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

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. 2. In vitro autoradiography showed dense binding of [125I]-PD 151242 (for ETA receptors) in the PAG area, with the binding sites being homogeneously distributed within the dorsal, lateral and ventral subregions. Tissues incubated with [125I]-BQ 3020 (for ETB receptors) had little binding. 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-1 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-1 min 100 g body weight) by 100 +/- 9% (n = 5) and reduced the cardiac output (CO; control, 94.7 +/- 3.1 ml min-1) 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 effect the responses to ET-1. 6. Thus, there are predominantly ETA binding sites within the PAG area and injection of ET-1 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.