Evaluation of the pituitary-gonadal response to GnRH, and adrenal status, in the leopard (Panthera pardus japonensis) and tiger (Panthera tigris)

Frequent blood samples were collected to study hormonal responses to GnRH in male and female leopards and tigers. Animals were anaesthetized with ketamine-HCl and blood samples were collected every 5 min for 15 min before and 160 min after i.v. administration of GnRH (1 micrograms/kg body weight) or saline. No differences in serum cortisol concentrations were observed between sexes within species, but mean cortisol was 2-fold greater in leopards than tigers. GnRH induced a rapid rise in LH in all animals (18.3 +/- 0.9 min to peak). Net LH peak height above pretreatment levels was 3-fold greater in males than conspecific females and was also greater in tigers than leopards. Serum FSH increased after GnRH, although the magnitude of response was less than that observed for LH. Basal LH and FSH and GnRH-stimulated FSH concentrations were not influenced by sex or species. Serum testosterone increased within 30-40 min after GnRH in 3/3 leopard and 1/3 tiger males. Basal testosterone was 3-fold greater in tiger than leopard males. LH pulses (1-2 pulses/3 h) were detected in 60% of saline-treated animals, suggesting pulsatile gonadotrophin secretion; however, in males concomitant testosterone pulses were not observed. These results indicate that there are marked sex and species differences in basal and GnRH-stimulated hormonal responses between felids of the genus Panthera which may be related to differences in adrenal activity.     

The frequency of gonadotropin-releasing hormone secretion regulates expression of alpha and luteinizing hormone beta-subunit messenger ribonucleic acids in male rats

The influence of GnRH pulse frequency on LH subunit mRNA concentrations was examined in castrate, testosterone-replaced male rats. GnRH pulses (25 ng/pulse) or saline to controls, were given via a carotid cannula at intervals of 7.5-240 min for 48 h. alpha and LH beta mRNA concentrations were 109 +/- 23 and 30 +/- 5 pg cDNA bound/100 micrograms pituitary DNA, respectively, in saline controls. GnRH pulse intervals of 15, 30, and 60 min resulted in elevated alpha and LH beta mRNAs (P less than 0.01) and maximum responses (4-fold, alpha; 3-fold, LH beta) were seen after the 30-min pulses. Acute LH release to the last GnRH pulse was seen after the 15-, 30-, and 60-min pulse intervals. In contrast, LH subunit mRNAs were not increased and acute LH release was markedly impaired after the rapid (7.5 min) or slower (120 and 240 min) pulse intervals. Equalization of total GnRH dose/48 h using the 7.5- and 240-min intervals did not increase LH subunit mRNAs to levels produced by the optimal 30-min interval. These data indicate that the frequency of the pulsatile GnRH stimulus regulates expression of alpha and LH beta mRNAs in male rats. Further, GnRH pulse frequencies that increase subunit mRNA concentrations are associated with continuing LH responsiveness to GnRH.     

Gonadotropin secretion and pituitary responsiveness to GnRH in mares with granulosa-theca cell tumor

Granulosa-theca cell tumors (GTCTs) are able to secrete variable amounts of sex steroids and immunoreactive inhibin (ir-INH). Although the pituitary appears to be affected by the presence of a GTCT, pituitary responsiveness to exogenous GnRH has not been examined. The aims of the present study were to: (i) assess the plasma hormone concentrations of ir-INH, gonadotropins and sex steroids in eight mares with GTCT and (ii) assess the responsiveness of pituitary gonadotroph cells to exogenous GnRH stimulus both before and after tumor removal. In seven mares, the contralateral ovary was firm, small and inactive. Histopathological observations of the tumors confirmed the presumptive diagnosis of a GTCT. Four mares, judged to be in vernal transition period (n=2) and in the breeding season (n=2), were used as controls. A single intravenous injection of 40 microg of GnRH agonist was given to each mare and blood samples were collected every 15 min from 2 h before to 4 h after injection. In four GTCT mares, this procedure was repeated 20 (n=2) and 90 (n=2) days after tumors removal. All plasma samples were analyzed for concentrations of ir-INH, LH, FSH, estradiol-17beta (E2), testosterone (T) by RIA and progesterone (P) by EIA. Results showed that E2 levels were significantly higher (P<0.001) in control animals compared to E2 levels in GTCT mares before and after surgery. P and T concentrations were not statistically different between the groups. Baseline levels of ir-INH were greater (P<0.05) in GTCT mares before surgery than in control mares, and decreased to undetectable levels after neoplasia ablation. Baseline FSH did not differ between control and GTCT animals either before or after the ovaries were removed. LH baseline values appeared to be higher for affected mares, but the difference was not statistically significant. Maximum release (MR) and area under the gonadotrophin release curve (AUC) after the GnRH challenge for both the gonadotrophins were similar between the groups.     

