Endothelin‐1 and the periaqueductal gray area of the rat: an autoradiographic and functional pharmacological study

Abstract

1 Endothelin-1 (ET-1) injected centrally induces pressor effects and associated haemodynamic changes.Here we have evaluated the effects on systemic and regional cardiovascular parameters of injection of ET-1 into the periaqueductal gray (PAG) area of anaesthetized rats. In addition, we have used the ETA receptor-selective antagonist, FR 139317, the ETB receptor-selective antagonist, BQ-788, and the ETA/ ETB receptor non-selective antagonist, SB 209670, to identify the receptor(s) mediating these effects. We have also used in vitro autoradiography to identify binding sites for ET-1 in the PAG. 3 Injection of ET-1 (0.1, 1 and 10 pmol per rat) in the dorsolateral PAG area significantly increased, in a dose-dependent manner the mean arterial blood pressure (MAP). The highest dose of ET-1 (10 pmol) also decreased the heart rate by 18+1%, n = 6 (P <0.05). Increases in blood pressure induced by ET-1(1 pmol; 31+6.6 mmHg, n = 6) were greatly reduced by pre-administration to the PAG area of FR 139317 (5 nmol per rat) or SB 209670 (3 nmol per rat) (97 and 94%, respectively), but were unaffected by BQ-788 (5 nmol per rat). Similarly, FR 139317 and SB 209670 prevented the decrease in heart rate induced by ET-1 while BQ-788 did not affect it. 4 Injection of ET-l to the PAG area caused falls in renal blood flow (RBF) as measured by an ultrasonic flow probe, and increased renal vascular resistance (RVR). Pre-treatment of the PAG with FR 139317 or SB 209670, but not with BQ-788, prevented this ET-1-induced effect. 5 Injection of ET-1 (10 pmol) also increased total peripheral resistance (TPR; control, 2.39 +0.2 mmHg ml-' min 100 g body weight) by 100+9% (n=5) and reduced the cardiac output (CO; control, 94.7 + 3.1 ml min-') by 30 + 3% (n = 5), as determined by radioactive microspheres. Vascular resistances were increased in other organs, such as skeletal muscle (88+5%, n=4), the colon (55+7%, n=4) and the stomach (47+3%, n=4). Pretreatment of the PAG area with FR 139317 or SB 209670 reduced the increases in TPR and vascular resistance, and the reduction in CO caused by ET-1. BQ-788 did not affect the responses to ET-1. 6 Thus, there are predominantly ETA binding sites within the PAG area and injection of ET-l into the PAG area causes complex haemodynamic changes which are sensitive to ETA receptor antagonism. ETA receptors are, therefore, the predominant mediators of the actions of ET-1 in the PAG of the rat.

Local administration of ETA (but not ETB) blockers into the PAG area of the brain decreases blood pressure of DOCA-salt rats

“…In accordance with our previous report (D'Amico et al 1996), microinjections of the various ET receptor antagonists into the PAG area of SD rats did not modify MABP and HR (Table 1).…”

“…The dosage of ET-1 antagonists (FR139317, 5 nmol; SB209670, 3 nmol; BQ-788, 5 nmol) was chosen in accordance with previous studies in which the same dosage effectively reduced cardiovascular effects elicited by ET-1 into the PAG area (D'Amico et al 1996).…”

“…In particular, the study has focussed on the dorso-lateral periaqueductal grey area (PAG) of the central nervous system. Autoradiographic studies have already shown that, in normotensive rats, endothelin-1 binds rather well to PAG (Kohzuki et al 1991;D'Amico et al 1996), and that it participates in the central control of the cardiovascular function mainly by activating the endothelin receptor subtype ETA (D'Amico et al 1996).…”

“…In contrast, no study has been conducted to assess whether, in DOCA-salt-treated rats, changes in the endothelin system also occur at CNS level, since a number of studies have underlined that endothelin-1 can be found in the central nervous system (CNS) with its own receptors and participating in the control of cardiovascular functions (D'Amico et al 1996).…”

“…In addition, since no data is available to support the influence that a DOCA-salt treatment has on the levels of the mRNA vis-à-vis the brain's most prevalent ET receptors, the ETA subtype that is (Kohzuki et al 1991;D'Amico et al 1996), we assessed the mRNA ETA levels by means of semiquantitative RT-PCR. Moreover, since there is no evidence of the influence some form of antagonism might have at central ET-receptor level on the hypertensive state of DOCA-salt rats, functional studies have been used to assess whether intracerebral endothelin receptor antagonists decrease arterial blood pressure levels and the heart rate (HR) values induced in rats by a DOCA-salt-based treatment.…”