Testosterone and photoperiod interact to regulate locomotor activity in male hamsters

Testicular size, plasma testosterone levels, copulatory behavior, and daily locomotor activity are reduced in male hamsters after 10 weeks of exposure to short days. The role of testosterone in the short day-induced decline in locomotor activity was investigated, determining whether or not photoperiod could alter the effect of testosterone on activity. Castrated adult hamsters were allowed to acclimate to running wheels (wired to digital counters) and then were kept on either long (L:D 14:10) or short (L:D 6:18) days for 60 days. On Day 60, half of the animals on each light cycle were implanted with 12-mm-long testosterone-filled Silastic capsules; half received empty capsules. Digital counting of wheel-running activity continued for another 140 days. Blood samples taken on Day 200 confirmed L:D 14:10 and L:D 6:18 testosterone-treated hamsters had equivalent plasma testosterone levels. After an initial decline in activity, L:D 14:10 animals exhibited a progressive rise in mean running activity (from ~2000 to ~5000 wheel revolutions per day) through 100 days after the initiation of testosterone treatment. In contrast, activity levels in testosterone-treated L:D 6:18 animals remained uniform (~2000 wheel revolutions per day) during this time, indicating exposure to short days rendered the hamsters less sensitive to the stimulatory effect of testosterone on activity. Of further interest was a marked increase in activity after 160–200 short days in animals treated with either testosterone-filled or empty capsules. It appears the total amount of daily locomotor activity in the hamster is modulated by circulating testosterone levels in a manner which is dependent upon the environmental photoperiod.     

Influence of photoperiod and 6-methoxybenzoxazolinone on the reproductive axis of inbred LSH/Ss Lak male hamsters

Adult male Syrian hamsters of the inbred LSH/Ss Lak strain were maintained under a 14L:10D light cycle until 13 weeks of age. At this point, they were implanted s.c. with elastomer capsules that were either empty or packed with 30-40 mg of 6-methoxybenzoxazolinone (6-MBOA), a compound found naturally in some monocotyledonous plants; half of the animals from each treatment group were then kept in long days (14L:10D) or transferred to short days (9L:15D). Testicular size was measured and blood samples collected from each hamster immediately before capsule implantation and again 2, 4, 6 and 8 weeks later. Within just 2 weeks of exposure to short days the mean plasma levels of LH and FSH had significantly declined, in both the control and 6-MBOA-treated animals, and were basal within 4 weeks. Testicular size closely followed these gonadotrophin changes; within 4-6 weeks the testes from all of the short-day hamsters had completely regressed to a prepubertal size. At the end of the experiment, at Week 8, the animals were killed and various components of the hypothalamo-pituitary-testicular axis were compared between the treatment groups. The pituitary content of FSH and LH, testicular weight, mean serum level of testosterone, but not hypothalamic LHRH content or pituitary gland weight, were considerably lower in the short-day than in the long-day hamsters, regardless of whether or not they had been chronically treated with 6-MBOA.(ABSTRACT TRUNCATED AT 250 WORDS)     

The relationship between sexual and aggressive behaviour, and pituitary and testicular activity during the seasonal sexual cycle of rams, and the influence of photoperiod

Six adult Soay rams were housed under artificial lighting conditions with alternating 16-week periods of long (16 h light: 8 h darkness) and short days (8L:16D) During long days the rams were reproductively quiescent: the abrupt change from long to short days induced a specific succession of responses in the reproductive system. Plasma LH and FSH levels began to increase after 2-4 weeks, followed almost immediately by a rise in plasma testosterone levels accompanied by growth of the testes. Testicular activity continued to increase during short days and the greatly elevated androgen levels apparent after 5-10 weeks caused changes in the peripheral target organs, including growth of the epididymides, development of the sexual flush on the exposed ventral skin and heightened genital sensitivity. High testosterone levels were also associated with an increase in aggressive (scored by a mechanical device) and sexual (incidence of Flehmen) behavior which was at peak about 1 month after the start of the peak androgen levels. The change to long days was associated with a decrease in plasma gonadotrophin levels within 2 weeks followed by a progressive decline in all reproductive parameters measured. Implantation of a low dose of testosterone (200 mg) during the period of reproductive quiescence induced the development of the sexual flush and an increase in genital tactile sensitivity, although behaviour was not significantly affected. The annual changes in reproductive physiology and behaviour of 12 Soay rams living under natural lighting conditions were recorded for comparison with the experimental situation. The nadir of the sexual cycle was in the spring and early summer, and the sequence of events culminating in the mating season in the autumn was similar to that induced experimentally.     

