Nicotine exerts both stimulant and depressant effects on the CNS which may modify its responses to stress. The ACTH axis has been used as a model to understand how nicotine acutely stimulates the rat brainstem-hypothalamic unit. Studies to further elucidate this stimulant effect of nicotine and its desensitization will focus on the neurochemical interaction between catecholamines originating in the brainstem, hypothalamic corticotropin releasing factor (CRF) and plasma ACTH. Investigations are also proposed to assess whether nicotine (i) affects brain function by altering genomic expression, and (ii) depresses neuronal genomic responses to non-nicotinic stimuli, such as CRF, which may be involved in coordinating CNS responses to stress. To localize and quantitate the depressive effects of nicotine, we plan to use the expression of c-fos mRNA, product of a rapid, early response gene, as an index of neuronal genomic responses to two stimuli: (i) metrazole, which as been shown to induce neuronal c-fos expression, and (ii) CRF, which we have shown induces c-fos. The neurochemical studies of nicotine stimulated ACTH secretion will correlate the turnover rates of hypothalamic CRF vs epinephrine and norepinephrine in rats pretreated with intracisternal colchicine or alpha-methyl-p-tyrosine, respectively, using HPLC with electrochemical detection of catecholamines. In these studies, a single dose of i.v. vs fourth ventricular (i.c.v.) nicotine will be compared. The effects of catecholamine depletion or adrenergic antagonists on CRF turnover and ACTH secretion in response to i.c.v. nicotine will be determine. Chronic, repetitive dosing with nicotine will also be studied to clarify the neurochemistry underlying desensitization of the ACTH response to nicotine. The effects of acute and chronic exposure to nicotine on the levels of hypothalamic CRF mRNA will be evaluated to determine whether chronic exposure causes neuronal adaptation. In many studies, the hypothalamic vs extra-hypothalamic effects of nicotine will be compared by in-situ hybridization or in tissue obtained by microdissection. Finally, the proximal site(s) mediating the effect of nicotine on ACTH secretion will be determine after local neurotoxic ablation or by infusing nicotine into selected brainstem cell groups. To determine whether chronic vs acute exposure to nicotine depresses the expression of c-fos mRNA in response to neuronal activation by metrazole or CRF, c-fos mRNA levels will be quantitated by dot-blot and Northern hybridization analysis of specific microdissected nuclei. The role of glucocorticoid feedback in mediating nicotine's affect on c-fos expression will be assessed in adrenalectomized vs glucocorticoid-replaced rats. Finally, intraparenchymal nicotine will be used to evaluate whether the depressive effect of nicotine is mediated locally. In summary, these studies test the hypothesis that a single exposure to nicotine stimulates the release of brain CRF, leading to ACTH secretion as part of a coordinated response similar to a stress response, whereas chronic exposure desensitizes CRF neurosecretory responses and depresses c-fos responses stimulated by CRF.
Showing the most recent 10 out of 55 publications
