Reductions in hamster serum thyroxine levels by melatonin are not altered by changes in serum testosterone

Daily melatonin injections reduce reproductive and thyroid hormones in male Syrian hamsters. The interrelationship between the decline in these hormones is not known. To explore this relationship, male Syrian hamsters were divided into four groups: castrated, implanted with testosterone (5-mm silastic implants), both treatments, or neither treatment. One-half of each group of hamsters (n = 7 or 8) were injected with melatonin (25 mg) daily at 1730 h. The other half of each group received daily vehicle injections. Ten weeks later, the hamsters were anesthetized and decapitated. Testes weights, serum testosterone, and serum thyroxine levels were measured. As expected, testes and serum testosterone levels were uniformly low in all of the melatonin-treated hamsters. All of the melatonin-treated groups also had lower than normal thyroxine values irrespective of gonadal treatment. Interestingly, in the non-melatonin-treated hamsters, serum thyroxine values were decreased in the castrated group and increased in the testosterone-implanted group. These results suggest that castration can reduce serum thyroxine levels in male Syrian hamsters and that replacement of testosterone restores these levels to normal. Notably, the declines in thyroxine levels produced by daily melatonin injections were not restored by testosterone implants in castrated or intact hamsters. Therefore, melatonin-induced reductions in thyroxine are not mediated by concurrent reductions in serum testosterone levels. It appears that melatonin-induced reductions in serum thyroxine levels do not use the same mechanism as castration-induced reductions.     

Hormonal Modulation of Pineal-Mediated Seasonal Events: Effects of Thyroid or Gonadal Manipulations

The ability of various manipulations of the endocrine system to affect pineal-mediated events was examined in the present studies. Male Syrian hamsters were analyzed for pineal-induced gonadal regression and depressions in serum thyroxine and testosterone levels after treatments which altered thyroid or gonadal function. Hamsters were thyroidectomized, received thyroxine implants (5 mg), or were thyroidectomized plus implanted with thyroxine. The animals were exposed to short photoperiods (10L:14D) for 9 wk, and plasma hormone levels and gonadal status were determined at the end of the experimental period. Likewise, hamsters were castrated, received testosterone implants (5 mg), or were castrated plus implanted with testosterone, and subsequently were exposed to short photoperiods for 9 wk. These animals' responses to photoperiod exposure were compared to animals which received identical hormonal treatments but remained in long photoperiod (14L:10D). All of the hamsters responded to the photoperiodic treatments equally, regardless of the hormonal treatment. The results of these studies indicate that experimentally induced alterations in plasma thyroxine or testosterone levels are unable to prevent or attenuate the ability of the pineal to elicit gonadal regression in response to short photoperiod exposure.     

Role of thyroid in the response of bill pigmentation to male hormone of the house sparrow, Passer domesticus

In the house sparrow (Passer domesticus), seasonal testicular growth was decreased, and fully developed testes regressed after thyroidectomy. Both castration and thyroidectomy resulted in a loss of bill color. Thyroidectomy had no effect on the body weight, the seasonal regression of the testes, or on regressed testes.Administration of testosterone at 25 μg day^?1 bird^?1 level had no appreciable effect on the bill color of either the castrated or thyroidectomized birds; and 50 μg day^?1 bird^?1 was partially effective only in castrates. Further, while 100 μg day^?1 bird^?1 fully counteracted the effects of castration on bill color during October/November and induced partial coloration during December, it could not suppress depigmentation or induce coloration in the bill of the thyroidectomized birds. Testosterone, however, had no effect on the gonads of thyroidectomized, but increased the weight of the vas deferens to the same extent as that of the castrated birds.It is suggested that the thyroid-gonad relationship in the house sparrow varies with the reproductive status of the bird, and thyroid activity is essential for the development of the gonads and pigmentation of the bill.     

Effects of castration and thyroidectomy on the annual biological cycles of the edible dormouse Glis glis

The annual cycle of hibernation, body weight, and plasma testosterone and thyroxine levels were compared in 7 orchidectomized and 16 intact dormice under natural conditions, and in 21 thyroidectomized and 23 intact dormice exposed to a 12-hr light and 12-hr darkness photoperiod under natural temperatures. Hibernation was not affected by either orchidectomy or thyroidectomy, nor did thyroidectomy alter the annual body weight cycle, which, however, was obliterated by orchidectomy. As regards endocrine cycles, orchidectomy had no significant effects on the annual thyroxine cycle, whereas thyroidectomy obliterated the annual testosterone cycle, which was replaced by short-lived, infradian plasma thyroxine cycles. As the latter resembled the fluctuations previously observed in intact animals exposed to constant ambient temperatures, it is proposed that in edible dormice, the annual thyroid cycle, which is thermoperiodically entrained, in turn entrains the testosterone cycle.     

