Effects of pinealectomy and melatonin or 5-methoxytryptamine on testicular LH and PRL receptors in Syrian hamsters (Mesocricetus auratus)

The pineal has been previously shown to be an important factor in the regulation of testicular function in photoperiodic mammals. The effects of lack or increase in pineal hormones on testicular hormonal receptors has, therefore, been examined. Pinealectomy decreased the concentration of testicular LH receptors in hamsters exposed to either a long or short photoperiod but had no effect on the concentration of testicular PRL receptors. In animals exposed to a short photoperiod, pinealectomy prevented testicular regression and the concomitant decreases in total LH and PRL receptor contents. Treatment for 12 weeks with either melatonin or 5-methoxytryptamine caused a decrease in testicular PRL receptor levels, whereas the only changes in LH receptor levels were due to melatonin-induced testicular regression. The present results indicate that some of the effects of pineal hormones on the testes are independent of the pineal-induced changes in testes mass and are the consequence of long-term action. Furthermore, testicular function appears to be affected by both the lack or the increase in pineal hormones.     

Seasonal variations in the testis and epididymis of vizcacha (Lagostomus maximus maximus).

Seasonal changes in reproductive activity in the adult male vizcacha (Lagostomus maximus maximus), a South American rodent, were investigated. Monthly, for 2 yr, the animals were killed and decapitated during the night near their burrows in the vicinity of San Luis, Argentina. The testes, epididymides, and pineal glands were removed and used for biochemical and structural studies. Significant changes associated with seasonal cycles were found. 1) In July-August (winter in South America), a short hibernal period of sexual quiescence, decline in testicular and epididymal weights, arrest of spermatogenesis, and decrease of serum testosterone were observed. The gonads regressed during this period, with regression most pronounced in August. 2) During September-November (spring), a recovery period--without arrest of spermatogenesis--was observed, with significant expression of gonadal activity during April-May (autumn). In this season, gonadal weight was increased and spermatogenesis was complete. These results indicate an increase in sexual activity as well as in the ability to secrete testosterone. A gradual reduction of testicular activity appeared in June-July (early winter). Conversely, in this period, the pineal hydroxyindole-O-methyl transferase activity decreased in contrast to the highest values observed in winter. Our findings indicate that the male adult vizcacha under natural conditions exhibits an annual reproductive cycle. A possible relationship between increased pineal activity and gonadal regression is also suggested.     

Testicular response to melatonin or suprachiasmatic nuclei ablation in the spotted skunk

Testes of the Western spotted skunk enlarge and regress seasonally. The pineal hormone, melatonin, may be important in timing this seasonal reproductive activity. Likewise, the suprachiasmatic nuclei (SCN) have been implicated as possible neural regulators of pineal and reproductive events. These experiments were conducted to determine whether ablation of the SCN or constant administration of melatonin alters timing of the seasonal pattern of testicular regression and recrudescence. Male skunks (n = 24) were treated as follows: six received two empty Silastic capsules, six received two melatonin-filled Silastic capsules, six received sham lesions to the SCN, and six received lesions to the SCN (SCNx). All skunks were exposed to a natural photoperiod and had regressed testes at the onset of the experiment. Four of six males from the SCNx group had an average of 94 +/- 11.3% of these nuclei destroyed. Sham SCNx, animals with less than 40% of the SCN ablated, and males with empty capsules did not have fully enlarged testes until October. SCNx and melatonin-treated skunks exhibited a hastening of testicular recrudescence with maximal testis size being reached in June. Skunks with lesions to the SCN maintained enlarged testes for 5 months while all other groups exhibited rapid regression after attaining maximal testis size. Testicular regression occurred from July through October in animals receiving continuous melatonin, while controls exhibited recrudescence during this same period. Our data suggest that the SCN, melatonin, and thus the pineal gland, play a role in regulating the seasonal testicular cycle of the spotted skunk.     

Relationship between body and testis temperatures in the European hedgehog, Erinaceus europaeus, during hibernation and sexual reactivation

Simultaneous telemetry of the body and testis temperatures of 8 hedgehogs was carried out during hibernation and during sexual reactivation in spring. Between October and January, when the testes were involuted, the body/testis temperature differential was variable, with mean daily testis temperatures up to 1 degrees C warmer than body temperatures. From mid-February onwards, when plasma testosterone approached maximal concentrations, mean testicular temperatures stabilized 1.4 +/- 0.2 degrees C below body temperatures. During spermatogenesis testicular temperature of hedgehogs was significantly lower than body temperature. Over the euthermic body temperature range of 34.7-36.2 degrees C, testicular temperatures varied from 34.0 to 34.9 degrees C. Only at body temperatures over 36.2 degrees C did testicular temperature reach 35 degrees C. During spermatogenesis hedgehog testis temperatures are similar to those of many scrotal mammals.     

