrochemical gradient.
Exocytosis: The rise in intracellular Ca 2+ is sensed by the protein synaptotagmin, which triggers the fusion of docked vesicles with the presynaptic membrane via the SNARE complex (synaptobrevin, syntaxin, and SNAP-25).
Diffusion: The neurotransmitter is released in fixed quantal packets and diffuses across the synaptic cleft, which is typically 20 to 40 nm wide.
Receptor Activation: The transmitter binds to specific postsynaptic receptors, which are classified as:
- Ionotropic: Ligand-gated ion channels that cause rapid changes in membrane potential, such as depolarizing EPSPs or hyperpolarizing IPSPs.
- Metabotropic: G-protein–coupled receptors that initiate slower intracellular signaling cascades, often involving second messengers like cAMP or IP3.
Termination: The signal is ended by removing the transmitter through diffusion, enzymatic degradation (e.g., acetylcholinesterase), or reuptake by Na+ -dependent transporters into neurons or glia.

Conventional neurotransmitters are intercellular messengers characterized by their storage in membrane-bound synaptic vesicles (SVs) and their release via Ca2+ -dependent exocytosis. They are typically categorized into three main groups based on their chemical structure and storage.

1. Small-Molecule Transmitters

These are stored in small, electron-lucent vesicles (approx. 40 nm) and typically mediate fast "wiring transmission" across narrow synaptic clefts.

Acetylcholine (ACh): The only transmitter used at the neuromuscular junction (NMJ) and by all autonomic preganglionic neurons. It can be excitatory (via nicotinic receptors) or inhibitory (via certain muscarinic receptors).
Amino Acids:
- Glutamate: The predominant excitatory neurotransmitter in the brain, essential for memory and learning.
- GABA and Glycine: The primary inhibitory neurotransmitters. GABA is widespread in the brain, while glycine is more common in the spinal cord. They act by opening Cl− channels to hyperpolarize the cell.
Purines: Includes ATP and adenosine, which often function as cotransmitters released alongside other messengers to modulate activity or regulate sleep/wake cycles.

2. Biogenic Monoamines

These are derived from amino acids and are often stored in small dense-core vesicles (40–70 nm). They frequently utilize "volume transmission," where the transmitter diffuses over a larger area to affect multiple cells.

Catecholamines: Includes Dopamine (movement and reward), Norepinephrine (arousal and blood pressure), and Epinephrine (hormonal stress response).
Serotonin (5-HT): Derived from tryptophan; regulates mood, appetite, and behavior.
Histamine: Derived from histidine; involved in arousal and attention.

3. Neuropeptides

Neuropeptides range from 3 to 100 amino acids long and are stored in large dense-core vesicles (100–150 nm). Unlike small molecules, they are synthesized in the cell body rather than the nerve terminal.

Release: They typically require high-frequency bursts of action potentials for exocytosis.
Examples: Substance P (pain transmission), enkephalins/endorphins (analgesia), and Cholecystokinin (CCK) (satiety).

Transmission and Termination

The lifecycle of these transmitters follows a fixed sequence: synthesis, storage, release triggered by Ca2+ influx, receptor binding, and termination. Termination is critical to stop the signal and occurs through:

Diffusion away from the cleft.
Enzymatic Deactivation (e.g., Acetylcholinesterase breaking down ACh).
Reuptake via Na+ -dependent transporters back into the presynaptic terminal or glia.

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