Influence of short days on diurnal patterns of serum gonadotrophins and prolactin concentrations in the male Djungarian hamster, Phodopus sungorus

Exposure to short days for 8 weeks suppressed mean serum concentrations of FSH, LH and prolactin compared to hamsters kept in long days. Hamsters in short days exhibited a small afternoon rise in serum FSH, but serum LH and prolactin did not exhibit 24-h variations. In hamsters under long days, a late afternoon-early evening increase was evident for circulating prolactin but none was detected for the gonadotrophins. A fall in testes weights rapidly occurred by 14-28 days after transfer to short days. This was accompanied or preceded by a decrease in serum gonadotrophins and prolactin. Reductions in serum FSH and LH occurred in short days in blood samples taken at 09:00 h or 15:00 h. However, the nadir in serum prolactin was first achieved (at 09:00 h), at least 7 days before that at 15:00 h (i.e. Day 14 versus Day 21 of short photoperiod, respectively). The ability to secrete gonadotrophins was further tested in hamsters that had undergone gonadal regression. Castration of hamsters exposed to short days or injected with melatonin in the afternoon, a treatment known to mimic short day effects, induced a 3- to 5-fold increase in serum gonadotrophins. However, this rise in FSH and LH was significantly attenuated compared to the 10-fold response in controls in long days. The results indicate that gonadal involution induced by short days may be mediated by the decline in mean gonadotrophin secretion which, in turn, is regulated by responsiveness to steroids, as well as a mechanism independent of the negative feedback action of gonadal steroids.     

Photoperiodic suppression of gonadotrophin secretion in castrated male hamsters

Male golden hamsters were transferred from long to short days and castrated on the day of transfer (Group SP0) or 1, 2 or 3 weeks afterwards (Groups SP1, SP2 & SP3). Animals in Group LP were castrated and maintained on long days. After 7 weeks of short days, plasma levels of LH and FSH were low in Groups SP1, SP2, and SP3. In Group SP0 LH output was also low and FSH secretion, although greater than in Groups SP1, SP2 and SP3, was lower than in the long-day controls (Group LP). This photoperiodic control of gonadotrophin secretion in castrated hamsters raises doubts about the significance of hypothetical alterations in hypothalamo-pituitary sensitivity to sex steroids.     

Profiles of luteinizing hormone, follicle-stimulating hormone, testosterone and prolactin in rams of diverse breeds: effects of contrasting short (8L:16D) and long (16L:8D) photoperiods