Interactions reciproques testo-thyroido-surrenaliennes chez la caille male

The effect of sexual and thyroid hormones on corticosteronemia, aldosteronemia, and the binding capacity of transcortin was studied in male Japanese quail. In euthyroid birds the plasma concentration of corticosterone was decreased by testosterone and increased by castration. In thyroidectomized animals, however, testosterone stimulates a corticosteronemia. Photostimulation per se stimulates a rise in the plasma concentration of the corticosterone. Testosterone inhibits the total transcortin capacity both in euthyroid and in hypothyroid birds. Aldosteronemia is lower in photostimulated birds but is not affected by changes in the plasma thyroxine level. In “short-day” birds the corticosteronemia was increased by thyroxine treatment. At the same time, in photo-gonado-stimulated quails only the free corticosterone fraction was increased by the same treatment. The corticosteronemia and the free corticosterone were lowered by the thyroidectomy. In “short-day” and “long-day” birds thyroxine injection decreases the binding capacity of the transcortin. The thyroidectomy stimulates the plasma transcortin concentration of the “short-day” birds. The synergy in the inhibitory effect of thyroxine and testosterone to the transcortin which has not yet been published must be stressed.     

Influence of thyroid hormones and testosterone on the activities of hepatic mitochondrial enzymes in the Indian garden lizard, Calotes versicolor

Thyroidectomy and castration in Calotes versicolor significantly decreased the activities of hepatic mitochondrial cytochrome oxidase, alpha-glycerophosphate dehydrogenase (alpha-GPDH), and succinate dehydrogenase (SDH) when compared to sham-operated controls. Administration of thyroid hormones in thyroidectomized lizards and testosterone in castrated specimens stimulated the activities of all three enzymes studied. Chloramphenicol, when injected with thyroxine prevented the hormone-stimulated activities of cytochrome oxidase and SDH, while actinomycin D and chloramphenicol, when administered along with testosterone propionate (low dose) prevented the testosterone-stimulated activities of cytochrome oxidase and alpha-GPDH.     

Role of thyroid and testicular hormones in the regulation of basal metabolic rate, gonad development, and body weight of spotted munia, Lonchura punctulata

Normal saline, L-thyroxine (L-T4, 0.5 micrograms/bird/day), and testosterone (100 micrograms/bird/day) were administered separately to intact, thyroidectomized, and castrated spotted munia over a 30-day period and the effects on basal metabolic rate (BMR), gonads, and body weight were recorded. Thyroidectomy and castration decreased the BMR. L-Thyroxine increased the BMR of intact and thyroidectomized but not of the castrated birds. Testosterone had no effect on BMR of either the intact or of the operated birds. L-Thyroxine and testosterone both had no effect on regressing gonads but inhibited thyroidectomy-induced testicular development. Body weight increased in intact, testosterone-treated birds, and in thyroidectomized and castrated spotted munia. L-Thyroxine inhibited body weight gain only in thyroidectomized birds. It is suggested that the physiological actions of L-T4 and testosterone depend on the gonadal status, thyroid activity, and nature of the thyroid-gonad relationship.     

N-acetyltransferase activity in the Harderian glands of the Syrian hamster, Mesocricetus auratus, is regulated by androgens and by hormones of the pituitary-thyroid axis

