Plasma LH, FSH and testosterone concentrations in adult rams which were homozygous carriers or non-carriers of the Booroola fecundity gene

No gene-specific differences were found with respect to LH or testosterone pulsatile secretion (over 12 h), or in 12 hourly mean FSH concentrations in adult Booroola FF and ++ rams. Also, no differences between genotypes in the LH response to an injection of testosterone propionate, the FSH response to an infusion of bovine follicular fluid, or the testosterone response to injections of PMSG were noted. However, during the phase of seasonal testicular development, mean testosterone pulse amplitude (over 12 h) and the FSH response to 25 micrograms GnRH were higher in FF than in ++ rams (P less than 0.05); there were also significant effects of sire (P less than 0.05 in FF genotype only) and litter size (P less than 0.05) on testosterone pulse amplitude and GnRH-stimulated FSH release, respectively. During the breeding season, mean LH, but not FSH, concentrations were higher in FF than in ++ rams, after an injection of 0.5 micrograms GnRH; LH release was not affected by sire or litter size (P greater than 0.05). Long-term studies revealed that the FF rams were born in significantly larger litters, they weighed significantly less than ++ rams (P less than 0.05), and that bodyweight was significantly correlated (P less than 0.05) with litter size. There were no differences in testis size, and testis size was not significantly correlated with bodyweight. There was a strong tendency (P = 0.056) for overall mean FSH concentrations, measured weekly for 9 months, to be highest more often in FF than in ++ rams.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Studies of pituitary-adrenal-testis interaction in sheep. II. The effects of repeated injections of adrenocorticotrophic hormone outside the breeding season

The administration of 0.5 mg of long-acting adrenocorticotrophic hormone (ACTH, Synacthen-Depot) twice daily for 5.5 d to four rams outside the breeding season caused marked rises in plasma cortisol without any evidence of adrenal depletion. This treatment also caused marked rises in basal plasma follicle stimulating hormone (FSH) concentrations which remained high even after cessation of treatment. Plasma FSH responses to 5 ug of gonadotrophin releasing hormone (GnRH) were consistently observed and ACTH treatment increased the FSH response to GnRH. In contrast, spontaneous fluctuations in the plasma luteinizing hormone (LH) and testosterone concentrations were abolished by ACTH treatment. The quantity of testosterone released after GnRH (estimated by the maximum values reached and by the area under the response curve) was also suppressed while that of LH was only slightly lower. A comparison of the results of this experiment with those obtained in rams during the breeding season showed that the effects of ACTH on LH and testosterone were more marked during the breeding season. In contrast, the effect of ACTH on FSH is to increase the latter during the nonbreeding season, whereas no effect was observed during the breeding season.     

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.     

Interaction of season and estradiol in the regulation of gonadotropin secretion in the adult ram

The effects of season and estradiol on the secretion of gonadotropic hormones in adult Dorset X Leicester X Suffolk rams were studied. Control groups of intact and castrate rams, and castrate rams given estradiol replacement (approximately 11.5 pg/mL) via polydimethylsiloxane capsules (sc) were assessed for 1 year, beginning in August. Mean concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) were determined every 2 weeks for all three groups of rams and measurements of testosterone concentration and scrotal circumference were taken on the intact rams. Pulsatile LH release and the LH response to a 2-micrograms dose (iv) of gonadotropin-releasing hormone (GnRH) were assessed for all rams when the testes of intact rams were redeveloped (late October), regressed (early February, late April), and redeveloping (early August). Season directly affected LH-pulse amplitude, which increased only in the control castrate rams between February and April. In October, LH-pulse frequency was the same in both groups of castrate rams, while in April, frequency in the estradiol-treated castrate rams was suppressed to intact ram values. Pituitary responsiveness to exogenous GnRH did not change throughout the year in either of the castrate groups, but along with LH-pulse amplitude, it was increased in August in the intact rams. Although FSH secretion was 14-fold higher in the control castrate rams than in the intact rams, seasonal-directional changes in mean concentration were similar. FSH concentration in the estradiol-treated castrate rams was stable throughout the year. PRL secretion never differed between the control castrate and intact rams but was enhanced in the estradiol-treated castrate rams, particularly during long days.     

Effects of season and testosterone treatment on gonadotrophin secretion and pituitary responsiveness to gonadotrophin-releasing hormone in castrated Romney and Poll Dorset rams.