Mature rams of Polled Dorset, Finnish Landrace, Rambouillet and Suffolk breeding were maintained in a temperature-controlled environment and exposed to two consecutive cycles of short (8L:16D) followed by long (16L:8D) days. Serum hormone concentrations were determined in weekly samples and in 24-h profiles characterized at the end of each lighting schedule (i.e., 12, 24, 36 and 48 weeks). In all four breeds, the pituitary-testicular axis was more active during short days as compared with long days and the magnitudes of changes in serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone concentrations were greater for the two most seasonal breeds, Finnish Landrace and Suffolks. In comparison to other breeds, Finnish Landrace rams had significantly (P less than 0.05) higher mean LH levels, showed the greatest number of LH peaks/24 h, and had the highest mean testosterone levels at the end of both periods of short days, while Rambouillet rams had significantly (P less than 0.05) lower testosterone. Rambouillets also showed the smallest changes in pulsatile LH and testosterone secretion and displayed the least number of LH peaks/24 h following short days. Serum FSH levels were significantly (P less than 0.05) higher in Finnish Landrace and Suffolk rams than in Polled Dorsets and Rambouillets after 12 weeks of short days. Breed differences in serum LH, FSH and testosterone were not apparent following long days. Prolactin levels in Rambouillet rams were significantly (P less than 0.05) lower than in the other breeds following both periods of long days. These results indicate that breed differences exist in mature rams with regard to hormone secretory profiles. Breed differences in serum gonadotropin and testosterone are only apparent during short days when the hypothalamo-pituitary-testicular axis in rams is considered most active. Likewise, breed differences in prolactin are noticeable only during long days when secretion of this hormone is enhanced. Breed differences in LH, FSH and testosterone secretion in rams during short days might be related to seasonality of mating and/or fecundity of breed types.     

Testicular involvement in peripubertal gonadotropin levels and accessory sex organ weight of male hamsters

This study evaluates the influence of testicular secretions during development in male hamsters on peripubertal gonadotropin levels. Castration or sham operations were performed on the day of birth (Day 1), Day 5, 10, or 20 of life. Repeated plasma samples on Days 20-60 at 10-day intervals were taken via orbital sinus puncture. Castrated animals received a subcutaneous testosterone capsule on Day 60 and were killed on Day 70. In addition, seminal vesicles and ventral prostate weights were taken in all animals at Day 70. Castrated animals, regardless of day of castration, had higher gonadotropin levels and suppressed sexual accessory organ weights. Animals castrated on the day of birth had lower luteinizing hormone (LH) levels than animals castrated on other days. Castration on Day 10 resulted in lower follicle stimulating hormone (FSH) levels. Males castrated on Day 20 were most sensitive to the negative feedback effect of testosterone on LH secretion, while Day 10 castrates had elevated FSH levels after testosterone exposure. Sexual accessory weights also differed depending upon the day of castration. Results point out the importance of testicular secretions on the developmental processes as well as the differing ages at which various systems may be influenced.     

The effects of a sudden decrease or increase in daylength on prolactin secretion in the ram

Six adult Soay rams were preconditioned to an artificial lighting regimen of alternating 4-month periods of long (16L:8D) and short days (8L:16D) for at least 10 months before blood samples were collected at hourly intervals for 24 h at various times. The abrupt change from long to short days resulted in a progressive decrease in plasma levels of prolactin, while that from short to long days had the reverse effect; the first response to the light changes was rapid, beginning within 6 days. During the periods of high secretion there was a 24-h cycle in plasma prolactin concentrations, with a peak in both the early dark and early light phases of each day. Changes in the relative magnitude of these peaks were observed in relation to the long-term alteration in prolactin secretion. Plasma levels of FSH were also measured and a close inverse relationship between gonadotrophin and prolactin secretion was observed.     

Gonadotrophin secretion and pituitary responsiveness to LHRH in castrated and intact male rabbits exposed to different photoperiods

Adult male wild rabbits were exposed to at least 16 weeks of 16L:8 D before experiments began. Plasma LH and FSH concentrations increased significantly (P less than 0.001) when rabbits were castrated in 16L:8D but declined when rabbits were transferred to 8L:16D. Concentrations had returned to normal for castrated rabbits in 16L:8D by 74 days after the start of the 8L:16D treatment. Treatment of intact male rabbits with an injection of LHRH before and after transfer to short daylengths caused a transient increase in plasma LH which lasted 50-80 min and this produced a concomitant rise in plasma testosterone. The daylength change had no effect on this response even though testicular size declined after the transfer to short daylengths. Rabbits moulted in response to exposure to 8L:16D. This suggests that hypothalamic activity responds to photoperiod and that changes in pituitary responsiveness to LHRH and steroid negative feedback are unimportant.     

