Disruption of pubertal onset by exogenous testosterone and estrogen in two species of rodents

The administration of adult physiological levels of testosterone (T) and 17beta-estradiol (E(2)) to male Siberian hamsters was previously shown to delay the onset of puberty. To examine whether this is a response common to other rodents, we investigated whether exogenous steroids also alter the onset of puberty in Syrian hamsters and mice. Juvenile male Syrian hamsters and mice were implanted with Silastic capsules containing T, E(2), or cholesterol control. After 15 days, plasma, pituitaries, and testes were processed for histological analysis or measurements of gonadotropins and circulating steroid hormones. T and E(2) implants reduced testis mass and gonadotropin stores in both species and arrested spermatogenesis in Syrian hamsters. In contrast, spermatogenesis in mice was unaffected by T and only modestly affected by E(2). Although E(2) inhibited circulating follicle-stimulating hormone (FSH) in both species, T inhibited circulating FSH in mice only. Overall, our results demonstrate that the hypothalamic-pituitary-gonadal axis of each rodent species responds uniquely to T and E(2) during the pubertal transition. Despite the highly varied effects of T and E(2) in these two species, the ability of steroid hormones to disrupt the onset of puberty appears to be a feature common to many rodents.     

Steroidal modulation of in vitro gonadotropin secretion in prepubertal and adult male Siberian hamsters

The effects of exogenous gonadal steroids, testosterone (T), and 17beta-estradiol (E(2)) upon the hypothalamo-pituitary-gonadal axis were reported to be different between prepubertal and adult Siberian hamsters. Utilizing an in vitro static culture system, we investigated if age-related differences in steroid responsiveness occurs at the pituitary. Prepubertal (20 days old) or adult (140 days old) male Siberian hamsters were implanted with 1 mm silastic capsules containing undiluted T, E(2) or cholesterol (Ch, control). After 15 days, pituitaries were removed, incubated in vitro, and subjected to the following treatments: two baseline measurements, one challenge with 10ng/ml of D-Lys(6)-gonadotropin-releasing hormone (GnRH), and three post-challenge washes. Fractions were collected every 30 minutes and measured for follicle-stimulating hormone (FSH) and luteinizing hormone (LH). T and E(2 )reduced basal secretion of LH and FSH in juveniles but not adults. In juveniles, E(2) increased GnRH-induced FSH and LH secretion, while T augmented GnRH-induced FSH secretion but attenuated GnRH-induced LH secretion. Steroid treatment had no effect on GnRH-stimulated LH or FSH release in adults. The only effect of steroid hormones upon adult pituitaries was the more rapid return of gonadotropin secretion to baseline levels following a GnRH challenge. These data suggest both basal and GnRH-induced gonadotropin secretion are more sensitive to steroid treatment in juvenile hamsters than adults. Further, differential steroidal regulation of FSH and LH at the level of the pituitary in juveniles might be a mechanism for the change in sensitivity to the negative effects of steroid hormones that occurs during the pubertal transition.     

Short day lengths affect perinatal development of the male reproductive system in the Siberian hamster, Phodopus sungorus.

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, seasonal breeding can be controlled by photoperiod, which affects the duration of nightly melatonin secretion. Winterlike, short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo pubertal gonadal development later than animals born into long days. However, to date there have been no reports of gestational photoperiod affecting fetal development of reproductive systems. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), compose a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8 h light, 16 h dark; 8L:16D) versus long-day (16L:8D) conditions. On the day of birth, and at postnatal (PN) days 2 and 18, the BC/LA muscles of hamsters gestated and raised in the short photoperiod were significantly reduced relative to those of their long-day counterparts. Testes weights were not significantly different between groups until day 18. Thus, photoperiod exposure during gestation and after birth affects perinatal development of the SNB system in this species, and these effects can be seen as early as the day of birth. Because photoperiod did not significantly affect testes weights until PN18, these results suggest that either perinatal photoperiod affects fetal androgen production without affecting testes weight or it influences BC/LA development independently from androgen.     

