Catecholamines mediate nicotine-induced adrenocorticotropin secretion via alpha-adrenergic receptors

Previous studies determined that iv nicotine stimulates ACTH secretion by acting on sites accessible from the fourth ventricle (IV), rather than directly on CRF-containing neurons in the hypothalamus. Brainstem catecholaminergic cell groups, which are accessible from the IV, project to the hypothalamus and stimulate the secretion of CRF and ACTH. Therefore, these studies investigated the role of catecholamines in nicotine-stimulated ACTH secretion. Experiments with the catecholaminergic neurotoxin, 6-hydroxydopamine, demonstrate that the ACTH response to nicotine delivered iv (0.03 or 0.05 mg/kg body wt) or instilled into the IV (1 or 2.5 micrograms) was significantly reduced in lesioned animals (P less than 0.01). Selective inhibitors of epinephrine synthesis, SKF 64139 and 2,3-dichloro-alpha-methylbenzylamine (DCMB), significantly reduced (P less than 0.01) the ACTH response to 0.05 mg/kg body wt nicotine iv, without affecting median eminence CRF content. Because controversy exists about the effect of DCMB as an alpha 2 adrenoreceptor antagonist in vivo, this was examined by administering norepinephrine into the third ventricle after DCMB ip; DCMB significantly reduced the ACTH response to norepinephrine 0.2 micrograms (P less than 0.05) but not to 0.5 micrograms. To determine whether alpha 2 receptors are indeed involved in the ACTH response to nicotine, yohimbine, an alpha 2 antagonist, was injected into the third ventricle before nicotine injection into the IV. Yohimbine significantly reduced the ACTH response. Thus, the secretion of both hypothalamic epinephrine and, to some extent, norepinephrine is involved in the ACTH response to the activation of catecholaminergic neurons in the IV. Further investigation of the adrenergic receptor(s) involved compared the ACTH response to nicotine (1 microgram) instilled into the IV after prazocin (alpha 1 antagonist), or propranolol (beta antagonist) was injected into the third ventricle. Prazocin significantly reduced (P less than 0.05) the ACTH response, whereas propranolol was ineffective. Thus, both alpha 1 and alpha 2 receptors are involved in mediating the ACTH response to nicotine.