Influence of gonadotropin-releasing hormone pulse amplitude, frequency, and treatment duration on the regulation of luteinizing hormone (LH) subunit messenger ribonucleic acids and LH secretion

The effects of GnRH pulse amplitude, frequency, and treatment duration on pituitary alpha and LH beta subunit mRNA concentrations were examined in castrate-testosterone replaced male rats. Experimental groups received iv GnRH pulses (5, 25, or 125 ng) at 7.5-, 30-, or 120-min intervals for 8, 24, or 48 h. Saline pulses were given to control rats. Acute LH secretion was measured in blood drawn before and 20 min after the last GnRH pulse. In saline controls, alpha and LH beta mRNAs (150 +/- 14, 23 +/- 2 pg cDNA bound/100 micrograms pituitary DNA) fell to 129 +/- 14 and 18 +/- 2, respectively, after 48 h. In animals receiving GnRH pulses (7.5-min intervals), the 125-ng dose stimulated a slight increase (P less than 0.01) in alpha mRNA levels after 8 and 24 h and both LH subunit mRNAs were increased by the 25- and 125-ng doses after 48 h. The 30-min pulse interval injections (25- and 125-ng doses) increased LH beta mRNA levels after 8 h, but alpha mRNAs were not elevated until after 24 h. Maximum (3-fold) increases in alpha and LH beta mRNAs were seen in rats receiving 25-ng pulses every 30 min for 48 h. Using 120-min pulses, LH subunit mRNAs were not increased by any GnRH dose through 48 h. Acute LH release was not seen in rats receiving 5 ng GnRH pulses at any pulse interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of gonadotropin-releasing hormone pulse-frequency modulation on luteinizing hormone, follicle-stimulating hormone and testosterone secretion in hypothalamo/pituitary-disconnected rams

The effects of changes in pulse frequency of exogenously infused gonadotropin-releasing hormone (GnRH) were investigated in 6 adult surgically hypothalamo/pituitary-disconnected (HPD) gonadal-intact rams. Ten-minute sampling in 16 normal animals prior to HPD showed endogenous luteinizing hormone (LH) pulses occurring every 2.3 h with a mean pulse amplitude of 1.11 +/- 0.06 (SEM) ng/ml. Mean testosterone and follicle-stimulating hormone (FSH) concentrations were 3.0 +/- 0.14 ng/ml and 0.85 +/- 0.10 ng/ml, respectively. Before HPD, increasing single doses of GnRH (50-500 ng) elicited a dose-dependent rise of LH, 50 ng producing a response of similar amplitude to those of spontaneous LH pulses. The effects of varying the pulse frequency of a 100-ng GnRH dose weekly was investigated in 6 HPD animals; the pulse intervals explored were those at 1, 2, and 4 h. The pulsatile GnRH treatment was commenced 2-6 days after HPD when plasma testosterone concentrations were in the castrate range (less than 0.5 ng/ml) in all animals. Pulsatile LH and testosterone secretion was reestablished in all animals in the first 7 days by 2-h GnRH pulses, but the maximal pulse amplitudes of both hormones were only 50 and 62%, respectively, of endogenous pulses in the pre-HPD state. The plasma FSH pattern was nonpulsatile and FSH concentrations gradually increased in the first 7 days, although not to the pre-HPD range. Increasing GnRH pulse frequency from 2- to 1-hour immediately increased the LH baseline and pulse amplitude. As testosterone concentrations increased, the LH responses declined in a reciprocal fashion between Days 2 and 7. FSH concentration decreased gradually over the 7 days at the 1-h pulse frequency. Slowing the GnRH pulse to a 4-h frequency produced a progressive fall in testosterone concentrations, even though LH baselines were unchanged and LH pulse amplitudes increased transiently. FSH concentrations were unaltered during the 4-h regime. These results show that 1) the pulsatile pattern of LH and testosterone secretion in HPD rams can be reestablished by exogenous GnRH, 2) the magnitude of LH, FSH, and testosterone secretion were not fully restored to pre-HPD levels by the GnRH dose of 100 ng per pulse, and 3) changes in GnRH pulse frequency alone can influence both gonadotropin and testosterone secretion in the HPD model.     