Effect of melatonin implants on gonadal weights and pineal gland fine structure of the golden hamster

Weekly subcutaneous implants of melatonin in a beeswax pellet prevented the testicular regression which normally occurs in hamsters exposed to short photoperiod for 8 weeks. Normal (14L:10D) hamster testes were indistinguishable from the testes of melatonin-treated (1L:23D) hamsters. The exogenous melatonin had varied effects on the fine structure of the golden hamster pineal gland. Pinealocyte nuclear characteristics of melatonin-treated hamsters (smaller average diameter, less polymorphism, and more heterochromatin) as well as apparent reductions in the amounts of hypertrophic SER and lipid moieties seemed to indicate that melatonin caused inhibition of pineal gland activity, and in this respect counteracted the effects of short photoperiod. However, an apparent increase in the number of large mitochondria, membrane whorls and dense-cored secretory vesicles in pinealocytes of melatonin-treated hamsters suggests enhanced pineal gland activity.     

Changes in the testicular binding of luteinizing hormone and plasma testosterone concentrations in the Djungarian hamster subjected to different photoperiods and temperatures and effects of long-term testosterone treatment on the binding

The effects of artificial photoperiod, temperature, and long-term testosterone treatment on testicular luteinizing hormone (LH) binding were studied in adult male Djungarian hamsters. In hamsters transferred to long-day (LD; 16 hr light, 8 hr dark) photoperiod 8 weeks after adaptation in short-day (SD; 8 hr light, 16 hr dark) photoperiod of 25 degrees C, testicular growth was associated with an increase in the total LH binding per two testes and a decrease in LH binding per unit testicular weight. Plasma testosterone levels reached a peak 47 days after transfer to LD and tended to decrease thereafter, while the testes continued growing. In contrast, when hamsters reared under LD conditions at 25 degrees C for 12 weeks were transferred to SD, testicular regression was associated with a decrease in plasma testosterone and the total LH binding per two testes and an increase in LH binding per unit testicular weight. A significant decrease in LH binding per unit weight compared to SD controls was observed in those hamsters exposed to SD with continuous testosterone treatment. The testosterone treatment tended to induce decrease in the total LH binding. Scatchard plot analyses of the binding suggested that changes in LH binding were due to changes in the number of binding sites. When sexually mature male hamsters were subjected for 8 weeks to two different ambient temperatures (7 degrees C and 25 degrees C) and photoperiods (LD and SD), the difference between the two temperature groups was statistically not significant regarding the weights of testes, epididymides, and prostates; plasma testosterone levels; and LH binding in either LD or SD group. These results suggest that photoperiod is a more important environmental factor than temperature for the regulation of testicular activity and LH receptors and that testosterone reduces the number of LH receptors per unit testicular weight in adult male Djungarian hamsters.     

Changes in the activity of acid glycosidases during posthatch development and regression after light reduction of Japanese quail testes and epididymides

In this study specific activities of four acid glycosidases: beta-N-acethylhexosaminidase (beta-HEX), beta-galactosidase (beta-GAL), alpha- and beta-mannosidase (alpha- and beta-MAN) were investigated in Japanese quail testes and epididymides during posthatch development and regression after light reduction. The specific activity of testicular beta-HEX and beta-GAL increased steadily during posthatch development and assumed maximum values for testes weighing 200-400 mg, when numerous spermatocytes appear in the testes of quail, and then decreased slowly. These enzymes showed much higher specific activity after 15 days of light reduction, and decreased to the control level after 30 days. Activity of alpha- and beta-MAN remained rather constant during testicular development and involution. The epididymal activity of the acid glycosidases was very low in immature individuals, whereas in sexually mature birds it was found to increase several-fold. Short photoperiod resulted in a decreased activity of these enzymes after 30 days to the values found in immature birds. A marked increase in the activity of acid glycosidases in the epididymides of sexually mature animals and a decrease in this activity during epididymidal regression indicate that these enzymes take part in reproductive processes. It is concluded that the activities of beta-HEX, beta-GAL, alpha- and beta-MAN in the development and regression of Japanese quail testes and epididymides change similarly as in mammals.     

