Hypothalamic follicle stimulating hormone releasing factor (FSHRF) content, pituitary and plasma follicle stimulating hormone (FSH) concentrations have been determined in female rats between 5 and 75 days of age. FSHRF was present by at least 10 days of age and at levels that exceeded those observed at any age thereafter. No fluctuations in FSHRF were detected during the estrous cycle. Plasma FSH levels determined by radioimmunoassay increased threefold between 5 and 15 days of age, at which time, values were approximately 1,300 ng/ml. At no other age were levels of this magnitude observed. During the estrous cycle, values ranged between 136 and 261 ng/ml, with the lowest during the morning of proestrus and the highest, the morning of estrus. Pituitary FSH stores during the cycle determined by bioassay were 16.5 (proestrus), 19.5 (estrus), 22.8 (metestrus), and 21.2 µg (diestrus) per pituitary. Neural regulation of FSH secretion exists by at least 10 days of age in the female rat. FSH secretion decreases between 15 days of age and puberty, and fluctuates at least twofold during the adult cycle.

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Previous studies from this laboratory suggested that the mechanism for control of prolactin release by the avian pituitary was different from that present in mammals. The effects of pigeon hypothalamic extracts on release of prolactin by the pigeon pituitary were determined when incubated together at 37 C for 2 or 4 hr. The effects of rat or pigeon hypothalamic extracts on release of prolactin by the pituitary of the other species were also tested. Extract equivalent to 1 hypothalamus from a parent pigeon, sacrificed on the day young were hatched, increased prolactin release 2-to 4-fold from each pituitary of 4-to 6-week-old pigeons during a 4-hr incubation period. Slight stimulatory activity was also found in hypothalamic extracts from 4-to 6-week-old pigeons, but at the dose level tested (2 hypothalami/pituitary) they did not elicit a statistically significant increase in prolactin release. Hypothalamic extracts from rats or pigeons had no effect on release of prolactin by the pituitaries of the other species. These experiments indicate that the hypothalamus of the parent pigeon on the day young are hatched can stimulate prolactin release by the pigeon pituitary. This is in contrast to the hypothalamus of mammals, which inhibits prolactin release by the mammalian pituitary. The results also suggest that the "prolactin-inhibiting factor" of the rat hypothalamus and the "prolactin-stimulating factor" of the pigeon hypothalamus are not the same. (Endocrinology 76:1169(Endocrinology 76: ,1965