In castrated rams (Romney and Poll Dorset, n = 8 for each breed), inhibition by testosterone treatment (administered via Silastic capsules) of luteinizing hormone (LH) pulse frequency, basal and mean LH concentrations, mean follicle-stimulating hormone (FSH) concentration, and the peak and total LH responses to exogenous gonadotrophin-releasing hormone (GnRH) were significantly (P less than 0.01) greater during the nonbreeding than during the breeding season. Poll Dorset rams were less sensitive to testosterone treatment than Romney rams. In rams not receiving testosterone treatment, LH pulse frequency was significantly (P less than 0.05) lower during the nonbreeding season than during the breeding season in the Romneys (15.8 +/- 0.9 versus 12.0 +/- 0.4 pulses in 8 h), but not in the Poll Dorsets (13.6 +/- 1.2 versus 12.8 +/- 0.8 pulses in 8 h). It is concluded that, in rams, season influences gonadotrophin secretion through a steroid-independent effect (directly on hypothalamic GnRH secretion) and a steroid-dependent effect (indirectly on the sensitivity of the hypothalamo-pituitary axis to the negative feedback of testosterone). The magnitude of these effects appears to be related to the seasonality of the breed.     

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.     

Abolition of seasonal testis changes in the Ile-de-France ram by short light cycles: relationship to luteinizing hormone and testosterone release

The purpose of this study was to examine the relationships between testis weight and the luteinizing hormone (LH) and testosterone releases in rams subjected to 14 successive 2-mo artificial light cycles in which daylength increased from 8 to 16 h in one month and decreased from 16 to 8 h the following month. Testis weights were measured fortnightly. Serial bleedings were performed during 6 to 8 h the last three 2-mo light cycles, when daylengths were 8, 12 (increasing), 16 and 12 h (decreasing) and plasma LH and testosterone were measured by radioimmunoassay. The overall mean testis weight, continuously high and equal to 314 g per testis, was not correlated with daylength. Mean LH levels and LH pulse height varied significantly according to daylength (P < 0.05 and P < 0.01, respectively) and were maximal on short days (8 h). Both mean and maximal testosterone levels presented variations according to daylength (P = 0.05). LH and testosterone pulsatility were quite high (equivalent to 7 to 10.3 pulses/day); however, mean plasma testosterone levels remained low. It is proposed that in rams under 2-mo light cycles, frequent but short stimulation of LH release by decreasing daylength phases allows maximal testis weight, while the interruption of LH stimulation by increasing daylengths prevents overstimulation of testosterone that can inhibit the LH release by negative feedback. Rams a under 2-mo light regimen presented a persistent high testis weight, indicating that seasonality had been broken down in the Ile-de-France male which is normally a marked seasonal breeder.     

Blockade of pulsatile LH, FSH and testosterone secretion in rams by constant infusion of an LHRH agonist

Adult Soay rams were infused for 21 days with 50 micrograms buserelin/day, using s.c. implanted osmotic mini-pumps. The continuous treatment with this LHRH agonist induced a supraphysiological increase in the blood concentrations of LH (15-fold) and testosterone (5-fold) followed by a decrease below pre-treatment values after 10 days. The blood concentrations of FSH showed only a minimal initial increase but the subsequent decrease was dramatic, occurring within 1 day. By Day 10 of treatment, the blood concentrations of all 3 hormones were low or declining, LH pulses were absent in the serial profiles based on 20-min blood samples and the administration of LHRH antiserum failed to affect the secretion of LH or testosterone. By Day 21, the secretion of FSH, LH and testosterone was maximally suppressed. The i.v. injection of 400 ng LHRH was totally ineffective at stimulating an increase in the blood concentrations of LH while the i.v. injection of 50 micrograms ovine LH induced a normal increase in the concentrations of testosterone; this confirmed that the chronic treatment with the LHRH agonist had desensitized the pituitary gonadotrophs without markedly affecting the responsiveness of the testicular Leydig cells. The ratio of bioactive: radioimmunoactive LH did not change during the treatment. The long-term effect of the infusion was fully reversible as shown by the increase in the blood concentrations of FSH, LH and testosterone and the return of normal pulsatile fluctuations in LH and testosterone within 7 days of the end of treatment.     

