Seasonal pattern of LH and testosterone secretion in adult male fallow deer, Dama dama

At monthly intervals during the year blood samples were collected every 20 min for 12 h from 4 entire and 2 prepubertally castrated adult fallow deer bucks. In the entire bucks there were seasonal changes in mean concentrations and pulse frequencies of plasma LH. Mean concentrations in late summer and autumn were 3-6 times higher than during other seasons. LH pulse frequency was low (0-1 pulses/12 h) during most of the year and increased only during the 2-month period (January and February) that marked the transition from the non-breeding season to the autumn rut. During this period there was a close temporal relationship between pulses of LH and testosterone. However, during the rutting period (March and April) episodic secretion of testosterone, manifest as surges in plasma concentrations of 4-6 h duration, was not associated with any detectable pulses in LH although mean plasma concentrations of LH remained elevated. During the rut, the surges of plasma testosterone occurred at similar times of the day. Plasma profiles in May indicated very low concentrations of LH and testosterone secretion in the immediate post-rut period. Castrated bucks exhibited highly seasonal patterns of LH secretion, with mean plasma LH concentrations and LH pulse frequency being lowest in November (early summer) and highest in February and March (late summer-early autumn). Mean concentrations and pulse frequency of LH in castrated bucks were higher than for entire bucks at all times of the year.     

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.     

Episodic release of luteinizing hormone and testosterone in Surti and Marwari bucks younger than one year of age

Studies on the episodic release of luteinizing hormone (LH) and testosterone in Surti (n=2) and Marwari (n=2) bucks younger than one year of age (6 to 8 months) were carried out by collecting blood plasma during the breeding season. The studies revealed that definite pulsatile releases of LH and testosterone occur in both breeds of bucks. The overall number of LH and testosterone pluses over a 24 hour period was 9.1+/-1.00 and 7.5+/-0.28, respectively. The peak, basal and mean LH and testosterone concentrations did not show significant differences between the two breeds. The duration and interval of LH and testosterone pulses differed during light and dark hours. The time interval between LH peak followed by the testosterone peak was significantly (P<0.05) longer during the night than the day hours for both the breeds. The physiological basis of the findings are discussed.     

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.     

Resumption of gonadotrophin release during the post-partum period in suckling and non-suckling ewes

The post-partum secretion of LH, FSH and prolactin was monitored in 15 suckling and 6 non-suckling Préalpes du Sud ewes lambing during the breeding season by measuring plasma hormone concentrations daily at 6-h intervals and also weekly at 20-min intervals for 6 h from parturition to resumption of regular cyclic ovarian activity. There was a constant phenomenon in the resumption of normal patterns of FSH and LH secretion: there was a rise in FSH values culminating on average on Day 4 post partum and returning subsequently to values observed during the oestrous cycle, and concurrently an increase in the frequency and amplitude of LH pulses more progressive in suckling than in non-suckling ewes which led to an elevation of LH mean concentrations and occurrence of an LH surge. Since neither the FSH secretory pattern nor FSH mean values differed between suckling and non-suckling ewes, the results suggested that LH pulsatile pattern was a major limiting factor for the resumption of normal oestrous cycles. Before regular oestrous cycles resumed other changes in preovulatory LH surges also occurred: (i) they increased in duration and probably in amplitude; (ii) they were preceded by an acceleration in LH pulse frequency and a large decrease in FSH values as in normal cyclic ewes; and (iii) at least in non-suckling ewes they occurred concurrently with a prolactin surge.     

Effects of acutely increased plasma testosterone concentrations on pulsatile luteinizing hormone secretion in the male rat

