Cholinergic Nerve Terminals in the Central Nervous System of

Synapses at which acetylcholine (ACh) is the primary chemical transmitter are widespread in the nervous tissues, glandular tissues and muscle tissues of man and animals. Much of our present knowledge of the structure and function of cholinergic synapses is based on studies performed on model systems, notably the electromotor synapses of fish and the neuromuscular junction of vertebrates. However, it is far from clear that the functional organization of these terminals can be extrapolated to the CNS where a much greater diversity of chemical interactions and a much more pronounced plasticity of nerve contacts is detected. The extremely (Fig. 1) high levels of ACh found in the ganglia of arthropods, almost as high as the concentration of the ACh-rich electric tissue of certain electric eels, may be ascribed to two unique properties of the functional organization of the arthropod nervous system. First, ACh appears to be the primary neurotransmitter for the sensory pathways. Secondly, the synapses made by the afferent fibres are in the CNS. Thus, the nervous tissue of insects appears to be an attractive system to explore molecular aspects of cholinergic synapses in the CNS. Recent technical advances in neurochemistry, neurophysiology and molecular neurobiology of insects have provided the stimulus for this review-the first comprehensive survey of the functional organization of cholinergic nerve terminals of these abundant and highly diverse organisms. The development of microfractionation procedures has permitted isolation of synaptosomal fractions from insect ganglia highly enriched in structurally intact, functionally viable, cholinergic nerve terminals which are uncontaminated by large postsynaptic membrane fragments. This has been of particular value for studying dynamic processes mediated by presynaptic membranes. In parallel, electrophysiological studies on a uniquely identified CNS cholinergic synapse have enabled presynaptic microelectrode recording closer to the presynaptic terminal than in any other synapse studied to date. Identification