Luteinizing hormone release by gonadotropin releasing hormone before and after castration in bulls

The magnitude of gonadotropin releasing hormone (GnRH) induced lutei nizing hormone (LH) release prior to castration, following castration, a nd during testosterone replacement in males, was compared, using 6 9-mon th-old Holstein bulls. Also, the effects of castration and testosterone replacement on patterns of episodic changes in serum LH were studied. Blood samples were collected at hourly intervals for 24 hours prior to castration, at 21 days after castration, and at 23 days postcastration a fter testosterone, 20 mg thrice daily, has been given for 24 hours. Each animal was given GnRH, 40 mcg iv, at 24 hours before castration, at 7 and 14 days after castration, and at 28 days postcastration following 6 days of testosterone treatment. GnRH caused LH release before and after castration. The LH increase was 2.5-fold at 14 days postcastratio n. Testosterone replacement did not reduce the magnitude of LH response to GnRH to precastration levels. The number of episodic increases in serum LH prior to castration averaged 3.7 daily and increased to 6.5 daily at 21 days after castration (p less than .05). The magnitude of increase in LH concentration in these epidsodic events was not affected by castration. Testosterone replacement failed to restore either the average number or change the magniture of LH increase above precastratio n levels. It was shown that LH is normally released episodically in bulls. The peaks of LH release were followed by increased testosterone in serum. Results suggest that LH release in bulls is controlled by gonadic factors other than testosterone.     

Pattern of LH and testosterone secretion of adult male fallow deer (Dama dama) during the transition into the breeding season

Pituitary secretion of LH and testicular secretion of testosterone were investigated during the transitional period from the non-breeding to breeding season of mature male fallow deer exhibiting either normal transitional patterns or shortened transitional patterns in response to summer melatonin treatment. Melatonin implants were administered to 4 bucks for a 150-day period starting 130 days after the winter solstice. Four contemporary bucks served as controls. Melatonin treatment advanced rutting activity, testis development and neck muscle hypertrophy by 6-8 weeks. Profiles of plasma LH and testosterone, based on a 30-min sampling frequency over 24 h, were obtained from 3 treated and 3 control bucks on 4 occasions over the period spanning the transition into the breeding season. In control bucks, LH and testosterone pulse frequency were low (0-2 pulses/24 h) in January and increased (5-7 pulses/24 h) in February. By March and April (pre-rut and rut periods respectively) there was a two-fold increase in basal plasma LH concentrations, a decline in LH pulse frequency (0-1 pulse/24 h) and episodic surges in plasma testosterone concentrations. Melatonin treatment resulted in a shift in hormone profiles, with highly pulsatile patterns of LH and testosterone secretion (7 pulses/24 h) occurring earlier in January. The subsequent post-rut profiles of treated bucks were characterized by lower basal plasma LH concentrations, and reduced frequency and amplitude of plasma testosterone surges.     

Seasonal changes in the endocrine responsiveness of the pituitary and tests of male sheep in relation to their patterns of gonadotropic hormone and testosterone secretion

Pituitary and testicular endocrine responses to exogenous gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH), respectively, were assessed for adult rams in an investigation of the regulation of seasonal changes in the patterns of episodic LH and testosterone secretion. Concurrent variations in testis size and in circulating levels of follicle stimulating hormone (FSH) and prolactin (PRL) were also examined. On 10 occasions throughout the year, serum hormone levels were assessed over 6- to 8-h periods during which time rams were left untreated (day 1) or were injected (iv) with single doses of either 10 micrograms synthetic GnRH (day 2) or 30 micrograms NIH-LH-S18 (day 3); blood samples were collected from the jugular vein at 10- to 20-min intervals. Testicular redevelopment during the summer, as indicated by increasing testis diameter measurements, was associated with increases in mean FSH level and was preceded by a springtime rise in mean PRL level; "spontaneously" occurring LH pulses and those produced in response to GnRH treatment were relatively large during this period. Increases in the magnitude of testosterone elevations in response to both endogenously and exogenously produced LH pulses occurred in August. Mean testosterone levels were elevated fourfold in the fall as a consequence of relatively frequent and small LH pulses stimulating a more responsive testis to produce more frequent and larger testosterone elevations; endogenous LH pulses, however, did not appear to stimulate the testes maximally at this time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of cortisol or adrenocorticotropic hormone on luteinizing hormone secretion by pig pituitary cells in vitro

