Opiate receptors and the endorphin-mediated cardiovascular effects of clonidine in rats: evidence for hypertension-induced mu-subtype to delta-subtype changes.

Publisher Website
Google Scholar
Abstract
Effects of opiate receptor antagonists on centrally mediated cardiovascular responses to clonidine and .beta.-endorphin were studied in urethane-anesthetized spontaneously hypertensive Okamoto-Aoki rats (SHR), normotensive Sprague-Dawley rats, and Sprague-Dawley rats made hypertensive with deoxycorticosterone pivalate/salt. Microinjection of 270 pmol of naloxone into the nucleus tractus solitarii (NTS) significantly inhibited the hypotensive and bradycardic response to 5 nmol of similarly administered clonidine in both SHR and normotensive Sprague-Dawley rats. In SHR, a similar inhibition was observed after the .delta.-opiate receptor antagonist ICI 174864, but not after the .mu.-receptor antagonist .beta.-funaltrexamine (both at 270 pmol, intra-NTS), whereas in normotensive Sprague-Dawley rats, .beta.-funaltrexamine, but not ICI 174864, was an effective inhibitor. The same pattern of differential inhibition was seen when clonidine was given i.v. and the opiate antagonists were given intracisternally in SHR and Sprague-Dawley rats. Intra-NTS microinjection of 280 fmol of .beta.-endorphin caused hypotension and bradycardia, and these effects were similarly inhibited by ICI 174864 in SHR and by .beta.-funaltrexamine in Sprague-Dawley rats. In Sprague-Dawley rats made hypertensive by chronic administration of deoxycorticosterone pivalate and salt, the hypotensive and bradycardic effects of intra-NTS clonidine were inhibited by ICI 174864, but not by .beta.-funaltrexamine, a pattern similar to that in SHR, but different from that in normotensive Sprague Dawley rats. These results support the hypothesis that .beta.-endorphin release and subsequent stimulation of opiate receptors in the NTS are involved in the cardiovascular effects of clonidine in rats. These results further suggest, however, that hypertension regulates the subtype of opiate receptors mediating these effects.