Maintenance of estrous cycle in female rats with anterior or posterior deafferentiation of the suprachiasmatic nucleus

The aim of the present study was to determine which of the anterior or posterior efferents of the suprachiasmatic nucleus (SCN) predominantly act for the maintenance of estrous cycle in the female rats. A small knife (0.8 mm in radius) was used for anterior (ASD), posterior (PSD) and complete suprachiasmatic deafferentiations (CSD). The rats subjected to CSD were divied into two groups, the first group bearing lesions on the medial preoptic nucleus (MPN; group CSD-MPN-L) and the other group bearing no lesions on the MPN (group CSD). For reference, anterior hypothalamic deafferentation (AHD) was carried out using a large knife (2.0 mm in radius). All rats with AHD or CSD-MPN-L and some of CSD rats showed persistent estrus, but all animals with ASD or PSD resumed regular estrous cycles after transient irregular periods. Ovaries of AHD, CSD and CSD-MPN-L rats contained very few, if any, corpora lutea, while the number of corpora lutea in ASD or PSD rats was not significantly different from that of sham-operated controls. Immunohistochemical examination revealed that in AHD rats LHRH immunoreactivity in the median eminence was markedly reduced as compared with that of sham-operated controls. In contrast, in ASD, PSD, CSD and CSD-MPN-L rats, the LHRH immunoreactivity in the median eminence was similar to that of sham-operated controls. These results indicate that estrous cycle and spontaneous ovulation can be maintained if about half of the SCN projections, either from the anterior or posterior routes, are left intact and that persistent anovulatory estrus induced by CSD is not due to the interruption of LHRH fibers, but is simply due to the complete isolation of the SCN from other parts of the brain. Additional observations on wheel-running activity showed that the behavioural circadian rhythm was abolished in CSD and CSD-MPN-L rats. Estrogen-induced daily LH surges were not observed in CSd and CSD-MPN-L rats. Progesterone-induced LH surge was not prevented, but was significantly reduced in CSD animals. However, in CSD-MPN-L rats the rise in LH level after progesterone treatment failed to occur. These results suggest that the SCN exerts its influence, through its anterior and/or posterior routes, on the neural mechanism participating in the positive feedback effect of estrogen.