The effect of cortisol or adrenocorticotropic hormone (ACTH) on basal and gonadotropin-releasing hormone (GnRH)-induced secretion of luteinizing hormone (LH) was studied in vitro using dispersed pig pituitary cells. Pig pituitary cells were dispersed with collagenase and DNAase and then grown in McCoy's 5a medium containing 10% dextran charcoal-pretreated horse serum and 2.5% fetal calf serum for 3 days. Cells were preincubated with cortisol or ACTH before GnRH was added. When pituitary cells were incubated with 400 micrograms cortisol/ml medium for 6 h or longer, increase basal secretion of LH was observed. However, GnRH-induced LH release was reduced by cortisol. The degree of this reduction was dependent on cortisol, and a concentration of cortisol higher than 100 micrograms/ml was needed. Cortisol also inhibited the 17 beta-estradiol-induced increase in GnRH response. ACTH-(1-24), ACTH-(1-39), or porcine ACTH had no influence on GnRH-induced LH secretion. Our results show that cortisol can act directly on pig pituitary to inhibit both normal and estradiol-sensitized LH responsiveness to GnRH.     

Influence of acute stress and the adrenal axis on regulation of LH and testosterone in the male rhesus monkey (Macaca mulatta)

In order to determine the mechanism by which stress may affect the secretion and function of luteinizing hormone (LH) in primates, the response of the adrenal and gonadal axes was followed in male rhesus monkeys during brief restraint in primate chairs and during various hormone treatments. To further assess the responsiveness of the gonadal axis, gonadotropin releasing hormone (GnRH) was administered during the experiments. Corticosteroid levels were elevated throughout the first restraint trial as compared to those in subsequent trials. LH was elevated in the first sample of the first trial as compared to that in the following trials. The responses of LH to GnRH were equivalent in all trials, while the testosterone response to GnRH was attenuated in the first trial. A single injection of adrenocorti-cotropin (ACTH, 40 IU), while increasing circulating corticosteroids similarly to that observed during the first restraint trial, failed to cause an acute initial release of LH. However, ACTH did lower the testosterone response to GnRH. Following 5 days of ACTH treatment (40 IU twice daily), basal LH was suppressed, and the testosterone response to GnRH was decreased. Following 5 days of cortisol injections (100 mg twice daily), basal LH and testosterone were suppressed, but again only the testosterone response to GnRH was attenuated. Acute restraint stress, acting by some mechanism other than the activation of adrenal axis, stimulates a transient release of LH. While the stress-stimulated release of corticosteroids failed to affect the LH response following GnRH administration, it did act directly on the testes to prevent the normal release of testosterone. Finally, chronic elevation of corticosteroids, produced by ACTH or cortisol administration, suppressed basal serum LH and attenuated the response of testosterone to GnRH.     

Phenotypic variation in testosterone and luteinizing hormone production among boars: differential response to gonadotropin releasing hormone and adrenocorticotropic hormone

Variation in ability of boars to produce testosterone and luteinizing hormone (LH) in response to both gonadotropin releasing hormone (GnRH) and adrenocorticotropic hormone (ACTH) stimulation, as well as quantitative relationships between pretreatment and posttreatment responses, were assessed in a population of 38 boars of similar age and breeding. Peripheral testosterone concentrations following either GnRH or ACTH increased (P less than 0.01) to peak circulating levels of 7.16 +/- 0.62 and 8.42 +/- 0.81 ng/ml by 120 and 45 min, respectively. Post-GnRH testosterone area varied from 7.44 to 50.84 ng/ml X h (CV = 47.44%) and post-ACTH testosterone area ranged from 3.05 to 28.78 ng/ml X h (CV = 46.09%). GnRH-induced increases in testosterone were preceded by elevations (P less than 0.01) in peripheral LH concentrations but ACTH had no effect upon LH levels. Post-GnRH area varied from 7.07 to 125.45 ng/ml X h (CV = 76.61%). Significant (P less than 0.01) correlations were obtained between pre-GnRH and post-GnRH testosterone areas (r = 0.58) and between pre-ACTH and post-ACTH testosterone areas (r = 0.67). Nonsignificant (P greater than 0.10) correlations were obtained between post-GnRH and post-ACTH testosterone areas (r = 0.006) and between post-GnRH testosterone and LH areas (r = 0.09). The testosterone producing ability of boars was highly variable and their innate ability to produce testosterone influenced their response to GnRH and ACTH. Additionally, the mechanisms by which GnRH and ACTH influence testosterone production in boars appear to differ. Variation in the ability of boars to produce testosterone could not be explained on the basis of differences in circulating levels of LH.     

