Hypothalamic-pituitary stalk portal blood was collected from 12 female rhesus monkeys. The pituitary stalk was approached transorbitally and cut at the level of the diaphragma sellae under direct visualization. After complete heparinization of the animal, stalk portal blood was obtained continuously, for periods of 30 minutes to 9 hours, using a constant exfusion pump at a rate of 30 to 40 mul/min. The mean GnRH in portal blood, as measured by radioimmunoassay, was 66 +/- 6.6 pg/ml (+/- SE) in 7 ovariectomized animals and 51 +/- 5.3 pg/ml (+/- SE) in 2 monkeys during the early follicular phase. Fluctuations in portal blood GnRH were most prominent in ovariectomized animals, with peak levels of 200-800 pg/ml and intervals of 1 to 3 hours between pulses. Peaks of GnRH during the early follicular phase did not exceed 200 pg/ml. The administration of estradiol (1000 ng, iv) to 3 monkeys did not decrease GnRH levels within the next 2 hours. These data provide direct evidence for a hypothalamic mediation of pituitary LH pulsatile release.

Gonadotropins-releasing hormone (Gn-RH) in selected regions of the female rat brain was measured by radioimmunoassay. Detectable immunoreactive Gn-RH was found in the anterior hypothalamic-septal region and in the mid-hypothalamic (arcuate-median eminence) region. Gn-RH was several times higher in the middle region than in the anterior region. Gn-RH was undetectable in the posterior hypothalamic region, frontal cerebral cortex and pineal glands, as well as in random blood samples, and low to undetectable in anterior pituitary glands. Gn-RH activity varied during the estrous cycle and after castration. In the mid-hypothalamic region, Gn-RH content was lowest throughout diestrus and in late morning and early afternoon of proestrus, and highest early in the morning of proestrus and during estrus. A significant decrease at mid-day was only found on the day of proestrus, a few hours prior to the critical period for LH release. In the anterior hypothalamic region, low Gn-RH activity was found from 1200 h of estrus to 1200 h of diestrus-2. A comparatively higher activity was seen at 1700 h of diestrus-2 and also from 1400 h of proestrus to 0800 h of estrus. Twenty-one days after ovariectomy, Gn-RH in the mid-hypothalamic region was significantly lower than the lowest values seen during the estrous cycle, while Gn-RH in the anterior hypothalamic region remained between low and high values seen during the cycle, being significantly higher than the low values. The changes observed during the estrous cycle and after castration suggest that gonadal steroids play a direct role in the control of hypothalamic Gn-RH. These data also demonstrate that Gn-RH varies in a different way in the anterior and mid-hypothalamic regions.

The developmental appearance of immunoreactive gonandotropin releasing hormone (Gn-RH) was measured by radioimmunoassay in male and female rats from the 12th day of gestation to adulthood. Gn-RH content of the whole hypothalamus of both sexes increased from birth to 21 days of age. By the 28th day, Gn-RH content in females had approximately reached adult levels, while that in males continued to increase. Adult male rats exhibited significantly higher levels than adult females. The ontogeny of the regional differences previously observed in cycling female rats (4) was also studied. Regional analysis of Gn-RH development was characterized in the anterior hypothalamus by the rapid attainment of adult levels by day 7. In striking contrast, in the mid-hypothalamic region, Gn-RH content increased gradually, rising dramatically just prior to vaginal opening. The developmental pattern of Gn-RH content is consistent with the maturation of those hypothalamic structures concerned with synthesis, transport and storage of Gn-RH. The regional patterns observed may thus represent sequential maturation of the neural pathways involved.