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Hypothalamic corticotrophin releasing (CR) activity and LH-releasing factor (RF) content, and pituitary and plasma LH, FSH and ACTH were measured in adult male and female Wistar rats maintained under 14 h light per day. Hypothalamic LH-RF and pituitary and plasma hormones were estimated by radioimmunoassay while CR-activity was assessed by the amount of ACTH released from hemipituitaries in vitro. Two experiments were carried out on male animals. In the first, some of the animals were kept in a room, distant from the animal house, in which the lighting was reversed with respect to the external environment. In animals exposed to the reversed lighting regime, hypothalamic LH-RF content and pituitary gonadotrophin concentrations were significantly lower than the values in male rats kept in the animal house where they were in close proximity to female rats. In the second experiment, which was carried out on animals which had all been kept in the animal house, there was no significant differences between the LH-RF contents measured at 3-4 h intervals throughout the day. Pituitary LH and FSH contents, but not concentrations, were significantly increased at 12.00 h. There was little differences between the experiments in CR-activity, plasma ACTH concentrations and profiles of pituitary ACTH content and concentration. As expected there was a diurnal rhythm in plasma corticosterone concentrations (determined by competitive protein-binding assay) with the peak occurring between 15.00 and 18.00 h. The profiles of plasma and pituitary ACTH were similar to that of plasma corticosterone. Corticotrophin releasing activity dropped significantly between 12.00 and 16.00 h, but remained steady at the other times. In female rats there were no significant differences between hypothalamic LH-RF content throughout the 4-day cycle. During pro-oestrus the mean LH-RF content rose to teach a high level at 18.00 h at which time plasma LH concentration had risen sharply to a level consistent with the peak of the preovulatory surge. Plasma FSH concentration also rose significantly between 15.00 and 18.00 h of pro-oestrus. At metoestrus and dioestrus, plasma FSH levels were lower in the morning than in the evening. These results suggest that (1) there is no diurnal rhythm in hypothalamic LH-RF, (2) there may be a diurnal rhythm in pituitary gonadotrophin content in the male and in plasma FSH concentration on the days of metoestrus and dioestrus in the female, (3) if a surge of LH-RF does occur on the afternoon of pro-oestrus, the rate of LH-RF synthesis exceeds its release, and (4) the mechanism which regulates gonadotrophin secretion in the male may be affected by factors in the environment other than daylength. The results provide further evidence for the view that the diurnal rhythm of corticosterone secretion is under hypothalamo-hypophysial control.

The effects of sex steroid hormones on the responsiveness of the neural mechanism responsible for the secretion of LH-RF have been examined in the female rat. Responsiveness was determined at pro-oestrus by measuring the increments in immunoreactive LH-RF of pituitary stalk blood produced by electrical stimulation of the medial preoptic area or median eminence. Ovariectomy on the morning of dioestrus reduced the LH-RF response to preoptic stimulation while oestradiol benzoate (OB) or testosterone propionate (TP) administered immediately after ovariectomy significantly augmented the response. The facilitatory effect of TP was possibly due to its conversion to an aromatized derivative since 5alpha-dihydrotestosterone monobenzoate was ineffective. Progesterone did not facilitate preoptic responsiveness, and, when administered to animals ovariectomized at 12.00 h of pro-oestrus, reduced the LH-RF response at 18.00 h the same day. Stimulation of the median eminence produced a significantly greater increment in LH-RF than stimulation of the preoptic area. The facilitatory action of OB on the LH-RF response was less marked for median eminence compared with preoptic stimulation. The administration of ICI 46474 at 17.00 h of dioestrus did not reduce preoptic responsiveness on the morning of the next day, suggesting that this compound does not act as an 'antioestrogen' at the level of the preoptic area.

The hypothalamic content of LH releasing factor (RF), pituitary ACTH and pituitary and plasma LH and FSH were measured by radioimmunoassay from foetal Day 15 to postnatal Day 65. Bioassayable corticotrophin releasing activity was also measured during the postnatal period. Hypothalamic LH-RF was detectable as early as foetal Day 15, increasing gradually until postnatal Day 2 and then steeply between Days 5 and 16. The levels of LH-RF were similar in both male and normal female rats until Day 41, after which the increase which had been occurring from Day 16 continued in the male but not the female. In female rats treated with testosterone propionate neonatally ('androgenized females') the hypothalamic content of LH-RF at Day 9 was significantly less than that in the male or normal female, levels reaching those found in the latter two groups by Days 16-22. The lower level of LH-RF in the androgenized female was associated with pituitary gonadotrophin and plasma FSH levels which were lower than in the normal female until Day 30. In the normal female, vaginal opening was associated with a marked drop in hypothalamic LH-RF content and in pituitary LH and FSH, but in the androgenized female, vaginal opening occurred while hypothalamic LH-RF and pituitary LH levels were still rising. The peaks in pituitary FSH and LH and in plasma LH seen on Days 22, 30 and 41, respectively, in the normal female were each delayed by 8-9 days in the androgenized female. In all three types of animal there was a significant drop in plasma FSH between Days 50 and 65 which was associated with a significant increase in pituitary FSH in the male and a significant decrease in pituitary FSH in the androgenized female rats. The day 17 foetal pituitary gland also contained ACTH, and again levels of this hormone rose steeply between Days 5 and 9. In contrast to the gonadotrophins, there was a marked divergence between the pituitary content and concentration of ACTH: content rose while concentration remained relatively steady after Day 9. There was no major difference in pituitary ACTH levels between the three types of animal throughout the study; however, around Days 16 and 50, corticotrophin releasing activity was higher in males and androgenized females compared with that in normal females.