To assess the role of testosterone (T) in regulating the minute-to-minute release of pulsatile luteinizing hormone (LH) secretion in the adult male rat, we investigated the negative feedback of acute increases in plasma T concentrations on pulsatile LH secretion in acutely castrated male rats. At the time of castration, we implanted T-filled Silastic capsules, s.c., which maintained plasma T concentrations at approximately 1.8 ng/ml and suppressed LH pulses. On the next day, the capsules were removed; blood sampling (every 6 min) was started 8 h after implant removal, thereby allowing LH pulses to be reinitiated. Immediately following a control bleeding interval of 2 h, either T or vehicle alone was infused s.c., and blood sampling continued for another 4 h. In animals receiving vehicle alone, LH pulse frequency and mean LH levels increased over the 6 h bleeding period. The administration of 200 ng T/min caused a rapid rise in plasma T concentrations of about 4 ng/ml ("physiological") and prevented the increase in pulse frequency that occurred in the control group; it did not, however, reduce pulse frequency over the 4 h infusion period. When T was infused at the rate of 400 ng/ml, plasma T concentrations rose to approximately 18 ng/ml ("supraphysiological") and LH pulse frequency was significantly reduced, but not completely inhibited, during the last 2 h of the infusion. The pulse amplitude of luteinizing hormone did not change significantly in any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fluctuation in responsiveness of LH and lack of responsiveness of FSH to prolonged infusion of morphine and naloxone in the ewe

In ewes in the mid-luteal phase, LH pulse frequency (P less than 0.01) and amplitude (P less than 0.05) increased during a 24 h infusion of naloxone (0.5 mg/kg/h) compared to a 24 h infusion of vehicle (mean +/- s.e.m.; 0.25 +/- 0.03 vs 0.14 +/- 0.01 pulses/h and 0.84 +/- 0.08 vs 0.55 +/- 0.08 ng/ml serum, respectively). The increase in pulse amplitude was immediate, but was less (P less than 0.05) during the second 12 h, compared to the first 12 h, of naloxone infusion (0.52 +/- 0.14 vs 0.98 +/- 0.08 ng/ml serum). Oestradiol concentrations were higher (P less than 0.01) during naloxone than during control infusion (5.63 +/- 0.26 vs 4.13 +/- 0.15 pg/ml serum). In ovariectomized ewes in the breeding season, LH pulse frequency was lower (P less than 0.01) during a 24 h infusion of morphine (0.5 mg/kg/h) than during a 24 h infusion of vehicle (mean +/- s.e.m.; 1.17 +/- 0.08 vs 1.71 +/- 0.06 pulses/h). We conclude that long-term infusion of naloxone results in a sustained increase in LH pulse frequency but only a transient elevation in pulse amplitude. No effects on FSH secretion were noted. LH secretion was sensitive to morphine in the absence of ovarian steroids, suggesting that ovarian steroids are not required for the presence of functional opioid receptors capable of modulating LH release.     

Oestradiol and the preovulatory surges of luteinising hormone and follicle stimulating hormone in ewes during the breeding season and transition into anoestrus

This study was designed to see if giving exogenous oestradiol, during the follicular phase of the oestrous cycle of intact ewes, during the breeding season or transition into anoestrus, would alter the occurrence, timing or magnitude of the preovulatory surge of secretion of luteinising hormone (LH) or follicle stimulating hormone (FSH). During the breeding season and the time of transition, separate groups of ewes were infused (intravenously) with either saline (30 ml h⁻¹; n = 6) or oestradiol in saline (n = 6) for 30 h. Infusion started 12 h after removal of progestin-containing intravaginal sponges that had been in place for 12 days. The initial dose of oestradiol was 0.02 μg h⁻¹; this was doubled every 4 h for 20 h, followed by every 5 h up to 30 h, to reach a maximum of 1.5 μg h⁻¹. Following progestin removal during the breeding season, peak serum concentrations of oestradiol in control ewes were 10.31 ± 1.04 pg ml⁻¹, at 49.60 ± 3.40 h after progestin removal. There was no obvious peak during transition, but at a time after progestin removal equivalent to the time of the oestradiol peak in ewes at mid breeding season, oestradiol concentrations were 6.70 ± 1.14 pg ml⁻¹ in ewes in transition (P < 0.05). In oestradiol treated ewes, peak serum oestradiol concentrations (24.8 ± 2.1 pg ml⁻¹) and time to peak (41.00 ± 0.05 h) did not differ between seasons (P > 0.05). During the breeding season, all six control ewes and four of six ewes given oestradiol showed oestrus with LH and FSH surges. The two ewes not showing oestrus did not respond to oestrus synchronisation and had persistently high serum concentrations of progesterone. During transition, three of six control ewes showed oestrus but only two had LH and FSH surges; all oestradiol treated ewes showed oestrus and gonadotrophin surges (P < 0.05). The timing and magnitude of LH and FSH surges did not vary with treatment or season. In blood samples collected every 12 min for 6 h, from 12 h after the start of oestradiol infusion, mean serum concentrations of LH and LH pulse frequency were lower in control ewes during transition than during mid breeding season (P < 0.05). Oestradiol treatment resulted in lower mean serum concentrations of LH in season and lower LH pulse frequency in transition (P < 0.05). We concluded that enhancing the height of the preovulatory peak in serum concentrations of oestradiol during the breeding season did not alter the timing or the magnitude of the preovulatory surge of LH and FSH secretion and that at transition into anoestrus, oestradiol can induce oestrus and the surge release of LH and FSH as effectively as during the breeding season.     

