THE DAILY PATTERN OF FOS SYNTHESIS BY HYPOTHALAMIC PROOPIOMELANOCORTIN NEURONS IS UNAFFECTED BY ADRENALECTOMY IN THE RAT

At the onset of dark, a large population of rat mediobasal hypothalami c (MBH) pro-opiomelanocortin (POMC) neurons starts spontaneously expre ssing Fos-immunoreactivity (Fos-IR). Here we studied the effect of adr enalectomy upon this expression since circulating corticosteroids, whi ch increase in the rat with the onset of behavioural wakening, are tho ught to modulate the basal expression of MBH POMC mRNA. Hence, groups of intact, adrenalectomised and sham-operated rats were sacrificed at times when Fos synthesis by POMC neurons is known to show either nadir (at light-offset) or peak (6 h after light-offset) values. Brains wer e processed for Fos- and/or POMC immunohistochemistry. This allowed us to show that, in all experimental groups, Fos-IR is hardly expressed in MBH POMC neurons at the onset of dark, whereas it is strongly induc ed 6h later. We concluded that such an induction is not triggered thro ugh the known evening rise of plasma corticosteroid levels. (C) 1998 E lsevier Science Ireland Ltd. All rights reserved.