Splenic hypertrophy and extramedullary hematopoiesis induced in male Syrian hamsters by short photoperiod or melatonin injections and reversed by melatonin pellets or pinealectomy

Adult male Syrian hamsters either placed in a short photoperiod alone or kept in a long photoperiod and given daily afternoon injections of the pineal indole melatonin (25 micrograms) exhibited splenic hypertrophy and extramedullary hematopoiesis in addition to a marked regression in testicular weight. The testicular regression as well as the changes in spleen weight and histology could be prevented if the animals in short photoperiod were either pinealectomized or implanted subcutaneously with a pellet containing 1 mg melatonin. Female Syrian hamsters given afternoon injections of melatonin for 7 or 12 weeks had ovaries devoid of corpora lutea; additionally, these animals had reduced relative spleen weights compared to the control animals. In conclusion, it is apparent that spleen weight varies with the functional status of the gonads. Splenic hypertrophy accompanied by pineal-induced testicular regression in males may be related to splenic extramedullary hematopoiesis.     

Role of the pineal and its hormone melatonin in the termination of photorefractoriness in golden hamsters

Continuous exposure of male hamsters to short day lengths induces testicular regression. This is followed many weeks later by spontaneous recrudescence of the testes with reinitiation of spermatogenesis and function of the accessory sexual glands. Hamsters at this stage of the annual reproductive cycle are refractory to short photoperiods--even continuous darkness will not induce another bout of testicular regression. Animals refractory to short days are also refractory to the pineal hormone melatonin and a number of investigators attribute spontaneous recrudescence and photo and melatonin refractoriness to a developed target cell insensitivity to endogenous melatonin from the pineal. Refractoriness is terminated by exposure to long days for at least 11 weeks. The pineal gland is reported to be essential for this process. We report here the effects of pinealectomy, daily melatonin injections, and constant-release melatonin implants on the ability of male hamsters to recover from the refractory state. In the absence of the pineal gland, refractory male hamsters did not discriminate (count?) 15 weeks of long days to terminate refractoriness. Daily melatonin injections at 1900 h, but not at 1200 h (lights 0600-2000 h) during the 15 weeks of long-day exposure blocked the recovery from refractoriness. Constant-release melatonin implants abolished the animals ability to measure 12 and 15 weeks of long days to terminate refractoriness. These results demonstrate that general target tissue insensitivity to melatonin cannot account for the refractory state in hamsters, that a multiplicity of target tissues may exist for melatonin to account for its varied roles throughout the annual reproductive cycle in hamsters, and that the pineal gland is intimately involved in the animals' ability to measure a prescribed duration of long days to terminate refractoriness.     

Seasonally and experimentally induced changes in testicular function of the Australian bush rat (Rattus fuscipes)

Serum concentrations of LH, FSH and testosterone were measured monthly throughout the year in male bush rats. Testicular size and ultrastructure, LH/hCG, FSH and oestradiol receptors and the response of the pituitary to LHRH were also recorded. LH and FSH rose in parallel with an increase in testicular size after the winter solstice with peak gonadotrophin levels in the spring (September). The subsequent fall in LH and FSH levels was associated with a rise in serum testosterone which reached peak levels during summer (December and January). In February serum testosterone levels and testicular size declined in parallel, while the pituitary response to an LHRH injection was maximal during late summer. The number of LH/hCG, FSH and oestradiol receptors per testis were all greatly reduced in the regressed testes when compared to active testes. In a controlled environment of decreased lighting (shortened photoperiod), temperature and food quality, the testes of sexually active adult males regressed at any time of the year, the resultant testicular morphology and endocrine status being identical to that of wild rats in the non-breeding season. Full testicular regression was achieved only when the photoperiod, temperature and food quality were changed: experiments in which only one or two of these factors were altered failed to produce complete sexual regression.     

Importance of photoperiods in determining temporal pattern of annual testicular events in rose-ringed parakeet (Psittacula krameri).