Seasonal pattern of luteinizing hormone and testosterone pulsatile secretion in young adult red deer stags (Cervus elaphus) and its association with the antler cycle.

Blood from stages aged 15 months (n = 6) was sampled at monthly intervals every 30 min for 24 h for 12 months, at 45 degrees S in New Zealand. Three extra samplings each for 24 h were carried out at about the anticipated time of antler casting. All samples were analysed for luteinizing hormone (LH) and testosterone and the resulting data further analysed by the Pulsar pulse detection routine. The animals were kept indoors under natural daylength and were fed ad libitum. All animals were weighed, antler status and size recorded and testes diameter was measured on each sampling day. Mean LH and testosterone pulsatily and plasma concentration varied seasonally. LH pulse frequency was low during autumn (2.5 pulses in 24 h), winter (1.0-1.5 pulses in 24 h) and early spring (1 pulse in 24 h) and lowest in late spring (0.2 pulse in 24 h) before rising in summer (1.0-4.0 pulses in 24 h). LH pulse amplitude and mean plasma concentration were low (< 1 ng ml-1) from March to November (autumn-spring); both rose to a peak in January (summer) of 3.4 and 1.6 ng ml-1, respectively. Testosterone pulse frequency was generally similar to LH except that slightly more pulses of testosterone than of LH were detected from March to November and more pulses of LH from November to February (summer). Testosterone pulse amplitude fell from March to November (5.3 ng ml-1 to undetectable) although there was a conspicuous peak in July (midwinter) of almost 5 ng ml-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal changes in LH secretion in normal ewes and ewes which grazed oestrogenic clover

Plasma luteinizing hormone (LH) concentrations were measured in normal (control) Corriedale X Merino (comeback) ewes and in clover-infertile comeback ewes which had grazed oestrogenic Yarloop clover (Trifolium subterraneum L. cv. Yarloop) for more than 4 years. Plasma LH concentrations were measured in samples taken at 20-min intervals for 6 h during the dioestrous stage of the oestrous cycle in the breeding season (BS) and during the anoestrous season (AS). In the control ewes during BS, transitory elevation in plasma LH concentration (pulses) occurred, reflecting secretory episodes, with a frequency of one per 5.2 h. This frequency fell to one per 16.5 h during the anoestrous season. In clover-infertile ewes, LH pulses occurred with a frequency of one per 4.5 h during BS and one per 4.9 h during AS (difference not significant). In the controls, plasma LH levels were higher (P less than 0.05) during BS (mean +/- s.d. = 1.2 +/- 0.4 ng/ml, n = 9) than in AS (0.7 +/- 0.3 ng/ml, n = 5). In the clover-infertile ewes, plasma LH levels in BS (1.3 +/- 0.6 ng/ml, n = 12) were similar to those of controls. During AS, plasma LH levels in the clover-infertile ewes (1.0 +/- 0.6 ng/ml, n = 10) remained similar to their BS levels, being significantly (P less than 0.05) higher than LH levels in the controls at this time. These studies indicate that the higher plasma concentrations of LH which have been reported in clover-infertile ewes arise from more frequent LH pulses. Furthermore, in contrast to normal ewes, average plasma LH, reflecting pulse frequency, is not reduced in AS. This supports the view that ingestion of phytooestrogens affects neural centres involved in regulating LH secretion.     