Reproductive effects of olfactory bulbectomy in the Syrian hamster

The effects of olfactory bulbectomy on circulating gonadotropin, prolactin and testosterone levels and on the testicular and pituitary responses to shortening of day length were studied in Syrian hamsters. Adult animals maintained on a 14L:10D cycle were sham-operated or sustained bilateral radical olfactory bulbectomies by aspiration to remove the main and accessory olfactory bulbs and the adjacent regions of the anterior olfactory nucleus. They were then maintained either on the long photoperiod or housed on a 10L:14D cycle. Testicular length was measured at weekly intervals over a 5-mo period. Sham-operated controls exhibited the normal pattern of testicular regression and eventual recrudescence on the short photoperiod. Testicular regression was significantly reduced in bulbectomized animals. Many of these animals showed no regression; others exhibited a reduced degree and/or shortened duration of regression. Serum levels of follicle-stimulating hormone (FSH) were substantially elevated in bulbectomized males maintained in long days. Their serum levels of luteinizing hormone (LH), prolactin and testosterone remained within the range for shams on long photoperiod. In short days, the bulbectomized animals showed the normal, pronounced decline in circulating prolactin levels. Serum FSH and LH levels also showed substantial declines, but the FSH levels were not reduced below the range for controls in long days, and the decline in LH levels was not as great as that for controls in short days.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of short photoperiod on the ability of golden hamster pituitaries to secrete prolactin and gonadotropins in vitro

Transfer of male golden (Syrian) hamsters from a 14L:10D (light:dark) to a 5L:19D photoperiod induced significant changes in pituitary function tested in vitro. Within 27 days after transfer to a 5L:19D photoperiod, basal prolactin (Prl) release was significantly depressed and response to dopamine (DA) was significantly enhanced as compared to Prl release by pituitaries from 14L: 10D hamsters. Follicle-stimulating hormone (FSH) release tended to be depressed after 9 or 27 days of 5L:19D exposure, but the effect was not significant. After 77 days of 5L:19D exposure, Prl release was further suppressed, while FSH release surpassed that seen in 14L:10D pituitaries. In vitro FSH response to luteinizing hormone releasing hormone (LHRH) was also enhanced at this time. After 15 weeks of exposure to a short photoperiod, FSH secretion was still elevated above control levels, but Prl release and Prl response to DA were no longer different from that of 14L: 10D controls. Secretion of luteinizing hormone (LH) in vitro, either basal or LHRH stimulated, was not affected by photoperiod at any time tested. From these results, we conclude that short photoperiod exposure does not reduce the pituitary's ability to secrete LH or FSH, although secretion of Prl is severely attenuated.     

Gonadotropin secretion and testicular function in golden hamsters exposed to skeleton photoperiods with ultrashort light pulses

Both sexually mature and sexually regressed male golden hamsters were transferred to asymmetric skeleton photoperiods with night interruptions of varying duration, the short pulses occurring 14 h after "dawn." Testicular function and accompanying changes in follicle-stimulating hormone (FSH), luteinizing hormone (LH), testosterone and spermatogenesis were observed. Sexually regressed animals exposed to a night-break of 6 seconds (sec) or longer exhibited maximal testicular development with a rapid rise in FSH secretion followed by a slower, more variable rise in LH. Full testicular size was achieved after 8 weeks. Night-breaks of 250 milliseconds (msec) or 1 sec induced testicular development and spermatogenesis but at a slower rate: levels of FSH and LH were still rising at the end of the experiment. Complete testicular maintenance was achieved by night-breaks of 1 sec or longer. Partial testicular regression was observed with a night-break of 250 msec. Night-breaks (60 sec) given less frequently than daily also stimulated testicular function and a night-break every 7 days increased FSH and LH secretion in sexually regressed hamsters, causing testicular development at a submaximal rate. Night-breaks given more frequently induced rapid testicular growth. Almost complete testicular maintenance of sexually mature hamsters was achieved with a 60-sec night-break at weekly intervals. Symmetric skeleton photoperiods also triggered testicular development in sexually regressed hamsters, with two 1-sec light pulses (14 h apart) being almost as effective as a normal long day. No difference in reproductive function was observed between animals on long days (14L:10D) and those exposed to maximally stimulatory skeleton photoperiods.     