Effects of a circadian mutation on seasonality in Syrian hamsters (Mesocricetus auratus).

In Syrian hamsters, exposure to short photoperiods or constant darkness induces a decrease in gonadotrophin secretion and gonadal regression. After 10-12 weeks, animals undergo spontaneous gonadal reactivation, gonadotrophin concentrations rise, and in males, testes size increases and spermatogenesis resumes. The tau mutation shortens the period of circadian wheel-running activity by 4 h in the homozygote. Here, we examine the impact of this mutation on the reproductive response to photoperiod change. Seventeen adult tau mutant and nine adult wild-type males were housed in complete darkness for 25 weeks and testes size determined at weekly intervals. Gonadal regression and subsequent recrudescence occurred in both groups of animals. Regression occurred more rapidly in tau mutants, with a nadir significantly earlier than wild-types but after a similar number of circadian cycles. Rates of testicular recrudescence were similar in both groups. Our data suggest that an acceleration of the circadian period increases the rate of reproductive inhibition in animals exposed to inhibitory photoperiods. Once initiated, the rate of spontaneous reactivation may be independent of the circadian axis.     

Effects of age, photoperiod and follicle-stimulating hormone on lactate production by cultured Sertoli cells from prepubertal Siberian hamsters (Phodopus sungorus).

To define a functional difference in Sertoli cells of animals exposed to different photoperiodic conditions, we isolated Sertoli cells from the testes of juvenile Siberian hamsters and cultured them in serum-free medium. In all age groups studied, Sertoli cells isolated from hamsters with delayed and normal puberty responded to follicle-stimulating hormone (FSH) with an increase in lactate production. The increase in lactate production induced by 1000 ng FSH ml-1 was significantly greater in Sertoli cells isolated from hamsters with delayed puberty than in those with normal puberty. These results suggest that Sertoli cells of Siberian hamsters exposed to short photoperiod in vivo may respond to increases in plasma FSH concentrations associated with photostimulation or spontaneous sexual maturation by an increase in secretory activity that may be critical for the initiation of spermatogenesis.     

Alteration of the hypothalamic-pituitary-gonadal axis in estrogen- and androgen-treated adult male leopard frog, <Emphasis Type="Italic">Rana pipiens</Emphasis>

Background  

Gonadal steroids, in particular 5 alpha-dihydrotestosterone (DHT) and 17 beta-estradiol (E2), have been shown to feed back on the hypothalamic-pituitary-gonadal (HPG) axis of the ranid frog. However, questions still remain on how DHT and E2 impact two of the less-studied components of the ranid HPG axis, the hypothalamus and the gonad, and if the feedback effects are consistently negative. Thus, the goal of the study was to examine the effects of DHT and E2 upon the HPG axis of the gonadally-intact, sexually mature male leopard frogs, Rana pipiens.     

Photoperiodic and hormonal influences on fur density and regrowth in two hamster species

Temperate and boreal mammals undergo seasonal changes in pelage that facilitate thermoregulation in winter and summer. We investigated photoperiodic influences on pelage characteristics of male Siberian and Syrian hamsters. Fur density (mg fur/cm2 skin) was measured by weighing the shavings of fur patches removed from the dorsal and ventral surfaces of hamsters maintained in long days (LDs) or transferred to short days (SDs). Patches were reshaved 3 wk later to assess fur regrowth (mg regrown fur/cm2 skin). Fur density was greater in SD than in LD Siberian hamsters after 11 wk of differential phototreatment. The onset of increased fur density in SDs was accompanied by a transient increase in fur regrowth (11-14 wk on the dorsal surface and 7-10 and 11-14 wk on the ventral surface), suggestive of a seasonal molting process. Fur density, body mass, and pelage color of Siberian hamsters returned to values characteristic of LD males after a similar duration of prolonged (>27 wk) SD treatment and appear to be regulated by a similar or common interval-timing mechanism. In Syrian hamsters, dorsal fur density, fur regrowth, and hair lengths were greater in SD than in LD males. Castration increased and testosterone (T) treatment decreased dorsal and ventral fur regrowth in LD and SD hamsters, but the effects of T manipulations on fur density were limited to a decrease in dorsal fur density after T treatment. Decreased circulating T in SDs likely contributes to the seasonal molt of male hamsters by increasing the rate of fur growth during the transition to the winter pelage.     