Behavioural and hormonal responses of sexually-experienced ile-de-France rams to oestrous females

Sexually-experienced Ile-de-France rams were tested for their behavioural and hormonal responses to oestrous females late in the breeding season (December) and again during the anoestrous/non-breeding season (February). Contact with oestrous ewes produced positive behavioural responses in most rams, including ano-genital sniffing, nudging, mounting and mating (ejaculation). Associated with this heterosexual contact was a rapid and significant increase in mean plasma testosterone levels. During the February test, fourfold-increased testosterone levels accompanied sixfold-increased LH pulse frequencies and threefold-increased mean plasma LH levels. In December, however, the spontaneous high frequency of LH pulses did not permit a significant female-induced change in LH secretion. Exposure of rams to the empty test-pen environment (i.e., females absent) failed to produce a change in plasma LH or testosterone. We conclude that the conditions necessary for demonstrating an “ewe effect” on LH-testosterone release in the ram are most favorable during the anoestrous/non-breeding season.     

Effect of transport on pulsatile LH release in ovariectomized ewes with or without prior steroid exposure at different times of year

The initial aim of the present study was to test whether the stress of transport suppresses LH pulsatile secretion in ewes. In a pilot experiment in the late breeding season, transport resulted in an unexpected response in three out of five transported, ovariectomized ewes pretreated with oestradiol and progesterone. Before transport, seasonal suppression of LH pulses had occurred earlier than anticipated, but LH pulsatility suddenly restarted for the period of transport. This finding was reminiscent of unexplained results obtained in ovariectomized ewes infused centrally with high doses of corticotrophin-releasing hormone after pretreatment with low doses of oestradiol with or without progesterone. Hence, an additional aim of the present study was to examine whether these latter results with corticotrophin-releasing hormone could be reproduced by increasing endogenous corticotrophin-releasing hormone secretion by transport. Subsequent experiments used groups of at least eight ovariectomized ewes at different times of the year with or without prior exposure to steroids to assess whether these unexpected observations were associated with season or the prevailing endocrine milieu. In the mid-breeding season, transport for 4 h in the absence of steroid pretreatment for 8 months reduced LH pulse frequency from 7.5 +/- 0.3 to 6.3 +/- 0.4 pulses per 4 h (P < 0.05) and LH pulse amplitude from 2.6 +/- 0.5 to 1.8 +/- 0.3 ng ml-1 (P < 0.05). Similarly, in the mid-breeding season, 34 h after the cessation of pretreatment with oestradiol and progesterone, transport suppressed LH pulse frequency from 6.1 +/- 0.4 to 5.5 +/- 0.3 pulses per 4 h (P < 0.05) with a tendency of effect on amplitude (6.2 +/- 2.7 to 2.61 +/- 0.6 ng ml-1; P = 0.07; note the large variance in the pretransport data). During mid-anoestrus, evidence of a suppressive effect of transport was only observed on LH pulse amplitude (4.7 +/- 0.6 versus 3.0 +/- 0.5 pulses per 4 h; P < 0.05) in ovariectomized ewes that had not been exposed to ovarian steroids for 4 months. Repetition of the pilot experiment with 12 ewes during the transition into anoestrus resulted in one ewe with LH pulses seasonally suppressed but increased by transport; 11 ewes had a distinct pulsatile LH pattern which was decreased by transport in six ewes. In anoestrus, there was no effect of transport on LH pulse frequency or amplitude in intact ewes, or those ovariectomized 2-3 weeks previously, with or without prior oestradiol and progesterone treatment. However, basal concentrations of cortisol were greater in anoestrus than in the breeding season, and the increment in cortisol during transport was similar in anoestrus and the breeding season but greater during the transition into anoestrus (P < 0.05). Progesterone concentrations increased from 0.31 +/- 0.02 ng ml-1 before transport to 0.48 +/- 0.05 ng ml-1 during the second hour of transport (P < 0.05). In conclusion, transport reduced LH pulse frequency and amplitude in ovariectomized ewes that had not been exposed to exogenous steroids for at least 4 months. In most animals, the previously observed increase in LH pulsatility induced by exogenous CRH was not reproduced by increasing endogenous CRH secretion by transport. However, in four ewes, transport did increase LH pulsatility, but only during the transition into anoestrus in ewes with seasonally suppressed LH profiles after withdrawal of steroid pretreatment.     

