The localization of gonadotrophs in normal adult male and female rats

We investigated the localization of LH and FSH cells within the pituitary glands of normal adult rats. Groups of four female rats were decapitated at one of five different times during the estrous cycle. Four male rats were also decapitated. Paired horizontal flip-flopped serial paraplast sections from the dorsal, middle, and ventral portions of each pituitary gland were stained. For each pair, one section was stained with antirat LH-S4 and the other section with antirat FSH-S7, by the unlabeled antibody peroxidase-antiperoxidase method. All immunoreactive cells were counted, and the area of pars distalis in each section was determined. We studied the spatial distribution of gonadotrophs within the sections and determined if a polarization along the antero-posterior axis existed. In the "sex zone" of the pars distalis, the cross-sectional area of LH cells and the percentages of LH cells that also contained FSH and vice versa were determined and compared with those obtained from the entire pars distalis. Additional sections were stained for TSH, ACTH, GH, or PRL, and the distribution of stained cells was compared with that of those that stained for LH or LH/FSH, particularly in the sex zone and in the pars intermedia. The results indicate that 1) gonadotrophs are more evenly distributed dorsoventrally within the pars distalis of male rats than in that of female rats; 2) an antero-posterior polarity in gonadotropic distribution is more pronounced in male rats than in female rats; 3) gonadotrophs containing only LH are less numerous in male than female rats, and in the female tend to be centrally located within the pars distalis; 4) the sex zone contains PRL cells and gonadotrophs, and the percentages of gonadotrophs that contain LH or LH and FSH are not different from those of the entire pars distalis; 5) LH, and occasionally LH/FSH cells, are present between lobules of immunoreactive ACTH cells in the pars intermedia; and 6) LH cells in the pars intermedia are smaller than those in the sex zone or entire pars distalis.

Dissociation between increased growth hormone and prolactin secretion during the morning hours of early pregnancy in the rat

We investigated whether serum growth hormone (GH) concentration changes in association with the rise in serum prolactin (PRL) concentration known to occur during the early morning hours in the pregnant rat. Animals were kept in a room with the lights on from 0500 to 1900 hours (hr) daily and decapitated for the collection of trunk blood at 2200 or 2400 hr on Day 6 of pregnancy or at 0200, 0400, 0800 or 1000 hr on Day 7 of pregnancy. Serum GH concentration rose more than 4-fold from low levels at 2200 and 2400 hr to higher levels at 0400 and 0800 hr and then declined by 1000 hr. Serum prolactin (PRL) concentration followed a similar pattern except that it returned to low levels earlier, by 0800 hr. Serum luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone concentrations showed no significant changes. Serum GH levels at 0800 hr in pregnant rats were higher than those observed in cyclic rats (13 time periods sampled). The results demonstrate that serum GH concentration is elevated during a circumscribed period in the 6- to 7-day pregnant rat. The time of onset of the rise is similar to that for serum PRL but the elevation in GH levels persists longer than that for PRL.

A quantitative immunocytochemical study of the luteinizing hormone and follicle-stimulating hormone cells in the adenohypophysis of adult male rats and adult female rats throughout the estrous cycle

We investigated whether 1) the absolute or the relative numbers of LH and FSH cells change during the rat estrous cycle, 2) the percentages of gonadotrophs that contain LH and/or FSH change during the estrous cycle, and 3) gonadotrophs change in size during the rat estrous cycle. Groups of four female rats were decapitated at one of five different times during the estrous cycle. Four male rats were also decapitated. Serum concentrations of LH and FSH were determined by RIA. Paired horizontal flip-flopped or nonflipped paraffin sections were mounted from the dorsal, middle, and ventral portions of each pituitary gland. In each pair of sections, one was stained with a-rat LH-S4 and the other with a-rat FSH-S7 by the unlabeled antibody peroxidase-antiperoxidase method. All immunoreactive cells were counted. Photographs were taken from randomly chosen corresponding areas, and the cells were individually matched to determine the percentage that contained one or both hormones. Correction factors had to be used because in paired flip-flopped or nonflipped sections stained with the same antibody (a-rat LH-S4), not all of the stained cells found in one section were found in the other section. The absolute numbers of LH and FSH cells did not change throughout the estrous cycle. The ratio of LH cells to FSH cells in the pars distalis of female rats was also constant throughout the estrous cycle. In female rats, 75.2% of LH cells also contained FSH, while 99.4% of FSH cells also contained LH. In the male rats, 88.6% of LH cells also contained FSH, while 98.6% of FSH cells also contained LH. Similar results were obtained in paired flip-flopped sections stained with a-rat LH beta and a-rat FSH beta. Sequential staining of additional individual tissue sections with a-rat LH-S4 and then a-rat FHS-S7 or vice versa revealed the following. Staining of LH-stained tissue for FSH revealed less than 1% new cells, but staining of FSH-stained tissue for LH revealed a 8.7% increase in gonadotrophs in males and a 25.4% increase in females. The gonadotrophs in female rats did not change in size during the estrous cycle and were significantly smaller than the gonadotrophs in male rats. The results suggest that in normal adult rats: 1) virtually all FSH-containing cells contain LH, 2) about 25% of the gonadotrophs in females and about 11% of the gonadotrophs in males contain LH but not FSH, 3) the number of cells containing LH or those containing LH and FSH does not change during the estrous cycle, 4) gonadotrophs in female rats do not change in size during the estrous cycle and are smaller than the gonadotrophs in male rats, and 5) FSH release during the early morning of estrus, when the serum FSH concentration is elevated and the serum LH concentration is low, occurs from cells that contain both LH and FSH.

