Neurotransmitter

Neurotransmitter is a chemical messenger that transmits signals across synapses from a neuron to another neuron, muscle, or gland cell. In most synapses, transmitters are synthesized in the presynaptic neuron, packed into synaptic vesicles, and released into the synaptic cleft in response to an action potential. They bind to receptors on the postsynaptic membrane to produce excitation or inhibition, or to modulate the strength and timing of synaptic transmission. After release, neurotransmitters are removed by reuptake into the presynaptic neuron, degraded by enzymes, or diffuse away from the synapse.

Major classes include small-molecule transmitters such as acetylcholine, glutamate, GABA, and glycine; monoamines such as dopamine, norepinephrine, epinephrine, and serotonin; and a variety of neuropeptides and gasotransmitters such as nitric oxide. Receptors are broadly divided into ionotropic receptors, which form ion channels, and metabotropic receptors, which act through G proteins and second messengers. The same neurotransmitter can have different effects depending on receptor type and cellular context.

Neurotransmission supports basic brain functions and higher processes. Glutamate and GABA balance excitatory and inhibitory signaling and are central to synaptic plasticity and learning. Monoamines influence mood, attention, and arousal. Acetylcholine regulates muscle contraction and memory. Dysregulation of neurotransmitter systems is implicated in numerous disorders, including Parkinson's disease (dopamine deficiency), Alzheimer's disease (acetylcholine loss), anxiety and depression (serotonin or norepinephrine dysregulation).

Pharmacologic agents target neurotransmitter systems, such as selective serotonin reuptake inhibitors, acetylcholinesterase inhibitors, and dopamine therapies, to alter signaling in disease. The term neuromodulator is used for signals that alter neuronal responsiveness over longer timescales or across larger brain regions, and not all signaling molecules fit neatly into one category.