Injection of endothelin-1 into the raphe obscurus of rats induces depressor responses predominantly through endothelin ET(A) receptors.

We used in vitro autoradiography to identify the endothelin-1 receptor subtype(s) in the nucleus raphe obscurus of rats. These studies showed dense binding of [125I]PD 151242 (for endothelin ET(A) receptors), while tissues incubated with [125I]BQ3020 (for endothelin ET(B) receptors) had low binding. In addition, we examined the effects of the endothelin receptor antagonists FR 139317 (endothelin ET(A) receptor-selective antagonist), SB 209670 (endothelin ET(A)/ET(B) receptor-non-selective antagonist) and BQ-788 (endothelin ETB receptor-selective antagonist) on the blood pressure responses following administration of endothelin-1 into the nucleus raphe obscurus. The basal mean arterial blood pressure (MABP) of the rats was 110+/-7 mmHg (n = 5). This was decreased in a dose-dependent manner by endothelin-1 (0.1, 1 and 10 pmol) microinjected into the nucleus raphe obscurus. This effect occurred within 1-6 s and recovered within 4+/-1.2 min at a dose of 10 pmol. The doses of 0.1 pmol and 1 pmol ET-1 had responses which lasted 1+/-0.4 min and 2+/-0.2 min, respectively. Small decreases in heart rate accompanied the MAP responses to endothelin-1. For instance, the heart rate decreased by 16+/-4 beats min(-1) after 10 pmol endothelin-1 (control, 366+/-6 beats min(-1), n = 5). Decreases in blood pressure induced by endothelin-1 were greatly reduced by pre-administration to the nucleus raphe obscurus of FR139317 (5 nmol/rat) or SB209670 (3 nmol/rat; 97+/-7\% and 95+/-6\%, P < 0.01, n = 5, respectively), but were not affected by BQ-788 (50 nmol/rat; 8+/-3\%, P > 0.05, n7 = 5). The antagonists did not influence heart rate when injected to the nucleus raphe obscurus prior to endothelin-1. FR 139317 (0.5 nmol) and SB209670 (0.3 nmol) had no effects on endothelin-induced changes in arterial blood pressure. Therefore, the autoradiographic study showed that there are binding sites for ET-1 within the nucleus raphe obscurus of rats, which are predominantly of ET(A) type. The in vivo study showed that ET(A) receptors are the predominant mediators of depressor responses induced by endothelin-1 injected into this nucleus.