Effect of testosterone and season on proenkephalin messenger RNA expression in the preoptic area of the hypothalamus in the ram

Enkephalin appears to exert an inhibitory action on LH secretion, but whether testosterone regulates enkephalin gene expression is unknown. This study tested the hypothesis that testosterone and/or season modulate preproenkephalin mRNA expression in specific areas of the hypothalamus. Romney Marsh rams were castrated (wethers) either during the breeding season or nonbreeding season and received intramuscular injections of either oil or testosterone propionate (five/group). Blood samples were taken for the assay of plasma LH and testosterone. Preproenkephalin mRNA expression was quantified in hypothalamic sections by in situ hybridization. Mean plasma LH concentrations were reduced and the interpulse interval for LH pulses was greater in testosterone propionate-treated wethers compared with oil-treated wethers, with no change in LH pulse amplitude. Testosterone propionate treatment reduced proenkephalin expression in the diagonal band of Broca, the caudal preoptic area, and the bed nucleus of the stria terminalis. Seasonal differences in proenkephalin expression were observed in the bed nucleus of the stria terminalis, lateral septum, periventricular nucleus, and paraventricular nucleus. No differences were observed between treatments in seven other regions examined. We conclude that testosterone and season regulate proenkephalin mRNA levels in the preoptic area/hypothalamus in the ram in a region-specific manner.     

Seasonal and hormonal control of pulsatile LH secretion in the dairy goat (Capra hircus)

In Exp. 1, the changes in pulsatile LH secretion at the onset of the breeding season were observed in 20 intact, mature Saanen does. Blood was sampled every 20 min for 6 h each week from the beginning of August until the onset of ovulatory activity, as evidenced by cycles in plasma progesterone. The first doe ovulated at the end of August and all were cycling by the end of September. As the first ovulation approached, LH pulse frequency increased by 67% and mean levels of LH increased by 47%. These changes were progressive rather than abrupt. In Exp. 2, seasonal changes in the inhibition of pulsatile LH secretion by ovarian steroids were studied in ovariectomized Saanen does. The animals were untreated (N = 4) or given subcutaneous oestradiol implants (N = 4) and blood was sampled every 10 min for 6 h, twice during the breeding season and twice during the anoestrous season. In each season, the second series of samples was taken after the animals had been treated with progesterone, administered by intravaginal implants. Season did not significantly affect LH secretion in goats not treated with oestradiol, but LH pulse frequency was 54% lower during the anoestrous season than during the breeding season in oestradiol-treated goats. Mean LH concentrations were affected in the same manner as pulse frequency, but pulse amplitude was increased by oestradiol treatment in both seasons. Progesterone had no detectable effect on LH secretion in either season. In Exp. 3, the response to repeated melatonin injections at a set time after dawn was investigated in 11 oestradiol-treated, ovariectomized goats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of time after ovariectomy, season and oestradiol on luteinizing hormone and follicle-stimulating hormone secretion in ovariectomized ewes.

During the breeding season, five groups of three ewes were implanted at ovariectomy with 0.36, 0.5, 1.0 and 6.0 cm oestradiol implants or implants containing no steroid. Eleven days after receiving implants, blood samples were taken every 10 min for 6 h; implants were then removed. Treatments were repeated three times during each of two consecutive breeding seasons and four times during the intervening anoestrus. In ovariectomized ewes without steroid treatment, luteinizing hormone (LH) pulse frequency increased from early to mid-breeding season, decreased to a minimum at mid-anoestrus and increased to reach a maximum at the mid-point of the second breeding season, subsequently declining. LH pulse amplitude was inversely related to frequency. Basal serum LH concentrations decreased gradually from the first breeding season to reach a minimum at mid-anoestrus and gradually increased to reach a maximum at the end of the second breeding season. Mean serum LH and follicle-stimulating hormone (FSH) concentrations were higher at the end of the second breeding season compared with the beginning of the first breeding season. All parameters of gonadotrophin secretion were decreased much more by oestradiol during the anoestrus than during the breeding season. LH pulse frequency was decreased during anoestrus and at high oestradiol concentrations during the first breeding season. Apart from LH pulse amplitude, the decreases in all parameters of gonadotrophin secretion were less during the second compared with the first breeding season. The minimum effective dose of oestradiol required to decrease mean and basal serum concentrations of LH during anoestrus was lower than in the breeding season. The minimum effective dose of oestradiol required to decrease mean serum concentrations of FSH was lower in the first compared with the second breeding season. Oestradiol depression of LH pulse amplitude and mean serum concentrations of LH and FSH showed a dose dependency during the breeding season. During anoestrus dose dependency was seen for basal concentrations of LH and mean serum concentrations of LH and FSH. We conclude that significant chronic changes in gonadotrophin secretion occur in the ewe with time after ovariectomy. Sensitivity to oestradiol also changes, and the effects of oestradiol are not always dose dependent. We suggest that the circannual pattern of LH pulse frequency and basal LH secretion are directly linked to the circannual cycle of photoperiod.(ABSTRACT TRUNCATED AT 400 WORDS)     

