Maturation of the hypothalamo-pituitary-gonadal axis of male Syrian hamsters.

Light-microscope immunocytochemistry (ICC) was used to investigate postnatal changes in the morphology of LHRH neurons in the brains of male Syrian hamsters and to relate these changes to more overt maturational developments within the hypothalamo-pituitary-gonadal axis. The animals were maintained under long-day photoperiods (14L:10D), and groups of 6-7 were killed at 10-day intervals from Day 15 to Day 65. Their brains were fixed with 4% paraformaldehyde, sectioned sagittally with a vibratome (75 microns), and processed for ICC using monoclonal LHRH antibody HU4H. Throughout the study period, the hamsters showed a progressive increase in plasma gonadotropin levels, closely followed by an increase in testicular weight and plasma testosterone levels. Histology of the testes revealed that spermatogenesis was already qualitatively completed by Day 35 and quantitative aspects were established by Day 45. Within the brain, LHRH neuronal perikarya were distributed primarily in the medial septal-preoptic area and the diagonal band of Broca; morphologically, these immunopositive neurons were either monopolar or bipolar. The total number of LHRH neurons detected in the areas examined was approximately 440 throughout the developmental period, and the relative proportions of monopolar and bipolar subtypes (86% and 14%, respectively) remained unchanged. In contrast, the area of the perikarya, as determined by autoimage analysis, showed a highly significant age-related increase, both for the monopolar and bipolar neurons. It is suggested that these developmental changes in the LHRH neurons reflect an increase in LHRH synthesis and may, therefore, provide a neuroendocrine trigger for the onset of puberty.     

Anabolic androgenic steroids differentially affect social behaviors in adolescent and adult male Syrian hamsters

Anabolic androgenic steroids (AAS) are synthetic derivatives of testosterone used by over half a million adolescents in the United States for their tissue-building potency and performance-enhancing effects. AAS also affect behavior, including reports of heightened aggression and changes in sexual libido. The expression of sexual and aggressive behaviors is a function of complex interactions among hormones, social context, and the brain, which is extensively remodeled during adolescence. Thus, AAS may have different consequences on behavior during adolescence and adulthood. Using a rodent model, these studies directly compared the effects of AAS on the expression of male sexual and aggressive behaviors in adolescents and adults. Male Syrian hamsters were injected daily for 14 days with either vehicle or an AAS cocktail containing testosterone cypionate (2 mg/kg), nandrolone decanoate (2 mg/kg), and boldenone undecylenate (1 mg/kg), either during adolescence (27-41 days of age) or in adulthood (63-77 days of age). The day after the last injection, males were tested for either sexual behavior with a receptive female or agonistic behavior with a male intruder. Adolescent males treated with AAS showed significant increases in sexual and aggressive behaviors relative to vehicle-treated adolescents. In contrast, AAS-treated adults showed significantly lower levels of sexual behavior compared with vehicle-treated adults and did not show heightened aggression. Thus, adolescents, but not adults, displayed significantly higher behavioral responses to AAS, suggesting that the still-developing adolescent brain is more vulnerable than the adult brain to the adverse consequences of AAS on the nervous system and behavior.     

Short duration testosterone infusions maintain male sex behavior in Syrian hamsters

In most mammalian species, reduced androgen availability is associated with marked reductions in male sexuality; conversely, androgen replacement in castrated males restores sex behavior within a few weeks. Testosterone (T) pulse duration, amplitude, frequency, and inter-pulse interval may be as important as total amount of hormone in determining target tissue responsiveness. We remain ignorant of the number and duration of daily T pulses necessary and sufficient to sustain male mating behavior. An in-dwelling infusion system was employed to vary T-pulse frequencies and durations. Daily 4 h infusions of aqueous T (100 microg/0.064 ml) and twice daily 4 h pulses of T (each 50 microg/0.064 ml) were sufficient to maintain ejaculatory behavior of sexually experienced castrated hamsters for 11 weeks post-castration; castrated hamsters infused with vehicle ceased to display the ejaculatory pattern 3 weeks after gonadectomy. Circulating T concentrations of hormone-infused hamsters declined markedly 7 h after the termination of each infusion. These results establish that male sex behavior can be sustained with infusions of relatively low T concentrations for 4 h/day and suggests that the basal concentrations of T sustained by the gonad during inter-pulse intervals may not be necessary for maintenance of sex behavior. 4 h T infusions were sufficient to maintain penile and seminal vesicles weights, but not ventral prostate weights or flank gland dimensions; the threshold for maintaining male sex behavior is lower than that for some androgen-dependent peripheral structures. Development of effective androgen replacement regimens that sustain sex behavior in castrated animals may be useful in the design of androgen replacement therapy for hypogonadal men.     

