THE EFFECT OF LACTATION ON INDUCED FOS-LIKE IMMUNOREACTIVITY IN THE RAT HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

Lactating rats display a period of blunted hypothalamo-pituitary-adren al (HPA) response to a variety of stressors. This hyporesponsiveness i s reported to be dependent upon continuous mother-pup interactions. In this study, computer-assisted densitometric methods were used to meas ure levels of induced Fos like immunoreactivity (FLI) in the hypothala mic 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 Fl;I in the PVN of lactating rats follows a U-shaped distribution; that the persistence of this phenomen on is dependent upon continued mother-pup interaction and that sustain ed 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 i ncreases in FLI in the PVN of lactating rats compared to non-lactating cohorts, and that the suppressing effect of lactation on Metrazole-in duced FLI does not extend to all brain regions. These results suggest that mechanisms responsible for the onset and maintenance of the so-ca lled lactational stress-hyporesponsive period (LSHRP) include altered function of glutamatergic pathways, (C) 1997 Elsevier Science B.V.