ANOMALOUS ADRENALECTOMY-INDUCED FOS-LIKE IMMUNOREACTIVITY IN THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS OF STRESS-HYPORESPONSIVE RATS

Cellular activity in the paraventricular nucleus of the hypothalamus ( PVN) in response to adrenalectomy (ADX) and sham-ADX was measured in a dult male rats, lactating females, and nursing pups using c-fos immuno cytochemistry. Increased Fos-like immunoreactivity (FLI) was seen in t he PVN of male rats at 4 h following ADX or sham-ADX but this increase was transient, and basal values were restored within 24 h. In sucklin g pups, ADX induced a marked FLI response in the PVN at 3 days and at 18 days postpartum. At 11 days postpartum, however, the FLI response w as attenuated relative to adult animals, and 3- and 18-day-old pups. S imilarly, in nursing mothers, ADX induced FLI at 3 days, but this resp onse disappeared by 7 days and did not reappear by the end of the suck ling period (day 21). These data indicate that c-fos expression is a s ensitive indicator of hyporesponsiveness in the hypothalamic-pituitary -adrenal (HPA) system.