Effects of acute ovariectomy on anterior pituitary gland follicle-stimulating hormone and luteinizing hormone secretion in the metestrous rat

Changes at the anterior pituitary gland level which result in follicle-stimulating hormone (FSH) release after ovariectomy in metestrous rats were investigated. Experimental rats were ovariectomized at 0900 h of metestrus and decapitated at 1000, 1100, 1300, 1500, 1700 or 1900 h of metestrus. Controls consisted of untreated rats killed at 0900 or 1700 h and rats sham ovariectomized at 0900 h and killed at 1700 h. Trunk blood was collected and the serum assayed for FSH and luteinizing hormone (LH) concentrations. The anterior pituitary gland was bisected. One-half was used to assay for FSH concentration. The other half was placed in culture medium for a 30-min preincubation and then placed in fresh medium for a 2-h incubation (basal FSH and LH release rates). The basal FSH release rate and the serum FSH concentration rose significantly by 4 h postovariectomy and remained high for an additional 6 h. The basal FSH release rate and the serum FSH concentration correlated positively (r=0.71 with 72 degrees of freedom) and did not change between 0900 and 1700 h in untreated or sham-ovariectomized rats. In contrast, the serum LH concentration and the basal LH release rate did not increase after ovariectomy. Ovariectomy had no significant effect on anterior pituitary gland FSH concentration. The results suggest that the postovariectomy rise in serum FSH concentration is the result, at least in part, of changes which cause an increase in the basal FSH secretion rate (secretion independent of the immediate presence of any hormones of nonanterior pituitary gland origin). The similarities between the selective rises in the basal FSH release rate and the serum FSH concentration in the ovariectomized metestrous rat and in the cyclic rat during late proestrus and estrus raise the possibility that an increase in the basal FSH release rate may be involved in many or all situations in which serum FSH concentration rises independently of LH.

A decrease of cytosol estrogen receptors in the hypothalamus as a result of treatment of neonatal rats with glutamate

Experiments were performed to determine whether the neuroendocrine dysfunctions of rats treated neonatally with monosodium glutamate (MSG) could be related to a loss of cytoplasmic estrogen receptors. Female rats treated with MSG as neonates were ovariectomized as adults and killed by decapitation 2 or 3 weeks after ovariectomy. Body, gonadal and anterior pituitary gland weights in MSG-treated rats were depressed when compared to that seen in their littermate controls. Serum prolactin concentration was elevated in the MSG-treated rats. Serum luteinizing hormone (LH) concentration was significantly lower in MSG-treated rats than in controls at 2 weeks, but not at 3 weeks after ovariectomy, suggesting a sluggish postovariectomy rise of serum LH concentration. Serum follicle-stimulating hormone (FSH) concentration was not altered by the MSG treatment. The concentration of cytosol estrogen receptors in the anterior pituitary gland was similar to that of controls, but hypothalamic concentration of estrogen receptors decreased as a result of the MSG treatment. After dissection of different hypothalamic regions, it was found that the greatest depletion of the cytosol estrogen receptors occurred in the arcuate-median eminence region. The results raise the possibility that some reproductive impairments of MSG-treated rats could stem from a decrease in cytosol estrogen receptors in the arcuate-median eminence region.

Anterior pituitary gland secretion after forebrain ablation: periovulatory gonadotropin release

We sought to determine whether the estrous phase of FSH release in cyclic female rats is dependent on the immediate presence of the diencephalon. A piece of forebrain, the diencephalon and part of the telencephalon, was surgically removed from female rats between 1130--1300 or 2000-2200 h on proestrus. Blood was withdrawn through indwelling venous cannulae during the afternoon and evening of proestrus and the early morning of estrus for RIA of plasma LH, FSH, and PRL concentrations. In rats sham operated at either time period, plasma LH, FSH, and PRL levels rose from 1345 to 1800 h on proestrus. Whereas the plasma LH and PRL concentrations fell from 1800 h on proestrus to 0300 h on estrus, the plasma FSH concentrations remained elevated during this period. The removal of the piece of forebrain around noon on proestrus blocked the rises in the plasma LH and FSH levels and caused high plasma PRL concentrations from 1345 h on proestrus to 0300 h on estrus. The removal of the piece of forebrain during the evening of proestrus did not interfere with the fall in plasma LH concentrations or the maintenance of elevated plasma FSH concentrations during either late proestrus or the early morning estrus, but did cause high plasma PRL levels during that time interval. Hypophysectomy combined with removal of the forebrain piece during the evening of proestrus resulted in a drop in plasma FSH and PRL concentrations. The results confirm that in the rat, 1) the prosencephalon plays an acute stimulatory role n causing the preovulatory LH surge and the proestrous phase of FSH release, 2) the prosencephalon exerts effects during the afternoon and/or early evening of proestrus that cause the estrous phase of FSH release, and 3) the estrous phase of FSH release occurs in the absence of acute diencephalic stimulation.

