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.     

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.     

Ewe effect: endocrine and testicular changes in experienced adult and inexperienced young Corriedale rams used for the ram effect

Changes in LH, FSH, and testosterone concentrations, testicular firmness and resilience, and scrotal circumference were monitored in 16 Corriedale rams (8 experienced adult and 8 inexperienced young) for 20 days during which they were used to stimulate ewes. The experiment was conducted during November (mid-non-breeding season). Increases (P<0.05) were observed in LH and testosterone concentrations and in testicular firmness and resilience during the first 4 days when rams were in permanent contact with estrual ewes. During the following 13 days, when rams were in contact with non-estrual ewes (i.e. initially estrual ewes were no longer in estrus), LH and testosterone concentrations decreased. When initially anestrous ewes exhibited estrus 17 to 20 days later, concentrations of testosterone increased. Testicular firmness and resilience remained high throughout the period. We conclude that: (1) rams used to stimulate anestrous ewes show an increase in LH and testosterone concentrations beginning at 12 h after joining, and greater concentrations are maintained while estrual ewes and mating are allowed; and (2) estrous and mating activity are probably the most important stimuli for the increase in hormone concentrations.     

Female effect in sheep. II. Role of volatile substances from the sexually receptive female; implication of the sense of smell

The importance of olfactory cues in inducing luteinizing hormone and testosterone secretion as a response to stimulation by sexually receptive ewes has been tested. In sexually experienced rams, olfactory stimulation with urine, wool and vaginal secretions from sexually receptive females placed in a mask did not induce an endocrine response. The female-induced secretion of LH and testosterone was similar in anosmic, sham-operated and in control rams. These results show that olfactory cues are not necessary in the mediation of interindividual stimulation of endocrine response in the sexually experienced ram.     

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.     

Effect of breed and age on sexual behaviour of rams

The objective of this study was to highlight the problems that arise during the reproduction between thin-tailed rams and fat-tailed ewes. At the same time, particular emphasis laid on the influence of sheep breed, sheep age, time after ram introduction and day of the ewe estrus cycle on ram and ewe sexual behaviour. Rams were subjected to sexual performance tests by being individually exposed to 12 ewes for 3 h daily, 19 consecutive days. The 16 rams of the experiment were separated according to their age (9 and 21 months old) and breed (Chios and Karagouniki), and the 96 ewes of Chios fat-tailed breed, were divided by age (9 and 21 months old). The main characteristics of courtship behaviour, like sniffing, nudging, flehmen response and following were recorded and studied in detail. Mature Chios rams, which were the only one with previous experience of Chios ewes, exhibited higher rates of sexual interest per ewe than the other rams (P < 0.05). On the other hand, rams sniffed and nudged more young than mature ewes (P < 0.05), probably due to the fact that young ewes did not express intense symptoms of estrus. Young rams exhibited substandard sexual interest towards mature ewes, when they first came in contact with them (P < 0.05). In general, Karagouniki thin-tailed rams exhibited reduced rates of mating behaviour when they courted with Chios fat-tailed ewes in comparison with Chios rams (P < 0.05). Moreover, as the time after ram introduction passed, the frequency and duration of sexual behaviour components decreased (P < 0.001). Finally, the effect of the day of the experiment was only significant in the case of sniffing, which increased during the first 2 days and then declined and stabilized (P < 0.01). As it was demonstrated, ram age and ram breed played a fundamental role in the exhibition of sexual interest elements.     

Sexual behaviour and LH secretion in spayed androgenized ewes after a s?ngle injection of testosterone or oestradiol-17beta

The behavioural and endocrine responses to single injections of 50 or 500 microgram oestradiol-17beta or 5 mg testosterone were recorded in spayed (control) ewes and in spayed ewes exposed to testosterone between Days 30 and 80 or Days 50 and 100 of prenatal life, The control ewes showed oestrus after injections on 17/18 occasions. The androgenized ewes showed poorer oestrous responses to each hormone although rams showed interest in the ewes. Masculine sexual and aggressive behaviour was shown by the androgenized ewes given either steroid. Both steroids caused a reduction in the plasma LH levels of all the ewes (negative feedback), followed by a preovulatory-type surge (positive feedback). The peak LH values were significantly lower (P is less than 0.05) in the Day 50-100 androgenized ewes than in the controls. It is concluded that prenatal androgenization causes a qualitative shift in the sexual behaviour of ewes from the female type to the male type and affects the sensitivity of the brain to "positive feedback" by steroids.     