Rapid cessation of estrous cyclicity and depressed castration response in short photoperiod-treated, inbred LSH/SsLaK hamsters

This study examined the effects of transfer from long photoperiod (LP) to short photoperiod (SP) on the cessation of ovarian cyclicity and the castration response in inbred LSH/SsLak golden Syrian hamsters. Forty-six 8 to 10-wk-old female hamsters were acclimatized in LP (14L:10D; lights on at 0600 h) during which time animals were monitored for regular ovarian cyclicity. Twenty-six animals were transferred to SP (8L:16D; lights on at 0600 h) and examined daily for vaginal discharges. One day after the day of the first missed ovulation, individual SP-exposed animals were bilaterally ovariectomized; concomitantly, an LP control animal in diestrus I underwent the same procedure. Thirty days after ovariectomy, the hamsters were fitted with intra-atrial silastic cannulae. On the following two postoperative days, 0.6 ml blood samples were collected at 0700, 1200, 1400, and 1600 h for SP animals and at 0700, 1400, 1600 and 1800 h for LP controls. On the third day, the animals were decapitated and sera and pituitaries saved for determination of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) by radioimmunoassay (RIA). All SP-exposed animals displayed their last estrous discharge 14-34 days after transfer to SP (mean = 23.0 +/- 0.8 days). Their ovaries were characterized by the absence of corpora lutea, the presence of large atretic antral follicles, few growing follicles, and interstitium that was stimulated to varying degrees. Total and adjusted pituitary weights were decreased by SP exposure (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of short photoperiod and cold exposure in regulating daily torpor in Djungarian hamsters

1. Male and female Djungarian hamsters (Phodopus sungorus) were gonadectomized or sham-operated after 12 weeks of exposure to short photoperiods (10L:14D). Half of the animals were single housed and transferred to a cold environment (7 degrees C) at week 13 of short days and half were transferred to cold at week 21. The time courses of short photoperiod induced seasonal changes in body weight, pelage color stage, and daily torpor were monitored periodically until the experiment was terminated after 34 weeks of short days. 2. The total duration of short photoperiod exposure was of primary importance compared to the duration of cold exposure in regulating seasonal changes in the frequency of daily torpor, body weight and pelage color exhibited by male and female Djungarian hamsters; that is, the change from long to short days was much more effective as a seasonal time cue than was the onset of cold exposure. 3. Gonadectomy did not prevent the occurrence of seasonal torpor in hamsters of either sex, indicating that these cycles are regulated by a time measuring mechanism (seasonal clock) that is largely independent of the gonadal cycle. However, castration did influence certain aspects of the body weight and torpor cycles exhibited by male hamsters. 4. Some castrated animals showed a delay in terminating the torpor season lending further support to the hypothesis that the spontaneous recrudescence of the testes which occurs toward the end of the torpor season may play a role in the termination of torpor in males.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoperiod entrainment of testosterone, luteinizing hormone, follicle-stimulating hormone, and prolactin cycles in rams in relation to testis size and semen quality