Effects of gonadotropin-releasing hormone administration on the pituitary-gonadal axis in male and female dogs before and after gonadectomy

GnRH-stimulation tests were performed in 14 female and 14 male client-owned dogs of several breeds, before and 4 to 5 mo after gonadectomy. The aim of the study was to obtain more insight into the pituitary-gonadal axis in intact and neutered dogs and to establish reference values. Basal plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) concentrations were increased significantly after gonadectomy in both bitches and male dogs. In both males and females ranges of the basal plasma FSH concentrations, before and after gonadectomy, did not overlap as opposed to the overlap in ranges of the basal plasma LH concentrations. Before gonadectomy basal plasma LH concentrations were lower and basal plasma FSH concentrations were higher in bitches than in male dogs. After gonadectomy these basal values did not differ significantly. GnRH administration before gonadectomy resulted in an increase in plasma LH and FSH concentrations in both genders. GnRH administration after gonadectomy produced an increase only in plasma LH concentrations in both genders, and a just significant increase in plasma FSH in castrated male dogs. GnRH administration before gonadectomy resulted in a significant increase in plasma testosterone concentration in both genders. In males ranges of basal and GnRH-stimulated plasma testosterone concentrations before and after gonadectomy did not overlap. Basal plasma estradiol concentrations were significantly higher in intact males than in castrated males and their ranges did not overlap. The basal estradiol concentrations in bitches before and after ovariectomy were not significantly different. At 120 min after GnRH administration, ranges of plasma estradiol concentration of intact and ovariectomized bitches no longer overlapped. In conclusion, basal plasma FSH concentration appears to be more reliable than basal plasma LH concentration for verification of neuter status in both male and female dogs. The basal plasma testosterone concentration appears to be reliable for verification of neuter status in male dogs. The plasma estradiol concentration at 120 min after GnRH administration can be used to discriminate between bitches with and without functional ovarian tissue.     

Effect of naloxone on GnRH-induced LH and FSH release in buffalo, Bubalus bubalis

The effect of naloxone on GnRH-induced LH and FSH release was measured in buffaloes in luteal phase of estrous cycle. Animals were administered intravenously, naloxone/saline (50 mg/injection) every 15 min for 3 hr followed by GnRH (100 micrograms). Peripheral plasma LH and FSH concentrations were measured in blood samples collected at 15 min intervals from 1 hr prior to beginning of naloxone/saline treatment up to 3 hr post GnRH administration and every 30 min for the subsequent 3.5 hr. Between the animals of Group I administered naloxone and those of Group II given saline, GnRH-induced peak LH and FSH concentrations, the total LH and FSH released in response to GnRH, and the time to peak LH and FSH concentrations were not significantly different. The results of the present study suggest the absence of a direct effect of naloxone on pituitary responsiveness to GnRH.     

