Synapses Between an Identified Giant Interneurone and A Filiform Hair Sensory Neurone in the Terminal Ganglion of First Instar Cockroaches (Periplaneta Americana L.)

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Abstract
The ultrastructure of input and output synapses of various identified insect neurones has recently been studied (for example Titmus & Hoyle, 1977; Altman, Shaw & Tyrer, 1980; Phillips, 1980; Watson & Burrows, 1981, 1982, 1983; Tolbert & Hildebrand, 1981) and connections between anatomically and physiologically defined axons have been described (King & Wyman, 1980). To date though, synapses have not been visualized in an insect central pathway in which aspects of the anatomy, physiology, pharmacology and associated behaviour have been investigated. The cereal afferent, giant interneurone synapses in the sixth abdominal (A6) ganglion of the cockroach Periplaneta americana are well suited for such studies. Using cobalt staining (Pitman, Tweedle & Cohen, 1972) of single neurones, seven giant interneurones (GIs 1–7) can be identified on the basis of their morphology (Harrow, Hue, Pelhate & Sattelle, 1980; Daley, Vardi, Appignani & Camhi, 1981). In adult cockroaches these interneurones receive sensory input from numerous cereal mechanoreceptor afferents (Callec, Guillet, Pichon & Boistel, 1971 ; Daley et al. 1981). There is considerable evidence for cholinergic monosynaptic transmission at these synapses (Callec, 1974; Sattelle, 1980) and postsynaptic acetylcholine receptors sensitive to the nicotinic cholinergic antagonist a-bungarotoxin are present on GI2 and GI 3 (Harrow, Hue, Pelhate & Sattelle, 1979; Sattelle, David, Harrow & Hue, 1980; Harrow & Sattelle, 1983 ; Sattelle et al. 1983). It has been shown that the cereal afferent, giant interneurone pathways play a part in mediating the escape response of the cockroach following stimulation of the cereal mechanoreceptors (Ritzmann & Camhi, 1978; Camhi, Tom & Volman, 1978).