Electrophysiological Pharmacology of the Nicotinic and Muscarinic Cholinergic Responses of Isolated Neuronal Somata from Locust Thoracic Ganglia

Abstract

1. Mechanically isolated neuronal somata from the thoracic ganglia of the locust Locusta migratoria remain electrophysiologically viable under current- or voltage-clamp in vitro for many hours. Nicotine and muscarine evoke different responses when pressure-microapplied to these somata. The response to acetylcholine is mainly nicotinic but contains a small muscarinic component. 2. The nicotinic (ACh1) response is a rapid depolarisation accompanied by a decrease in membrane resistance. In voltage-clamped somata, the current mediating the ACh1 response is inward over the membrane potential range −30 to −110 mV, decreasing with depolarisation and with a projected reversal potential of about +20 mV. 3. The muscarinic (ACh2) response is a slow depolarisation accompanied by a decrease in membrane resistance. In voltage-clamped somata, the current mediating the ACh2 response is inward, decreasing to zero at potentials of −80 to −90 mV. 4. The ACh1 response is evoked by nicotine, anabasine, tetramethylammonium, DMPP and relatively high concentrations of the nitromethylene heterocycle insecticide, PMNI. Suberyldicholine or decamethonium evoke the response only when acetylcholine is present in the bathing saline. Nicotinic antagonists of the ACh1 response, in descending order of potency, are PMN1 >> α-bungarotoxin ≥ lobeline≥mecamylamine>trimethaphan camsylate>chlorisondamine>d-tubocurarine≥hexamethomium≥gallamine triethiodide≥tetraethylammonium. This response is also potently blocked by strychnine and more weakly blocked by δ-philanthotoxin, bicuculline and picrotoxin. 5. The ACh2 response is evoked by muscarine, oxotremorine, arecoline, pilocarpine and, very weakly, by the M1-selective agonist McN-A-343. Muscarinic antagonists of the ACh2 response, in descending order of potency, are QNB≥scopolamine>atropine>4-DAMP (M3)≥benactyzine≥HHSiD (M1/M3)≥ pirenzepine (M1). QNX (M1), AF-DX116 (M2), gallamine triethiodide (M2) and methoctramine (M2) are almost or completely inactive.