Presynaptic muscarinic M1 and M2 receptor modulation of auriculotemporal nerve transmission in the rat

Abstract

Parotid secretory and vascular responses to electrical stimulation of the parasympathetic innervation were measured in anaesthetized rats. Stimulation was performed at 1, 10 and 40 Hz. Atropine (1.5 μmol/kg I.V.) almost abolished the secretion to stimulation of peptide depleted nerves at 40 Hz, thus confirming the existence of a pure cholinergic response. Atropine also reduced secretion by 74% during stimulation of non-depleted nerves at the same frequency. Selective blockade by the muscarinic M1 receptor antagonist pirenzepine and by the muscarinic M2 receptor antagonist methoctramine was found to occur at doses (50 nmol/kg I.V. and of 300 nmol/kg I.V., respectively) that did not inhibit the responses to exogenous acetylcholine. In the presence of methoctramine, the nerve-evoked fluid responses were increased by 200% at 1 Hz independently of the total number of impulses (10–300), suggesting that M2 receptor activation normally has an inhibitory effect on transmitter release. The magnitude of the increase was inversely related to frequency of stimulation, and changes in the secretory responses occurred at 40 Hz only when non-depleted nerves were stimulated over the longest period employed. The fluid response then increased by 35% and protein concentration by 200%. The vasodilator responses increased at 1 and 10 Hz, but not at 40 Hz. Pirenzepine reduced the secretory and vascular responses at 10 and 40 Hz but only during stimulation over short periods of time. This suggests that M1 receptor activation normally has a facilitatory effect on neurotransmitter release. During stimulation of non-depleted nerves at 10 Hz for 10 impulses, the fluid response was reduced by 29% and the protein concentration by 26%. When the peptide depleted nerves were stimulated at 10 Hz, pirenzepine also reduced the fluid response (by 43%), but not the protein concentration. It is concluded that the release of transmitter from postganglionic nerve fibres in the rat auriculotemporal nerve is modulated by presynaptic muscarinic receptors. Muscarinic M1 receptors normally facilitate cholinergic and peptidergic transmission during short, intense stimulation. On the other hand, muscarinic M2 receptors normally inhibit cholinergic transmission at low frequencies; at higher frequencies, peptidergic transmission is also inhibited, but only after some delay.