Temporal relationships among peripheral blood concentrations of corticosteroids, luteinizing hormone and testosterone in bulls

Interrelationships among peripheral blood concentrations of corticosteroids (CS), luteinizing hormone (LH) and testosterone (T) were evaluated over a 24-hr period in four Angus bulls (18 months of age and 450 kg in body weight). Concentrations of LH and T were determined by radioimmunoassay and concentrations of CS by competitive protein binding assay of blood samples collected via jugular cannula at hourly intervals for 24 consecutive hr. A positive temporal relationship was observed between LH and T as significant positive correlations were obtained between concentrations of LH at one hour and concentrations of T at the subsequent hour in 3 of 4 bulls. Although LH peaks preceded T peaks by 1 hr, variation in this temporal relationship was observed as LH peaks occurred which were not accompanied by T peaks in some bulls. LH peaks were usually preceded by basal or declining concentrations of CS and prolonged elevations in concentrations of CS were often coincident with basal concentrations of LH and T. Negative correlations were obtained between concentrations of CS at one hour and concentrations of LH and T at the subsequent hour. These data describe the positive regulatory role of LH in testicular T production in the bull and suggest that alterations in endogenous concentrations of CS may influence peripheral concentrations of LH and T in the bull.     

Corticotropin releasing hormone and gonadotropin secretion in physically active males after acute exercise.

Plasma concentrations of corticotropin releasing hormone (CRH) and the serum concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone, adrenocorticotropic hormone (ACTH) and cortisol were measured in seven physically active males after acute exercise on a treadmill using the Bruce protocol. Measurements were made in the basal pre-exercise state, immediately after exercise, and at 30-min intervals for 3 h after exercise. Serum LH concentrations declined following exercise reaching nadir values between 60 and 180 min after exercise (90 min post exercise in the group). The nadir values in individual volunteers were significantly lower than both the baseline and post-exercise levels. This fall in serum LH concentration appeared to follow a slight but significant elevation of the plasma concentration of CRH which reached peak levels when measured immediately post exercise. Plasma ACTH concentrations paralleled the rise in CRH, but fell to undetectable levels of below 13.8 nmol.l-1 (less than 5 ng.l-1) 60 min after exercise. Plasma cortisol concentrations peaked approximately 30 min after the rise in ACTH, after which they gradually declined to baseline levels. Plasma testosterone concentrations paralleled the concentrations of LH. The data suggest that CRH, on the basis of its previously described gonadotropin-depressant property, may be the hormone involved in the exercise-mediated decline in serum LH. Alternatively, some as yet unidentified factor(s), may be involved in producing the altered concentrations of both LH and CRH.     

Testosterone response of cryptorchid and hypophysectomized rats to human chorionic gonadotrophin (hCG) stimulation

The testosterone responses to a single injection of hCG (100 i.u.) in hypophysectomized (hypox.), cryptorchid or sham-operated rats were followed over a 5-day period. In sham-operated rats, hCG induced a biphasic rise in serum testosterone, peaks being observed at 2 and 72 h. Reduced testis weights, elevated FSH and LH levels and reduced serum testosterone levels were found after 4 weeks of cryptorchidism, but hCG stimulation resulted in a normal 2 h peak in serum testosterone. However, the secondary rise at 72 h in cryptorchid rats was significantly lower than sham-operated rats. Reduced testis weight and undetectable serum FSH and LH levels together with decreased testosterone levels were found 4 weeks after hypophysectomy. Serum testosterone levels rose 2 h after hCG in comparison to hypox. controls but this peak was significantly reduced compared with sham-operated rats. The second rise in serum testosterone began on day 2, peaking on day 4 at levels comparable to that seen in sham-operated rats after hCG. The in vitro basal and hCG stimulated secretion of testosterone by cryptorchid testes was greater than that secreted by normal rat testes (518.0 +/- 45.9 and 3337.6 +/- 304.1 pmol per testis per 4 h compared with 223.6 +/- 24.9 and 1312.9 +/- 141.4 pmol per testis per 4 h for normal rat testes). In cryptorchid animals a single injection of 100 i.u. hCG resulted in a pattern of in vitro refractoriness similar to normal rats, lasting from 12 h to 2 days, during which testosterone secretion was reduced to near basal levels. The in vitro basal and hCG-stimulated secretion of testosterone by hypox. rat testes was severely diminished compared with normal rat testes. The temporal pattern of in vitro secretion of testosterone from hypox. rat testes mimicked the in vivo serum testosterone pattern seen in these animals. This study demonstrates important differences in the in vivo and in vitro testosterone response to hCG after testicular damage.     

