Neurotransmitters

Abstract

Basically three criteria can be used for identifying a neurotransmitter: (a) At the presynaptic level, the substance should show depolarizaton-induced release and/or selective uptake of that agent, (b) At the postsynaptic level, there must be specific receptor(s) that should be selectively blocked by appropriate antagonists, (c) Its postsynaptic effects should be similar to those elicited by stimulating the corresponding projection pathway, in terms of antagonist sensitivity and synaptic current characteristics (i.e., conductance changes or reversal potential). When all these criteria are fulfilled, a synaptic pathway may be considered as using this substance as a transmitter. In the central visual structures, some substances, such as ACh and NE, fulfill most of the above criteria. Other substances, such as some amino acids, only partially fulfill the criteria (Krnjevic 1974; Parnavelas and McDonald 1983; Mayer and Westbrook 1987).