Studies of pituitary-adrenal-testis interaction in sheep. III. The effects of repeated injections of adrenocorticotrophic hormone in recently castrated rams

Twice daily for 5.5 d, 0.5 ug of long-acting adrenocorticotrophic hormone (ACTH, Synacthen-Depot) was administered to four rams castrated 17 d earlier. There was a progressive diminution in basal plasma follicle stimulating hormone (FSH) during and after treatment. ACTH also suppressed basal plasma luteinizing hormone (LH) concentrations and the maximum LH values reached and the quantity of LH released in response to the injection of 5 ug of gonadotrophin releasing hormone (GnRH). There was, however, evidence that the LH concentrations returned to pretreatment levels after ACTH treatment ceased. This experiment demonstrated that the effects of ACTH on LH are modulated by castration, but throughout this series of experiments ACTH always depressed LH activity. In contrast, FSH is affected by ACTH in different ways, depending on the season and the presence or absence of a testis.     

Induced ovulation and changes in pituitary responsiveness to continuous infusion of gonadotropin-releasing hormone during the ovarian cycle in the bullfrog, Rana catesbeiana

Chronic (2-4 days) constant-rate infusions of mammalian gonadotropin releasing hormone (GnRH) were performed in female bullfrogs, Rana catesbeiana. The magnitude and temporal relationship of profiles of plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH) and sex steroids [testosterone (T), estradiol-17 beta (E2) and progesterone (P)] during GnRH infusion were dependent on ovarian stage. However, in all females, the same biphasic increase in plasma gonadotropins was apparent and initial elevations in gonadotropins were accompanied by correlated increments in plasma T and E2. Complete pituitary "desensitization" to chronic GnRH infusion was not observed. Females in early follicular stages were relatively unresponsive to infusions of 1.0-10.0 micrograms/h GnRH; elevations in plasma LH were marginal and FSH was unchanged. Females with fully developed (preovulatory) ovaries were more responsive: infusion of 1.0 micrograms/h GnRH produced significant elevations in plasma LH by 2 h followed by even larger increases ("surges") after 12 h. This LH "surge" was preceded by a decline in plasma T and E2 and was accompanied by abrupt elevations in plasma P and by ovulation. Postovulatory females showed a more gradual and smaller increase in plasma LH. Infusion of GnRH in the female bullfrog establishes a clear relationship between pituitary responsiveness and the ovarian cycle not evident from acute GnRH injection; GnRH was most effective immediately before ovulation. These data are also the first to detail periovulatory changes in plasma gonadotropins and ovarian steroids in an amphibian.     

The reproductive--endocrine response of adult rams to sexual encounters with estrual ewes is season dependent

An experiment was conducted with four adult, sexually inexperienced Finnish Landrace rams during the ovine nonbreeding (July) and breeding (October) seasons to determine the influence of components of the rams' mating behavior on the secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), and testosterone. On four occasions in both seasons, blood was collected by jugular venipuncture at 20-min intervals during an 8-hr period while rams were (1) separated from, (2) observing with minimal direct physical contact, (3) mounting without intromission, or (4) mating estrous-induced ewes. In comparison with separation periods, mating activity in July was associated with increased mean LH (P less than 0.05) and testosterone levels and number of LH peaks, while in October, obvious increases were detected in only baseline LH levels (P less than 0.05). Circulating LH and testosterone levels either did not change (July) or were depressed (October) during the mounting and observation periods. FSH levels generally remained unaffected by engagement in the various sexual activities. Although a clear relationship between type of sexual activity and mean PRL levels was not observed in July, activities which appeared to involve the most physical exertion tended to be associated with much higher circulating PRL levels in October. These data suggest (1) the act of ejaculation is important in the induction of increases in LH and testosterone secretion that occur in rams in response to mating activity during the nonbreeding season and (2) excessively stressful sexual activities during the breeding season may alter the pattern of secretion of some reproductive hormones.     

Episodic gonadotropin secretion in the mature fowl: serial blood sampling from unrestrained male broiler breeders (Gallus domesticus)

Forty-week-old male broiler breeders were used in two experiments. Males were reared as recommended by the breeder, housed in individual cages, and cannulated to facilitate blood sampling. In experiment 1, blood samples were collected at 10- min intervals for 4 h commencing the day of cannulation (Day 0) and for 12 h on each of Days 1 and 2. In experiment 2, blood samples were collected at 10-min intervals for 8 h on Day 1. After centrifugation, plasma was stored at -20 degrees C until LH, FSH (experiment 1 and 2), testosterone, and corticosterone (experiment 1) concentrations were determined by RIA. Different statistical methods used to identify hormone secretion profiles revealed a characteristic pulsatile pattern of LH and FSH in plasma. However, LH pulses were more frequent and had greater amplitude than FSH pulses. Less than 32% of the FSH pulses were associated with LH episodes. Conversely, the association between LH and testosterone pulses averaged 83% in birds with testis weight greater than 10 g. Concentrations of corticosterone tended to increase after cannulation and remained elevated for only 3-4 h. Our data indicate that LH, FSH, and testosterone secretion is pulsatile in male broiler breeders. Additionally, LH pulses are associated with testosterone episodes but not with FSH pulses. The pulsatile pattern of FSH secretion, which is unique from those of LH, in adult males suggests that FSH secretion is independently regulated in the adult male fowl.     