Male rose-ringed parakeets (Psittacula krameri) were transferred to a long photoperiod (LP; LD 16:8) or a short photoperiod (SP; LD 8:16) for 45 or 90 days on four dates corresponding to the beginnings of different reproductive phases in an annual testicular cycle, and testicular responsiveness was evaluated by comparison with the testicular volume, weight, seminiferous tubular diameter, and germ cell profiles of birds in a natural photoperiod (NP). Exposure of birds to LP during the progressive phase (November) led to precocious maturation of testes after 45 days, but induced regression at 90 days. After showing retarded gametogenic functions at 45 days, parallel (November) SP birds exhibited an accelerated rate of germ cell formation at day 90. During the prebreeding phase (January), there were no remarkable differences in any features of testes among NP. LP, and SP birds at 45 days, but gonadal involution in LP parakeets and active spermatogenesis in SP birds occurred after 90 days. The testes did not show any response to LP or SP for 45 and 90 days when the birds were transferred to altered photoperiods during the breeding (March) and preparatory (June) phases, indicating that the parakeets were photorefractory for at least 6 months (March through September). The results also suggest that initiation and termination of seasonal gametogenic activity in parakeets are possibly functions of endogenous rhythmicity or extraphotoperiodic environmental factors. Duration of light may have certain influences on the attainment of annual peak in spermatogenesis, but in all probability the species has a low photoperiod threshold for induction of testicular growth.     

Inverse relationship between testicular proliferation and apoptosis in mammalian seasonal breeders

Seasonal cycles of testicular activity occur in many mammals and can include transitions between total arrest and recrudescence of spermatogenesis. We hypothesize that involution and reactivation of testis result from two antagonistic processes, proliferation and programmed cell death (apoptosis), which are activated at different times. To test this hypothesis, quantitative measurements of both proliferation-specific marker and apoptotic produced nucleosomes have been compared with sperm and testosterone production in testes from adult roe deer during breeding and non-breeding seasons (May to September). Testes of brown hare were included from periods of testes regression (June to August) and recrudescence (November to December). The highest testicular weights in roe deer were found in the rutting period from late July to early August (27.25 +/- 8.56 g), corresponding with the highest number of testicular sperm/g parenchyma. The peak of sperm production coincided with a peak in testosterone concentration (1.19 +/- 0.53 microg/g testis). The maximum level of proliferation-specific marker was also found during the breeding season (98.6 +/- 58.2 U/g testis in comparison to 20.1 +/- 22.0 U/g in the prerutting period). In contrast, the most significant apoptosis was observed in the nonbreeding season than the breeding period (71.11 +/- 5.79 U/mg testis and 18.88 +/- 6.79 U/mg, respectively). Testicular proliferation was low in the brown hare (0.061 +/- 0.062 U/g) during involution of the testes. It was newly activated in November and December (0.85 +/- 0.33 U/g), preceding the increase in testicular volume. Testosterone production increased in conjunction with testicular proliferation. At this time, testicular apoptosis was significantly lower (14.16 +/- 2.12 U/mg testis) than during the period of pronounced testicular regression (30.16 +/- 19.95 U/g). These results suggest that regulation of seasonal testicular activity is characterized by an inverse relationship of proliferation and apoptosis.     

Testicular concentration of meiosis-activating sterol is associated with normal testicular descent

In the cryptorchid stallion, spermatogenesis is arrested at various levels before the completion of meiosis. In men, infantile cryptorchidism is also often associated with oligo- and azoospermia during adulthood. An impairment of spermatogenesis might be reflected in the level of locally produced factors. Formerly, a meiosis-activating sterol (T-MAS) has been isolated in murine and bovine testes. This sterol possesses the potential to trigger resumption of meiosis in cultured mouse oocytes, indicating that it might play an important role in the regulation of the meiotic process in the female gamete. The function of T-MAS in the testis is still unclear, but T-MAS may be associated with spermatogenesis. The objectives of this study were 1) to demonstrate the presence of T-MAS in equine testes, 2) to compare the contents of T-MAS in testicular tissue of stallions with complete and incomplete testicular descent and 3) to compare testicular T-MAS concentration before and after puberty Testes were collected from 16 normal and cryptorchid stallions submitted for castration and stored at -80 degrees C until the content of T-MAS was measured quantitatively with an HPLC-assay. In stallions > or = 2 years of age, the content of T-MAS was higher (P < 0.001) in normal testes (19.3+/-1.1 microg T-MAS/g, n=7) than in inguinally (4.1+/-2.4 microg T-MAS/g, n=4) or abdominally located testes (1.6+/-0.2 microg T-MAS/g, n=2). The contents of T-MAS in normal testes from stallions < 2 years of age (2.8+/-1.5 microg T-MAS/g, n=4) was lower than in normal testes from stallions > or =2 years of age (P < 0.001) From the present study it can be concluded that T-MAS is present in equine testicular tissue. Furthermore, the present study demonstrates that the production of T-MAS in testicular tissue is, concurrently with spermatogenesis, associated with normal testicular descent and is temporarily related to the onset of puberty.     

