Objectives: The magnocellular neurons in the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON) either contain vasopressin or oxytocin. Even though both hormones are released after systemic administration of nicotine, the mechanism through which the two populations of neurons are activated is not known. This study was carried out in the rat to investigate the effect of increasing doses of nicotine on subsets of magnocellular neurons containing either oxytocin or vasopressin.
Methods: The activated neurons were identified by means of Fos immunohistochemistry and the induction of Fos in magnocellular subdivisions was investigated by means of dual-immunohistochemistry.
Results: While oxytocinergic neurons were sensitive to systemic administration of 0.5 mg/kg of nicotine, vasopressinergic neurons were not affected at doses up to 1 mg/kg. The vast majority (85%) of oxytocinergic neurons in the PVN was affected by nicotine, whilst only about half of the vasopressinergic neurons were stimulated, and only at maximal doses. Notably, the sensitivity of oxytocinergic neurons to nicotine was found to be different in the PVN and SON, because only about 55% of the SON oxytocinergic neurons co-stored Fos even after the highest dose of nicotine.
Conclusion: These data show that magnocellular neurons are differentially regulated by nicotine and that their sensitivity is dependent on both their peptidergic phenotype and their location within the hypothalamus.
Keywords: acetylcholine, vasopressin, oxytocin, Fos, stress, cell counting.