Nicotinic actions of oxotremorine on murine skeletal muscle. Evidence against muscarinic modulation of acetylcholine release

Abstract

The effects of oxotremorine, arecoline and muscarine on neuromuscular transmission of mouse or rat phrenic nerve-diaphragm were investigated. For some studies of endplate potentials (e.p.p.s) the preparation was immobilized by cutting muscle fibers. Oxotremorine (0.3–10 μM) depolarized endplate membranes, reduced miniature e.p.p. amplitudes but increased frequency, induced spontaneous neural discharges and muscle fasciculations, and produced contracture of denervated mouse diaphragm. In mouse and young rat preparations pretreated with Mn 2+, Co 2+, Ni 2+, Cd 2+ or low Ca 2+ Tyrode to depress evoked acetylcholine release, oxotremorine 0.3–1 μM increased indirect twitches as well as amplitudes and quantal contents of e.p.p.s. These increases were not observed when the synaptic transmission was not depressed, nor in adult rat preparations. The augmentation by oxotremorine of evoked acetylcholine release persisted in preparations pretreated with neostigmine (1 μM) and tetrodotoxin (20 nM), which inhibited acetylcholinesterase and oxotremorine-induced spontaneous neural discharges. These effects of oxotremerine were mimicked by arecoline but not by muscarine and were antagonized by tubocurarine (0.3 μM) but not by atropine (0.1–10 μM). Atropine alone did not affect indirect twitches, synaptic transmission, tetanic responses evoked by direct stimulation of diaphragms, nor the durations of muscle action potentials. The direct twitch responses were only slightly increased by oxotremorine at 2–3 μM. Oxotremorine at high concentrations (>2 μM), depressed indirect twitches and e.p.p. amplitude, and accelerated the run-down of trains of e.p.p.s. The IC 50 on indirect twitches was reduced by pretreatment with diltiazem or proadifen, which are known to promote receptor desensitization. These results suggest that oxotremorine affects synaptic transmission of neuromuscular junction biphasically by an agonist action on pre- and postsynaptic nicotinic receptors. No muscarinic modulation of acetylcholine release could be demonstrated.