Effects of complete hypothalamic deafferentation on the estrous phase of follicle-stimulating hormone release in the cyclic rat

We investigated whether neural afferents to the medial basal hypothalamus play an acute role in the estrous phase of FSH release in the 4-day cyclic rat. A cannula was inserted into the right atrium of the heart under brief ether anesthesia during the early afternoon of proestrus for subsequent blood collections and injection of LHRH. In some of the rats, the medial basal hypothalamus was surgically isolated from the rest of the brain with a small knife under brief ether anesthesia between 2000 h and 2130 h of proestrus. Control groups consisted of naive rats which were not treated during the night of proestrus and sham-operated animals in which the knife was lowered to the corpus callosum between 2000 h and 2130 h or proestrus. Rats were bled at 2200 h of proestrus and at 0200 h, 0600 h and 1000 h of estrus for radioimmunoassay of plasma FSH and LH. The plasma FSH levels in all 3 groups between 2200 h of proestrus and 1000 h of estrus were elevated above levels observed in other cannulated rats bled to the onset of the proestrous phase of FSH release at 1400 h of proestrus. There were no statistically significant differences in plasma FSH or LH concentrations at any of the time periods between the 3 groups of serially bled rats. The deafferentation procedure did not appear to impair the pituitary gland's ability to secret gonadotrophins as injection of 50 ng of LHRH after the bleeding at 1000 h of estrus caused substantial elevations in plasma FSH and LH concentrations which were not different between the 3 groups. The results suggest that neural afferents to the medial basal hypothalamus play no acute role in the estrous phase of FSH release in the cyclic rat.

Catecholestrogens and release of anterior pituitary gland hormones. II. Prolactin

We investigated the effects of the peripheral administration of 17 beta-estradiol (E2), estrone (E1), and the catecholestrogens, 2-hydroxyestradiol (2-OHE2) and 2-hydroxyestrone, (2-OHE1), on anterior pituitary gland PRL release in the prepuberal rat. Steroids in oil were injected sc into 25-day-old female and 35- to 40-day-old male rats. The injection of E2, E1, or 2-OHE2, but not of 2-OHE, caused a surge in serum PRL levels in female rats 48 h later, during the afternoon hours. Only E1 induced a PRL surge 24 h after injection. In male rats, the injection of E1 or 2-OHE2, but not of 2-OHE1, elevated serum PRL levels on a chronic basis. The results suggest that 2-OHE1 plays no discernible role in PRL release in either sex, but that 2-OHE2 might play a role in the tonic release of PRL in the male and in the preovulatory release of PRL in the female.

Catecholestrogens and release of anterior pituitary gland hormones. I. Luteinizing hormone

We investigated the effects of peripheral administration of 17 beta-estradiol (E2), estrone (E1), and the catecholestrogens, 2-hydroxyestradiol (2-OHE2) and 2-hydroxyestrone (2-OHE1), on anterior pituitary gland LH release in the prepuberal rat. Steroids in oil were injected sc into 25-day-old female and 35- to 40-day-old male rats. The injection of E2, E1, or 2-OHE2 caused a surge in serum LH levels in female rats 48 h later, during the after hours. Only E1 induced a LH surge 24 h after injection. The positive effects of 2-OHE2 in the females were only observed if a massive dose was administered, the steroid was injected on 2 consecutive days, or E2 or progesterone was given to 2-OHE2-primed rats. The 2-OHE1 was totally ineffective in causing a serum LH surge under a variety of experimental protocols. In male rats, the injection of any one of the four steroids decreased serum LH levels. Even the injection of E2 or 2-OHE2 for 2 days or the injection of E2 in 2-OHE2-primed rats failed to elevate the serum LH concentration in male rats. The results suggest that 2-OHE2 and E1 could play a role in the preovulatory release of LH in the female; 2-OHE2 and 2-OHE1 could play a role in the negative feedback control of LH release in the male.

Effects of hypothalamic arcuate nucleus lesions on pulsatile luteinizing hormone concentration in ovariectomized rats

We investigated the effects of hypothalamic arcuate nuclei destruction on the postovariectomy rise in plasma luteinizing hormone (LH) concentration and the pulsatile LH release mechanism in ovariectomized rats. Rats were injected with 1, 2, or 4 mg/g body wt of L-monosodium glutamate (MSG) on Days 1, 3, 5, 7, and 9 of life to lesion the arcuate nuclei. They were then ovariectomized as adults and used 2 or 4-8 weeks later. Serial blood sampling at 10-min intervals and measurement of plasma LH concentration revealed elevated plasma LH levels which fluctuated in a pulsatile fashion in all control and 1 and 2 mg/g/MSG-treated rats. In 4 mg/g MSG-treated rats, plasma LH levels were lower than in controls of both time periods after ovariectomy due to a decrease in the amplitude and/or frequency of LH pulses. Phenobarbital was administered to all 4- to 8-week ovariectomized rats to block endogenous pulsatile LH release. In phenobarbital-blocked rats, three sequential iv injections of LH releasing hormone (LHRH) caused substantial elevations in plasma LH concentrations even in the 4 mg/g MSG-treated animals which had low plasma LH concentrations during control bleedings prior to the injection of phenobarbital. The results indicate that the postovariectomy rise in plasma LH concentration and the associated pulsatile LH release mechanism are functional in rats with extensive arcuate nucleus lesions. The diminution in the rise in plasma LH levels and the decreased amplitude and/or frequency of LH pulses in the MSG-treated rats is likely due to a diminution in hypothalamic LHRH release.