Infrequent low dose testosterone treatment maintains male sexual behavior in Syrian hamsters

Testosterone (T) secreted in short pulses several times each day is essential for the maintenance of male sex behavior (MSB) in mammals. Blood T concentrations are relatively low during inter-pulse intervals. Assessment of androgenic influences on MSB of rodents has, with very few exceptions, involved either injections of pure or esterified hormones dissolved in oil or implantation of constant release capsules that generate supraphysiological and/or constantly elevated T concentrations. The minimum daily concentration of T necessary to maintain and restore MSB when T is delivered as a discrete short pulse remains unspecified; nor is it known whether infrequent T pulses in the physiological range sustain MSB. To address these questions, we varied T injection concentrations and frequencies in castrated, sexually-experienced Syrian hamsters. All males injected daily with an aqueous vehicle failed to display the ejaculatory reflex 5 weeks after castration. Once daily 15 μg subcutaneous T injections both maintained and restored MSB, whereas once daily 5 μg T injections resulted in fewer males ejaculating and longer ejaculation latencies. Substantially higher T doses were required to restore MSB in previous studies when T was administered in an oil vehicle. 50 μg T maintained MSB in most hamsters injected once every 4 or 7 days, despite long intervals between injections during which circulating T was undetectable or well below physiological concentrations. Some T regimens that maintained MSB were associated with subnormal seminal vesicle and ventral prostate weights. The demonstration that relatively brief, infrequent elevations of T are sufficient to support MSB provides a useful model to assess the neuroendocrine basis of MSB and raises the possibility that infrequent low dose androgen replacement protocols may restore sex behavior to hypogonadal men without inducing some of the negative side-effects associated with more frequent, higher dose treatments.     

Photoperiodic influences on sexual behavior in male Syrian hamsters

The effect of photoperiodic conditions on sexual behavior was investigated in male Syrian hamsters that were either gonadally intact, or castrated and treated with low doses of testosterone throughout the experiment. Hamsters were exposed to long (LD 16:8) or short (LD 8:16) days for 7 weeks; for the next 8 weeks, either they were exposed to an intermediate daylength (LD 12:12), or daylength conditions remained unchanged. Sexual behavior was affected by photoperiod conditions in both gonadally intact animals and testosterone-treated castrates, but to different degrees. Intact males exposed to short days for 15 weeks exhibited gonadal regression, and their copulatory performance was impaired. The percentage of animals that intromitted or ejaculated was significantly reduced. Additional measures of sexual performance among the copulating males were also affected. In contrast, among the castrates with testosterone clamped at low but stable levels, the proportion of males that mounted, intromitted, or ejaculated was not affected by photoperiod. However, among the males that continued to copulate, sexual performance changes were present in the short-day castrates that resembled those displayed by the intact males. We infer that these behavioral effects in both hormonal conditions reflect primarily a difficulty in the attainment of intromission. Gonadal regression alone cannot easily account for the behavioral deficits of the intact males, because circulating testosterone levels at the end of the experiment were not significantly different between the gonadally intact hamsters and the castrated, testosterone-treated hamsters exposed continuously to short days. Males transferred from either long or short days to the intermediate-daylength condition responded behaviorally to this photoperiod as if it were a short day, that is, their ejaculatory frequency declined. We conclude that male hamsters exposed to photoinhibitory daylengths exhibit deficits in their sexual behavior, not only because endogenous levels of testosterone decrease, but also because the substrates on which this hormone acts become less responsive. We hypothesize that under physiological conditions, the episodic secretion of testosterone imposes constraints on the maintenance or restoration of copulation, and that the potent behavioral effects achieved by constant-release implants of testosterone may mask the presence of photoperiodically induced alterations in the hamster's sensitivity to this gonadal hormone.     

