Changes in somatostatin-like activity in discrete areas of the rat hypothalamus during different stages of proestrus and diestrus and their relation to serum gonadotropin, prolactin, and growth hormone levels.

To investigate physiological changes in the somatotropin release-inhibiting factor (SRIF) content of the hypothalamus during the estrous cycle, groups of 8-10 rats were decapitated every 2 h from 0800-1800 h on proestrus and diestrus. SRIF was assayed in the preoptic suprachiasmatic area, posterior hypothalamus, arcuate nucleus area, and median eminence. Gonadotropins, PRL, and GH were measured in trunk blood and correlated with concentrations of SRIF. A marked decrease in SRIF occurred between 0800-1400 h of proestrus in the preoptic suprachiasmatic area (1.00 +/- 0.28 vs. 0.09 +/- 0.03 micrograms/g wet wt, P less than 0.001) and the posterior hypothalamus (0.43 +/- 0.10 vs. 0.02 +/- 0.004 micrograms/g, P less than 0.001). A similar pattern of decrease in SRIF also occurred in the tissue containing the arcuate nucleus (2.09 +/- 0.37 vs. 0.98 +/- 0.13 micrograms/g, P less than 0.01). In the median eminence the lowest SRIF concentration was measured at 1600 h (31.4 +/- 3.4 vs. 22.0 +/- 1.32 micrograms/g, P less than 0.02). Diestrous values for SRIF did not change in the posterior hypothalamus and arcuate area; however, values increased slightly in the median eminence and decreased in the preoptic suprachiasmatic area between 1400-1800 h. Spearman's correlation coefficient (rs) between PRL and SRIF in the preoptic suprachiasmatic area and the posterior hypothalamus was rs = -0.66 and rs = -0.55 (P less than 0.001), respectively.