Effects of oestradiol, hypothalamic extracts and luteinizing hormone releasing hormone on rat anterior pituitary gland gonadotrophin release in vitro before and after the preovulatory luteinizing hormone surge at pro-oestrus

Changes at the anterior pituitary and/or hypothalamic levels which result in selective FSH release during late pro-oestrus in the cyclic rat were investigated. The possible involvement of decreasing serum concentrations of oestrogen during pro-oestrus in such changes was studied. Rats were decapitated at 12.00 h on pro-oestrus, before the onset of the LH surge and first phase of FSH release, or at 24.00 h on pro-oestrus, shortly after the onset of the second or selective phase of FSH release. Other rats were given oestrogen (OE2) at 14.00 h and killed at 24.00 h pro-oestrus. Paired hemi-anterior pituitary glands were incubated with vehicle or OE2 with or without synthetic LH-releasing hormone (LH-RH) or hypothalamic acid extracts prepared from rats killed at 12.00 or 24.00 h on pro-oestrus. At 24.00 h pro-oestrus, serum FSH concentration was high while serum LH concentration was low regardless of whether rats were given OE2. Glands collected and incubated at 24.00 h released more FSH and less LH than did glands collected and incubated at 12.00 h pro-oestrus. Administration of OE2 in vivo and/or in vitro did not affect these responses. The increments in LH and FSH release attributed to LH-RH or hypothalamic extracts in the glands incubated at 24.00 h were not different from those of the glands incubated at 12.00 h. Also, the hypothalamic extracts prepared from rats killed at 24.00 h were no more effective than the extracts prepared from rats killed at 12.00 h in releasing LH or FSH from glands incubated at 12.00 or 24.00 h pro-oestrus. Administration of OE2 in vivo caused a small suppression of LH-RH-induced FSH release. We suggest that a change occurs at the level of the anterior pituitary gland during the period of the LH surge and first phase of FSH release to increase basal FSH secretion selectively and cause, at least in part, the second phase of increased serum FSH. This change is not mediated by a decrease in serum oestrogen concentration. We failed to observe any evidence that LH-RH causes preferential FSH release during late pro-oestrus or that a hypothalamic peptide with a preferential FSh releasing ability is involved in FSH release at this time.

Plasma prolactin and progesterone responses to mating are altered in ages rats

The effects of varying amounts of copulatory stimulation on patterns of plasma concentrations of prolactin and progesterone were evaluated in 3- and 12-month-old female rats. The 12-month-old group included rats which still exhibited oestrous cycles and rats in persistent vaginal oestrus (PVO). The extent of copulatory stimulation was defined by the number of intromissions received during mating: less than or equal to 5, 15 or greater than 50. Blood samples were drawn over the 8 days after mating through a cannula inserted into the right external jugular vein. Plasma from the samples was assayed for prolactin and progesterone. In aged but still cyclic rats, pregnancy rates were positively correlated with the number of intromissions received during mating. Only one rat in PVO became pregnant. All animals which became pregnant and rats in PVO which, after mating, exhibited a disruption of the pattern of PVO, showed the nocturnal surge of plasma prolactin characteristic of pregnant and pseudopregnant rats. While these surges persisted until day 8 after mating in pregnant animals, they were absent by this time in the rats in PVO. Prolactin surges were present in some but not all of the aged rats which did not become pregnant. Progesterone concentrations were raised in all pregnant animals except the one pregnant rat in PVO and, while not related to the number of intromissions, concentrations were higher 8 days after mating in young compared with those in aged pregnant rats. Plasma progesterone was low in rats in PVO regardless of disruption of the pattern of PVO. We have concluded that the failure of limited copulatory stimulation to induce pregnancy in older rats results, at least in part, from its failure to initiate nocturnal prolactin surges. Nevertheless, our data suggest that matings which are not experimentally limited should provide ample stimulation to establish such surges. Although reduced plasma concentrations of prolactin and progesterone at pro-oestrus and reduced plasma progesterone through part of gestation may contribute to decreasing fertility in aged rats, other unidentified factors appear to be involved in mediating the capacity of extensive copulatory stimulation to induce pregnancy in these animals.

Further studies on the effectiveness with which exogenous luteinizing hormone and follicle-stimulating hormone stimulate the release of endogenous follicle-stimulating hormone during the rat oestrous cycle

The effects of exogenous rat LH or FSH on the release of endogenous FSH in the cyclic rat have been investigated. Rats were administered phenobarbitone to block the spontaneous increases in gonadotrophins in plasma during pro-oestrus and oestrus and then cannulated through the jugular vein or cannulated and hypophysectomized during the late morning or early afternoon of pro-oestrus. Comparison of patterns of plasma FSH in hypophysectomized and intact rats after i.v. injection of 0.5 micrograms FSH at 17.00 h suggested that exogenous FSH stimulated the release of endogenous FSH in less than 5 h. Intravenous LH (2 micrograms at 16.00 and at 18.00 h) raised the level of FSH in plasma between 2 and 6 h after the first injection of LH. Both gonadotrophins stimulated FSH release by the pituitary gland during the morning of oestrus. Comparison of patterns of plasma FSH in hypophysectomized and intact rats after i.v. injection of 0.25 or 0.05 micrograms FSH at 14.00 h suggested that the latency between FSH injection and stimulation of some FSH release by the pituitary gland is as short as 2 h. Intravenous LH (3, 4 or 9 micrograms) at 14.00 h did not increase the level of FSH in plasma within 2 h and was only minimally effective in raising the level within 4 h. Intravenous LH (2 micrograms at 16.00 and at 18.00 h) on the afternoon of dioestrus day 2 was nearly as effective in increasing the levels of FSH in plasma as it was when administered to pro-oestrous rats. This procedure did not raise the plasma levels of FSH in rats used on dioestrus day 1. The results suggest that in the phenobarbitone-blocked, pro-oestrous rat (1) a small increase (less than that observed spontaneously) in plasma rat FSH during pro-oestrus is effective in stimulating FSH release by the pituitary gland, (2) an increase in plasma rat FSH can exert positive feedback on its own secretion within 2 h and (3) a large increase in plasma rat LH is not very effective in increasing the plasma level of FSH over a period of 4 h. The results also suggest that the spontaneous increase in plasma levels of FSH and, to a lesser extent, of LH is involved in causing the selective phase of FSH release which occurs during late pro-oestrus and the morning of oestrus, and that LH and FSH act differently, but not necessarily by way of a different mechanism, to stimulate release of FSH by the pituitary gland.