Regulatory roles of leptin at the hypothalamic-hypophyseal axis before and after sexual maturation in cattle

Studies assessed, either directly or indirectly, the role of GnRH in leptin-mediated stimulation of LH release in cattle before and after sexual maturation. In experiment 1, the objectives were to determine whether leptin could acutely accelerate the frequency of LH pulses, and putatively GnRH pulses, in prepubertal heifers at different stages of development. In experiment 2, we determined directly whether acute, leptin-mediated increases in LH secretion in the fasted, mature female are accompanied by an increase in GnRH secretion. Ten-month-old prepubertal heifers (experiment 1) fed normal- (n = 5) and restricted-growth (n = 5) diets received three injections of saline or recombinant ovine leptin (oleptin; 0.2 microg/kg body weight, i.v.) at hourly intervals during 5-h experiments conducted every 5 wk until all normal-growth heifers were pubertal. Leptin increased mean concentrations of circulating LH regardless of diet, but pulse characteristics were not altered at any age. In experiment 2, ovariectomized, estradiol-implanted cows (n = 5) were fasted twice for 72 h and treated with either saline or oleptin i.v. (as in experiment 1) on Day 3 of each fast. Leptin increased plasma concentrations of LH and third ventricle cerebrospinal fluid concentrations of GnRH, and increased the amplitude of LH and the size of GnRH pulses, respectively, on Day 3 of fasting compared to saline. Overall, results indicate that leptin is unable to accelerate the pulse generator in heifers at any developmental stage. However, leptin-mediated augmentation of LH concentrations and pulse amplitude in the nutritionally stressed, mature female are associated with modifications in GnRH secretory dynamics.     

Effect of maternal treatment with altrenogest on pituitary response to exogenous GnRH in pubertal stallions

The pituitary response to exogenous GnRH was studied in 8 colts of Quarter Horse phenotype from 32 to 96 weeks of age. Colts were from dams treated daily from Day 20 to 325 of gestation with (1) 2 ml neobee oil per 50 kg body weight (controls); or (2) 2 ml altrenogest per 50 kg body weight. GnRH challenges (5 micrograms/kg body weight) were administered every 8 weeks from 32 to 96 weeks of age to estimate pituitary content of LH. Blood samples were collected every 20 min for 4 h before GnRH and 15, 30, 45, 60, 90, 120, 180, 240 and 360 min after GnRH. Serum concentrations of LH and FSH were determined for the 2 pre-GnRH and all post-GnRH samples. Baseline concentrations (mean of 2 pre-GnRH samples) of LH and FSH were not affected by treatment (P greater than 0.05). Serum concentrations of LH declined from 40 to 56 weeks and rose again between 72 and 80 weeks. Basal concentrations of FSH declined from 32 to 56 weeks, and varied widely after 56 weeks. The maximum LH response to GnRH (highest concentration after GnRH minus baseline) declined steadily in both groups for 48 to 64 weeks but remained relatively constant in both groups after 64 weeks. The maximum FSH response to GnRH declined from 32 to 64 weeks then remained relatively constant in both groups. The GnRH-induced gonadotrophin release remained low with a transient increase at 72 weeks for both hormones.(ABSTRACT TRUNCATED AT 250 WORDS)     

Testosterone inhibits gonadotropin-releasing hormone pulse frequency in the male sheep

The objective was to determine the effect of chronic testosterone (T) treatment on GnRH and LH secretion in wethers. Rams were either castrated only or castrated and immediately treated with Silastic implants containing T. Several weeks later, a device for collecting hypophyseal-portal blood was surgically implanted. Six to seven days later, blood samples were collected simultaneously and continuously from the portal vessels and jugular vein of pairs of conscious animals. Samples were divided at 10-min intervals for 6-12 h. One hour before the end of collection, all animals received i.v. injections of 250 ng of GnRH. In samples collected simultaneously from 6 pairs of animals, T reduced the frequency of both GnRH pulses (1.8 +/- 0.2 vs. 0.9 +/- 0.3/h, p less than 0.03) and LH pulses (1.6 +/- 0.1 vs. 0.8 +/- 0.3/h, p less than 0.03). T did not alter amplitude of either GnRH or LH pulses. Testosterone reduced mean GnRH (9.7 +/- 0.6 vs. 7.9 +/- 0.5 pg/ml, p less than 0.05), whereas mean LH was not significantly reduced (9.6 +/- 1.4 vs. 6.1 +/- 1.8 ng/ml, p = 0.16). These results support the hypothesis that T reduces GnRH pulse frequency.     

