ABSTRACT
Classical neurotransmitters are small molecules of neuroactive agents actively involved in synaptic transmission and modulation. They are synthesized in the neurons and are released at the presynaptic terminals in sufcient amounts to affect the membrane potential or conductance of the postsynaptic neurons, producing inhibition or excitation. Their effect is commonly associated with the selective opening of specic ion channels in the postsynaptic membrane/and or phosphorylation of intracellular protein. They may bind directly to a receptor and cause second messenger-mediated changes in the neurotransmission. Exogenous administration of neurotransmitters may mimic the actions of the endogenous transmitters. Certain neurotransmitters may be released after a more sustained activation. Inactivation of these agents may occur locally at the terminals by enzymatic uptake and degradation, or diffusion and release. Some neurotransmitters do not act upon the postsynaptic membrane but affect its response to other neuromediators by enhancing or inhibiting their activities. Classical (small-molecule) neurotansmitters are comprised of amino acid neurotransmitters, acetylcholine, and biogenic amines.