Adult rams were exposed to photoperiod treatments over 2 years to study the influence of light regimes on pituitary-testicular activity and semen quality. Initially, all rams (12 per group) were exposed to 3 months of long days (16L:8D). Group 1 was then exposed to a regime of continuous short days (8L:16D) and Groups 2, 3, and 4 were exposed to 4 months of short days alternated with 1, 2, or 4 months, respectively, of long days. Every 2 weeks, serum hormone levels and scrotal circumference were determined and semen quality was evaluated. Regular cycles in pituitary and testicular activities corresponding to the period of the lighting regime resulted in Groups 2, 3, and 4, but not in Group 1. In general, the change from long days to short days induced increases in follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone levels, scrotal size and sperm numbers and a decrease in prolactin. The reverse occurred after subsequent exposure to long days. After 4 months of long days, testicular regression was complete, but when long-day exposure was reduced, less regression occurred. With continuous exposure to short days, FSH and testosterone remained above basal levels, prolactin levels were depressed, scrotal size remained near the maximum, and elevated numbers of motile sperm were sustained.     

Neuroendocrine changes in male hamsters following photostimulation

Transfer of gonadally regressed male golden hamsters from a short (5 L:19 D) to a stimulatory (14 L:10 D) photoperiod elicits, within 24 hr, significant changes in hypothalamic dopamine, serotonin, and possibly norepinephrine metabolism. Hypothalamic LHRH content was significantly elevated in short-photoperiod animals, but within 24 hr of transfer to a 14:10 photoperiod, LHRH declined to levels not different from those in hamsters maintained continuously in a long photoperiod. Plasma FSH levels were also significantly elevated within 24 hr of transfer, but increases in plasma LH were somewhat slower. Chronic treatment with the tyrosine hydroxylase inhibitor, alpha-methyl tyrosine (alpha MPT), which inhibits catecholamine synthesis, blocked the effect of a stimulatory photoperiod on plasma FSH levels, while treatment of 5:19 hamsters with the catecholamine precursor, L-dopa, mimicked the effects of photostimulation on plasma FSH levels. Testicular weights were not affected by alpha MPT or L-dopa treatment for 1 week. From these data, it appears that endocrine events associated with photoperiod-induced testicular recrudescence are under the control of hypothalamic neurotransmitters.     

Lighting conditions affect testosterone feedback sensitivity in castrated rats

It has been shown in the Syrian hamster that a short photoperiod sensitizes the hypothalamo-hypophyseal axis of castrated animals to the negative feedback effect of testosterone. There is some evidence that even the reproductive system of the rat, which is generally considered not to be very sensitive to light, can respond to changes in illumination. Therefore, we found it of interest to examine whether alterations in lighting conditions produce changes of sensitivity in the negative feedback effect of testosterone in the rat. We kept intact, castrated, and castrated testosterone-treated animals either in periodic (L:D 12:12) or constant light for 7 days starting 4 weeks after castration. In all 3 testosterone-injected groups, serum luteinizing hormone (LH) was lower in constant than in periodic light. Exogenous testosterone did not decrease the castration-induced elevations of pituitary LH and follicle-stimulating hormone (FSH). On the contrary, testosterone increased the pituitary contents of LH and FSH, especially in constant light. We conclude that, in constant light, the hypothalamo-hypophyseal axis of the castrated rat becomes more sensitive to the negative feedback action of testosterone.     

Changes in plasma gonadotrophin and prolactin concentrations following castration of the pony stallion

