Evidence for cholinergic vagal afferents and vagal presynaptic M 1 receptors in the ferret

Abstract

The distribution of muscarinic receptor binding was examined in the ferret brainstem vagal nuclei using the non-selective ligand [ 3H]quinuclidinyl benzilate and the relatively M 1 receptor-selective ligand [ 3H]pirenzepine. The highest density of receptor sites are found in the subnucleus gelatinosus and lower levels in the other subnuclei of the nucleus of the tractus solitarius and in the area postrema and dorsal motor nucleus of the vagus nerve. Dense binding was also seen in the adjacent hypoglossal nucleus. Following unilateral cervical nodose ganglion excision binding in the subnucleus gelatinosus was attenuated ipsilateral to the lesion compared with the contralateral side. In contrast, [ 3H]pirenzepine binding was only seen in the subnucleus gelatinosus and in no other region at this level of the brainstem. This binding was reduced in the subnucleus as a whole by 52% ipsilateral to a cervical vagotomy. In the more rostral parts of the subnucleus gelatinosus, binding was undetectable ipsilateral to the lesion but more caudally, appreciable levels of binding persisted. This distribution parallels the known rostro-caudal variation in cross-over of vagal afferent fibres in the ferret dorsal vagal complex and indicates a presynaptic localization of [ 3H]pirenzepine binding sites on vagal afferent terminals. The distribution of binding of the high affinity choline uptake site blocker, [ 3H]hemicholinium-3, was also examined in the ferret brainstem using autoradiography. High densities of [ 3H]hemicholinium-3 binding were seen in the hypoglossal nucleus, the subnucleus gelatinosus and in the area postrema, with lower levels in the dorsal motor nucleus of the vagus, the trigeminal nucleus and other subnuclei of the nucleus of the tractus solitarius. After unilateral vagal denervation, binding of [ 3H]hemicholinium-3 was attenuated in the ipsilateral subnucleus gelatinosus but not in other regions of the caudal medulla oblongata. These data provide evidence that a proportion of vagal afferent fibres are cholinergic and that pirenzepine binding sites, which are likely to be M 1 receptors, are located presynaptically on vagal afferent terminals.