Conversion of testosterone to estradiol may not be necessary for the expression of mating behavior in male Syrian hamsters (Mesocricetus auratus)

Male sexual behavior is mediated in part by androgens, but in several species, mating is also influenced by estradiol formed locally in the brain by the aromatization of testosterone. The role of testosterone aromatization in the copulatory behavior of male Syrian hamsters is unclear because prior studies are equivocal. Therefore, the present study tested whether blocking the conversion of testosterone to estradiol would inhibit male hamster sexual behavior. Chronic systemic administration of the nonsteroidal aromatase inhibitor Fadrozole (2.0 mg/kg/day) for 5 or 8 weeks did not significantly increase mount latency or reduce mount frequency, intromission frequency, ejaculation frequency, or anogenital investigation relative to levels shown by surgical controls. However, Fadrozole effectively inhibited aromatase activity, as evidenced by the suppression of estrogen-dependent progesterone receptor immunoreactivity in the male hamster brain. The JZB39 anti-progesterone receptor antibody labeled significantly more neurons in brains of sham-treated hamsters than in brains of Fadrozole-treated hamsters. These data suggest that aromatization of testosterone to estradiol is not necessary for normal mating behavior in Syrian hamsters.     

Cell proliferation and survival in the mating circuit of adult male hamsters: effects of testosterone and sexual behavior

The transient actions of gonadal steroids on the adult brain facilitate social behaviors, including reproduction. In male rodents, testosterone acts in the posterior medial amygdala (MeP) and medial preoptic area (MPOA) to promote mating. Adult neurogenesis occurs in both regions. The current study determined if testosterone and/or sexual behavior promote cell proliferation and survival in MeP and MPOA. Two experiments were conducted using the thymidine analog BrdU. First, gonad-intact and castrated male hamsters (n = 6/group) were compared 24 h or 7 weeks after BrdU. In MeP, testosterone-stimulated cell proliferation 24 h after BrdU (intact: 22.8 ± 3.9 cells/mm², castrate: 13.2 ± 1.4 cells/mm²). Testosterone did not promote cell proliferation in MPOA. Seven weeks after BrdU, cell survival was sparse in both regions (MeP: 2.5 ± 0.6 and MPOA: 1.7 ± 0.2 cells/mm²), and was not enhanced by testosterone. In Experiment 2, gonad-intact sexually-experienced animals were mated weekly to determine if regular neural activation enhances cell survival 7 weeks after BrdU in MeP and MPOA. Weekly mating failed to increase cell survival in MeP (8.1 ± 1.6 vs. 9.9 ± 3.2 cells/mm²) or MPOA (3.9 ± 0.7 vs. 3.4 ± 0.3 cells/mm²). Furthermore, mating at the time of BrdU injection did not stimulate cell proliferation in MeP (8.9 ± 1.7 vs. 8.1 ± 1.6 cells/mm²) or MPOA (3.6 ± 0.5 vs. 3.9 ± 0.7 cells/mm²). Taken together, our results demonstrate a limited capacity for neurogenesis in the mating circuitry. Specifically, cell proliferation in MeP and MPOA are differentially influenced by testosterone, and the birth and survival of new cells in either region are not enhanced by reproductive activity.     

Sex difference in thermal preference of adult mice does not depend on presence of the gonads

Background

The thermoneutral zone (TNZ) is a species-specific range of ambient temperature (T _a), at which mammals can maintain a constant body temperature with the lowest metabolic rate. The TNZ for an adult mouse is between 26 and 34 °C. Interestingly, female mice prefer a higher T _a than male mice although the underlying mechanism for this sex difference is unknown. Here, we tested whether gonadal hormones are dominant factors controlling temperature preference in male and female mice.

Methods

We performed a temperature preference test in which 10-week-old gonadectomized and sham-operated male and female C57BL/6J mice were allowed to choose to reside at the thermoneutral cage of 29 °C or an experimental cage of 26, 29, or 32 °C.

Results

All mice preferred a T _a higher than 26 °C, especially in the inactive phase. Choosing between 29 and 32 °C, female mice resided more at 32 °C while male mice had no preference between the temperatures. Hence, the preferred T _a for female mice was significantly higher (0.9?±?0.2 °C) than that for male mice. However, gonadectomy did not influence the T _a preference.