Photoperiodic regulation of compensatory testicular hypertrophy in hamsters

In mammals, removal of one testis results in compensatory testicular hypertrophy (CTH) of the remaining gonad. Although CTH is ubiquitous among juveniles of many species, laboratory rats, laboratory mice, and humans unilaterally castrated in adulthood fail to display CTH. We documented CTH in pre- and postpubertally hemi-castrated Syrian and Siberian hamsters and tested whether day length affects CTH in juvenile and adult Siberian hamsters. Robust CTH was evident in long-day hemi-castrates of both species and was preceded by increased serum FSH concentrations in juvenile Siberian hamsters. In sharp contrast, CTH was undetectable in short-day hemi-castrated Siberian hamsters for several months and only made its appearance with the development of neuroendocrine refractoriness to short day lengths; serum FSH concentrations of juveniles also did not increase above sham-castrate values until the onset of refractoriness. Long-day hemi-castrated Siberian hamsters with hypertrophied testes underwent complete gonadal regression after transfer to short days, albeit at a reduced rate for the first 3 weeks of treatment. Blood testosterone concentrations of adult hamsters did not differ between long-day hemicastrates and sham-castrates 9-12 weeks after surgery. We conclude that CTH is suppressed by short day lengths in Siberian hamsters at all ages and stages of reproductive development; in short day lengths, but not long day lengths, the remaining testis produces sufficient negative feedback inhibition to restrain FSH hypersecretion and prevent CTH.     

Low ambient temperature accelerates short-day responses in Siberian hamsters by altering responsiveness to melatonin

Exposure to low ambient temperatures (Ta) accelerates appearance of the winter phenotype in Siberian hamsters transferred from long to short day lengths. Because melatonin transduces the effects of day length on the neuroendocrine axis, the authors assessed whether low Ta promotes the transition to winterlike traits by accelerating the onset of increased nocturnal melatonin secretion or by enhancing responsiveness to melatonin in short day lengths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h light/day) photoperiods and held at 5 degrees C or 22 degrees C. Locomotor activity was recorded continuously, and body mass, testis size, and pelage color were determined biweekly for 8 weeks. The duration of nocturnal locomotion (alpha), a reliable indicator of the duration of nocturnal melatonin secretion, lengthened significantly earlier in hamsters exposed to a Ta of 5 degrees C than 22 degrees C. Cold exposure increased the proportion of hamsters that were photoresponsive: gonadal regression in short days increased from 44% at 22 degrees C to 81% at 5 degrees C (p < 0.05); low Ta did not, however, accelerate testicular regression in animals that were photoresponsive. Nonphotoresponsive animals at 5 degrees C temporarily had longer alphas during the first 4 weeks in short days and significant decreases in body mass and testicular size that were reversed during the ensuing weeks when alpha decreased. In a 2nd experiment, pinealectomized male hamsters infused for 10 h/day with melatonin for 2 weeks had significantly lower body and testes masses when maintained at 5 degrees C but not 22 degrees C. Low-ambient temperature appears to accelerate the appearance of the winter phenotype primarily by increasing target tissue responsiveness to melatonin and to a lesser extent by augmenting the rate at which the duration of nocturnal melatonin secretion increases in short day lengths.     

