SUMMARYThe possibility that luteinizing hormone releasing factor (LH-RF) not only stimulates the secretion of luteinizing hormone (LH) but also has the ability to prime the anterior pituitary gland so that further exposure to LH-RF enhances the responsiveness of the gonadotrophs has been investigated.

The responsiveness of the anterior pituitary gland to synthetic luteinizing hormone releasing factor (LH-RF) was tested at various times ofthe oestrous cycle of the rat in order to assess the relative importance of changes in pituitary sensitivity in the timing and magnitude of the preovulatory surge of luteinizing and follicle-stimulating hormones (LH and FSH). The minimal dosage of LH-RF which consistently induced ovulation in rats anaesthetized with sodium pentobarbitone (Nembutal) immediately before the critical period of pro-oestrus was 50 ng/100 g body weight, and this was used as the standard dose throughout the study. The LH-RF was injected either intravenously into animals anaesthetized with Nembutal 30 min to 1 h before, or into conscious animals through a cardiac catheter which had been inserted several days before. Most experiments were carried out on rats which had exhibited regular 4-day oestrous cycles, although results from animals with 5-day cycles are also presented and discussed. Blood samples were taken immediately before and at frequent intervals after the injection of LH-RF, and the concentrations of LH and FSH in these samples were determined by radioimmunoassay.With respect to the secretion of LH, in both anaesthetized and conscious animals with regular 4-day cycles a tenfold increase in pituitary sensitivity occurred between the early afternoon of dioestrus and pro-oestrus. This was followed by a marked (50-fold) increase which reached a peak at 17.00\p=n-\ 18.00 h of pro-oestrus. The phase of marked increase in sensitivity appeared to commence at about the time of the onset of the pre\ x=req-\ ovulatory surge of LH and continued for some hours after. Pituitary sensitivity then declined through oestrus and metoestrus, reaching a nadir at 14.00 h of dioestrus. From 15.00 to 22.30 h of pro-oestrus there was a significant positive correlation between the pre-injection levels of and maximal increments in plasma LH, and this, together with the fact that the marked increase in sensitivity could be abolished by administering Nembutal before the critical period, raised the possibility that endogenous LH-RF, presumably secreted in greater amounts during the early afternoon of pro-oestrus, may exert a priming effect on the anterior pituitary gland.Pituitary responsiveness to LH-RF with respect to the secretion of FSH was also at its highest at 17.00\p=n-\18.00 h of pro-oestrus. However, in contrast to LH, the mean level of FSH before injection reached a peak at 05.00 h of oestrus, 11 h after the maximum FSH response to LH-RF.These results suggest that a change in pituitary sensitivity to LH-RF may play an important role in determining the magnitude of the pre\ x=req-\ ovulatory surge of LH, but it would appear that the timing and duration of the surge depend on other factors as well. The relationship between the secretion of LH and ovulation is discussed with respect to the finding that there appeared to be no correlation between the number of ova shed and the maximal levels of plasma LH after inje...

SUMMARY The role of ovarian hormones in the development of increased sensitivity of the anterior pituitary gland to synthetic luteinizing hormone releasing factor (LH-RF) which occurs before and during the preovulatory surge of luteinizing hormone (LH) in the rat has been examined. The response of the pituitary gland was determined, with respect to the secretion of LH and follicle-stimulating hormone (FSH), after the intravenous injection of 50 ng LH-RF/100 g body weight. The LH-RF was injected 30–60 min after the administration of sodium pentobarbitone at either 13.30 h or 18.80 h of pro-oestrus. Blood samples were collected immediately before and at frequent intervals after the injection of LH-RF, and the concentration of LH and FSH in these samples was measured by radioimmunoassay. Ovariectomy at 10.00–11.00 h of dioestrus reduced the LH response to LH-RF injected at 14.00 h of pro-oestrus, while oestradiol benzoate administered immediately after ovariectomy restored and even augmented this response. These data together with the finding that administration of the antioestrogen, ICI 46 474, at 17.00 h of dioestrus reduced the LH response to LH-RF injected on the afternoon of pro-oestrus indicates that the initial phase of increased pituitary sensitivity to LH-RF is dependent upon the marked rise in the concentration of oestradiol-17β in plasma which precedes the preovulatory surge of LH. The abrupt, marked increase in pituitary sensitivity to LH-RF, which, in the normal cycle, occurs between 14.00 and 18.30 h of pro-oestrus, failed to develop in rats ovariectomized on the morning of dioestrus whether or not oestradiol benzoate was administered after the operation. However, the LH response to LH-RF injected on the evening of pro-oestrus increased significantly when progesterone was administered at 13.00 h of pro-oestrus in rats ovariectomized and treated with oestradiol benzoate at 10.00–11.00 h of dioestrus. This suggests that the development of the second phase of increased pituitary sensitivity to LH-RF depends, at least partially, on progesterone acting on an oestrogen-primed pituitary gland. The concentrations of FSH in blood samples taken before injection of LH-RF at either 14.00 or 18.30 h of pro-oestrus were significantly greater in ovariectomized compared with those in sham-operated rats. In contrast the FSH responses, in terms of the mean maximal increments, were not significantly different in the various groups irrespective of the nature or time of operation or the time of injection of LH-RF. The FSH response to LH-RF was not appreciably altered by treatment with either oestradiol benzoate or progesterone immediately after ovariectomy although it was increased significantly by the sequential administration of oestrogen and progesterone. The significance of the findings that under certain conditions there were considerable differences between the LH and FSH responses to synthetic LH-RF is discussed with respect to the hypothesis that there is a common releasing factor for both gonadotrophins.