Conclusions

Female mice prefer a warmer environment than male mice, a difference not affected by gonadectomy. This suggests that thermal-sensing mechanisms may be influenced by sex-specific pathways other than gonadal factors or that the thermoregulatory set point has already been determined prior to puberty.

     

Spontaneous and GnRH-induced pulsatile LH and testosterone release in pubertal, adult and aging male beagles

Baseline concentrations of LH and testosterone (T) in blood, their pulses, and LH and T response to GnRH (5mug/kg) treatment were compared in 19 sexually sound male beagles and in 2 sexually dysfunctional dogs. The intact beagles were allocated to 4 groups according to age, which ranged from pubertal 7-mo-old animals to 11-yr-old adults. Baseline concentrations of LH and T were measured every 15 min for a period of 6 h and for a further 3 h following challenge with GnRH. Both LH and T were released in a pulsatile fashion with a wide range of pulse frequency and amplitude. The time intervals between the LH and T pulses ranged from 30 to 60 min, with no significant difference between groups. However, LH concentrations were significantly higher (P<0.01) and T values were markedly lower in the 7-mo-old pubertal dogs than in the other age groups. Following GnRH administration, LH peaked within 15 to 30 min in all the animals, with a significantly higher increase occurring in the pubertal group (P < 0.05). Peak T values occurred 15 to 105 min after the LH peaks, with no clear increases occurring in the pubertal dogs. In the 2 sexually dysfunctional animals, LH levels increased following GnRH treatment; however, T values remained extremely low both before and after treatment, indicating loss of Leydig cell function.     

Leptin inhibits the reproductive axis in adult male Syrian hamsters exposed to long and short photoperiod

The aim of the study was to investigate the effects of acute leptin treatment of adult Syrian hamsters exposed to a long (LP, eugonadal males) and short photoperiod (SP, hypogonadal males). Animals were exposed to LP (L:D 14:10) or SP (L:D 10:14) for 10 weeks. Afterwards, both LP and SP hamsters were allocated to a control (SP-C, LP-C) or leptin-treated group (SP 3, SP 10, SP 30 or LP3, LP 10, LP 30). One hour before sacrifice, a single dose of leptin (3, 10 or 30 μg/kg) or vehicle was administered (i.p.) to the males. Testis weight, serum and pituitary luteinizing hormone (LH) concentrations, as well as the hypothalamic concentration of gonadotropin-releasing hormone (GnRH) were recorded. Histological analysis of the testis was performed and GnRH concentration in the culture medium of hypothalamic explants was examined. A dramatic regression of testicular weight and histological atrophy of seminiferous tubules, as well as a decrease in serum and pituitary LH concentrations were found in SP males. All doses of leptin significantly reduced serum LH levels and medium GnRH concentrations in both photoperiod groups. Pituitary LH and hypothalamic GnRH concentrations were not affected by leptin. In conclusion, we demonstrated that leptin inhibited the reproductive axis of Syrian male hamsters exposed to LP and SP and fed ad libitum.     

Beta-adrenergic blockers prevent short photoperiod-induced gonadal regression, but not melatonin-induced regression in male Syrian hamsters

Pineal melatonin synthesis is regulated by norepinephrine at beta-adrenergic receptors on pinealocytes. Melatonin released from the pineal is believed to be responsible for short photoperiod-induced gonadal regression. By blocking melatonin production at the beta-adrenergic receptor with beta-adrenergic antagonists, short photoperiod-induced gonadal regression was prevented. Propranolol or nadolol injected daily in the late afternoon prohibited short photoperiod-produced testicular regression in male Syrian hamsters. Likewise, propranolol and nadolol pellets were able to block short photoperiod-induced gonadal regression. Interestingly, in the presence of beta-adrenergic antagonists, gonadal regression was still produced by daily afternoon injections of melatonin. These results indicate that propranolol or nadolol can be used to "pharmacologically pinealectomize" hamsters and block the physiological action of the pineal.     