This study tested the hypothesis that activity of the enzyme N-acetyltransferase (NAT) in the Harderian gland of the Syrian hamster is regulated both by androgens and by hormones of the pituitary-thyroid axis. To test the effects of castration and hypothyroidism, intact or castrated male hamsters were given either tap water or methimazole in their drinking water for 3 weeks. Methimazole suppresses iodination of thyroglobulin, thereby decreasing circulating levels of thyroid hormones and increasing TSH levels. Hypothyroidism or castration caused elevated or depressed Harderian gland NAT activities respectively, compared with euthyroid controls. When castration and hypothyroidism were combined, the animals exhibited high NAT activity compared with castrated euthyroid males. To test the effects of castration and hyperthyroidism, male hamsters were given daily injections of thyroxine (T4) or diluent and were either castrated or left intact for 4 weeks. Intact animals given T4 had depressed Harderian NAT activity; serum thyroid hormone levels were elevated and TSH levels were depressed compared with those of intact controls. Castrated animals had depressed NAT activity below that of intact controls; serum thyroid hormone levels were normal but TSH levels were depressed. Castrated animals given T4 injections had NAT activity similar to that of euthyroid castrated hamsters; thyroid hormone levels were elevated but TSH levels were similar to those seen in euthyroid castrated hamsters. In another experiment, both T4 and tri-iodothyronine (T3) were equally effective in decreasing NAT activity in intact males. To determine the effects of the removal of pituitary influences, male hamsters were hypophysectomized. NAT activity in the Harderian glands of these animals was reduced compared with intact controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reproductive hormone secretion and spermatogenic function in thyroidectomized rams receiving graded doses of exogenous thyroxine

This study aimed to obtain a better understanding of the relationship between circulating thyroxine (T4) concentrations and reproductive endocrine function in the ram. Mature Merino rams were thyroidectomized and supplemented with 0, 30, 100 and 300% of normal T4 for 10 weeks. Thyroidectomy had no apparent effect on spermatogenic function but interfered with sperm maturation, the latter being returned to normal by 30% T4 replacement. Circulating testosterone levels were reduced by thyroidectomy and restored to control levels by 30% T4; when T4 levels were supranormal (300%), circulating testosterone levels were again reduced. The lowered circulating testosterone levels in thyroidectomized rams occurred as a result of suppressed testosterone secretion from the testis, observed under basal conditions and also following LH-releasing hormone (LHRH) and human chorionic gonadotrophin injection. In thyroidectomized rams, sex hormone binding globulin (SHBG) levels were depressed without changes in testosterone clearance rate (TCR), while in rams with supranormal T4 levels, TCR was increased without changes in SHBG levels. Subnormal levels of T4 also restored to normal the reduced LH pulse frequency in thyroidectomized rams. Reduced LH pulse frequency, together with diminished LH release following LHRH injection in thyroidectomized rams, suggested effects of T4 at the hypothalamo-pituitary axis. The present study demonstrates that complete lack of thyroid hormones suppresses normal reproductive endocrine function in the ram, but that this can be restored to normal by 30% T4 replacement. The results support the theory that T4 plays a permissive rather than a regulatory role in reproductive function in males.     

A year-long study of changes induced by castration in the plasma lipid and lipoprotein spectrum in the European badger

In man, an influence of male sex hormones on plasma lipid transport is well established; however, recent data on this subject in the literature are both relatively lacking and occasionally conflicting. The male European badger exhibits seasonal variations of large amplitude in its gonadic function. We have therefore attempted to establish the influence of male sex steroids on plasma lipids and lipoproteins in this species. For this purpose, we have examined the plasma lipid and lipoprotein spectrum in a group of castrated male badgers every month for a year, non-operated animals being used as controls. Our analyses included measurement of plasma lipid levels, density gradient ultracentrifugation of lipoproteins, electrophoresis of lipoproteins and apolipoproteins, and evaluation of plasma testosterone and thyroxine levels. The differences observed between the 2 groups of animals were maximal during the months when plasma testosterone was elevated in intact badgers (January to July). For this period, castration resulted in higher plasma concentrations of cholesterol, phospholipids and triglycerides, while the latter alone remained significantly more elevated in operated animals until the end of our experiments. With regard to lipoproteins, the main effect of castration consisted of a large augmentation in the concentration of lipoproteins with d approximately equal to 1.027-1.065 g/ml which were responsible for the transport of most of the increased amounts of triglycerides present in the plasma of castrated badgers. The proportion of apoprotein B in the protein moiety of these lipoprotein components was enhanced after castration. Other changes in the lipoprotein spectrum included (1) a moderate increase in the concentration of lipoproteins with d less than 1.015 g/ml and 1.019-1.027 g/ml, and (2) a modification of the respective proportions of high density lipoproteins with d 1.065-1.100 g/ml and d 1.100-1.162 g/ml. Finally, no considerable differences between the 2 groups of animals were noted in the respective percentages of the various chemical constituents in each lipoprotein subfraction assayed, except for those with d 1.023-1.027 g/ml, which, in castrated badgers, did not exhibit the enrichment in triglycerides usually noted during late winter and spring in intact animals.     

