Differential activation of c-fos immunoreactivity after hypophysectomy in developing and adult rats

The aim of the present study is to compare c-fos expression in identified hypothalamic vasopressin (AVP) and oxytocin (OT) neurons in developing (PN7 and PN14) and adult rats following hypophysectomy using dual-labeled immunostaining. Our results showed that hypophysectomy induced c-fos expression in supraoptic (SON) and paraventricular (PVN) nuclei in both the developing and adult rats. Few or no positive cells were observed in the same nuclei in sham-operated animals. Quantitative analysis for c-fos and either of the above named neuropeptides revealed that almost all AVP and OT neurons in the adult and PN14 groups expressed c-fos in response to hypophysectomy. In PN7, hypophysectomy also induced all AVP neurons to express c-fos in SON and PVN. However, few OT neurons in the SON and PVN produced c-fos after hypophysectomy. In addition, the time course of c-fos expression was different in the developing and adult rats after hypophysectomy. The c-fos expression in the developing rats exhibited a more prolonged induction in which staining for c-fos persisted for at least 3 days after hypophysectomy compared with that in the adult in which c-fos immunoreactivity disappeared within 24 hr post-lesion. This study demonstrates that c-fos expression after hypophysectomy is regulated differently during development.