Photoperiodic elevation of testicular zinc protects seminiferous tubules against fluoride toxicity in the bank vole Clethrionomys glareolus

Recent work has shown that a high fluoride (F) intake in rodents leads to histopathologic changes in the germinal epithelium of testes and to zinc deficiency in the testis and several other organs. The purpose of the present study was to determine whether an elevation of testicular zinc concentration during fluoride exposure could protect the testes of bank vole from damage. The elevation of testicular zinc was achieved by exposing the bank voles to a long photoperiod (16 hr light/8 hr dark). The zinc concentration in the testes of bank voles kept under the long photoperiod was 38% higher than that in animals exposed to a moderate photoperiod (12 hr light/12 hr dark). Fluoride exposure (200 μg F/ml drinking water) during 4 months decreased additionally p > 0.05 zinc concentration in the testes of bank voles kept under the moderate photoperiod. The same animals also exhibited histopathologic changes in the germinal epithelium. By contrast, these disturbances were not observed in animals maintained in the long photoperiod. This experiment suggests that an increase in testicular zinc due to a long photoperiod prevents seminiferous tubules from a damage induced by fluoride in bank voles. The protective effects of zinc (or a long photoperiod) did not appear to be related to a decrease in testicular fluoride accumulation or lipid peroxidation.     

Effect of melatonin implants on reproduction in the male silver fox (Vulpes vulpes)

In June 1987, when the testes were fully regressed, 5 males were given s.c. implants of 40 mg melatonin. The same treatment was repeated in August and October 1987. Five males served as controls. Plasma concentrations of melatonin increased significantly in treated males and were still elevated at the end of the study in April 1988. The changes in testicular volume and blood plasma concentrations of testosterone in response to GnRH indicated that melatonin administration promoted testicular development. However, testicular regression was observed earlier in treated than control animals, perhaps because of refractoriness to melatonin or to a down-regulation of melatonin receptors. Semen was collected and frozen in November 1987, 2 months ahead of the natural breeding season, from the melatonin-treated males, and 10 blue fox vixens were inseminated the following breeding season: 9 vixens conceived, and the average litter size was 7.6 +/- 0.5. The results demonstrate that melatonin treatment initiated during exposure to naturally long days (a) promotes testicular development in a way similar to an artificial short photoperiod and (b) may induce a refractory condition after an extended period of treatment.     

Seasonal effects of pinealectomy on testicular recrudescence and secretory activity of seminal vesicles in the catfish Heteropneustes fossilis (Bloch)

The effects of pinealectomy on testicular activity and secretory activity of seminal vesicles were examined in the catfish Heteropneustes fossilis under various combinations of photoperiod and temperature during different periods of the annual reproductive cycle. Pinealectomy had no effect on gonadal activity during the preparatory, prespawning and spawning periods of the reproductive cycle. However, during the postspawning period, under long (LD 14:10) or short (LD 9:15) photoperiod at 25° C or at gradually increasing ambient temperature, pinealectomy accelerated testicular recrudescence and secretory activity of the seminal vesicles. Nevertheless. during this period the presence of the pineal facilitated the recrudescence of testes and seminal vesicles in catfish exposed to continuous light (LL), continuous darkness (DD) and 12 hL:l2 hD (LD) at 25° C. These findings suggest that the role of the pineal in catfish reproduction is variable and depends upon the photoperiod and temperature regimes to which the fish are exposed, as well as on the time of the year and the state of the reproductive cycle. The results also suggest that the effects of pinealectomy in catfish are mediated through an influence on the hypothalamo-hypophyseal gonadal axis.     