In vivo and in vitro studies on the effect of adrenocorticotropic hormone or cortisol on the pituitary response to gonadotropin releasing hormone

Experiment I: Hyperadrenal states were induced in intact heifers (N = 3) or adrenalectomized (ADRX) heifers (N = 3) by constant infusion of ACTH (20.8 micrograms, 1-24 ACTH/h) or hydrocortisone succinate (HS) (30 mg/h), respectively. Control infusions consisted of the saline vehicle. All infusions began on Day 2 of a normal estrous cycle. Exogenous gonadotropin releasing hormone (GnRH) was given as a 100-micrograms bolus i.v. on Days 7, 9, and 11 (intact) or 5, 7, and 9 (ADRX) of the cycle. In intact heifers, the cumulative luteinizing hormone (LH) response was reduced (P less than 0.05) by the ACTH treatment. In ADRX heifers, the HS treatment did not alter the cumulative response but did alter the qualitative response with a time X treatment interaction (P less than 0.01). The LH response in the HS-ADRX animals had a slower onset and lower peak concentrations with a more prolonged response. Experiment II: Dispersed bovine pituitary cells were prepared and incubated at concentrations of 2 X 10(6) viable cells in 2.0 ml per dish. Cells were exposed to cortisol at concentrations of 0.01, 0.10, 0.21 and 1.03 X 10(-6) M for time periods of 8, 14, 20 or 26 h for basal LH secretion studies and 10, 16, 22 and 28 h for GnRH-stimulated LH secretion. Both dosage of cortisol and length of exposure had a depressing effect on basal LH release. The cortisol pretreatment also decreased (P less than 0.001) the LH release following addition of GnRH (8.5 X 10(-8) M) in cultures at all dosages and exposure times of cortisol. However, there was no decrease in LH or protein content of cells. These experiments indicate a direct action of cortisol on the pituitary gland to depress both basal and stimulated LH release.     

Effects of a gonadotropin-releasing hormone agonist implant on reproduction in a male marsupial, Macropus eugenii

This study evaluated the potential of slow-release GnRH agonist (deslorelin) implants to inhibit reproductive function in the male tammar wallaby. The specific aim was to measure the effects of graded dosages of deslorelin on testes size and plasma LH and testosterone concentrations. Adult male tammar wallabies were assigned to four groups (n = 6 per group) and received the following treatment: control, placebo implant; low dose, 5 mg deslorelin; medium dose, 10 mg; high dose, 20 mg. All dosages of deslorelin induced acute increases (P < 0.001) in plasma LH and testosterone concentrations within 2 h, with concentrations remaining elevated during the first 24 h but returning to pretreatment levels by Day 7. Thereafter, there was no evidence of a treatment-induced decline in plasma testosterone concentrations. There was no detectable difference in basal LH concentrations between treated and control animals, nor was there a significant change in testes width or length (P > 0.05). These results suggest that the male tammar wallaby is resistant to the contraceptive effects of chronic GnRH agonist treatment. Despite the maintenance of testosterone secretion, the majority of male tammars (10 of 17) failed to respond to a GnRH challenge with a release of LH between Days 186 and 197 of treatment. The failure of animals to respond to exogenous GnRH suggests a direct effect of deslorelin on the pituitary, resulting in a level of desensitization that was sufficient to inhibit a LH surge but insufficient to inhibit basal LH secretion. The variation between animals is believed to result from earlier recovery of some individuals, in particular those that received a lower dose, or individual resistance to the desensitization process.     

