Effects of systemic and intracerebroventricular phenylephrine and clonidine on corticosterone secretion in rats.

A site of action of adrenergic drugs and type of receptors involved in stimulating the pituitary-adrenocortical activity, assessed indirectly through corticosterone (CS) secretion, was investigated in conscious rats. Adrenergic drugs were administered intraperitoneally (i.p.) or intracerebroventricularly (i.c.v.), the antagonists always 15 min prior to the agonist and 1 h later the trunk blood was collected for CS determination. Phenylephrine (PHE) (alpha 1-agonist) given by either route increased dose-dependently the serum CS levels. Such effect of PHE given i.p. was abolished by i.p. pretreatment with prazosin (PRA) (alpha 1-antagonist), and similar effect of i.c.v. administered PHE was considerably suppressed by i.c.v. pretreatment with PRA. I.c.v. pretreatment with yohimbine (YO) (alpha 2-antagonist), did not influence the effect of PHE. Clonidine (CLON) given i.c.v. significantly increased the serum CS level and such effect was halved by i.c.v. pretreatment with either YO and PRA. I.p. administered PRA did not decrease the CLON-induced CS response. YO given i.p. considerably reduced the CLON-induced CS response in both low and high doses thus indicating the interaction with pre- and post-synaptic alpha-adrenoceptors. These results suggest that PHE activated the pituitary-adrenocortical axis by a selective stimulation of alpha 1-adrenoceptors located in the hypothalamus and possibly on anterior pituitary corticotrophs. CLON seems to stimulate the secretion of CS by activating both alpha 2- and alpha 1-adrenoceptors in the hypothalamus.