The continuous presence of ewes in estrus in spring influences testicular volume,testicular echogenicity and testosterone concentration,but not LH pulsatility in rams

The continuous presence of active male small ruminants prevents seasonal anestrus in females, but evidence of the same mechanism operating from the females to the males is scarce. This study assessed the effects of the continuous presence of ewes in estrus in spring on ram sexual activity, testicular size and echogenicity, and LH and testosterone concentrations. On 1 March, 20 rams were assigned to two groups (n = 10 each): isolated (ISO) from other sheep, or stimulated (STI) by 12 ewes, which were separated from the rams by an openwork metal barrier, allowing contact between sexes. Each week, four ewes were induced into estrus by intravaginal sponges. Live weight, scrotal circumference, testicular width (TW) and length (TL) were recorded at the beginning and at the end of the experiment, and testicular volume (TV) was calculated; at the same time, testicular ultrasonography and color Doppler scanning were performed. Blood samples (March to May) were collected once per week for testosterone determinations, and at the end of the experiment, blood samples were collected for 6 h at 20-min intervals for LH analysis. Rams were exposed to four estrous ewes in a serving-capacity test. Scrotal circumference, TW and TL were higher in the STI than in the ISO rams (P < 0.05) in May, and TV was higher (P < 0.05) in the STI (391 ± 17 cm³) than in the ISO rams (354 ± 24 cm³). In ISO rams, the number of white pixels was higher (P < 0.01) in May (348 ± 74) than in March (94 ± 21) and differed significantly (P < 0.01) from that of the STI rams in May (160 ± 33). In ISO rams, the number of grey pixels was higher (P < 0.05) in May (107 ± 3) than it was in March (99 ± 1). Stimulated and ISO rams did not differ significantly in mean LH plasma concentrations (0.8 ± 0.5 v. 0.9 ± 0.4 ng/ml), LH pulses (2.1 ± 0.5 v. 2.2 ± 0.2) and amplitude (2.0 ± 0.4 v. 3.2 ± 0.7 ng/ml, respectively). Stimulated rams had significantly higher testosterone concentrations than ISO rams from April to the end of the experiment. Stimulated rams performed more (P < 0.05) mountings with intromission (3.0 ± 0.4) than did ISO rams (1.5 ± 0.5). In conclusion, after 3 months in the continuous presence of ewes in estrus in spring, rams had higher TV and some testicular echogenic parameters were modified than isolated rams. Although exposed rams also had higher levels of testosterone after 2 months in the presence of estrous ewes, their LH pulsatility at the end of the study was not modified.     

Can audio–visual or visual stimuli from a prospective mate stimulate a reproductive neuroendocrine response in sheep?

Stimuli from a prospective mate increase the secretion of luteinising hormone (LH) in sheep. This 'male effect' in ewes and 'female effect' effect in rams is predominantly mediated by olfactory signals, though it is thought that non-olfactory signals play synergistic or substitutive roles. In this study, we tested whether exposure to visual or audio-visual stimuli from a prospective mate would stimulate an increase in LH secretion in ewes (Experiment 1) and rams (Experiment 2). In Experiment 1, groups of eight Merino ewes were exposed to one of three stimuli midway through a frequent blood-sampling regimen: full ram contact, still images of rams, a video of ewes and rams mating. Control ewes (n = 8) were completely isolated from rams. Exposure to still images of rams appeared to stimulate an increase in mean LH concentrations (P < 0.05) and tended to increase LH pulse frequency (P < 0.1), but the response was significantly smaller than that observed in ewes exposed to rams (P < 0.01). Audio-visual stimuli had no effect on any parameters of LH secretion (P > 0.1). In Experiment 2, Merino rams were allocated to either an Exposure (n = 7) or a Control (n = 7) group. Exposure rams underwent two exposure periods midway through a frequent blood-sampling regimen; exposure to still images of ewes and audio recorded during mating of ewes and rams (audio-visual exposure); exposure to oestrous ewes (ewe exposure). Control rams were sampled at the same frequency but remained isolated from ewe stimuli. Exposure of rams to the audio-visual stimuli did not affect any parameters of LH secretion (P > 0.1). In contrast, exposure to oestrous ewes increased LH pulse frequency (P < 0.05) and advanced the onset of the next LH pulse (P < 0.05). In conclusion, visual signals appear to be involved in eliciting the neuroendocrine response of ewes to rams and are of greater importance to this phenomenon in ewes (male effect) than rams (female effect). However, overall the visual and audio-visual signals used in this study were far less effective than stimulus animals, suggesting that these stimuli are less important than olfactory signals, or a combination of olfactory and audio-visual signals.     

