What are the criteria for a substance in a neurotransmitter at a synapse?

Answer 1

Criteria required identifying a substance as a neurotransmitter 1- ) Synthesis of the substance: • It is synthesized in the neuron • Some substances are synthesized from the Tryptophan • Rate limiting step in its synthesis • Enzymes required in the synthesis of that substance are themselves synthesized in the cell body of neuron ,then distributed throughout the neuron • Mainly found in the cell body and nerve terminal • These enzymes are stored in the presynaptic terminal of neuron, inside the synaptic vesicles. 2- ) Storage of that substance: • The neutransmitter is stored inside the synaptic vesicle of nerve terminal 3- ) Release of the neurotransmitter: • It is released from the nerve terminal usually through Ca++ dependent process following the stimulation of the nerve fiber by action potential. • Fusion of the synaptic vesicle with the nerve membrane releasing neurotransmitter outside the nerve fiber into the synaptic cleft. 4- ) Exogenous analogue (drug) of the neurotransmitter: • If drug analogue of the neurotransmitter is injected exogenously, it should mimic the same mechanism of action as done by the endogenously released neurotransmitter.

Answer 2

For a substance to be considered a neurotransmitter at a synapse, it must be synthesized within the presynaptic neuron, released in response to an action potential, be present in sufficient quantities to produce a physiological response, and have specific receptors on the postsynaptic neuron that it can bind to in order to transmit a signal. Additionally, it should be capable of being broken down or removed from the synaptic cleft to terminate its signaling action.