Gonadotrophin release in ovariectomized ewes fed different amounts of coumestrol

Three experiments were conducted to study changes in pulsatile secretion of LH and FSH during the breeding season or anoestrus in ovariectomized Ile-de-France ewes fed different amounts of the phyto-oestrogen coumestrol. In Exp. 1, conducted during the breeding season, ewes (3-4 per group) were fed lucerne supplying 4, 18 or 30 mg coumestrol per ewe per day for 15 days. Experiments 2 and 3 were conducted during seasonal anoestrus. In Exp. 2, ewes (4 per group) were fed lucerne supplying coumestrol concentrations ranging from 4 to 38 mg/ewe/day for 15 days. In Exp. 3, ewes (10 per group) were fed lucerne supplying 14 or 125 mg coumestrol/ewe/day for 15 days. During the breeding season, an increased concentration of coumestrol in the diet significantly decreased the amplitude of LH pulses. There were no effects on LH pulse frequency or on FSH concentrations. During seasonal anoestrus, there were no significant effects on LH pulse frequency, or amplitude and no significant effect on FSH concentration. These results show that high concentrations of coumestrol in lucerne diets would not explain seasonal variation in LH pulse frequency in ovariectomized ewes. However, lucerne diets with increased coumestrol concentrations can influence LH release during the breeding season.     

Temporal variation in LH and testosterone responses of rams after the introduction of oestrous females during the breeding season

The response of sexually experienced Ile-de-France rams to the presentation of oestrous females in October at sunrise (Subgroup S) or at 11:00 h (Subgroup N) was studied and compared with unstimulated controls (Subgroup C). Animals (12 per group) were bled for 7 h at 20-min intervals, starting 3 h before stimulation by oestrous females (3 per group). Eight rams from Subgroup S showed an increase of LH pulse frequency and only 3 in Subgroup N (P less than 0.03). In Subgroup S the introduction of females led to 2- and 3-fold increases in LH pulse frequency during the stimulation period compared with values in Subgroup C or before the stimulation period (3, 1.6 and 1 peaks/rams/6 h respectively; P less than 0.05). The presence of females also led to an increase in mean testosterone concentrations, and small increases in basal and mean LH values. No differences were found in LH peak amplitudes. In Subgroup N only inconsistent evidence of increases in mean LH and testosterone values was found. No differences between Subgroups S and N in behavioural patterns during stimulation were detected. We conclude that the presence of females affects LH pulse frequency at sunrise but not at noon during the breeding season and this effect is at least partly independent of sexual behaviour. These results suggest a possible circadian variation in CNS sensitivity involving the hypothalamic regulation of LH secretion in response to the presence of oestrous females.     

Circadian and pulsatile variations in plasma levels of inhibin, FSH, LH and testosterone in adult Murrah buffalo bulls

The present study investigated pulsatile and circadian variations in the circulatory levels of inhibin, gonadotrophins and testosterone. Six adult buffalo bulls (6 to 7 yr of age) were fitted with indwelling jugular vein catheters, and blood samples were collected at 2-h intervals for a period of 24 h and then at 15-min interval for 5 h. Plasma concentrations of inhibin, FSH, LH and testosterone were determined by specific radioimmunoassays. Plasma inhibin levels in Murrah buffalo bulls ranged between 0.201 to 0.429 ng/mL, with a mean of 0.278 +/- 0.023 ng/mL. No inhibin pulses could be detected during the 15-min sampling interval. Plasma FSH levels ranged between 0.95 to 3.61 ng/mL, the mean concentration of FSH over 24 h was 1.66 +/- 0.25 ng/mL. A single FSH pulse was detected in 2 of 6 bulls. The LH levels in peripheral circulation ranged between 0.92 to 9.91 ng/mL, with a mean concentration of 3.33 +/- 1.02 ng/mL. Pulsatility was detected in LH secretion with an average of 0.6 pulses/h. Plasma testosterone levels in 4 buffalo bulls ranged from 0.19 to 2.99 ng/mL, the mean level over 24 h were 1.34 +/- 0.52 ng/mL. Testosterone levels in peripheral circulation followed the LH secretory pattern, with an average of 0.32 pulses/h. The results indicate parallelism in inhibin, FSH and LH, and testosterone secretory pattern. Divergence in LH and FSH secretory patterns in adult buffalo bulls might be due to the presence of appreciable amounts of peripheral inhibin.     