Differential effects of glucocorticoids on energy homeostasis in Syrian hamsters

Syrian hamsters, like many humans, increase food intake and body adiposity in response to stress. We hypothesized that glucocorticoids (cortisol and corticosterone) mediate these stress-induced effects on energy homeostasis. Because Syrian hamsters are dual secretors of cortisol and corticosterone, differential effects of each glucocorticoid on energy homeostasis were investigated. First, adrenal intact hamsters were injected with varying physiological concentrations of cortisol, corticosterone, or vehicle to emulate our previously published defeat regimens (i.e., 1 injection/day for 5 days). Neither food intake nor body weight was altered following glucocorticoid injections. Therefore, we investigated the effect of sustained glucocorticoid exposure on energy homeostasis. This was accomplished by implanting hamsters with supraphysiological steady-state pellets of cortisol, corticosterone, or cholesterol as a control. Cortisol, but not corticosterone, significantly decreased food intake, body mass, and lean and fat tissue compared with controls. Despite decreases in body mass and adiposity, cortisol significantly increased circulating free fatty acids, triglyceride, cholesterol, and hepatic triglyceride concentrations. Although corticosterone did not induce alterations in any of the aforementioned metabolic end points, Syrian hamsters were responsive to the effects of corticosterone since glucocorticoids both induced thymic involution and decreased adrenal mass. These findings indicate that cortisol is the more potent glucocorticoid in energy homeostasis in Syrian hamsters. However, the data suggest that cortisol alone does not mediate stress-induced increases in food intake or body mass in this species.     

Acute and chronic social defeat suppresses humoral immunity of male Syrian hamsters (Mesocricetus auratus).

Stressors, both physical and psychological, can activate the hypothalamic-pituitary-adrenal (HPA) axis, leading to a wide range of physiological responses including increased glucocorticoid release and suppression of immune function. The majority of studies published to date have focused on the effects of physical stressors (e.g., cold exposure, electric shock) on immunity. The present study examined the role of a stressor, social defeat, on humoral immune function of Syrian hamsters (Mesocricetus auratus). Specifically, adult male Syrian hamsters experienced social defeat (i.e., exposure to a dominant animal in that animal's home cage) that was either acute (i.e., a single exposure) or chronic (i.e., daily exposures across 5 days). A control group of animals was placed in a resident's home cage without the resident animal present and did not experience defeat. After the last encounter, blood samples were drawn and animals were subsequently injected with keyhole limpet hemocyanin (KLH). Blood samples were again taken 5 and 10 days postimmunization and serum was analyzed to determine serum cortisol and anti-KLH immunoglobulin G (IgG) concentrations. Cortisol concentrations were elevated in both acutely and chronically defeated hamsters compared with control animals. In contrast, serum IgG concentrations were significantly reduced in both groups of defeated hamsters compared with control animals. Collectively, these results demonstrate that both acute social defeat and chronic social defeat lead to activation of the HPA axis and suppression of humoral immune function. These data suggest that social defeat is an important, ecologically relevant model with which to examine stress-induced immune suppression in rodents.     

Winter day lengths counteract stimulatory effects of apomorphine and yohimbine on sexual behavior of male Syrian hamsters

Yohimbine and apomorphine selectively act on noradrenergic and dopaminergic neural substrates to augment male sexual behavior (MSB) in several rodent species. The present study assessed whether these drugs can overcome the suppressive effects of short winter-like day lengths on MSB. Yohimbine treatments that markedly increase copulatory behavior of male hamsters in long days were completely ineffective in facilitating MSB when injected after gonadal regression induced by 16 wks of short day lengths and after complete gonadal recrudescence after 32 wks of short days; apomorphine was similarly ineffective. The brain circuit that mediates MSB either may be less responsive to yohimbine and apomorphine in short than long days, or these drugs may not produce equivalent neurotransmitter changes in the two day lengths. After 32 wks of short-day treatment, all males had undergone testicular recrudescence and successfully ejaculated on initial tests with sexually receptive females after a hiatus of at least 4 mo during which they were denied mating opportunities. This suggests that overwintering males in the field are in a state of reproductive readiness at the outset of spring conditions favorable for survival of offspring.     