A plastic interval timer synchronizes pubertal development of summer- and fall-born hamsters

Summer and fall decreases in day length induce reproductive regression in adult hamsters and delay reproductive maturation of their young. The following year pubertal development is triggered by an interval timer (IT) that renders animals refractory to inhibitory short day lengths after approximately 25 wk. Timing of gonadal and somatic development was examined among offspring born to Siberian hamsters in early-August vs. late-September day lengths. Pubertal maturation was delayed in both groups until late winter. Gonadal growth occurred at significantly later ages among August- vs. September-born males as did late-winter spurts in ponderal growth of both sexes. Timing of reproductive and somatic development depended on postnatal rather than prenatal photoperiod exposure and was unrelated to the circadian entrainment status of dams. When developmental patterns were assessed in relation to time of year, group differences were largely eliminated. Because the IT triggers these developmental events, its duration must be plastic. This plasticity facilitates a relative synchronization or entrainment of developmental milestones in hamsters born into different late-summer/early-fall photoperiods.     

Exogenous T3 mimics long day lengths in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit seasonal cycles of reproduction driven by changes in day length. Day length is encoded endogenously by the duration of nocturnal melatonin (Mel) secretion from the pineal gland. Short-duration Mel signals stimulate reproduction and long-duration signals inhibit reproduction. The mechanism by which Mel signals are decoded at the level of neural target tissues remains uncharacterized. In Siberian hamsters, exposure to short day lengths or injections of Mel in long days results in a decrease in hypothalamic expression of type 2 iodothyronine deiodinase (Dio2) mRNA. Dio2 catalyzes the conversion of the thyroid hormone thyroxine to triiodothyronine (T3). Thus exposure to short and long day lengths should decrease and increase hypothalamic T3 concentrations, respectively. We tested the hypothesis that exogenous T3 administered to short-day hamsters would mimic exposure to long day lengths with respect to gonadal stimulation. Hamsters gestated and raised in short day lengths that exhibited photoinhibition of the testes were given daily subutaneous injections of T3 or saline vehicle for 4 wk beginning at week 12 of life. The results indicate that exogenous T3 induced gonadal growth in short-day hamsters and delayed spontaneous gonadal development by an interval equal to the number of weeks during which T3 was administered. T3 injections delayed gonadal regression if given coincident with the transfer of hamsters from long to short day lengths. These results suggest that T3 mimics long day exposure in Siberian hamsters and may serve as an intermediate step between the Mel rhythm and the reproductive response.     

Social cues attenuate photoresponsiveness of the male reproductive system in Siberian hamsters (Phodopus sungorus)

Transfer of adult Siberian hamsters (Phodopus sungorus) from long (16 h light and 8 h dark, 16L:8D) to short (8L:16D) daily photoperiods induces an involution of the gonads and a cessation of reproductive behavior 8 to 10 weeks later. However, when male and female long-day hamsters were paired on transfer to short photoperiods, the males' gonads did not undergo the typical short-day response. Similarly, when male long-day hamsters were paired with refractory females (i.e., females housed in short photoperiods for at least 28 weeks so that they became unresponsive to short photoperiods), the response of the males' reproductive system to short photoperiods also was attenuated. Thus, social cues can override or delay the effects of photoperiod on the testes of this species. These results suggest that the inhibitory effects of long durations of melatonin secretion (in response to short photoperiods) on the male hypothalamic-pituitary-gonadal axis may be attenuated by social cues such as contact with the opposite sex.     

Juvenile Siberian hamsters display torpor and modified locomotor activity and body temperature rhythms in response to reduced food availability

Siberian hamsters as young as 16 and 28 d displayed torpor in response to treatment with 2,500 mg/kg 2-deoxy-D-glucose and reduced food availability, respectively. In addition, most food-restricted hamsters displayed increased locomotor activity and elevated body temperatures in the 3 h immediately preceding daily food delivery. This anticipatory activity disappeared within a few days of reimposition of ad lib. feeding. Torpor first appeared spontaneously at approximately 13 wk of age in hamsters fed ad lib. and maintained in short day lengths. The onset of this "spontaneous" torpor was unaffected by the hamsters' history of food restriction before age 2 mo. Siberian hamsters born late in the breeding season can conserve energy by undergoing torpor immediately after weaning when they contend with food shortages and concurrent energetic challenges imposed by growth requirements and low ambient temperatures.     

