The Study of Neuromodulators-Opening Pandora’s Box

Abstract

Only four decades ago, neuroscientists still thought that communication between neurons in the central nervous system or between neurons and their peripheral targets such as muscles is mediated by only a few chemical signalling molecules termed transmitters. The action of a transmitter is usually fast, it is exerted at specialized regions of contact, the synapses, and can be either excitatory or inhibitory through effects on membrane potential. Although for many animal phyla to date not a single transmitter substance has been identified, most of the transmitters employed by the nervous system are known at least for the larger phyla of invertebrates and for all classes of vertebrates. However, the number of substances used as transmitters is surprisingly small and their distribution is wide among the different phyla. Throughout the entire vertebrate phylum, neurons of the same functional type are neurochemically alike and produce the same transmitter: all motoneurons, all preganglionic autonomic and postganglionic parasympathetic neurons, but not sensory neurons, use acetylcholine as their transmitter. In the very large phylum of arthropods, the situation is the opposite: sensory neurons are cholinergic, motor neurons are not. However, with regard to the number and the substances used as transmitters, there is good evidence that invertebrates share the consistency and uniformity of the neurochemical pattern found in vertebrates.