Roles of estradiol and gonadotropin-releasing hormone in controlling negative and positive feedback associated with the luteinizing hormone surge in ovariectomized pigs.

Ovariectomized gilts (n = 63) were given estradiol benzoate (estradiol), antiserum to neutralize endogenous GnRH, and pulses of a GnRH agonist (GnRH-A) to stimulate release of LH. GnRH-A was given as 200-ng pulses hourly from 0 to 54 h and as 100- or 200-ng pulses every 30 or 60 min from 54 to 96 h after estradiol. Estradiol alone suppressed LH from 6 to 54 h and elicited an LH surge that peaked at 72 h. When GnRH-A was given every 30-60 min from 0 to 96 h, estradiol suppressed LH for 6-12 h, but then LH returned to pre-estradiol concentrations. When pulses of GnRH-A were given only between 54 and 96 h after estradiol, the surge of LH was related positively to dose and frequency of GnRH-A. We conclude that 1) estrogen acts at the hypothalamus to inhibit release of GnRH for 54 h and then causes a synchronous release of GnRH; 2) estrogen acts at the pituitary to block its response to GnRH for 6-12 h and enhances the accumulation of releasable LH; and 3) magnitude of the LH surge is dependent on the amount of GnRH stimulation.     

Opiatergic inhibition of pulsatile luteinizing hormone release during the menstrual cycle of rhesus macaques

The endogenous opioid peptides (EOPs) may inhibit the rate of hypothalamic gonadotropin-releasing hormone (GnRH) release and hence the frequency of pulsatile luteinizing hormone (LH) release, particularly in the luteal phase of the menstrual cycle. Our objectives were to compare the effects of an opiate antagonist, naloxone (NAL), on the patterns of LH, estradiol-17 beta (E2), and progesterone (P4) secretion during the follicular and luteal phases of the macaque menstrual cycle. Plasma levels of E2, P4, and bioactive LH were measured in serial, 15-min blood samples during 8-hr infusions of NAL (2 mg/hr) or saline, either on Days 5 or 6 of the follicular phase (FN and FS, n = 5 and 4, respectively) or on Days 8, 9, or 10 of the luteal phase (LN and LS, n = 5 each) of a menstrual cycle. The pulsatile parameters of each hormone were determined by PULSAR analysis and the correspondence of steroid pulses with those of LH were analyzed for each cycle stage in each animal. As expected, LH mean levels and pulse frequencies in LS monkeys were only about one-third of those values in FS animals. NAL had no effects on pulsatile LH, E2, or P4 release during the follicular phase. In contrast, luteal phase NAL infusions increased both LH mean levels and pulse frequencies to values which were indistinguishable from those in FS animals. LH pulse amplitudes did not differ among the four groups. Mean levels and pulse frequencies of P4 secretion in LS monkeys were about 4- and 14-fold greater than those values in FS animals. Mean levels and pulse amplitudes of P4 release in LN animals were greater than those values in all other groups. LH and E2 pulses were not closely correlated in follicular phase animals, and this pulse association was not altered by NAL. In FS monkeys, LH and P4 pulses were not correlated; however, NAL increased this LH-p4 pulse correspondence. LH and P4 pulses were closely correlated in luteal phase animals and this association was not affected by NAL. Our data suggest that the EOPs inhibit the frequency of pulsatile LH secretion in the presence of luteal phase levels of P4. During the midfollicular phase when LH pulses occur every 60 to 90 min, the opioid antagonist NAL alters neither the pulsatile pattern of LH release nor E2 secretion, but NAL may directly affect P4-secreting cells.     

The effect of neonatal exposure to diethylstilbestrol, genistein, and zearalenone on pituitary responsiveness and sexually dimorphic nucleus volume in the castrated adult rat.

