An AF-DX 116 sensitive inhibitory mechanism modulates nicotinic and muscarinic transmission in cat superior cervical ganglion in the presence of anticholinesterase

The effect of the muscarinic receptor antagonist AF-DX 116 on the inhibitory action of muscarinic agonists and on responses mediated by nicotinic or muscarinic ganglionic transmission was studied in the superior cervical ganglion of the anesthetized cat. The postganglionic compound action potential evoked by cervical sympathetic trunk stimulation was depressed by methacholine or acetylcholine (ACh) injected into the ganglionic arterial supply. The depression was blocked by AF-DX 116. The compound action potentials evoked by preganglionic stimulus trains were also depressed when the intratrain frequency was 2 Hz or greater. This intratrain depression was, however, insensitive to AF-DX 116. The anticholinesterase drug physostigmine markedly enhanced the intratrain depression of the compound action potential. This effect was reversed by AF-DX 116. During nicotinic receptor block with hexamethonium, preganglionic stimulus trains with intratrain frequencies of 5 Hz or greater produced nictitating membrane contractions that could be blocked by the M1 muscarinic receptor antagonist pirenzepine. The amplitude of the contractions increased with frequency and reached a maximum at 20–40 Hz. AF-DX 116 had no effect on these responses. After administration of physostigmine, the amplitude of the nictitating membrane responses decreased with increasing intratrain frequency. AF-DX 116 reversed this effect. The data suggest that, in the superior cervical ganglion, AF-DX 116 sensitive muscarinic receptors which depress synaptic transmission are activated by exogenous agonists but not by the ACh released by the preganglionic axon terminals unless cholinesterase activity is inhibited. This may result from the fact that these receptors, although accessible to agonists injected into the ganglionic arterial supply, are remote from synaptic release sites and that, under normal conditions, cholinesterase activity prevents sufficient activation of these receptors by the nerve-released ACh to result in modulation of synaptic transmission.Key words: muscarinic inhibition, M2 receptors, sympathetic ganglion modulation.