Postjunctional alpha 2-adrenoceptors in pial arteries of anesthetized newborn pigs

The purpose of this study was to characterize the nature of the postjunctional alpha-adrenoceptors (alpha 1 versus alpha 2) in the cerebral circulation of anesthetized newborn pigs. Diameters of pial arteries in anesthetized piglets, 1-6 days old, were examined using a 'closed' cranial window. We examined sympathetically mediated constriction of pial arteries in the presence and absence of alpha 1- (prazosin) or alpha 2- (yohimbine) adrenoceptor antagonists (1 mg/kg i.v.). Initial experiments demonstrated that prazosin and yohimbine attenuated constriction of pial arteries during topical administration to the pial surface of phenylephrine and clonidine, respectively. In one group, electrical stimulation of the ipsilateral superior cervical ganglion reduced pial arterial diameter from 176 +/- 26 (mean +/- SEM) to 153 +/- 24 microns, and this response was unaffected by prior administration of prazosin. In a second group, sympathetic nerve stimulation reduced pial arterial diameter from 175 +/- 23 to 148 +/- 19 microns, and this response was abolished by prior administration of yohimbine. After administration of yohimbine, pial arteries still constricted in response to topical phenylephrine and arterial hypertension. We conclude that sympathetic nerve stimulation in anesthetized newborn pigs constricts pial arteries via alpha 2-adrenoceptors.