The neonatal hormone environment determines the sexually differentiated pattern of brain growth. Estrogens, derived from intracerebral aromatization of testosterone, promote male sexual central nervous system (CNS) development. Developing animals may also encounter estrogens from plant, fungal, and xenobiotic sources (environmental estrogens). The purpose of this study was to assess the effects of environmental estrogens on the physiology and morphology of the hypothalamus and pituitary. Neonatal rats received injections of either corn oil, 0.1 microgram diethylstilbestrol (DES), 100 micrograms genistein (G100), 1000 micrograms genistein (G1000), 100 micrograms zearalenone (Z100), or 1000 micrograms zearalenone (Z1000) on Days 1-10 of life and were castrated on Day 21. On Day 42, right heart catheters were placed, GnRH (50 ng/kg) was administered, and blood was sampled for LH at 0, 5, 10, 15, and 30 min. Females exposed neonatally to DES, G1000, Z100, and Z1000 showed significantly decreased pituitary responsiveness to GnRH, whereas G100 increased GnRH-induced LH secretion. Males exposed neonatally to G100 also showed increased pituitary response to GnRH, and the remaining estrogen-exposed groups of males exhibited either decreased tonic LH or attenuated GnRH-stimulated LH secretion. The animals were killed by decapitation on Day 49. Volumes of the sexually dimorphic nucleus of the preoptic area (SDN-POA) of the exposed groups were compared. In females, DES, G1000, and Z1000 increased SDN volume; Z100 and G100 had no effect. There was no difference in SDN size among the male groups. These data show that exposure to environmental estrogens early in development alters postpubertal pituitary response to GnRH and "androgenizes" the SDN-POA.     

Changes in circulating levels of immunoreactive follicle stimulating hormone, luteinizing hormone, and testosterone during sexual development in the rhesus monkey, Macaca mulatta

Basal serum levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) and the responsiveness of these hormones to a challenge dose of luteinizing hormone releasing hormone (LHRH), were determined in juvenile, pubertal, and adult rhesus monkeys. The monkey gonadotrophins were analyzed using RIA reagents supplied by the World Health Organization (WHO) Special Programme of Human Reproduction. The FSH levels which were near the assay sensitivity in immature monkeys (2.4 +/- 0.8 ng/ml) showed a discernible increase in pubertal animals (6.4 +/- 1.8 ng/ml). Compared to other two age groups, the serum FSH concentration was markedly higher (16.1 +/- 1.8 ng/ml) in adults. Serum LH levels were below the detectable limits of the assay in juvenile monkeys but rose to 16.2 +/- 3.1 ng/ml in pubertal animals. When compared to pubertal animals, a two-fold increase in LH levels paralleled changes in serum LH during the three developmental stages. Response of serum gonadotrophins and T levels to a challenge dose of LHRH (2.5 micrograms; i.v.) was variable in the different age groups. The present data suggest: an asynchronous rise of FSH and LH during the pubertal period and a temporal correlation between the testicular size and FSH concentrations; the challenge dose of LHRH, which induces a significant rise in serum LH and T levels, fails to elicit an FSH response in all the three age groups; and the pubertal as compared to adult monkeys release significantly larger quantities of LH in response to exogenous LHRH.     

Comparative response of rams and bulls to long-term treatment with gonadotropin-releasing hormone analogs

The objective was to compare the relative response between rams and bulls in characteristics of LH, FSH and testosterone (T) secretion, during and after long-term treatment with GnRH analogs. Animals were treated with GnRH agonist, GnRH antagonist, or vehicle (Control) for 28 days. Serial blood samples were collected on day 21 of treatment, and at several intervals after treatment. Injections of natural sequence GnRH were used to evaluate the capacity of the pituitary to release gonadotropins during and after treatment. Treatment with GnRH agonist increased basal LH and T concentrations in both rams and bulls, with a greater relative increase in bulls. Endogenous LH pulses and LH release after administration of GnRH were suppressed during treatment with GnRH agonist. Treatment with GnRH antagonist decreased mean hormone concentrations, LH and T pulse frequency, and the release of LH and T after exogenous GnRH, with greater relative effects in bulls. Rams previously treated with antagonist had a greater release of LH after administration of GnRH compared with control rams, while rams previously treated with agonist showed a reduced LH response. Bulls previously treated with agonist had reduced FSH concentrations and LH pulse amplitudes compared with control bulls while bulls previously treated with antagonist had greater T concentrations and pulse frequency. The present study was the first direct comparison between domestic species of the response in males to treatment with GnRH analogs. The findings demonstrated that differences do occur between rams and bulls in LH, FSH and testosterone secretion during and after treatment. Also, the consequences of treatment with either GnRH analog can persist for a considerable time after discontinuation of treatment.     