Pineal, Photoperiod and Gonadal Function in the Indian Palm Squirrel, Funambulus pennanti

Effects of morning and evening injections of pineal 5-methoxyindoles (MI), melatonin (aMT) and 5-methoxytryptamine (MT), for 60 continuous days, were observed on the testes of sham-operated (SO) and pinealectomized (Px) Indian palm squirrel, Funambulus pennanti maintained under different photoperiods during the gonad active phase. Long photoperiod (LP) of 14L:10D appeared stimulatory to the testes and caused a significant increase in the weight and seminiferous tubule diameter of both SO and Px animals, as compared to the animals under natural day-length (NDL). Short photoperiod (SP) of 10L:14D had an inhibitory influence and reduced the testes weight and its tubule diameter. aMT and MT injections during evening hours significantly reduced testes weight and tubule diameter of SO and Px animals under NDL, LP and SP. However morning injections, under all conditions, were without any significant effect. The results suggest an inhibitory effect of aMT and MT, under above photoperiodic conditions, on the testes of this tropical mammal. The time of administration of the MI is important in the expression of the effect.     

Effect of photoperiod on the size of the Leydig cell population and the rate of recruitment of Leydig cells in adult Syrian hamsters

The number of Leydig cells was determined by stereologic procedures in adult Syrian hamsters housed in long days (14L:10D) to maintain testicular activity (active), in short days (5L:19D) for 12-13 wk to induce testicular regression (photoperiod-induced regressed), or in short days for a period of 21 wk or more to allow spontaneous gonadal recrudescence (spontaneously recrudesced). Testes were removed, sliced, fixed, embedded in Epon 812, and observed by bright-field microscopy. Testicular and seminal vesicle weights, plasma testosterone concentration, total Leydig cell volume per testis, and volume of single Leydig cell were greater (p less than 0.01) in active and recrudesced animals than in regressed animals. The density of Leydig cells was greater in the regressed testes, but the total number per testis was not influenced by photoperiod. In Experiment 2, the rate of recruitment of Leydig cells was determined in 5 adult hamsters exposed to long days (active) or 5 hamsters whose testes were regressed by exposure of animals to short days for 13 wk followed by long-day exposure to initiate testicular growth (photoperiod-induced recrudescing). Hamsters were injected for 3 days/wk for 3 wk with tritiated thymidine, 0.5 or 1 microCi/g body weight. Testes were fixed and tissues prepared, as above, and processed for autoradiography. Again, the photoperiod did not influence the number of Leydig cells per testis. Labeling of Leydig cell nuclei revealed that recruitment of new Leydig cells occurred at approximately 1.3% per day in recrudescing testes but also occurred at approximately 0.6% per day in active testes. Without change in the total number of Leydig cells, new Leydig cells were added continually to the existing population in adult hamsters with either recrudescing or active testes.     

Effect of hexachlorobenzene on some male reproductive parameters in Meriones unguiculatus

Organochlorine pesticides are known to cause disturbances in many physiological functions. The effects of in vivo administered hexachlorobenzene (HCB) on the male testicular function were studied in Meriones unguiculatus. Three groups of sexually mature meriones were orally exposed to 1.6, 4, and 16 mg/kg of body weight in olive oil for 30 days. Morphological and morphometrical estimations were applied to quantify some structural constituents of the testes. Testicular weight was significantly decreased in all treated groups, while no change was noted in seminal vesicle weight. A decrease in the spermatozoid content of the seminiferous tube was noted and appeared correlated with a modification of the process of spermatogenesis. Spermatic activity in HCB-treated animals testes decreased significantly, particularly in the group treated with the higher dose (60+/-3.16% vs. 88+/-4.89% in controls). Plasma testosterone levels were decreased significantly in the groups treated with 4 and 16 mg[HCB]/kg BW (0.48+/-0.08 ng/ml and 0.54+/-0.07 ng/ml) comparatively to controls (1.08+/-0.1 ng/ml) p<0.01.     

Temporal difference of thyrotropin-releasing hormone in prevention of testicular regression in golden hamsters exposed to short photoperiods: possible involvement of beta-endorphin

Male golden hamsters were exposed to long photoperiod or short photoperiod (SP) and injected with 1 microgram TRH and/or 1 microgram LHRH at lights on (LO) or lights off (LX) for a total of 8 weeks. Both TRH and LHRH prevented testicular regression if they were injected at LO. Injected at LX, TRH did not prevent testicular regression, and LHRH was only partially effective. Plasma beta-endorphin levels were significantly higher in groups with atrophic testes. These results indicate that TRH like LHRH can prevent SP-induced testicular regression in hamsters by some unknown mechanism and that beta-endorphin may be involved in the control of testicular function in hamsters.