Short Communication Do Presynaptic Muscarinic Receptors Regulate Acetylcholine Release in the Central Nervous System of the Cockroach Periplaneta Americana?

Although insects have proved to be suitable subjects for neuropharmacological investigations, the role of muscarinic acetylcholine (ACh) receptors remains unresolved. Up to now, this problem has mainly been investigated by studying the characteristics of ACh release from insect synaptosomes (Breer & Knipper, 1984) and by characterizing the binding to central nervous system (CNS) extracts from the cockroach of two muscarinic receptor-blocking drugs, L-[benzilic-4,4'-H] quinuclidinyl benzilate (Meyer & Edwards, 1980; Lummis & Sattelle, 1985) and [7V-meffcy/-H]scopolamine (Lummis & Sattelle, 1986). These parallel approaches have shown that muscarinic receptors exist in the insect CNS. The functional role of these receptors is Investigated in the present study by electrophysiological experiments on a synaptic preparation from the CNS of the cockroach, Periplaneta americana. The synaptic preparation was carefully dissected from the nerve cord of adult males, and consisted of a cercus, the corresponding cereal nerve XI, the sixth abdominal (A6) ganglion (containing the studied synapses) and the ventral nerve cord. The electrophysiological experiments employed the single-fibre oil-gap technique pioneered by Callec (1974), which allows reliable recordings of postsynaptic potentials in an isolated giant interneurone in response to presynaptic stimulation. Unitary excitatory postsynaptic potentials (uEPSPs) were evoked by mechanical stimulation of a single mechanoreceptor (long bristle hair) on the cercus. Composite EPSPs (cEPSPs) were triggered by electrical stimulation of the ipsilateral cereal nerve XT. In the cockroach, EPSPs are known to result from the interaction of ACh with nicotinic ACh receptors of giant interneurones (Sattelle et al. 1983). Sensitivity of nicotinic ACh receptors was tested by ionophoretic injection of carbamylcholine (CCh) according to the method developed by Callec et al. (1982). Direct measurements of postsynaptic membrane resistance were achieved using a balanced Wheatstone bridge circuit to apply hyperpolarizing square current pulses to the membrane. The saline superfusing the desheathed A6