Role of High-Affinity Receptors and Membrane Transporters in Nonsynaptic Communication and Drug Action in the Central Nervous System

Neurochemical and morphological evidence has shown that some neurotransmitters or substances may be released from both synaptic and nonsynaptic sites for diffusion to target cells more distant than those observed in regular synaptic transmission. There are functional interactions between neurons without synaptic contacts, and matches between release sites and localization of receptors sensitive to the chemical signal are exceptions rather than the rule in the central nervous system. This also indicates that besides cabled information signaling (through synapses), there is a "wireless" nonsynaptic interaction between axon terminals. This would be a form of communication transitional between discrete classical neurotransmission (in Sherrington's synapse) and the relatively nonspecific neuroendocrine secretion. Recent findings indicate that in addition to monoamines (norepinephrine, dopamine, serotonin), other transmitters, such as acetylcholine and nitric oxide (NO), may also be involved in these nonsynaptic interactions. It has been shown that NO, an ideal mediator of nonsynaptic communication, can influence the function of uptake carrier systems, which may be an important factor in the regulation of extracellular concentration of different transmitters. This review will focus on the role of nonsynaptic receptors and transporters in presynaptic modulation of chemical transmission in the central nervous system. The nonsynaptic interaction between neurons mediated via receptors and transports of high affinity not localized in synapses has the potential to be an important contributor to the properties and function of neuronal networks. In addition, it will be suggested for the first time that the receptors and transporters expressed nonsynaptically and being of high affinity are the target of drugs taken by the patient.