Neurotransmitters in the Avian Visual System

Increasing evidence that amino acids are involved in chemical synaptic transmission has been obtained by subjecting them to the criteria established for neurotransmitters like acetylcholine and the catecholamines. Their concentration relative to other regions of the nervous system should be high, their synthetizing enzyme(s) must show a high activity and the nerve endings should possess a selective high affinity uptake mechanism All those presynaptic characteristics would then diminish or disappear when the terminals degenerate following a lesion of the afferent neuron or axon. Further, the transmitter candidate should be released from the nerve terminals, either through the effect of high K+ concentration or drugs like veratridine, or through electrical stimulation, diffuse or, better, applied to the corresponding afferent pathway. Microiontophoretic application of the amino acid should mimic the effect of the physiological synaptic action of the transmitter on the postsynaptic element and specific antagonists should prevent this action.