Thyroxine treatment facilitates prolactin secretion and induces a state of photorefractoriness in thyroidectomized starlings

Sexually immature male starlings were radiothyroidectomized while held under short day-lengths. They were then subjected to long (18-h) photoperiods and the testes developed rapidly and apparently normally to full maturity. As expected, thyroidectomy prevented an onset of photorefractoriness and after 140 days the testes were still fully developed (testicular width 6.5 +/- 0.8 mm). Half of the birds were then given thyroxine (0.011 mmol/l) in the drinking water for 14 days while the others were maintained as controls. At the end of this time the birds still had fully developed gonads (testicular width 7.8 +/- 0.3 mm) but after a further 14 days testicular regression began in the thyroxine-treated birds. The testes of all individuals in this group were fully regressed (width 1.8 +/- 0.1 mm) by 56 days after the end of thyroxine administration and moulting of the flight feathers had begun. No recrudescence of the gonads was subsequently noted in the next 2 months and the birds were apparently photorefractory. The short period of thyroxine treatment also caused a rapid and prolonged increase in plasma prolactin levels from 2.0 +/- 0.3 to 16.8 +/- 2.6 micrograms/l. No testicular regression or moulting was observed in the control birds and their plasma prolactin levels remained very low (below 4.0 micrograms/l) throughout the experiment. In thyroidectomized and castrated starlings held on 18-h daylengths, 14 daily injections of thyroxine (100 micrograms/bird per day) caused a rapid and permanent decrease in circulating FSH to basal levels (reached about 36 days after thyroxine treatment began, at which time the birds moulted). The treatment also caused a marked increase in plasma prolactin lasting for about 50 days.     

Role of thyroid and testicular hormones in the oxidative metabolism of the Indian garden lizard, Calotes versicolor

L-Thyroxine (L-T4) and testosterone were administered separately to intact, thyroidectomized, and castrated male Indian garden lizards, Calotes versicolor (maintained under natural climatic conditions) throughout the year to study their effects on the whole-body oxygen consumption and on tissue (liver, muscle, kidney, and brain) respiration. The findings indicate that the calorigenic actions of L-T4 and testosterone are independent of ambient temperature, and different tissues have different, temperature-independent, annual rhythms of sensitivity to L-T4 and testosterone. Internal factors seem to play a major role in the determination of these annual tissue sensitivity rhythms. Effects of thyroidectomy and castration are brought about by decreased levels of both thyroidal and testicular hormones. It is suggested that gonadal hormones are directly involved in the oxidative metabolism of Calotes which is regulated jointly by thyroid and gonadal steroids, and the degree of involvement of the glands depends upon the energy demand and sensitivity of tissues during different months of the year.     

Increased sensitivity to the negative feedback effects of testosterone induced by hyperprolactinemia in the adult male rat

High plasma levels of PRL induced by transplants of two donor pituitaries under the kidney capsule of adult male rats resulted in a prolonged suppression of plasma levels of LH and FSH although testosterone levels were maintained within normal limits. Castration of rats with pituitary transplants resulted in a normal though delayed rise in serum levels of both LH and FSH to levels equivalent to those in normal castrated controls. This increase in gonadotropin levels occurred in spite of maintenance of elevated PRL levels. Two experiments were carried out in which testosterone was restored after castration by Silastic testosterone-containing implants of various lengths (Exp 1:60, 30, and 10 mm; Exp 2: 30, 20, 10, 5, and 2 mm). In both experiments 60- and 30-mm testosterone implants prevented the postcastration rise in LH and FSH in both control and hyperprolactinemic rats. However, although the shorter testosterone implants delayed this rise in control rats, levels of LH and FSH increased by 4 days and were not significantly different from castrated rats without testosterone implants by 15 days after castration. In contrast, this rise in gonadotropins was abolished or considerably delayed by the shorter implants in hyperprolactinemic rats, demonstrating an increase in sensitivity of the hypothalamic pituitary axis to the negative feedback effects of testosterone in these animals. These results suggest that 1) to maintain suppression of gonadotropin secretion in hyperprolactinemia high levels of PRL alone are insufficient and gonadal steroids are required, and 2) high levels of PRL appear to sensitize the hypothalamic-pituitary axis to the negative feedback effects of gonadal steroids.     