Concentrations of gonadotrophins and prolactin were recorded in pony stallions castrated during the early breeding season, to examine the regulatory role of the gonad at a time when testosterone has been postulated to exert positive feedback on LH secretion. Further, gonadotrophin concentrations in geldings are reported to return to values within the normal range of the entire stallion. In an attempt to characterize this species-specific reversal, the gonadotrophin concentrations of 6 male ponies castrated on 25 March were monitored for 4 months, and 4 stallions were used to generate control data. Blood samples were collected daily, from 3 d before to 10 d after castration (Day 0), and weekly thereafter until Day 122. The pituitary response to castration was immediate. Castration resulted in a previously unreported, dramatic (13-fold) but transient (3 d) surge in circulating concentrations of LH. Concentrations of LH and FSH increased in a logarithmically scaled (LH, R2 = 0.77; FSH, R2 = 0.93) manner over the subsequent 5 wk, during which temporal changes in concentrations of both hormones were strongly correlated (R2 = 0.97). The ratio of plasma gonadotrophin concentrations was consistent throughout (LH:FSH, 1.43 +/- 0.04). Maximal concentrations of LH (20.58 +/- 1.97 ng/mL, Day 34.8 +/- 3.2) were attained approximately 2 wk before the peak in FSH (16.99 +/- 1.97 ng/mL, Day 49.7 +/- 3.0). Plasma gonadotrophin concentrations exceeded those of entire stallions throughout the study. The equine testes inhibited LH secretion during the early breeding season, and no chronic decrease in plasma gonadotrophin concentrations was recorded. However, the LH surge evident for 3 d immediately afer castration, may be related to the dynamic seasonal interaction between gonadal steroids and the regulation of pituitary gonadotrophin release.     

Interrelationships of porcine X and Y chromosomes with pituitary gonadotropins and testicular size

Endocrine and testicular responses to unilateral castration on 1, 10, 56, or 112 days of age were characterized in 132 Chinese Meishan (MS) x White composite (WC) crossbred boars in which testicular size associates with a quantitative trait locus (QTL) on X chromosome. At 220 days of age, testicles of boars unilaterally castrated on Day 1 or 10 weighed more and had greater total daily sperm production (DSP) than one testicle of bilaterally intact boars (P < 0.05); compensation did not double these two responses. Boars with MS alleles at the X chromosome QTL had smaller testicles, darker colored parenchyma, and lower total DSP than boars with WC alleles (P < 0.05). The MS alleles engendered greater (P < 0.05) plasma FSH and LH during puberty than WC alleles. Plasma FSH increased (P < 0.05) within 48 h of unilateral castration on Days 1, 10, and 56. Subsequent increases occurred earlier during puberty (P < 0.05) after unilateral castration at younger ages than after unilateral castration at older ages. Pubertal increases in plasma FSH and LH were greater (P < 0.05) in boars with MS alleles than in those with WC alleles for the X chromosome QTL. Breed of Y chromosome had no effect on testicular traits, FSH, testosterone, or estrone. For LH, boars with an MS Y chromosome had greater (P < 0.01) plasma LH across all ages than boars with a WC Y chromosome. We conclude that a gene or groups of genes that reside on the porcine X chromosome regulate testicular development and pubertal gonadotropin concentrations.     

LH and testosterone responses to GnRH in red deer (Cervus elaphus) stags kept in a manipulated photoperiod

Six red deer stags from age 4 months were kept in a light-proof room under an artificial photoperiod consisting of 5.5 cycles of alternate 2-month periods of 16 h light and 8 h dark (16L:8D) and 8L:16D. At 2 or 3 weekly intervals from 10 months of age through 4 cycles, the stags were anaesthetized with xylazine and challenged i.v. with 10 micrograms GnRH. Blood samples were withdrawn immediately before and 10 and 60 min after injection. LH and testosterone concentrations were measured in all samples by RIA. Antler status was recorded daily. Peak LH values on each sampling day occurred in the sample taken 10 min after GnRH stimulation while peak testosterone occurred in the sample taken at 60 min. There were 4 cycles of LH and testosterone secretion accompanied by 4 antler cycles in the stags. The highest LH responses were detected during short days (8L:16D), and the highest testosterone responses were detected around the time of the change from short to long days. The responses of both hormones were lowest at the end of periods of long days or the beginning of short days. The increased pituitary LH response to GnRH was evident 4 weeks after the change to short days which are stimulatory for gonadal development. Antler casting occurred at the end of long days and cleaning at the end of short days. It is considered that antler cycles were due to the ability of the stags to vary release of LH and testosterone in response to changes in the artificial photoperiod.