Timing of testicular recrudescence in siberian hamsters is unaffected by pinealectomy or long-day photoperiod after 9 weeks in short days

In this study, the authors asked whether pinealectomy or temporary exposure to a stimulatory photoperiod affects the timing of spontaneous testicular recrudescence in adult Siberian hamsters chronically exposed to short days (9:15 light:dark). In Experiment 1, hamsters were pinealectomized after 6, 9, or 12 weeks in short days. Pinealectomy after 9 or 12 weeks did not affect the timing of spontaneous gonadal growth (27.7 +/- 1.9 and 25.4 +/- 1.3 weeks, respectively) compared to sham-operated controls (28.6 +/- 0.9 weeks). Enlarged testes occurred earlier in animals that were pinealectomized after 6 weeks in short days (21.8 +/- 2.1 weeks). In Experiment 2, adult hamsters were exposed to short days for 9 weeks, transferred to long days (16:8 light:dark) for 4 weeks, and then returned to short days for 23 additional weeks. Although long-day interruption caused gonadal growth in 15 out of 19 hamsters, the temporary long-day exposure did not affect the timing of spontaneous gonadal growth following return to short days (28.2 +/- 0.9 weeks) in 10 of the 15, relative to the timing observed in control hamsters continuously maintained in short days (28.2 +/- 1.1 weeks). Four out of 19 hamsters did not show gonadal growth following long-day exposure. Spontaneous gonadal growth in these hamsters (28.0 +/- 1.4 weeks) also occurred at the same time as controls. The remaining 5 hamsters exhibited enlarged testes following long-day exposure (12.0 +/- 0.0 weeks) but were refractory to the second short-day exposure. All hamsters exhibited entrainment of wheel-running activity following the change in photoperiod. A final group of 13 animals were pinealectomized before long-day transfer. They exhibited gonadal growth (at 17.2 +/- 0.8 weeks) but failed to regress a second time when returned to short days. The timing of gonadal growth in these animals was delayed relative to the sham-operated hamsters temporarily transferred to long days (Experiment 2) but accelerated relative to the hamsters pinealectomized at 9 weeks, which remained continuously in short days (Experiment 1). The results of both experiments suggest that a pineal-independent process mediates the timing of spontaneous gonadal growth in Siberian hamsters chronically exposed to a short-day photoperiod.     

Variation in angiotensin-converting enzyme activity in the testes of prepubertal, pubertal and adult cockerels

The variation in the level of angiotensin-converting enzyme in the testes of prepubertal, pubertal and adult cockerels were investigated. Maximum levels of angiotensin-converting enzyme were observed at 8 wk of age, followed by a progressive decrease (P / 0.01) until 14 wk of age. After this stage, however, a rapid growth rate of the testes and comb was noted. The concentration of angiotensin-converting enzyme in pubertal and adult birds remained at a significantly lower level than in the prepubertal birds. The level of activity of this enzyme was significantly but negatively correlated with age and with the weights of the testes and comb. The high level of this enzyme found in prepubertal cockerels suggests it may be more highly involved in the early rather than later stages of spermatogenesis.     

Precocious puberty: growth and genetics

The precise neuroendocrine mechanisms underlying activation of hypothalamic-pituitary-gonadal (HPG) axis maturation are elusive. The wide age range of pubertal onset among normal individuals throughout the world may suggest that both genetic and environmental factors modulate the timing of puberty. Early activation of the HPG axis, termed central precocious puberty (CPP), causes psychosocial difficulties and may lead to compromised final height, especially if medical intervention is delayed. Although CPP is considered to be idiopathic in the majority of patients, we have recently reported a 27.5% prevalence of familial cases among 147 patients with idiopathic CPP. Segregation analysis of this cohort suggested an autosomal dominant transmission with incomplete sex-dependent penetrance. Allelic variants of candidate genes that regulate the timing of puberty may cause familial CPP. Detection of these genes will provide a tool for identification of children at risk of developing CPP, enabling early intervention with the aim of preventing its distressing outcomes.     

