We studied the effect of glucocorticoid pretreatment, mediobasal hypothalamus lesion (MBHL) and the interaction between clonidine and yohimbine in male Wistar rats to elucidate the sites and/or mechanisms of endocrine actions of α2-antagonists. The pretreatment of 1 mg/kg s.c. dexamethasone for 4 days effectively prevented the stimulatory effect of α2-antagonists yohimbine (5 mg/kg i.p.) and CH-38083 (1 mg/kg i.p.) on adrenocorticotropin (ACTH) secretion, while the action of these antagonists on prolactin (PRL) and β-endorphin (βE) remained unchanged. The central (i.c.v.) pretreatment of 5 µg/ rat clonidine failed to antagonize the prolactin (PRL) and βE releasing effect of yohimbine. However, it inhibited the yohimbine-induced ACTH secretion. MBHL resulted in a significant enhancement in basal plasma PRL and β-endorphin (βE) levels. But basal plasma ACTH levels have not been changed. Yohimbine failed to stimulate ACTH secretion in MBH-lesions rats, while PRL and βE response to the yohimbine was maintained in these animals. This study confirms that the α2-antagonists stimulate ACTH secretion by a cortico-steroid-sensitive mechanism which is located centrally. In contrast, α2-antagonists affect PRL and βE secretion via a corticosteroid-insensitive mechanism located at the periphery, possible within the pituitary gland.

Keywords:
Adrenal steroids, Catecholamine receptors, Corticotropin, Prolactin, Beta-endorphin
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© 1995 S. Karger AG, Basel
1995
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