CHANGING PATTERNS OF FOS EXPRESSION IN BRAIN-STEM CATECHOLAMINERGIC NEURONS DURING THE RAT ESTROUS-CYCLE

Brainstem catecholaminergic neurons are believed to play an important role in the activation of luteinising hormone-releasing hormone (LHRH) neurons on the afternoon of proestrus which results in the luteinising hormone (LH) surge. To examine the respective roles of brainstem A1 and A2 neurons and the adjoining C1 and C2 adrenergic cells at this time, we have examined the patterns of Fos-immunoreactivity within tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl transferase (PNMT) neurons during diestrus and proestrus. Initial studies demonstrated that the LH surge commenced at approximately 15:00 h in proestrous animals and that peak plasma levels of LH were observed between 16:00 and 17:00 h. Groups of cycling female rats (n = 6) were then perfused between 09:00 and 11:00 (diestrus early) and 18:00 to 19:30 h (diestrus late) on diestrus and at the same times on proestrus (proestrus early and proestrus late). Double-labelling immunocytochemistry revealed little Fos expression by adrenergic neurons of the C1 or C2 cell groups and this did not change significantly between any of the experimental groups. Analysis of the A2 region was divided into rostral, middle and caudal divisions and all regions showed a significant (P < 0.01) increase in the number of Fos-expressing TH neurons (up to 35% of TH cells) in proestrus early animals compared with diestrus and proestrus late rats. In the A1 region, a significant increase in the number of TH neurons expressing Fos (similar to 33%) was detected in both proestrus early (P < 0.05) and diestrus early (P < 0.01) rats compared with animals perfused in the late afternoon (similar to 12%). These results indicate that TH-immunoreactive neurons in both A1 and A2 cell groups are activated on the morning of proestrus prior to the LH surge whilst the C1 and C2 adrenergic neurons express little Fos throughout. The morning increase in Fos-expression by TH neurons within the A1 region on both diestrus and proestrus indicates a circadian pattern of activation for A1 noradrenergic cells and suggests different roles for the A1 and A2 cell groups in regulating the activity of LHRH neurons on proestrus.