Ability of cortisol and progesterone to mediate the stimulatory effect of adrenocorticotropic hormone upon testosterone production by the porcine testis

The influence of corticosteroids and progesterone upon porcine testicular testosterone production was investigated by administration of exogenous adrenocorticotropic hormone (ACTH), cortisol and progesterone, and by applying a specific stressor. Synthetic ACTH (10 micrograms/kg BW) increased (P less than 0.01) peripheral concentrations of testosterone to peak levels of 5.58 +/- 0.74 ng/ml by 90 min but had no effect upon levels of luteinizing hormone (LH). Concentrations of corticosteroids and progesterone also increased (P less than 0.01) to peak levels of 162.26 +/- 25.61 and 8.49 +/- 1.00 ng/ml by 135 and 90 min, respectively. Exogenous cortisol (1.5 mg X three doses every 5 min) had no effect upon circulating levels of either testosterone or LH, although peripheral concentrations of corticosteroids were elevated (P less than 0.01) to peak levels of 263.57 +/- 35.03 ng/ml by 10 min after first injection. Exogenous progesterone (50 micrograms X three doses every 5 min) had no effect upon circulating levels of either testosterone or LH, although concentrations of progesterone were elevated (P less than 0.01) to peak levels of 17.17 +/- 1.5 ng/ml by 15 min after first injection. Application of an acute stressor for 5 min increased (P less than 0.05) concentrations of corticosteroids and progesterone to peak levels of 121.32 +/- 12.63 and 1.87 +/- 0.29 ng/ml by 10 and 15 min, respectively. However, concentrations of testosterone were not significantly affected (P greater than 0.10). These results indicate that the increase in testicular testosterone production which occurs in boars following ACTH administration is not mediated by either cortisol or progesterone.     

Effects of active immunization of ram lambs and bull calves against synthetic luteinizing hormone releasing hormone

Ram lambs and bull calves were immunized against LH-RH by injections given in weeks 0, 6, 12 and 28 (ram lambs, week 0 = 16 to 20 weeks of age) and weeks 0, 6, 12 and 18 (bull calves, week 0 = approximately 4 weeks of age). The testis size of LH-RH-immunized animals was significantly less than that of controls from week 13 onwards in ram lambs and from week 15 onwards in bull calves. When ram lambs were sampled in week 17 and bull calves in week 20, mean plasma gonadotrophin and testosterone concentrations were consistently lower in LH-RH-immunized animals than in controls. Single intravenous injection of synthetic LH-RH or an analogue of LH-RH in week 27 failed to induce LH or testosterone responses in LH-RH-immunized ram lambs. Motile semen samples could not be obtained from any of the LH-RH-immunized ram lambs in weeks 24, 25 and 26 or from 7 of 10 in week 72, but samples of moderate motility were obtained in week 72 from three rams in which LH-RH antibody titres had fallen. No attempt was made to obtain semen from bull calves. After castration there was no increase in plasma LH in LH-RH-immunized rams and only a small increase in LH-RH-immunized bull calves. Mean testis weight was significantly lower in LH-RH-immunized animals than in controls of both species. Thus the normal development of the reproductive system in ram lambs and bull calves was blocked by active immunization against LH-RH. Some evidence was obtained for natural reversal of the effects with time and falling antibody titres. These findings demonstrate the potential of LH-RH immunization as an alternative to castration.     

Effect of vitamin A deficiency upon gonadotropin response to gonadotropin-releasing hormone

There is a monotypic change in basal serum gonadotropin levels following retinol treatment of chronically vitamin A-deficient (VAD) male rats. The present study was undertaken to investigate the hypothesis that the specific increase in serum follicle-stimulating hormone (FSH) represents a change in gonadotrope responsiveness to gonadotropin-releasing hormone (GnRH). To this end, a test dose of GnRH was given to VAD rats pre-, 5 days post-, and 10 days postreplacement of vitamin A (PVA). In VAD rats, basal serum FSH and luteinizing hormone (LH) levels were higher than those of controls. Increased LH/testosterone ratios, both in basal levels and in the secretory response to GnRH, suggested Leydig cell hyporesponsiveness in VAD animals. Both the FSH and LH responses to GnRH were maximal at 1 h, declining thereafter. Although the absolute increments in FSH and LH 1 h after GnRH in VAD rats were greater than in controls, the percent increase in FSH tended to be lower in VAD rats and to increase after vitamin A replacement. The specific enhancement of FSH release PVA became evident only when assessing total secretion of FSH and LH after GnRH. Luteinizing hormone response to GnRH increased PVA, but not significantly, while FSH secretion after GnRH increased both 5 and 10 days PVA, times during which basal FSH levels were also increasing. These changes in FSH secretion could not be attributed either to increases in endogenous GnRH or to changes in testosterone or estradiol levels. Basal serum androgen binding protein levels, elevated in VAD animals, did not respond to the acute increases in FSH after GnRH and remained high PVA, suggesting no acute change in Sertoli cell function. Thus, the PVA increase in FSH secretion unmasks a partial inhibition of the gonadotrope present in the retinol-deficient, retinoic acid-fed male rat.     

