Detailed neuroanatomical information is an essential prerequisite for functional analysis of specific neurones in the central nervous system. In this investigation we have combined the dissection of single axons with cobalt-backfilling to reveal the structural organisation of a class of interganglionic giant intemeurones in the central nervous system of the cockroach, Periplaneta americana (L). Each giant interneurone possesses a cell body, neurite and dendrites within the sixth abdominal ganglion and also has a giant axon which ascends the ventral nerve cord (Harris & Smyth, 1971). Cereal sensory neurones provide monosynaptic input to at least some giant interneurones (Callec et al., 1974). Furthermore the cereal afferent, giant interneurone pathway has proved to be a useful system for quantitative pharmacology of putative cholinergic synapses (Callec & Sattelle, 1973; Callec, 1974).

1. Ionophoresis of acetylcholine (ACh) onto the cell body membrane of an identified giant interneurone (GI2) in the central nervous system of the cockroach Periplaneta americana induced a depolarizing response at resting potential which was attributed to a population of extrasynaptic ACh receptors. 2. The sensitivity of the cell body membrane of GI 2 to ionophoresis of ACh was determined. 3. Perfusion of 1.0 × 10−6M neostigmine, an inhibitor of acetylcholinesterase, potentiated the ACh sensitivity of the cell body membrane of GI2. This indicated that a high acetylcholinesterase activity was present in the periphery of the sixth abdominal ganglion (A6). 4. The nicotinic antagonist, α-bungarotoxin (at a concentration of 1.0 × 10−7M) was found to block the ACh response of the cell body membrane of GI2. By contrast, the muscarinic antagonist, quinuclidinyl benzilate, (at concentrations up to 1.0 × 10−5 M) had no detectable effect on the ACh response. 5. It is suggested that an extrasynaptic nicotinic type of ACh receptor is present on the cell body membrane of GI2.

1. Autoradiographic localization of an 125I-α-bungarotoxin binding component revealed that specific binding was distributed mainly in the neuropile and to some extent in the periphery of the terminal abdominal ganglion of the cockroach, Periplaneta americana (L.). 2. Action potentials recorded from the axon of GI2 under current-clamp conditions were not affected by exposure to 1.0×10−5M α-bungarotoxin. 3. Excitatory postsynaptic potentials recorded from GI2, evoked by stimulation of cereal sensory neurones, were sensitive to block by relatively low concentrations (1.0×10−9-1.0×10−7 M) of α-bungarotoxin. The time-course for blockade was found to depend on toxin concentration and frequency of afferent stimulation. 4. Excitatory postsynaptic potentials recorded from GI 2 were not affected by exposure to 1.0×10−6M quinuclidinyl benzilate. 5. We conclude that at least a portion of the 125I-α-bungarotoxin binding component represents cholinergic receptors which have a postsynaptic function at synapses between cereal sensory neurones and GI 2.