SUCKLING-INDUCED FOS-IMMUNOREACTIVITY IN SUBGROUPS OF HYPOTHALAMIC POMC NEURONS OF THE LACTATING RAT - INVESTIGATION OF A ROLE FOR PROLACTIN

Attention has recently been focused on lactation-induced modifications of activity of neuronal populations in the arcuate nucleus (ARC) of t he mediobasal hypothalamus. The ARC hosts the tubero-infundibular dopa minergic (TIDA system) responsible for the neuroendocrine control of p rolactin (PRL), and other non-neuroendocrine neuronal populations, suc h as neuropeptide Y (NPY)- and proopiomelanocortin (POMC)-containing s ystems that are important modulators of hypothalamic gonadoliberin (Gn RH) secretion. Our longstanding interest in the functional anatomy of the ARC led us to investigate whether the suckling stimulus would trig ger an expression of Fos-ir in specific arcuate neuronal populations a nd to possibly characterize responsive neurons by using double-labelin g immunohistochemistry. Freely nursing lactating females expressed str ong Fos-ir in neurons of the ARC compared to diestrous females. Fos-ir was encountered in neurons not belonging to the TIDA system and that was for a targe proportion identical to the POMCergic neurons. We show ed that, in lactating females submitted to suppression of the suckling stimulus by removal of the pups, the pattern of expression of Fos-ir is similar to that seen in diestrous females and that, a pattern of ex pression of Fos-ir indistinguishable from that observed during free la ctation is reinstated a short time after the return of the pups and re storation of the suckling stimulus, suggesting that this expression of Fos-ir strictly depends upon the presence of the newborns and the suc kling stimulus, By lowering circulating levels of the PRL with bromocr yptine-or PRL antiserum-treatment, we noticed a decrease in the number of (beta-endorphin + Fos)-ir neurons compared to noninjected freely n ursing lactating females, By maintaining high levels of circulating PR L with haloperidol-treatment, we observed a number of colocalizations close to that observed in freely nursing lactating females, Our result s suggest that during lactation a rostral subgroup of the arcuate POMC ergic neuronal population is activated at least partially in response to the suckling-induced secretion of PRL and that this activation part icipates in maintaining the endocrine and/or metabolic demands of the lactational status.