The effect of nutrition on sexual development of bulls

Most bulls that are managed for sale as yearlings are fed high-energy diets in the post-weaning period to maximize rates of gain in body weight. High-energy diets with adequate protein, vitamins and minerals result in a larger scrotal circumference at 1 y of age, however, part of this increase in size is likely due to scrotal fat. It is unclear whether testis size and spermatogenesis is significantly affected by nutritional intake in the post-weaning period. There are indications of an effect of calfhood nutrition on age at puberty and testis size. Scrotal circumference was smaller in yearling bulls raised by first-parity dams, compared to those raised by older dams. This may have been due to lower milk production by first-parity dams, an in utero effect, or both. The effect of reduced calfhood nutrition may be mediated through gonadotropin secretion. Calves destined to become later maturing bulls with smaller testes had lower amounts of LH secretion during the period of the early gonadotropin rise (8-16 wk of age). Furthermore, augmenting circulating LH concentrations at this time by treating calves with GnRH hastened pubertal development. In addition, FSH treatments in calfhood also increased scrotal circumference and hastened spermatogenesis. In that regard, FSH has been considered a main driver of Sertoli cell proliferation in prepubertal animals. Since Sertoli cell multiplication ceases at 20-25 wk of age in bulls, final testis size in bulls is likely determined in calfhood. Four experiments were done to investigate the effects of calfhood nutrition on pubertal development. These studies confirmed that superior calfhood nutrition augmented gonadotropin secretion (which is probably mediated by metabolic hormones); this resulted in larger testes at 1 y of age and an earlier onset of spermatogenesis.     

Delayed reproductive regression in male hamsters bearing intrarenal pituitary homografts and kept under natural winter photoperiods.

The testes and accessory sex organs of adult male hamsters regressed to an infantile state when the animals were maintained under natural photoperiodic and temperature conditions beginning November 13. The atrophy was evident after both five and ten weeks exposure to these conditions. Either one or two prolactin-secreting pituitary grafts placed under the kidney capsule greatly delayed the testicular and accessory organ regression. Animals bearing pituitary grafts also had higher circulating levels of immunoreactive prolactin while plasma luteinizing hormone values were similar in all groups of hamsters.     

The influence of short photoperiod on testicular and circulating levels of testosterone precursors in the adult golden hamster

The in vivo effects of short photoperiod (SPP, 6L:18D) for 8 and 12 wk on plasma and testicular levels of testosterone (T) precursors in adult golden hamsters were evaluated. Plasma and testicular progesterone (P), 17 alpha-hydroxyprogesterone (17 alpha-OHP), androstenedione (A-dione), and T were measured after 5 injections of saline or human chorionic gonadotropin (hCG) (5 or 25 IU/day). The basal levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL) in circulation were also determined. There were significant reductions in the weight of the testes in animals exposed to SPP. After 12 wk in SPP, circulating levels and testicular content of 17 alpha-OHP, A-dione, and T were significantly reduced, suggesting that the decrease in T secretion may be associated with the impairment of synthesis and/or action of 17 alpha-steroid hydroxylase, C17-20 steroid lyase, and 17 beta-hydroxysteroid dehydrogenase enzymes in the testes. Exposure to SPP for 8 wk resulted in decreased plasma and testicular content of T. Although there were reductions in testicular content of 17 alpha-OHP and A-dione, this was not reflected in plasma levels of these steroids. After 8 and 12 wk of exposure to SPP, hCG treatment increased the total amounts of T precursors (except P at 8 wk) in the testes, but the values attained in animals exposed to 12 wk of SPP remained below those observed in hamsters kept in a long photoperiod (14L:10D), suggesting that gonadotropin replacement alone may be insufficient to normalize testicular steroidogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)