THE CONCEPT OF CHEMICAL NEUROTRANSMISSION - VARIATIONS ON THE THEME

The present concept of chemical neurotransmission occurring purely thr ough synaptic transmission has dominated neurobiological. thinking for about the last 40 years. According to this conventional view neurotra nsmitters are substances that are synthesized within the neurones, lib erated into the synaptic cleft after stimulation of the nerve, and tha t finally elicit a biologically plausible response in the postsynaptic target cell or the nerve terminal itself. This concept undoubtedly co mprises the main body of interneuronal chemical signalling. However, a large amount of evidence, obtained during the last two decades, sugge sts that there are a number of parallel mechanisms, which may essentia lly participate in neuronal signalling, or at least modulate it. Thus, the recent progress of research has provided the following compelling evidence: 1) a large variety of substances, some of them synthesized in non-neuronal cells, actually participate actively in neuronal signa lling; 2) functional connections in brain are not determined by the sy naptic connections only; 3) glial cells have an active and fundamental role in signal transmission; and 4) the signalling properties and mec hanisms of each neurone are constantly under functional and structural regulation. The aim of this review is to present shortly some of the central concepts and/or mechanisms that have risen during the last two decades. Also the functional and/or clinical relevance of these mecha nisms is addressed briefly.