Influence of season and low-level oestradiol immunoneutralization on episodic LH and testosterone secretion and testicular steroidogenic enzymes and steroidogenic acute regulatory protein in the adult ram

The regulation of LH-dependent and -independent increases in testosterone secretion by key proteins in the testes of adult rams was investigated. Serial blood samples were collected from groups of four control and passively immunized (oestradiol antiserum for 3 weeks) rams and the animals were gonadectomized in either the non-breeding season (April) or the breeding season (September). LH pulse frequency and basal (interpulse) concentrations were several times greater (P < 0.01) in the breeding season than in the non-breeding season. Neither of these parameters nor LH pulse amplitude were affected by oestradiol immunization. Parameters of testosterone episodic secretion and response to an injection (i.v.) of 15 micrograms NIH-LH-S25 were also greater (P < 0.05) in the breeding season and, with the exception of pulse frequency, in immunized rams versus controls. Substrate utilization established that testosterone biosynthesis was predominantly via the 5-ene pathway. Increases in blood testosterone concentration in the breeding season were associated with a fivefold higher (P < 0.01) activity of cytochrome P450 17alpha-hydroxylase/C-17,20 lyase (P450(17alpha)) and a 65% higher (P < 0.05) relative amount of mRNA for cytochrome P450 cholesterol side-chain cleavage enzyme complex (P450scc) in the testis. Of the steroidogenic enzyme activities examined, only that for 17beta-hydroxysteroid dehydrogenase (17beta-HSD) tended to be increased by oestradiol immunization. Blood concentrations of cholesterol lipoproteins and expression of the testicular low density lipoprotein receptor were not affected by season or immunization. The amount of steroidogenic acute regulatory protein (StAR) mRNA was 65% higher (P < 0.01) in the breeding season and 20% higher (P < 0.01) in immunized rams versus controls. These results indicate that greater LH stimulation may increase testosterone biosynthesis in the breeding season by increasing StAR mRNA (and presumably delivery of cholesterol to P450scc) and the activity of P450(17alpha), and possibly that of P450scc (activity not measured). More moderate increases in StAR mRNA and 17beta-HSD activity may explain, in part, the increases in testosterone secretion with oestradiol immunization.     

Does the seasonal increase in estradiol negative feedback prevent luteinizing hormone surges in anestrous ewes by suppressing hypothalamic gonadotropin-releasing hormone pulse frequency?

To test the hypothesis that the anestrous increase in estradiol negative feedback prevents estrous cycles by suppressing hypothalamic gonadotropin-releasing hormone (GnRH) pulse frequency, a variety of regimens of increasing GnRH pulse frequency were administered to anestrous ewes for 3 days. A luteinizing hormone (LH) surge was induced in 45 of 46 ewes regardless of amplitude or frequency of GnRH pulses, but only 19 had luteal phases. Estradiol administration induced LH surges in 6 of 6 ewes, only 3 having luteal phases. Anestrous luteal phase progesterone profiles were similar in incidence, time course, and amplitude to those of the first luteal phases of the breeding season, which in turn had lower progesterone maxima than late breeding season luteal phases. In the remaining ewes, progesterone increased briefly or not at all, the increases being similar to the transient rises in progesterone occurring in most ewes at the onset of the breeding season. These results demonstrate that increasing GnRH pulse frequency induces LH surges in anestrus and that the subsequent events are similar to those at the beginning of the breeding season. Finally, they support the hypothesis that the negative feedback action of estradiol prevents cycles in anestrus by suppressing the frequency of the hypothalamic pulse generator.