Fig. 1.

Induction of daily LH surges by estradiol in mice. (A) Bars represent serum LH concentrations (mean ± SEM). Open bars show samples obtained at 7 a.m., and filled bars show samples obtained at 4 p.m. (B) Serum LH levels (mean ± SEM) sampled in OVX+E mice at various times on d 2 after OVX+E. Gray shading indicates time during which lights were off. LH surge reliably begins ≈1.5 h before lights off (4:30 p.m.). (C) Serum LH levels show no diurnal difference in OVX mice, and estradiol induces negative feedback in the a.m. and positive feedback in the p.m. A, a.m. same-day control; B, 7 a.m. control; C, OVX control. *, P < 0.05; ***, P < 0.001; #, P < 0.01 vs. d 2 p.m.