Hypothalamic-pituitary interactions during the periovulatory secretion of follicle-stimulating hormone in the rat

We investigated the importance of anterior afferents to the medial basal hypothalamus (MBH) on the increases in plasma FSH during the periovulatory period in the 4-day cyclic rat. We served the anterior connections to the MBH either at 1200 h on proestrus (before the time of onset of the normal spontaneous LH surge in plasma and the associated first phase of FSH release) or near the end of the LH surge and first phase of FSH release at 2000 h on proestrus (before the onset of the second or selective phase of FSH release). Analyses of FSH and LH in blood collected through indwelling atrial catheters or from the trunk after decapitation showed that anterior deafferentation of the MBH at 1200 h on proestrus blocked the proestrous LH surge, the elevations in plasma FSH during proestrus and estrus, and ovulation. In contrast, when brain surgery was delayed until 2000 h on proestrus, the second phase of FSH release and ovulation occurred. In rats with retrochiasmatic transections made at 1200 h, a constant rate iv infusion of LHRH from 1500-1800 h on proestrus restored the LH surge, both phases of increased plasma FSH, and ovulation. The results suggest that 1) the prevolutory LH surge and the first phase of FSH release are dependent on rostral afferents to the MBH which result in hypothalamic LHRH release and 2) the role of rostral afferents to the MBH in the second phase of FSH release is solely to result in hypothalamic LHRH release during proestrus.

Monosodium glutamate disruption of behavioral and endocrine function in the female rat

Experiments were conducted to determine the effects of neonatal administration of L-monosodium glutamate (MSG) on behavioral and endocrine function in the female rat. Administration of MSG (4 mg/kg body weight) at days 1, 3, 5, 7 and 9 in neonates results in a delay of vaginal opening (VO) and the absence of ovulation at the time of VO. However, some rats were observed to ovulate after VO if they were subjected to sequential laparotomies. MSG-treated rats also fail to exhibit compensatory ovarian hypertrophy. Ovariectomized MSG-treated rats injected with estradiol benzoate (EB) followed by a progesterone injected 2 days later did not exhibit sexual beahvior to male rats, while all the control rats displayed lordosis. Chronic treatment with EB for 12 days, followed by a progesterone injection on the 12th day, resulted in a marked improvement of the sexual receptivity of the MSG-treated rats. The body weight of the MSG-treated animals was lower than that of the controls during development although the MSG animals looked obese. Food intake is normal in the MSG-treated rats, but when expressed as intake/100 g body weight, the MSG-treated rats appeared slightly hyperphagic, MSG-treated rats respond with increased food intake after ovariectomy and EB treatment suppresses the increased food intake. Thus, the control of food intake by estrogen does not seem to be affected by the MSG treatment; in fact, these animals seem to be more sensitive than control rats to the anorectic effects of EB. Neonatal MSG treatment appears to affect the neural control for the tonic secretion of gonadotropins by destroying arcuate nuclei. This undoubtedly reduces the reproductive capacity of the animals by impeding the growth and secretions of their ovaries. The findings that chronic estrogen followed by progesterone treatment can reinstate sexual receptivity in MSG-treated animals suggests that the arcuate nuclei are not needed for the expression of sexual behavior and that estrogens might remedy the fertility problems of MSG-treated animals.

Effects of long-term adrenalectomy on periovulatory increases in serum gonadotrophins and ovulation in rats

The effects of chronic adrenalectomy on the oestrous cycle, periovulatory gonadotrophin surges in sera and ovulation in the rat were assessed. Rats which were exposed to a 14 h light : 10 h darkness lighting schedule for at least 3 weeks and those which showed at least two consecutive 4 day oestrous cycles were divided into three groups. One group (controls) was left untreated. The rest were adrenalectomized (ADX) or sham-adrenalectomized (Sham) during oestrus and then placed in a different animal room with the same lighting schedule but with the onset and end of the photoperiod delayed by 4 h. These operations did not interrupt the length of the oestrous cycles. Rats were decapitated for collection of trunk blood on the fifth pro-oestrous or oestrous day after the operation. Both Sham and ADX rats synchronized to the new lighting schedule in terms of the temporal patterns of their periovulatory surges in serum LH and FSH and the timing of ovulation. Neither adrenalectomy nor the sham-operation altered the magnitude of the pro-oestrous or oestrous increases in serum LH or FSH when compared with those in control rats. Body, uterine and ovarian weights as well as the number of ova shed were similar in the ADX and Sham groups. The results indicate that adrenal secretions are not necessary for rats to synchronize the timing of the periovulatory surges in serum LH and FSH or ovulation to a 14 h light : 10 h darkness schedule and the periovulatory surges of LH and FSH are unaltered in chronically adrenalectomized rats.

Plasma LH patterns after LHRH infusion in long-term, unanesthetized ovariectomized rats. Evidence for neural control of the pulsatile LH phenomenon

Experiments were conducted in vivo to investigate further if the control of the pulsatile plasma LH phenomenon in ovariectomized (OVX) rats is located in the brain or in the adenohypophysis. Luteinizing hormone releasing hormone (LHRH) was infused at a constant rate (2--100 ng/h) through an indwelling venous cannula in unanesthetized, unrestrained OVX rats. Blood samples were collected at 5-min intervals through a second venous cannula prior to and during LHRH infusion for subsequent radioimmunoassay of plasma LH. LHRH infusion at 12.5, 50 and 100 ng/h did not interfere with the magnitude or the periodicity of LH pulses in plasma but the range within which plasma LH fluctuated was elevated. Phenobarbital (75 mg/kg BW; i.p.) blocked the pulsatile plasma LH and maintained the plasma LH nearly constant at reduced levels. Pulse i.v. injections of LHRH but not constant rate i.v. infusions restored pulsatile LH patterns in phenobarbital-treated OVX rats. The results are consistent with the view that pulsatile LHRH release is responsible for the pulsatile nature of plasma LH in OVX rats. The results do not support the concepts of a short-loop feedback of LH or an ultra-short-loop feedback of LHRH on LH secretion at least on an acute basis.