Suppression of pulsatile luteinizing hormone secretion by gonadotrophin-releasing hormone antagonist does not affect episodic progesterone secretion or corpus luteum function in ewes.

Progesterone secretion has been observed to be episodic in the late luteal phase of the oestrous cycle of ewes and is apparently independent of luteinizing hormone (LH). This study investigated the effects of suppressing the pulsatile release of LH in the early or late luteal phase on the episodic secretion of progesterone. Six Scottish Blackface ewes were treated i.m. with 1 mg kg-1 body weight of a potent gonadotrophin-releasing hormone (GnRH) antagonist on either day 4 or day 11 of the luteal phase. Six ewes received saline at each time and acted as controls. Serial blood samples were collected at 10 or 15 min intervals between 0 and 8 h, 24 and 32 h, and 48 and 56 h after GnRH antagonist treatment and daily from oestrus (day 0) of the treatment cycle for 22 days. Oestrous behaviour was determined using a vasectomized ram present throughout the experiment. Progesterone secretion was episodic in both the early and late luteal phase with a frequency of between 1.6 and 3.2 pulses in 8 h. The GnRH antagonist abolished the pulsatile secretion and suppressed the basal concentrations of LH for at least 3 days after treatment. This suppression of LH, in either the early or late luteal phase, did not affect the episodic release of progesterone. Daily concentrations of progesterone in plasma showed a minimal reduction on days 11 to 14 after GnRH antagonist treatment on day 4, although this was significant (P < 0.05) only on days 11 and 13. There was no effect of treatment on day 11 on daily progesterone concentration, and the timing of luteolysis and the duration of corpus luteum function was unaffected by GnRH antagonist treatment on either day 4 or day 11. These results indicate that the episodic secretion of progesterone during the luteal phase of the oestrous cycle in ewes is independent of LH pulses and normal progesterone secretion by the corpus luteum can be maintained with minimal basal concentrations of LH.     

Circadian variations of plasma LH and testosterone in adult swamp buffalo bulls

Three swamp buffalo bulls aged 1.5, 1.10 and 2 years were submitted to frequent blood sampling every 15 m during a period of 25 h using an indwelling infusion set. Plasma LH and testosterone were quantified by radioimmunoassay technique. The levels of the two hormones in each individual exhibited episodic and nonrhythmic patterns. The number of LH peaks varied according to individval, ranging from no peak in one bull to 2 in the other two bulls. The mean LH concentrations during the period of study for each bull were 0.74, 0.33 and 1.17 ng/ml. Whereas the number of testosterone peaks varied between 1-10 and the average testosterone concentrations were 0.1, 0.33 and 0.55 ng/ml for the younger to the older bulls respectively. The testosterone peaks related to the LH peaks in each individual bull.     

Impact of season on seminal characteristics and endocrine status of adult free-ranging African buffalo (Syncerus caffer)

