A Daily Neural Signal for Luteinizing Hormone Release in the Untreated Ovariectomized Rat: Changes in Gonadotropin-Releasing Hormone Content of the Preoptic Area and Hypothalamus Throughout the Day1

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Abstract
Gonadotropin-releasing hormone (GnRH) content of preoptic areas (POA) and hypothalami, and serum gonadotropins of rats ovariectomized 6 wk earlier were measured throughout the day in 2 experiments. In the first, rats were decapitated at 2 h intervals between 0800 and 1800 h. The entire preoptic-hypothalamic region was removed and extracted for radioimmunoassay (RIA) of GnRH. Serum gonadotropins measured by RIA were highly variable but mean concentrations were not significantly different throughout the day. Preoptic hypothalamic content of GnRH declined markedly between 1000 and 1200 h. In the 2nd experiment, 75 rats were divided into 3 groups and were untreated or were implanted s.c. with empty Silastic capsules or capsules containing estradiol-17.beta. (E2). Two days later, groups of 5 rats from each of the 3 treatment groups were decapitated at 0800, 1100, 1400, 1700 and 2000 h. The preoptic area was separated from the hypothalamus by a transverse cut at the caudal aspect of the optic chiasm. POA and hypothalamic content of GnRH correlated well (r = 0.74, P < 0.001, n = 75). Two-way analysis of variance failed to reveal any effect of treatment on the GnRH content in either the POA or hypothalamus. GnRH content of both regions decreased significantly between 1100 and 1700 h regardless of whether E2 was administered. In striking contrast, gonadotropin surges occurred in the late afternoon only in the E2-treated rats. Serum GnRH was undetectable (< 5 pg/ml) in all groups of animals. In the untreated ovariectomized rat GnRH content of the POA and hypothalamus decreases during the early afternoon. This study supports the concept of a daily neural signal for LH [luteinizing hormone] release and E2 is necessary for expression of the daily LH surge in the ovariectomized rat.