Non-synaptic Interneuronal Communication: Physiological and Pharmacological Implication

Abstract

This chapter discusses non-synaptic interneuronal communication and its physiological and pharmacological implication. Intercellular communication in the nervous system generally involves the release of chemical transmitters from axon terminals. From the data presented in the chapter, it seems convincing that intercellular communication in both peripheral and central nervous systems is likely to be mediated by substance released from (a) non-synaptic axon terminals, or (b) from axon terminals making synaptic contact but acting both on synaptic and non-synaptic target cells, or (c) from regions of the nerve cell other than the presynaptic axon terminal. The chapter further describes the semantic aspects of neuroactive substances. These include neurotransmitter, neuromodulator, and neurohormone. The neuromodulator communication, which does not require direct synaptic contact between the nerve cell releasing the transmitter and the target cells, can be compared to a radio broadcasting system, where there is public transmission of a signal that can be picked up by any properly equipped receiver; any cell properly equipped with receptors, within range. In addition to neurotransmitter substances acting at close range in chemical synaptic neurotransmission, there exist chemical interactions between neurons without any close synaptic contact; interneuronal modulation of transmission which operates over some distance. Release of transmitters from axon terminals devoid of synaptic membrane specialization has long been known in the peripheral autonomic nervous system. The chapter explains that the presynaptic modulation, via α-adrenoceptor, of chemical neurotransmission seems to be of physiological and pharmacological importance.