The mechanism of the clonidine-induced reduction in peripheral parasympathetic submaxillary salivation

The mechanism of the clonidine induced reduction in submaxillary salivation evoked by electrical stimulation of the chorda tympani was investigated in anaesthetised cats. This effect of clonidine was found to be dose and frequency dependent. In addition to clonidine, tramazoline, also a preferential presynaptic alpha-adrenoceptor agonist, produced a reduction in electricallly evoked salivation. Methoxamine, noradrenaline and naphazoline, which are less potent presynaptic alpha-receptor agonists, caused increases in salivation. Phentolamine only partially antagonized the decrease in salivation produced by clonidine whereas it was virtually abolished by yohimbine. Clonidine increased salivation evoked by intra-arterial injections of carbachol. These findings suggest that clonidine reduces peripheral parasympathetically evoked submaxillary salivation by activation of presynaptic alpha-adrenoceptors which inhibit cholinergic transmission.