The ovine sexually dimorphic nucleus,aromatase, and sexual partner preferences in sheep

We are using the domestic ram as an experimental model to examine the role of aromatase in the development of sexual partner preferences. This interest has arisen because of the observation that as many as 8% of domestic rams are sexually attracted to other rams (male-oriented) in contrast to the majority of rams that are attracted to estrous ewes (female-oriented). Our findings demonstrate that aromatase expression is enriched in a cluster of neurons in the medial preoptic nucleus called the ovine sexually dimorphic nucleus (oSDN). The size of the oSDN is associated with a ram's sexual partner preference, such that the nucleus is 2–3 times larger in rams that are attracted to females (female-oriented) than in rams that are attracted to other rams (male-oriented). Moreover, the volume of the oSDN in male-oriented rams is similar to the volume in ewes. These volume differences are not influenced by adult concentrations of serum testosterone. Instead, we found that the oSDN is already present in late gestation lamb fetuses (～day 135 of gestation) when it is ～2-fold greater in males than in females. Exposure of genetic female fetuses to exogenous testosterone during the critical period for sexual differentiation masculinizes oSDN volume and aromatase expression when examined subsequently on day 135. The demonstration that the oSDN is organized prenatally by testosterone exposure suggests that the brain of the male-oriented ram may be under-androgenized during development.     

Effects of recent sexual experience and melatonin treatment of rams on plasma testosterone concentration, sexual behaviour and ability to induce ovulation in seasonally anoestrous ewes

The aim of this study was to determine whether advancing the seasonal changes associated with rams by treatment with exogenous melatonin and allowing the rams previous sexual experience would increase the proportion of anoestrous ewes ovulating in early July. North Country Mule ewes (n = 225) were grouped by live body weight and body condition score and allocated randomly to the following treatments: (i) isolated from rams (control; n = 25); (ii) introduced to rams (treatment 2); (iii) introduced to rams that had mated with ewes during the previous 2 days (treatment 3); (iv) introduced to rams implanted with melatonin (treatment 4); and (v) introduced to rams that were implanted with melatonin and had mated with ewes during the previous 2 days (treatment 5). Treatments 2-5 were replicated (2 x 25 ewes) and two rams were introduced to each replicate group. Introductions began on 4 July and were completed by 11 July. The rams were withdrawn from the ewes after 8 days. Melatonin was administered as a subcutaneous implant (Regulin((R))) on 22 May and again on 20 June. Blood samples were taken from all rams to determine plasma melatonin and testosterone concentrations (19 samples in 6 h). The behaviour of the sheep was videotaped continuously during the first 3 h after the ram was introduced. Ovulation was detected by an increase in plasma progesterone concentrations from < 0.5 ng ml(-1) to > 0.5 ng ml(-1). Mean +/- SE plasma melatonin concentrations were 649.7 +/- 281.4 and 18.3 +/- 2.4 pg ml(-1) in rams with and without melatonin implants, respectively (P < 0.001). Melatonin implants also increased plasma testosterone concentrations from 4.30 +/- 1.88 to 10.10 +/- 1.10 ng ml(-1) (P < 0.01), the libido of the rams and the proportion of ewes that ovulated in response to the rams (43 and 56% (treatments 4 and 5) versus 24% (treatments 2 and 3)). In conclusion, implanting rams with melatonin before introducing them to seasonally anoestrous ewes increases the proportion of ewes that ovulate in response to introduction of a ram, but previous sexual experience of rams appears to have little or no effect.     

Effects of sexual stimulation on the sexual performance of rams

The sexual performance of bulls and male goats is improved if they are allowed to view the hetero-sexual behavior of other males as a prelude to mating. The purpose of the following study was to determine whether sexual stimulation enhances the sexual performance of rams. In Experiment 1, 11 sexually experienced ram lambs ( 9 months of age) and 18 sexually experienced yearling and 2-year-old rams were individually exposed to 4 unrestrained, hormone-induced estrous ewes for 60 min after viewing the courtship and mounting behaviors of a male conspecific for 20 min (two tests) and in the absence of stimulator animals (two tests). In contrast to the results with bulls and bucks, the rams were hardly influenced by the sexual stimulation treatment. Latencies for first mount and first ejaculation were shorter for sexually stimulated ram lambs; otherwise, treatment differences were negligible.

