PRESYNAPTIC MUSCARINE RECEPTORS AND INHIBITION OF RELEASE

Abstract

This chapter discusses the presynaptic muscarine receptors and inhibition mechanism. Inhibition of the stimulation-evoked noradrenaline output could be attributed to the activation of a muscarine receptor affecting the neuronal release. The chapter presents certain experiments in which an electrical stimulation of the vagus nerves was employed for releasing endogenous acetylcholine onto adrenergic nerve fibers. The results of these experiments showed that muscarinic drugs do not interfere with the propagation of action potentials along the terminal sympathetic fibers but inhibit a process that is confined to the actual site of release. A pharmacological significance of the muscarinic inhibition of noradrenaline release is rendered likely by the possibility that atropine-like drugs relieve the restriction by an endogenous parasympathetic activity of a sympathetic tone and that muscarinic drugs inhibit the effect of a sympathetic drive. Adrenergic nerve fibers endowed with presynaptic inhibitory muscarine receptors occur not only in organs innervated by the autonomic ground plexus but also in organs without a functionally important cholinergic innervation such as veins, abdominal arteries, or spleen.