Oestrogen receptor alpha and beta immunoreactive cells in the suprachiasmatic nucleus of mice: distribution, sex differences and regulation by gonadal hormones

Oestrogen regulates various aspects of circadian rhythm physiology. The presence of oestrogen receptors within the suprachiasmatic nucleus (SCN), the principal circadian oscillator, indicates that some actions of oestrogen on circadian functions may be exerted at that site. The present study analysed sex differences, topographic distribution, and neurochemical phenotype of neurones expressing the alpha and beta subtypes of oestrogen receptors (ERalpha and ERbeta) in the mouse SCN. We found that relatively few neurones in the SCN are immunoreactive (IR) for ERalpha (approximately 4.5% in females and 3% in males), but five- to six-fold more SCN neurones express ERbeta. ER-IR neurones are primarily in the shell subdivision of the nucleus and show differences between the sexes, significantly greater numbers being found in females. Treatment of male or female gonadectomised mice with oestradiol benzoate for 24 h substantially reduced the number of ERbeta-IR neurones, but not ERalpha-IR neurones. Double-labelling immunocytochemical experiments to characterise the phenotype of the oestrogen-receptive neurones showed the presence of the calcium-binding proteins calretinin or calbindin D28K in approximately 12% and 10%, respectively, of ERalpha-IR neurones. A higher proportion (approximately 38%) of ERbeta-IR neurones contains calbindin D28K; a few (approximately 2%) express calretinin or vasopressin. These double-labelled cells appear primarily in the shell subdivision of the SCN. Neither vasoactive intestinal polypeptide- nor gastrin releasing peptide-immunoreactivity was observed in ER-IR neurones. These data indicate that the primary target cells for oestrogen are in the shell subdivision of the nucleus. The sexually differentiated expression and distribution of ERalpha and ERbeta in various cell populations of the SCN suggest multiple modes of oestrogen signalling within this nucleus, which may modulate circadian functions.