Effects of intermittent pulsatile infusion of luteinizing hormone-releasing hormone on dihydrotestosterone-suppressed gonadotropin secretion in castrate rams

The feedback effects of dihydrotestosterone (DHT) on gonadotropin secretion in rams were investigated using DHT-implanted castrate rams (wethers) infused with intermittent pulsatile luteinizing hormone-releasing hormone (LHRH) for 14 days. Castration, as anticipated, reduced both serum testosterone and DHT but elevated serum LH and follicle-stimulating hormone (FSH). Dihydrotestosterone implants raised serum DHT in wethers to intact ram levels and blocked the LH and FSH response to castration. The secretory profile of these individuals failed to show an endogenous LH pulse during any of the scheduled blood sampling periods, but a small LH pulse was observed following a 5-ng/kg LHRH challenge injection. Dihydrotestosterone-implanted wethers given repeated LHRH injections beginning at the time of castration increased serum FSH and yielded LH pulses that were temporally coupled to exogenous LHRH administration. While the frequency of these secretory episodes was comparable to that observed for castrates, amplitudes of the induced LH pulses were blunted relative to those observed for similarly infused, testosterone-implanted castrates. Dihydrotestosterone was also shown to inhibit LH and FSH secretion and serum testosterone concentrations in intact rams. In summary, it appears that DHT may normally participate in feedback regulation of LH and FSH secretion in rams. These data suggest androgen feedback is regulated by deceleration of the hypothalamic LHRH pulse generator and direct actions at the level of the adenohypophysis.     

The temporal relationship between plasma levels of FSH and LH in the ram.

Blood samples were collected from adult Soay rams at frequent intervals during the regressed, developing and active phases of the sexual cycle, or after i.v. injections of synthetic LH-RH or standard preparations of ovine FSH and LH, and were assayed for FSH, LH and testosterone. The highest FSH and LH levels occurred during the developing stage of the cycle, and the highest testosterone levels during the active stage. At each phase there were conspicuous short-term fluctuations in the concentrations of all three hormones, and episodic peaks of plasma LH were associated with increases in both FSH and testosterone. Synthetic LH-RH (5 microgram) induced a rise in the plasma values of FSH and LH at all stages and the magnitude and duration of the response could be predicted from a knowledge of the endogenous short-term fluctuations. When 50--250 ng LH-RH were given i.v. plasma LH increased in a way similar to that occurring spontaneously. The ratio of FSH:LH released after LH-RH changed with the stage of the sexual cycle, but the clearance rate of the two hormones was not affected. These findings are consistent with the control of FSH and LH by a single releasing hormone which is secreted in pulses. The different temporal patterns in the circulating FSH and LH result from differences in secretion and clearance.     

Effects of aproteic diet on hypothalamic-pituitary- gonadal regulation in the male rat

The effect of an aproteic diet (Ap) on the reproductive axis in young male rats was studied. Also the refeeding effect at different times after the aproteic diet was studied. The Ap diet was given during 21 days. In refeeding groups, the control diet was given during 2, 4 and 6 weeks after the aproteic diet. We studied the plasmatic testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels. Also the hypothalamic GnRH concentration and in vitro hypothalamic GnRH secretion in basal and induced condition was studied. The total protein deficit produced significant reduction in body, testis, seminal vesicles and prostate weights. This was accompanied with decreased levels of plasmatic testosterone (P<0.02). In this aproteic group there was a significant reduction in LH (P<0.05) and FSH (P<0.05) plasmatic levels. Refeeding with control diet reversed this situation, producing significant increment in LH (P<0.05) and FSH levels (P<0.01) at the fourth and second weeks, respectively. The basal hypothalamic GnRH secretion did not differ from the control; nevertheless the induced secretion was significantly (P<0.05) greater in the aproteic group. Also the hypothalamic GnRH concentration was increased (P<0.05) in animals fed with the aproteic diet. The minor testis, prostate, and seminal vesicles" weight, and a decreased plasmatic testosterone in rats fed with an aproteic diet, are produced by a decrease in gonadotrophin secretion. This decrease in turn is caused by a reduction in GnRH secretion, since hypothalamic GnRH concentration is increased in rats fed with the aproteic group, and induced secretion is greater in this group. All these alterations produced by an aproteic diet are reversible, since-with contol diet refeeding-the gonadotrophin secretion returned at control levels.     

