Presynaptic modulation of transmitter release via alpha2-adrenoceptors: nonsynaptic interactions

It is generally accepted that neurochemical transmission occurring at the synapse is the primary way of sending messages from one neuron to another. Neurotransmitters released from axon terminal in a [Ca2+]0-dependent manner act transsynaptically on the postsynaptic site. The past 30 years have witnessed something of a revolution in the understanding of how neurons communicate with each other. It has been shown that the exocytotic release of transmitters from axon terminals is subject to presynaptic modulation via presynatic hetero- and auto-receptors. For example via stimulation of alpha2-adrenoceptors expressed on varicosities and coupled to G-protein the stimulation-evoked release of different transmitters can be inhibited. This review will focus on nonsynaptic interactions between axon terminals. The present data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.