Opiate receptor subtypes in the nucleus tractus solitarii of the cat: the effect of vagal section.

The distribution of mu, delta and kappa opioid receptors in the lower brainstem of the cat has been determined autoradiographically by studying the binding of tritiated [D-Ala2,MePhe4,Glyol5][tyrosyl-3,5-3H]enkephalin (DAGO), [D-Pen2,D-Pen5][tyrosyl-3,5-(n)-3H]enkephalin (DPDPE) and [9-3H]ethylketazocine (EKC), respectively. General opiate receptor binding was established using [3H]naloxone (NX). High densities of [3H]NX and DAGO binding sites were found most prominently in the nucleus tractus solitarii. There was no DPDPE and very weak EKC binding within this nucleus, although both these ligands bound to the cerebellum. The effect of unilateral vagotomy on receptor density was examined. Sectioning the cervical vagus had no effect on the density of mu receptors in the brainstem. Sectioning the vagus, accompanied by nodose ganglion excision, led to a marked depletion of mu receptors which was restricted to dorsal and medial regions of the ipsilateral nucleus tractus solitarii at, and rostral to, the obex. These results suggest that mu opiate receptors are located presynaptically on vagal afferents terminating within a restricted region of the nucleus tractus solitarii.