Studies of pituitary-adrenal-testis interaction in sheep. I. The effects of repeated injections of adrenocorticotrophic hormone during the breeding season

The administration of 0.5 mg of long-acting adrenocorticotrophic hormone (ACTH, Synacthen-Depot) twice daily for 5.5 d to four adult rams during the breeding season had no consistent effect on plasma follicle stimulating hormone (FSH) concentrations. In contrast, it suppressed both plasma luteinizing hormone (LH) and testosterone concentrations. The responses to injections of 5 ug of gonadotrophin releasing hormone (GnRH), as measured by maximum concentrations reached and areas under the response curves, were also suppressed. These findings suggest that ACTH exerts its suppressive efects on LH at the pituitary level.     

Effects of sexual experience, season, and mating stimuli on endocrine concentrations in the adult ram.

Two behavioral trials were conducted to determine the endocrine response of cortisol (C), luteinizing hormone (LH), testosterone (T), prolactin (PRL), and growth hormone (GH) in adult rams during exposure to estrous ewes during the breeding and nonbreeding seasons. One-half of the rams in each season were sexually experienced (SE) and the remainder were sexually inexperienced (SI). All SE rams (100%) achieved at least one ejaculation, but only 33% (summer) and 67% (fall) SI rams achieved ejaculation. In the fall, mean C, T, and GH concentrations were elevated (P less than .001) compared to values measured in the summer, whereas LH and PRL levels were higher (P less than .01) in the summer. Overall levels of C, LH, T, and PRL were higher (P less than .05) in SE rams than in SI rams. Mean GH concentration was higher (P less than .10) in SI than in SE rams during restricted and complete access to estrous ewes. In general, LH, PRL, and GH responses were similar during restricted and complete access to females for both SE and SI rams. Cortisol levels were higher (P less than .06) during periods of mating and T levels were higher (P less than .001) during periods where activity was limited to courtship behavior (nasogenital investigation). Correlations of hormones to reproductive behaviors indicated that mounting and intromission were associated with elevations in C and PRL, whereas elevated LH and T tended to be associated with courtship behaviors. Correlations between GH and behaviors were inconsistent. However, there was an increased coincidence between time of female exposure and hormonal response that occurred in the fall; brief exposure to estrous ewes resulted in increases in concentrations of all hormones examined. The most consistent response was observed in sexually experienced rams during restricted access to females during the breeding season. These results provide new information on the effects of season and level of sexual experience upon hormonal and behavioral characteristics of the ram during mating activity.     

Influence of the degree of stimulation of the pituitary by gonadotropin-releasing hormone on the action of inhibin and testosterone to suppress the secretion of the gonadotropins in rams

This experiment determined if the degree of stimulation of the pituitary gland by GnRH affects the suppressive actions of inhibin and testosterone on gonadotropin secretion in rams. Two groups (n = 5) of castrated adult rams underwent hypothalamopituitary disconnection and were given two i.v. injections of vehicle or 0.64 microg/kg of recombinant human inhibin A (rh-inhibin) 6 h apart when treated with i.m. injections of oil and testosterone propionate every 12 h for at least 7 days. Each treatment was administered when the rams were infused i.v. with 125 ng of GnRH every 4 h (i.e., slow-pulse frequency) and 125 ng of GnRH every hour (i.e., fast-pulse frequency). The FSH concentrations and LH pulse amplitude were lower and the LH concentrations higher during the fast GnRH pulse frequency. The GnRH pulse frequency did not influence the ability of rh-inhibin and testosterone to suppress FSH secretion. Testosterone did not affect LH secretion. Following rh-inhibin treatment, LH pulse amplitude decreased at the slow, but not at the fast, GnRH pulse frequency, and LH concentrations decreased at both GnRH pulse frequencies. We conclude that the degree of stimulation of the pituitary by GnRH does not influence the ability of inhibin or testosterone to suppress FSH secretion in rams. Inhibin may be capable of suppressing LH secretion under conditions of low GnRH.