A similar follow-up experiment was administered to 12 mature rams using restrained females in the sexual performance tests. Again, treatment differences were minor.

It was concluded that sexual stimulation does not functionally enhance the sexual performance of rams. Species differences in response to sexual stimulation are discussed in terms of female sexual behaviors that may result in a selective (competitive) advantage to males that are stimulated to locate and mate with females early in the estrous period.     

Lack of sexual experience does not reduce the responses of LH, estrus or fertility in anestrous goats exposed to sexually active males

We investigated whether LH secretion, estrous behavior and fertility would differ between sexually inexperienced and experienced anestrous goats exposed to the males. Male goats were rendered sexually active during the reproductive rest season by exposure to 2.5 months of artificial long days. Two groups of anovulatory sexually inexperienced and sexually experienced does were exposed to males during 15 days (n = 20 per group). LH pulsatility was determined every 15 min from 4 h before to 8 h after introducing males (Day 0). Estrous behavior was recorded twice daily. Pregnancy rates were determined on Day 50. Fertility was determined at parturition. Male sexual behavior was registered on days 1 and 2 during 1 h. Before introducing the males, the number of LH pulses did not differ between groups. After introduction of the males, all females increased their LH pulsatility, but the number of pulses did not differ between sexually inexperienced and experienced goats. The proportion of females displaying estrous behavior with a high pregnancy rate and fertility did not differ between inexperienced and experienced goats. The sexual behavior of the males did not differ significantly between those interacting with sexually inexperienced or experienced goats. We conclude that goats can show substantial endocrine and reproductive responses to males, even in the absence of previous sexual experience, when sexually active bucks are used.     

A note on ram preference by oestrous ewes: Influence of rams’ age and sexual performance

Ewes in oestrus, besides being receptive to rams, develop an active search for them. We performed two experiments to determine if oestrous ewes prefer: (1) high or low sexual-performance rams, and (2) 2-year-old or aged (>8-years-old) rams. In the first experiment, the preference of ewes for rams with high or low sexual performances was compared in 16 tests. In the second experiment, 17 tests were performed comparing ewes’ preference for aged rams (>8-years-old) or young rams (2-years-old). In each test, two rams were tied at equal distances from the point of entrance of the ewe, with a maximum mobility for each ram of 2 m. The first approach to a ram, the quantity of approaches toward each ram (less than one body length), and the total time that the ewe remained near each ram were registered during 5 min. Oestrous ewes did not display any preference according to sexual performance of rams, but spent significantly more time (P = 0.02), and did it in more tests (P = 0.025), with young than with aged rams. Ewes also tended to approach first (P = 0.07), and more times in more tests (P = 0.06), to young than to aged rams. We conclude that ram's sexual performance seems not to influence oestrous ewes preference, and that ewes’ prefer young (2-years-old) rather than aged rams (>8-years-old).     

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)     

Prior exposure of maiden ewes to rams enhances their behavioural interactions with rams but is not a pre-requisite to their endocrine response to the ram effect

In this study, we tested whether prior experience with rams would modify the behavioural and endocrine responses of maiden ewes to rams. During mid-anoestrus, sexually na?ve, maiden ewes were exposed to rams for 7 days (ram experienced, RE; n=61) or isolated from rams (ram na?ve, RN; n=63). All ewes were subsequently isolated from rams. In Experiment 1, RE (n=55) and RN (n=57) ewes were introduced to rams during late anoestrus. RE ewes had more total and positive interactions with rams than RN ewes (P<0.001). RE ewes showed more ram seeking behaviour and spent more time in proximity of rams than RN ewes (at least; P<0.05). In Experiment 2, RE (n=6) and RN (n=6) ewes were introduced to rams midway through a frequent blood sampling regime in late anoestrus. Ram introduction stimulated an increase in LH pulse frequency and basal LH in both RE and RN ewes (at least P<0.05). RE ewes had an increase in mean LH concentrations (P<0.01) that failed to reach significance in RN ewes (P<0.1). There was no significant effect of prior experience with rams on LH pulse frequency, amplitude or whether ewes had an LH surge. In conclusion, prior experience with rams is important in developing appropriate ewe-ram interactions but is not a pre-requisite to the endocrine response to the ram effect.     