Pituitary, gonadal and adrenal activity were compared in free-living, adult African buffalo bulls during the breeding and nonbreeding seasons. Frequent blood samples were collected for 2 h from anaesthetized bulls treated intravenously with saline, gonadotrophin-releasing hormone (GnRH, 200 micrograms), human chorionic gonadotrophin (hCG, 10,000 i.u.) or adrenocorticotrophic hormone (ACTH, 1.5 mg). Electroejaculates also were collected from anaesthetized bulls during the breeding and nonbreeding seasons. Pretreatment testosterone concentrations among bulls varied more during the breeding (0.17-23.0 ng/ml) than the nonbreeding (0.15-2.21 ng/ml) season. The variation within the breeding season was attributed to 8 of 25 bulls producing higher (P less than 0.05) serum testosterone (High-T; 16.28 +/- 2.03 ng/ml) and testicular LH receptor (1.53 +/- 0.22 fmol/mg testis) concentrations compared with their seasonal counterparts (Low-T; 0.95 +/- 0.26 ng/ml; 0.38 +/- 0.04 fmol/mg) or with all bulls during the nonbreeding season (0.90 +/- 0.27 ng/ml; 0.31 +/- 0.04 fmol/mg). The magnitude of GnRH- and hCG-induced increases in serum testosterone was similar (P greater than 0.05) between Low-T bulls and bulls during the nonbreeding season. In the High-T animals treated with GnRH or hCG, serum testosterone did not increase, suggesting that secretion was already maximal. Peak serum LH concentrations after GnRH were greater (P less than 0.05) in bulls during the nonbreeding than the breeding season; FSH responses were similar (P greater than 0.05). ACTH treatment did not increase serum cortisol concentrations above the 2-fold increase measured in bulls treated with saline, hCG and GnRH (P greater than 0.05).(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.     

Evaluation of the possible direct effects of gonadotrophin-releasing hormone analogues on the monkey (Macaca mulatta) testis

In Exp. 1, the effect of treatment with a GnRH agonist on basal concentrations of serum testosterone and peak values of serum testosterone after administration of hCG was determined. One group of adult male monkeys was treated with a low dose (5-10 micrograms/day) and a second group with a high dose (25 micrograms/day) of a GnRH agonist for 44 weeks. Basal and peak testosterone concentrations were both significantly reduced by GnRH agonist treatment in all groups compared to untreated control animals, but the % rise in serum testosterone above basal values in response to hCG administration was unchanged by agonist treatment. In Exp. 2, the GnRH agonist (100 or 400 ng) or a GnRH antagonist (4 micrograms) was infused into the testicular arteries of adult monkeys. The agonist did not alter testosterone concentrations in the testicular vein or testosterone and LH values in the femoral vein. In Exp. 3, testicular interstitial cells from monkeys were incubated with three concentrations (10(-9), 10(-7) and 10(-5)M) of the GnRH agonist or a GnRH antagonist with and without hCG. After 24 h, neither basal nor hCG-stimulated testosterone production was affected by the presence of the GnRH agonist or antagonist. The results from all 3 experiments clearly suggest that GnRH agonist treatment does not directly alter steroid production by the monkey testis.     

Age-related changes in secretion of luteinizing hormone and metabolism of hypothalamic amines in bull calves prior to puberty

Effects of age and castration on secretion of luteinizing hormone (LH) and metabolism of hypothalamic monoamines were determined in Holstein bulls. Calves were assigned to be intact or castrated and killed at 8, 12, or 24 wk of age. Animals were castrated and bled every 10 min for 6 h at 96 and 24 h prior to slaughter, respectively. The stalk median eminence (SME), medial basal (MBH), and anterior-preoptic (AHA-POA) hypothalamic regions were obtained at slaughter and assayed for norepinephrine (NE), dopamine (DA), dihydroxy-phenylacetic acid (DOPAC), homovanillic acid (HVA), serotonin (5-HT), and 5-hydroxyindole-acetic acid (5-HIAA) using high performance liquid chromatography with electrochemical detection (HPLC-EC). Concentrations of LH and testosterone in plasma were determined by radioimmunoassay (RIA). In intact calves, LH pulse frequency (pulses/6 h) increased between 8 and 12 wk (1.4 vs. 3.4) and then declined (1.6 at 24 wk of age). Frequency of LH discharges did not change during the first 72 h post-castration in calves 8 (1.4 vs 1.0) and 12 (3.4 vs. 3.8) wk of age, but increased in 24-wk-old calves during this time (1.6 vs. 6.4). The amplitude of LH pulses increased with age (p less than 0.05) and after castration (p less than 0.05). There were marked regional differences in concentrations of monoamines. However, effects of age and castration on concentrations of monoamines were observed only within the SME where DA, DOPAC and NE increased significantly with age. Plasma concentrations of testosterone were correlated with concentrations of NE and DOPAC within the SME. Changes in 5-HT with age were biphasic; at each age, 5-HT increased after castration. From these data, it is concluded that 1) different mechanisms regulate LH pulse frequency and amplitude in calves as early as 8 wk of age, and 2) differences in hypothalamic metabolism of monoamines may be related to maturational changes in secretion of LH in bull calves.