Effect of prior gonadotrophin injection on the rising phases of simulated preovulatory surges of luteinizing hormone and follicle-stimulating hormone in the plasma of phenobarbitone-blocked, pro-oestrous rats

Experiments were performed to determine whether the initial rise in the level of LH in peripheral plasma which occurs during its preovulatory surge in plasma may modulate the subsequent patterns of rising of LH and FSH which occur during pro-oestrus. Two cannulae were inserted into the right atrium of the heart of pro-oestrous rats previously given phenobarbitone to block the preovulatory surges of LH and FSH in plasma. One cannula was used for blood collection and injections and the other for infusions. Vehicle, 0·2 or 2·0 μg rat LH, or 1·0 μg rat FSH was injected 60 min before a 90 min infusion of 0·9% saline or of LH releasing hormone (LH-RH) which was infused at a rate of 50 ng/h to simulate the LH and FSH surges. Plasma LH rose more than 12-fold or 125-fold 5 min after injection of 0·2 or 2·0 μg rat LH respectively. At 60 min, the level of LH in plasma was basal or raised about fourfold respectively. The plasma LH and FSH responses to LH-RH in these rats were similar to those of control animals. Infusion of LH-RH caused levels of LH in plasma to increase about fourfold during an initial rising phase. This was followed by a marked increase in levels of plasma LH during a rapid rising phase, an effect attributed to self-priming by LH-RH. Plasma FSH was not raised after 25 min but was increased 60 min after the start of infusion of LH-RH. Injection of FSH did not alter the plasma LH response to LH-RH. The results suggest that the delayed rise in plasma FSH in response to LH-RH is not influenced by, nor is it the result of, a prior increase in levels of LH in peripheral plasma and that the marked increase in plasma LH attributed to self-priming by LH-RH is not the result of a positive feedback of peripheral plasma LH on its own secretion.

Paradoxical effects of drugs acting on the central nervous system on the preovulatory release of pituitary luteinizing hormone in pro-oestrous rats

There are many drugs which act on the central nervous system to block the spontaneous, preovulatory surge of LH in the plasma of the rat when administered just before the start of the LH surge at pro-oestrus. Several of these drugs have been tested for their ability to inhibit LH release after the start of the LH surge. Four day cyclic rats were each fitted with an indwelling right atrial cannula during the morning of pro-oestrus. Serial blood samples were collected through the cannula during the afternoon of pro-oestrus, starting at a time when the level of LH in the plasma was expected to be raised. Drugs were then administered at doses known to block the surge of LH. Additional blood samples were collected and subsequently the plasma concentration of LH was measured by radioimmunoassay. In rats with a raised level of LH, i.p. injection of phenobarbitone or pentobarbitone, s.c. injection of atropine or i.v. injection of ethanol caused the concentration of LH to fall rapidly starting within 15 min after the injection. The mean half-lives for the disappearance of LH from plasma ranged from 23·5 to 28·6 min over a 30 min period. In contrast, s.c. injection of nicotine, i.p. injection of urethane or continuous exposure to ether fumes resulted in a biphasic effect on the concentration of LH in the plasma. The value rose rapidly, reached a peak within 5–20 min and declined rapidly (range of mean half-lives 25·3– 28·6 min). Nicotine, urethane and ether caused a small, transient increase in the plasma level of LH when administered before the start of the spontaneous surge of LH. These results and the fact that luteinizing hormone releasing hormone (LH-RH) is far more effective at raising the concentration of LH in the plasma when given after the start of the LH surge than just before it, suggest that LH-RH must prime the pituitary gland before nicotine, urethane or ether can cause a substantial increase in the plasma level of LH in pro-oestrous rats.

Changes in plasma luteinizing hormone-releasing hormone and gonadotropin concentrations during constant rate intravenous infusion of luteinizing hormone-releasing hormone in cyclic rats

Further analysis has been made of the response of the rat pituitary gland to LHRH during the 4-day estrous cycle. LHRH was infused iv at a constant rate (50 ng/h) into phenobarbital-treated rats at different times during the estrous cycle. Infusion at this rate in proestrous rats simulates the rising and plateau phases of the spontaneous proestrous surges of LH and FSH in plasma. Plasma LH rose to similar heights during the "initial phase" of LH release (during the first 40 min of infusion) on the afternoons of estrus, diestrous day one, and proestrus and during the morning of proestrus. The increase during the afternoon of diestrous day two was significantly less than that in all the other groups. A similar response was seen in the case of FSH release. A "rapid rising" or "augmented" phase of LH release (during 40-120 min of infusion) was present in all groups and the magnitude of the response was greatest during the afternoon of proestrus. In the case of FSH, an augmented phase of release started 60 min after the start of infusion, and the response during the afternoon of proestrus was slightly greater than the responses measured at the other times tested. The responses on diestrous day one were not altered when phenobarbital was omitted or when rats were ovariectomized shortly before LHRH infusion. Other differences in the LH and FSH responses during both initial and augmented phases of release were seen in rats tested at different times during the estrous cycle with an LHRH infusion rate which caused a supraphysiological response on proestrus. The results suggest that 1) the initial rising phases in plasma LH and FSH during the spontaneous surges during proestrus are not the result of an increase in pituitary responsiveness to LHRH during the estrous cycle, 2) augmented phases of LH and FSH release can be elicited on all days of the estrous cycle, and 3) the increases in magnitude of the augmented phases of LH and FSH release on proestrus, as compared to those on other days of the cycle, are the result of an increase in pituitary responsiveness to LHRH during the estrous cycle.