Contribution of increasing and decreasing daylength to the photoperiodic control of LH secretion in the Ile-de-France ram

Sexually mature Ile-de-France rams were exposed to an 8-month light regimen in which the daily light increment and decrement were constant and equal to 7 min/day. Daylength therefore varied from 6 to 20 h. The animals were allotted to two groups of 12 rams each and submitted to the same light regimen but 4 months out of phase. Blood was collected every 40 min for 10 h, on 15 occasions at various intervals during the second light cycle. Plasma LH and on some occasions plasma testosterone concentrations were measured by radioimmunoassay. In both groups the number of LH pulses increased significantly as daylength increased from 11:40 to 20:00 h (P less than 0.01) but because their amplitude was low the mean plasma LH increase, although significant, was moderate. As daylength started to decrease (from 20:00 to 18:30 h), the frequency of LH pulses further increased (P less than 0.05) and the pulse amplitude doubled so that mean plasma LH values increased abruptly and remained high until the decreasing light photoperiod reached 11:40 h. Thereafter, the frequency and amplitude of LH pulses decreased and the mean plasma LH dropped to one-fourth of its maximal value. Mean plasma testosterone concentrations were low during most of the 8-month cycle but increased steeply when daylength decreased from 11:40 to 6:00 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of hyperprolactinemia on the control of luteinizing hormone and follicle-stimulating hormone secretion in the male rat

Experiments were conducted to determine the effects of acute hyperprolactinemia (hyperPRL) on the control of luteinizing hormone and follicle-stimulating hormone secretion in male rats. Exposure to elevated levels of prolactin from the time of castration (1 mg ovine prolactin 2 X daily) greatly attenuated the post-castration rise in LH observed 3 days after castration. By 7 days after castration, LH concentrations in the prolactin-treated animals approached the levels observed in control animals. HyperPRL had no effect on the postcastration rise in FSH. Pituitary responsiveness to gonadotropin hormone-releasing hormone (GnRH), as assessed by LH responses to an i.v. bolus of 25 ng GnRH, was only minimally effected by hperPRL at 3 and 7 days postcastration. LH responses were similar at all time points after GnRH in control and prolactin-treated animals, except for the peak LH responses, which were significantly smaller in the prolactin-treated animals. The effects of hyperPRL were examined further by exposing hemipituitaries in vitro from male rats to 6-min pulses of GnRH (5 ng/ml) every 30 min for 4 h. HyperPRL had no effect on basal LH release in vitro, on GnRH-stimulated LH release, or on pituitary LH concentrations in hemipituitaries from animals that were intact, 3 days postcastration, or 7 days postcastration. However, net GnRH-stimulated release of FSH was significantly higher by pituitaries from hyperprolactinemic, castrated males. To assess indirectly the effects of hyperPRL on GnRH release, males were subjected to electrical stimulation of the arcuate nucleus/median eminence (ARC/ME) 3 days postcastration. The presence of elevated levels of prolactin not only suppressed basal LH secretion but reduced the LH responses to electrical stimulation by 50% when compared to the LH responses in control castrated males. These results suggest that acute hyperPRL suppresses LH secretion but not FSH secretion. Although pituitary responsiveness is somewhat attenuated in hyperprolactinemic males, as assessed in vivo, it is normal when pituitaries are exposed to adequate amounts of GnRH in vitro. Thus, the effects of hyperPRL on pituitary responsiveness appear to be minimal, especially if the pituitary is exposed to an adequate GnRH stimulus. The suppression of basal LH secretion in vivo most likely reflects inadequate endogenous GnRH secretion. The greatly reduced LH responses after electrical stimulation in hyperprolactinemic males exposed to prolactin suggest further that hyperPRL suppresses GnRH secretion.     

