PREOPTIC RECESS ALPHA-ADRENOCEPTORS CONTROL CARDIOVASCULAR-RESPONSES TO HYPEROSMOLALITY

The roles of alpha-adrenoceptors in the anteroventral third ventricle (AV3V) and diagonal band of Broca (DBB) in cardiovascular responses to peripheral hypertonicity were investigated in conscious rats. Normal artificial cerebrospinal fluid (aCSF) or aCSF containing phentolamine (alpha(1)- and era-antagonist), yohimbine (alpha(2)-antagonist), or pr azosin (alpha(1)-antagonist) was perfused through microdialysis probes in the DBB, AV3V, or lateral ventricle during a 30-min infusion of is otonic (0.17 M; 0.1 or 1.7 ml.kg(-1).min(-1) iv) or hypertonic (2.5 M; 0.1 ml.kg(-1).min(-1) iv) NaCl. Hypertonic infusion increased blood p ressure [mean arterial blood pressure (MAP); 17 +/- 2 mmHg] and decrea sed heart rate (HR; 36 +/- 6 beats/min). Both responses were abolished by AV3V administration of phentolamine or yohimbine, whereas prazosin selectively prevented the bradycardia. Phentolamine in the DBB or lat eral ventricle did not alter either response. Stimulation of AVSV alph a(1)-adrenoceptors (phenylephrine) decreased HR and MAP, whereas alpha (2)-adrenoceptor stimulation (clonidine) produced bradycardia but incr eased MAP. Data suggest that alpha-adrenoceptors in the AV3V, but not the DBB, regulate cardiovascular responses to hyperosmolality.