The response of rams and ewes of three breeds to artificial photoperiod

Rams and ewes of the Romney Marsh (N = 6), Dorset Horn (N = 8) and Australian Merino (N = 8) breeds were subjected to 4 successive periods of alternating 6 h light/18 h dark ('short' days) and 18 h light/6 h dark ('long' days) preceded by 16 weeks of 12 h light/12 h dark. The initial period was of 32 weeks (16 weeks 'short' days; 16 weeks 'long' days) and the next 3 were of 24 weeks (12 weeks 'short' days; 12 weeks 'long' days). Rams of all breeds showed a cyclic pattern of growth and regression of testes associated with plasma testosterone concentration, influenced by the change in light regimen 15-19 weeks previously. Sexual behaviour was also cyclic but lagged by some 6-7 weeks. The changes were greatest in the Romneys and least in the Merinos in which a higher degree of sexual activity was evident even when the testes were regressed (P less than 0.001). This was the major breed difference. All ewes of the Romney and Dorset breeds showed marked seasonality related to the imposed light regimen, whereas only 1 of the 4 Merinos did so. The mean peak of ovarian activity in the former 2 breeds coincided with that of maximum sexual activity of rams housed with them; that is, some 6 weeks after maximum scrotal volume. The rams and ewes were subjected to serial blood sampling episodes for plasma LH and testosterone and tested for plasma LH release following GnRH administration. There was little variation between breeds in LH concentration. Testosterone concentration varied greatly in the ram, highest levels associated with the developed phase of the testes and with maximum LH pulse frequency. The LH response to GnRH changed with respect to the state of the gonads. Maximal responses were observed in the developing phase of testicular growth although this variation was greater in the Romney and Dorsets than in the Merinos (P less than 0.001). In the ewes, maximal responses were seen in the follicular phase (P less than 0.001), with no difference between the luteal and acyclic phases. There were no breed differences. Plasma pooled from the serial blood sampling episodes was assayed for prolactin.(ABSTRACT TRUNCATED AT 400 WORDS)     

Response of pre-pubertally castrated rams and ewes to artificial photoperiod--changes in plasma LH and prolactin

Castrate rams and ovariectomized ewes were maintained in the presence of entire rams and ewes and subjected to successive periods of alternating 6 h light:18 h darkness ('short' days) and 18 h light:6 h darkness ('long' days) preceded by a period of 12 h light:12 h darkness ('constant' light days). Plasma concentrations of LH and prolactin were measured in the castrate animals in order to determine how LH and prolactin secretion responded to the artificial light regime and corresponding periods of elevated or depressed testicular and ovarian activity in the entire rams and ewes. There was no variation in mean plasma LH concentrations or LH pulse frequency with either the changes in photoperiod or the phases of gonadal activity in the entire animals. However, there was a highly significant (P less than 0.001) relationship between prolactin secretion and the artificial photoperiod in both castrate groups with high and low levels coinciding with long and short days respectively. In addition, there was a marginally significant (P less than 0.1) relationship between prolactin secretion in the castrate ram and the stage of testicular activity in the entire rams with elevated levels associated with regressed activity. Prolactin secretion in the ovariectomized ewes was significantly (P less than 0.05) related to the phase of ovarian development with high levels associated with acyclic activity. It is concluded that LH secretion and pituitary responsiveness to exogenous GnRH were not modified by the artificial light regime. However, the changing light pattern was physiologically 'perceived' by the castrate animals as indicted by a concomitant variation in plasma prolactin concentrations.     

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.     

Luteinizing hormone, testosterone and cortisol responses in rams upon presentation of estrous females in the nonbreeding season

The physiological responses of luteinizing hormone, testosterone and cortisol in sexually experienced Ile de France rams to the introduction of estrous females were studied during the nonbreeding season. Blood sampling were collected from males for 7 h at 20-min intervals, starting 3 h before stimulation by estrous females. The differences in hormonal secretions were tested by comparisons between pretreatment and treatment Periods in 45 stimulated rams. Comparisons were conducted between rams that had increased LH pulse frequency and those that did not, between rams that ejaculated and those that did not, and between rams that were in direct physical contact and those that were kept at a distance of 30 cm from estrous females. Twenty-five rams (55% of the total) showed significant increases in LH pulse frequency (range, 0.80 to 4.00 peaks/ram/6 h, P<0.05), in basal and mean LH levels (1.5- and 2.5-fold, respectively), and in mean testosterone levels (3.5-fold). More frequent LH pulses had been found during the pretreatment period in 20 rams without increased LH pulse frequency. Eight ejaculating rams showed higher cortisol and mean, basal, and peak LH amplitude levels. Deprivation of physical contact with estrous females was associated with an absence of endocrine response. These results suggest that olfactory and/or tactile cues may be involved in the female effect on hormone levels.