[Possible functional significance of the simultaneous release of several putative neurotransmitters by the same...

At least three criteria should be fulfilled before it can be concluded that one neurone may use more than one neurotransmitter (or neuromodulator) for synaptic transmission: Such putative neurotransmitters must be located in this particular neurone: They must be released from the neurone under physiological conditions, and: They must exert separately or together pre- and/or post-synaptic effects. Critical analysis of the literature reveals that the coexistence and corelease of neuroactive substances are rarely firmly established. However, more convincing data have been reported on the interaction at the pre- and/or post-synaptic levels between neurotransmitters possibly located in the same neuronal population. Thus, it has been shown that VIP coreleased with acetylcholine from the same parasympathetic fibres can facilitate the interaction of this neurotransmitter with muscarinic receptors on target cells. In contrast, substance P coreleased with serotonin from bulbo-spinal fibres can reduce the effect of the amine on specific presynaptic autoreceptors. The functional significance of such interactions between neurotransmitters coexisting in the same neurone seems extremely important. In particular, they might well be involved in the physiological control of hormone release from the adenohypophysis.