Sexual stimulation does not affect plasma LH nor testosterone levels in New-Zealand male rabbits

Young sexually naive (3-4 months) and sexually experienced (2-3 years) male rabbits were subjected to various sexual stimulation procedures. Blood samples were taken just before and 30 min after mounting (unreceptive females) or coitus (receptive females). Testosterone and luteinizing hormone were assayed using specific radioimmunoassays. Sexual stimulation did not affect postcoital testosterone and LH levels in naive and experienced males while in females LH levels were significantly increased. We may conclude that the postcoital neuroendocrine reflex which causes a massive release of pituitary LH in female rabbit does not exist in males.     

Experimental induction of social instability during early breeding does not alter testosterone levels in male black redstarts,a socially monogamous songbird

Testosterone plays an important role in territorial behavior of many male vertebrates and the Challenge Hypothesis has been suggested to explain differences in testosterone concentrations between males. For socially monogamous birds, the challenge hypothesis predicts that testosterone should increase during male–male interactions. To test this, simulated territorial intrusion (STI) experiments have been conducted, but only about a third of all bird species investigated so far show the expected increase in testosterone. Previous studies have shown that male black redstarts (Phoenicurus ochruros) do not increase testosterone during STIs or short-term male–male challenges. The aim of this study was to evaluate whether black redstarts modulate testosterone in an experimentally induced longer-term unstable social situation. We created social instability by removing males from their territories and compared the behavior and testosterone concentrations of replacement males and neighbors with those of control areas. Testosterone levels did not differ among replacement males, neighbors and control males. Injections with GnRH resulted in elevation of testosterone in all groups, suggesting that all males were capable of increasing testosterone. We found no difference in the behavioral response to STIs between control and replacement males. Furthermore, there was no difference in testosterone levels between replacement males that had expanded their territory and new-coming males. In combination with prior work these data suggest that testosterone is not modulated by male–male interactions in black redstarts and that testosterone plays only a minor role in territorial behavior. We suggest that territorial behavior in species that are territorial throughout most of their annual life-cycle may be decoupled from testosterone.     

Chemoattractant-stimulated calcium influx in Dictyostelium discoideum does not depend on cGMP

Chemoattractant stimulation of Dictyostelium cells leads to the opening of calcium channels in the plasma membrane, causing extracellular calcium to flux into the cell. The genetically uncharacterised mutants stmF and KI8 show strongly altered chemoattractant-stimulated cGMP responses. The aberrant calcium influx in these strains has provided evidence that the chemoattractant-stimulated calcium influx is potentiated by cGMP. We have tested this hypothesis in genetically defined mutants by measuring the calcium influx in a strain that lacks intracellular cGMP due to the disruption of two guanylyl cyclases, and in a strain with increased cGMP levels caused by the disruption of two cGMP-degrading phosphodiesterases. The results reveal that the calcium influx stimulated by cAMP or folic acid is essentially identical in these strains. We conclude that cGMP is not involved in chemoattractant-stimulated calcium influx.     

Ventromedial hypothalamic mediation of photoperiodic gonadal responses in male Syrian hamsters.

Short day lengths induce testicular regression in seasonally breeding Syrian hamsters. To test whether the ventromedial hypothalamus is necessary to maintain reproductive quiescence once testicular regression has been achieved, photoregressed male hamsters were subjected to lesions of the ventromedial hypothalamus (VMHx), pinealectomy (Pinx), or sham operation (Sham). VMHx hamsters underwent accelerated gonadal recrudescence compared to Pinx and Sham hamsters. Recovery of prolactin concentrations (PRL) to values characteristic of long-day hamsters was hastened in the VMHx animals compared to Sham hamsters. Concentrations of follicle stimulating hormone (FSH) increased prematurely in both the VMHx and Pinx animals, beginning a few weeks after surgery. By the time the gonads had undergone recrudescence and the hamsters were refractory to melatonin, PRL and FSH concentrations had returned to baseline long-day values in all groups; there was no evidence of hypersecretion of either hormone in any of the animals with lesions. Melatonin concentrations of VMHx hamsters did not differ from those of sham-operated animals, but because only a single determination was made, it remains possible that VMH damage altered the duration of nightly melatonin secretion. An intact VMH appears to be essential for the continued maintenance of reproductive suppression induced by exposure to short day lengths; these and earlier findings suggest that the VMH-dorsomedial hypothalamic complex mediates regression of the reproductive apparatus during decreasing day lengths of late summer and early autumn and also is necessary to sustain regression during the winter months.