Neurotransmittermolecule that carries signals between neurons (nerve cells) at synapses in the nervous system.
Neurotransmitters may be either excitatory (EPSPs) or inhibitory (IPSPs). That is, they may be of a type that fosters the initiation of a nerve impulse in the receiving neuron, or they may inhibit such an impulse. GABA and glycine are well-known inhibitory neurotransmitters.
Within the cells, neurotransmitter molecules are packaged in vesicles and released by rapid exocytosis upon the arrival of a nerve impulse. Then they diffuse across the synaptic gap to bind neurotransmitter receptors or other ligand gated ion channels, and stimulate or inhibit the firing of the postsynaptic neuron.
Many neurotransmitters are removed from the synaptic gap, after they have activated their specific receptors, by transport proteins residing in neuronal and glial plasma membranes. This process is called reuptake (or often simply uptake). Without reuptake, the molecules might (counterproductively) continue to stimulate or inhibit the firing of the postsynaptic neuron. At cholinergic synapses, acetylcholine (ACh) is the neurotransmitter; the enzyme acetylcholinesterase - rather than a transport protein - removes the ACh.
Drugs may alter the way neurotransmitters function. Cocaine, for example, blocks the reuptake of dopamine, leaving it in the synaptic gap longer. AMPT prevents the conversion of tyrosine to L-DOPA; reserpine prevents dopamine storage within vesicles; and monoamine oxidase (MAO) in the axon terminal degrades dopamine (deprenyl, blocks MAO-B and increases dopamine levels).
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