Effects of mating and injection of luteinizing hormone releasing hormone on serum luteinizing hormone and testosterone concentrations in rabbits

The effects of coitus and injection of luteinizing hormone releasing hormone (LH-RH) on serum concentrations of LH, testosterone and dihydrotestosterone (17β-hydroxy-5α-androstan-3-one; DHT) were tested in male rabbits. Before experimentation, male and female rabbits were housed in individual cages in the same room. Male rabbits were then bled by cardiac puncture before and after placement with female rabbits or intravenous injection of LH-RH. Serum LH, testosterone and DHT were measured by radioimmunoassay. Sexual excitement (sniffing, chasing and mounting), with or without intromission, caused a marked rise in serum testosterone and DHT concentrations in only some of the bucks. These increases were accompanied or preceded by a small, transient increase in serum LH. In the rest of the bucks, sexual excitement with or without intromission had either no effect on serum levels of all three hormones, or only serum testosterone and DHT decreased during the collection period. Similar responses were measured in bucks which were housed in a room without does for 2–4 weeks before experimentation. Injection of 10, 30 or 100 ng or 50 μg LH-RH caused serum LH, testosterone and DHT to rise in all bucks tested, but the magnitude of the rises in serum testosterone and DHT were not related to the magnitude of the LH rise. In both mated and LH-RH-injected bucks, the rises in serum testosterone and DHT were greatest in animals with low initial testosterone and DHT values. Under the conditions of this study, the data suggest that: (1) serum testosterone and DHT rise in only some male rabbits after sexual excitement (with or without intromission), (2) the rises in serum testosterone and DHT are dependent on a small transient increase in serum LH and (3) sexual excitement is less likely to cause release of LH-RH in bucks with raised serum testosterone and DHT concentrations.

Effects of pinealectomy on the rat oestrous cycle and pituitary gonadotrophin release

In 4-day cyclic rats kept in a room with the lights on from 05.00 to 19.00 h, sham pinealectomy or pinealectomy on the morning of pro-oestrus did not alter the length of the oestrous cycle for 44 days or the time and magnitude of the rises in LH, FSH and prolactin in the circulation in the afternoon on pro-oestrous days 0, 20 or 44. On day 45, the light schedule was set forward 4 h to run from 09.00 to 23.00 h. The rats continued to have seven additional consecutive 4-day oestrous cycles. On day 27 after the resetting of the light schedule, the pro-oestrous rises in serum LH, FSH and prolactin were delayed 4 h in all rats and a normal quota of eggs was ovulated that night. Other 4- and 5-day cyclic rats which had been made persistently oestrous by anterior deafferentation of the medial basal hypothalamus (AC) underwent pinealectomy. These AC-pinealectomized rats were ovariectomized 60 days later and histological examination of the ovaries revealed no evidence of recent ovulation. Five to six weeks after ovariectomy, sequential blood samples were withdrawn through indwelling atrial cannulas in the AC-pinealectomized-ovariectomized rats and in ovariectomized, pinealectomized-ovariectomized and AC-ovariectomized rats. Regular pulsatile rhythms in plasma LH were measured in all rats. Subcutaneous injection of 50 mug mug oestradiol benzoate in oil lowered plasma LH levels in all four groups but caused an LH surge in the afternoon 2 days later only in the ovariectomized and pinealectomized-ovariectomized rats. The results indicate that the pineal gland in rats kept on a 14 h light: 10 h darkness schedule does not play an active or premissive role in the timing or magnitude of LH, FSH or prolactin release at pro-oestrus, the length of the oestrous cycle, or LH release in ovariectomized rats.

A detailed characterization of the proestrous luteinizing hormone surge

Four-day cycling rats were kept in a room with the light on from 0500-1900 h. Plasma LH concentrations in blood withdrawn through atrial cannulas at hourly intervals from 1400-2000 h on proestrus were very similar to serum LH concentrations in blood collected from uncannulated rats by decapitation at these times. Additional cannulated rats were bled (0.1 ml) at 5 min intervals from 1400-1600, 1600-1800, or 1800-2000 h. After each bleeding, 0.1 ml of heparinized saline was injected through the cannula. Following a rise (rate unknown) to detectable levels of about 200 ng/ml, plasma LH displayed a rapid linear increase to 1538 +/- 118 ng/ml, starting between 1445 and 1650 h and lasting 20 to 50 min. Over the next 110 min plasma LH at first rose erratically by about another 200 ng/ml and then fluctuated around an apparent plateu. It then dropped rapidly but the declines were commonly interrupted by one or more rapid increases in plasma LH. A generalized pattern of the proestrous LH surge has been constructed from the data.