Inhibition of pituitary-gonadal function in male rats by a potent GnRH antagonist

The inhibitory effects of the potent GnRH antagonist, [Ac-D-pCl-Phe1,2,D-Trp3,D-Arg6,DAla10]GnRH (GnRHant) upon pituitary-gonadal function were investigated in normal and castrated male rats. The antagonist was given a single subcutaneous (s.c.) injections of 1-500 micrograms to 40-60 day old rats which were killed from 1 to 7 days later for assay of pituitary GnRH receptors, gonadal receptors for LH, FSH, and PRL, and plasma gonadotropins, PRL, and testosterone (T). In intact rats treated with low doses of the antagonist (1, 5 or 10 micrograms), available pituitary GnRH receptors were reduced to 40, 30 and 15% of the control values, respectively, with no change in serum gonadotropin, PRL, and T levels. Higher antagonist doses (50, 100 or 500 micrograms) caused more marked decreases in free GnRH receptors, to 8, 4 and 1% of the control values, which were accompanied by dose-related reductions in serum LH and T concentrations. After the highest dose of GnRHant (500 micrograms), serum LH and T levels were completely suppressed at 24 h, and serum levels of the GnRH antagonist were detectable for up to 3 days by radioimmunoassay. The 500 micrograms dose of GnRHant also reduced testicular LH and PRL receptors by 30 and 50% respectively, at 24 h; by 72 h, PRL receptors and LH receptors were still slightly below control values. In castrate rats, treatment with GnRHant reduced pituitary GnRH receptors by 90% and suppressed serum LH and FSH to hypophysectomized levels. Such responses in castrate animals were observed following injection of relatively low doses of GnRHant (100 micrograms), after which the antagonist was detectable in serum for up to 24 h. These data suggest that extensive or complete occupancy of the pituitary receptor population by a GnRH antagonist is necessary to reduce plasma gonadotropin and testosterone levels in intact rats. In castrate animals, partial occupancy of the available GnRH receptor sites appears to be sufficient to inhibit the elevated rate of gonadotropin secretion.     

Roles of pulsatile release of LH in the development and maintenance of corpus luteum function in the goat

The roles of the pulsatile release of LH in the functional development and maintenance of the corpus luteum (CL) during the estrus cycle in the goat were examined using a potent GnRH antagonist. In Experiment 1, to assess the inhibitory effects of the GnRH antagonist on the release of LH during the estrus cycle, 9 goats were divided into 3 groups. Goats in Group I received only saline on Days 0 (day of ovulation), 5, 10 and 15. Goats in Group II received the GnRH antagonist (50 microg/kg, s.c.) on the days mentioned for Group I to inhibit endogenous LH during the periods of luteal development and maintenance. Goats in Group III received saline on Days 0 and 5 and then the GnRH antagonist on Days 10 and 15 to inhibit LH during the period of luteal maintenance. Serial blood sampling took place on Days 1, 3, 5, 8, 13 and 18 to characterize the LH pulses. The LH pulses were observed throughout the estrus cycle in Group I but were completely abolished in Group II. In Group III, the pulsatile release of LH was observed from Day 1 to 8, but the LH pulses were completely abolished on Days 13 and 18. In Experiment 2, 16 goats were divided into the same 3 groups as in Experiment 1 to examine the effects of the GnRH antagonist on the luteal function. The concentration of progesterone in the plasma in Group I increased after ovulation, reached a maximum level around Day 12, and subsequently returned to the basal level on Day 17. The concentrations of progesterone in Group II rose after ovulation, but reached a plateau around Day 6 and maintained the level up to Day 9, then rapidly decreased from Day 9 to 10 to the basal level. The concentrations of progesterone in Group II were lower on Days 7 to 15 than those in Group I (P<0.01). The concentrations of progesterone in Group III increased after ovulation, reached a maximum level around Day 8, then dropped from Day 10 to 13 to the basal level. The concentrations of progesterone in Group III on Days 11 to 15 were lower than those in Group I (P<0.05 on Day 11, P<0.01 on Days 12 to 15). These results demonstrate that endogenous LH is essential for normal development and maintenance of the CL function during the estrus cycle in the goat. Further, this study suggests that while the functional maintenance of the caprine CL depends entirely on LH support, such functional dependence during early CL development is only partial.