The effect of lactation on induced Fos-like immunoreactivity in the rat hypothalamic paraventricular nucleus

Abstract

Lactating rats display a period of blunted hypothalamo-pituitary-adrenal (HPA) response to a variety of stressors. This hyporesponsiveness is reported to be dependent upon continuous mother–pup interactions. In this study, computer-assisted densitometric methods were used to measure levels of induced Fos-like immunoreactivity (FLI) in the hypothalamic paraventricular nucleus (PVN) of lactating and non-lactating rats. Adrenalectomy (ADX) induces elevated levels of FLI in the PVN of non-lactating rats. We have observed that, between post-partum day (pd) 4 and pd 21, the level of ADX-induced FLI in the PVN of lactating rats follows a U-shaped distribution; that the persistence of this phenomenon is dependent upon continued mother–pup interaction and that sustained mother–pup interaction beyond the end of the normal suckling period (pd 21) does not extend the period of refractoriness. We have further determined that both the non-specific neural activator Metrazole, and the glutamate agonist N-methyl- d, l-aspartate (NMA), induced smaller increases in FLI in the PVN of lactating rats compared to non-lactating cohorts, and that the suppressing effect of lactation on Metrazole-induced FLI does not extend to all brain regions. These results suggest that mechanisms responsible for the onset and maintenance of the so-called lactational stress-hyporesponsive period (LSHRP) include altered function of glutamatergic pathways. © 1997 Elsevier Science B.V. All rights reserved.