Simulation of the proestrous luteinizing hormone (LH) surge after infusion of LH-releasing hormone in phenobarbital-blocked rats

Four-day cycling rats were kept in a room with the lights on from 0500-1900 h. Injection of an ovulation-blocking dose of phenobarbital at 1230 h on proestrus did not alter the rise in plasma LH concentration in response to rapid injection of 12.4, 124, or 1240 ng of LHRH at 1300 h. In additional blocked rats, blood was rapidly withdrawen through one of two indwelling atrial cannulas while LHRH was infused at a constant rate through the other. Administration of a given amount of LHRH by slow infusion was much more effective in elevating plasma LH than was rapid injection of the same amount of releasing hormone. The pattern of plasma LH concentration after infusion of approximately 50 ng of LHRH per hour from 1500-1810 h was remarkably similar to that of the spontaneous LH surge, i.e., a gradual rise in plasma LH concentration followed by a steep linear increase to high levels which remained elevated for a period of approximately 2 h before declining rapidly soon after the end of infusion. In rats given a second 3 h infusion of LHRH at the same rate, from 2200 to 0100 h, a different response pattern was seen: the initial increase in plasma LH was greater, but the linear rise, which ensued after a lag period of similar duration (about 45 min), was less marked. Prolongation of the first infusion beyond 1800 h did not prolong the plateau: plasma LH levels declined before the infusion was terminated. This decline was less rapid than that seen at the end of a spontaneous LH surge, which in turn was less rapid than that seen after termination of a 3 h and 10 min infusion starting at the same time (1500 h), suggesting that both LHRH release and pituitary responsiveness are diminished (but not abolished) at this time. These findings clearly indicate that, in phenobarbital-blocked proestrous rats, the major part of the proestrous LH surge (including rising and plateau phases) can be simulated by a constant-rate infusion of about 150 ng of LHRH over the 3 h and 10 min period beginning at 1500 h. Since the data also suggest that LHRH release is reduced and/or occurs sporadically during the terminal phase of the surge (when plasma LH levels are declining), these experiments imply that the LH surge is caused by a properly timed, nearly constant-rate release of LHRH for about 3 h (beginning about 1500 h of proestrus) followed by a period of diminished LHRH release.

Simulation of the early phase of the proestrous follicle-stimulating hormone rise after infusion of luteinizing hormone-releasing hormone in phenobarbital-blocked rats

Four-day cycling rats were kept in a room with the lights on from 0500-1900 h. Plasma FSH concentrations in blood withdrawn through atrial cannulas at hourly intervals from 1400-2000 h on proestrus were very similar to serum FSH concentrations in blood collected by decapitation. Additional cannulated rats were bled at 20 min intervals from 1400-1600, 1600-1800, or 1800-2000 h. In most rats, plasma FSH concentration rose gradually to approximately three times pre-rise levels by 1900 h. It then decreased slightly by 2000 h. Injection of an ovulation-blocking dosage of phenobarbital at 1345 h blocked the FSH rise. Rapid injection of 124 or 1240 ng of LHRH at 1300 h did not elevate plasma FSH by 1315 h but the 1240 ng dose did by 1400 h. Phenobarbital injection at 1230 h did not alter this response. In additional blocked rats, blood was rapidly withdrawen through one of two indwelling atrial cannulas while LHRH was infused at a constant rate through the other. Administration of LHRH by infusion was much more effective in elevating plasma FSH than was rapid injection of the releasing hormone. The pattern of plasma FSH concentration after infusion of about 50 ng of LHRH per hour from 1500-1810 h and then about 12 ng of LHRH per hour from 1810-1920 h was remarkably similar to that of the spontaneous FSH rise. These experiments imply that the early phase of the FSH surge (the one associated with the proestrous LH surge) is caused by a properly timed, nearly constant-rate release of LHRH for about 3 h (beginning about 1500 h of proestrus) followed by a period of diminished LHRH release.

Effects of intravenous infusion of catecholamines on rat plasma luteinizing hormone and prolactin concentrations

Catecholamines were infused through an atrial cannula in unanesthetized rats on the afternoon of proestrus and blood was withdrawn through a second cannula for radioimmunoassay of LH and prolactin. Infusion of epinephrine, but not of norepinephrine or dopamine, blocked spontaneous pituitary LH release and ovulation. Ultimately, this effect appears to be exerted on the brain and not on the pituitary or through changes in pituitary blood flow. Pituitary LH release in response to exogenous LHRH, when administered in an amount that simulated the proestrous LH surge in phenobarital-treated rats, was unaltered by epinephrine infusion. In addition, epinephrine infusion did not alter the timing of the rise in plasma prolactin. Infusion of dopamine blocked the spontaneous rise in plasma prolactin and depressed basal prolactin levels. After the end of infusion, plasma prolactin rose rapidly. Infusion of norepinephrine or epinephrine partially suppressed the prolactin rise but only after 2 h of infusion.

THE RESULTS

1) point out the possibility that chronic release of epinephrine from the adrenal medulla may be involved in phenomena in which "stress" inhibits reproductive function; and 2) are consistent with the view that dopamine, but not norepinephrine, may be PIF.

Radioimmunoassay of serum LH and testosterone in male rabbits actively immunized against testosterone

Five male rabbits were actively immunized against testosterone to determine if this procedure could permanently inactivate circulating testosterone. To assess the response to immunization, serum LH, serum testosterone and antitestosterone titer (titer) were measured by radioimmunoassay at various times after immunization. All rabbits produced antisera to testosterone and developed serum LH levels similar to those measured in castrated males. The rise in serum LH was particularly pronounced after a booster immunization but these elevated levels were not maintained which suggests that biological neutralization was only transient. Serum testosterone rose dramatically after immunization and was directly correlated with titer. High serum testosterone concentrations were associated with both high and low serum LH. No correlation existed between titer ans serum LH. It is concluded that active immunization against testosterone does not necessarily result in a permanent neutralization of circulating testosterone and that titer alone is an inadequate criterion of neutralization.