We have investigated whether the priming effect of LH-RF can be elicited by electrical stimulation of the medial preoptic area, or by i.v. infusion or multiple i.v. injections of the synthetic decapeptide. All experiments were carried out on animals anaesthetized with sodium pentobarbitone at 13.30 h. In pro-oestrous rats, the LH response to the second of two electrical stimuli, 15 min in duration and separated by 60 min, was significantly greater than the response to the first stimulus. When synthetic LH-RF was infused at a constant rate for 90 min, plasma LH increased gradually for the first 45-60 min after which it increased markedly. This enhanced secretion of LH did not occur in rats which were infused with the same total dose of LH-RF, either 15 or 75 ng/100 g body wt, over periods of 45 min or less. When a dose of 15 ng LH-RF/100 g body wt was administered in six divided doses by i.v. injections, each separated by 15 min, there was a marked increase in plasma LH after 75 min. The profile of the mean plasma LH concentration in rats subjected to preoptic stimulation for 90 min was similar to that in rats infused for 90 min with LH-RF, but the variation in response was much greater in the stimulated rats. These results indicate that the priming effect can be elicited by endogenous as well as synthetic LH-RF, and that whether LH-RF reaches the pituitary at a constant rate or in a pulsatile manner the factor is capable of significantly increasing the responsiveness of the gonadotrophs. The relevance of these findings with respect to the development of the spontaneous preovulatory LH surge is discussed. A priming effect could not be elicited by constant LH-RF infusion in dioestrous rats; this supports the view that steroid hormones, especially oestradiol-17phi, determine the magnitude of the effect. The LH response in male rats subjected to i.v. infusion of LH-RF was much lower than in females. Pre-treatment with oestradiol benzoate did not increase the response significantly, suggesting that this sex difference cannot be ascribed simply to low levels of plasma oestrogen in the male.

SUMMARY The responsiveness of the hypothalamo-hypophysial axis to electrical stimulation of the medial preoptic area was tested at various times during the oestrous cycle of the rat. Animals were anaesthetized with sodium pentobarbitone, and glass-insulated bipolar platinum electrodes were stereotaxically implanted in the medial preoptic area. The stimulus consisted of accurately balanced biphasic rectangular pulses, height 500 μA, duration 2 ms and frequency 60 Hz, applied in trains of 30 s intervals. The concentration of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in blood samples withdrawn from the external jugular vein immediately before and at frequent intervals up to 1 h after application of the stimulus was determined by radioimmunoassay. In all animals the stimulus was applied for a period of 15 min, for it was found that under these conditions the profiles of LH and FSH produced in individual animals were similar to those which followed the i.v. injection of 50 ng synthetic luteinizing hormone releasing factor (LH-RF)/100 g body wt, the minimal ovulatory dose of LH-RF in our laboratory. Both for LH and FSH, the profiles of responsiveness of the hypothalamo-hypophysial axis to electrical stimulation through the oestrous cycle resembled closely the profile of responsiveness of the anterior pituitary gland to LH-RF administered intravenously. There was a relatively gradual increase in pituitary sensitivity, in terms of the mean maximal increments, between the early afternoon of dioestrus and pro-oestrus, followed by an abrupt and marked rise which reached a peak at 18.00 and 21.00 h of pro-oestrus for LH and FSH, respectively. Sensitivity of the hypothalamo-hypophysial axis declined through oestrus and metoestrus reaching a nadir at 13.30 h of dioestrus. While these results do not exclude a change in sensitivity of the neural component of the hypothalamo-hypophysial axis during the oestrus cycle, they do indicate that the timing and magnitude of changes in sensitivity of the anterior pituitary gland to both endogenous and exogenous LH-RF are similar. This raises the possibility that a marked increase in the secretion of LH-RF during the afternoon of pro-oestrus may not be necessary for the occurrence of the surge of LH and FSH, for conceivably the latter could depend on a change in sensitivity of pituitary gonadotrophs to constant or only slightly increased levels of LH-RF in hypophysial portal blood. Our findings are compared with those of workers who have employed electrochemical stimulation of the preoptic area.