Light, thyroid, gonad, and photorefractory state in the migratory redheaded bunting, Emberiza bruniceps

When exposed to constant long photoperiods the redheaded bunting, Emberiza bruniceps, was shown to become absolutely photorefractory and the condition was observed to be accelerated by thyroidectomy. Photoinduced gonadal growth was inhibited partially and the photoinduced increase in body weight was inhibited completely by thyroidectomy. Injection of thyroxine reversed the effects of thyroidectomy if administered during the nonbreeding photosensitive or breeding states, but had no effect in birds which had entered the photorefractory state. Gonadal growth was stimulated by pregnant mare serum gonadotrophin in intact and thyroidectomized photorefractory birds, suggesting that thyroidectomy does not lead to loss of response of the gonads to gonadotrophins but directly affects the secretion of gonadotrophins. Injection of prolactin increased the body weight of intact but not of thyroidectomized photorefractory birds, suggesting that metabolic photorefractoriness is due to a decreased secretion of prolactin and that the effects of T4 on body weight depend on its synergism with prolactin. It is suggested that thyroid hormone(s) may be part of the coupling mechanism(s) for body weight and gonad development cycles and that the beneficial role of thyroid hormones may consist in an increased threshold of the central nervous system control of neuroendocrine functions towards the negative feedback regulation of increased sex steroid levels.     

Substance P and luteinizing hormone-releasing hormone levels in the brain of the male golden hamster are both altered by castration and testosterone replacement

The effects of castration and testosterone (T) replacement on levels of substance P (SP) and luteinizing hormone-releasing hormone (LHRH) were assessed in discrete areas of the male hamster brain. The animals were either castrated, castrated and given a chronically low or high dose of T by Silastic implant, or sham-operated. Brain tissues and trunk blood were collected 3 weeks after surgery. Plasma T levels were maintained within the normal range by the implants but at significantly lower or higher levels than the mean for sham-operated males. Levels of SP and LHRH were quantified in the olfactory bulbs, rostral basal forebrain, anterior hypothalamic and preoptic area, medial basal hypothalamic area, medial basal hypothalamic area and median eminence, and brain stem. In general, castration and T replacement effected opposite changes in levels of SP and LHRH. In the medial basal hypothalamic area and median eminence SP levels were found to be inversely related to the chronic T levels, whereas the LHRH levels were directly correlated. In the anterior hypothalamic and preoptic area, castration reduced levels of SP. Conversely, castration elevated levels of LHRH in this area. This inverse dynamic relationship between changing peptide levels was also observed in the rostral basal forebrain but not in the olfactory bulbs. In most of these forebrain regions, the dose-response curves for the experimental groups could not incorporate the peptide levels in the sham-operated control group. SP levels in the brain stem showed a monotonic inverse relationship to circulating T levels which did include the control group values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal changes in testicular size and in plasma follicle-stimulating hormone and prolactin concentrations in thyroidectomized male and thyroidectomized castrated starlings (Sturnus vulgaris)

Testicular size and plasma concentrations of FSH and prolactin were measured frequently in thyroidectomized male and thyroidectomized castrated starlings kept in outdoor aviaries during 1 year. In thyroidectomized males, testes increased from minimal to maximal size during March, remained large until June, and then decreased slowly during the next 6 months. Plasma FSH increased to a peak in April (126 micrograms/litre) and then declined slightly. Plasma FSH in thyroidectomized castrates was very high (360-760 micrograms/litre) and did not change significantly during the year. In both groups of birds, plasma prolactin concentration remained low (less than 5 micrograms/litre) and there was no molt. These results demonstrate that while thyroidectomy prevents the onset of photorefractoriness in starlings, short days nevertheless cause gonadal regression. However, this regression is not analogous to the short-day-induced gonadal regression shown by species, such as quail, which do not exhibit absolute photorefractoriness.     

Testosterone raises neuropeptide-Y concentration in selected hypothalamic sites and in vitro release from the medial basal hypothalamus of castrated male rats

Although neuropeptide-Y (NPY)-containing neurons are widely distributed in the hypothalamus, castration decreased NPY concentrations only in the median eminence (ME), arcuate nucleus (ARC), and ventromedial nucleus (VMN). We have now examined the effects of testosterone (T) replacement in 2-week castrated male rats on NPY levels in hypothalamic and preoptic area regions and in vitro NPY release in three experiments. In the first experiment we studied the effect of T on NPY concentration in castrated rats. Two-week castrated rats were implanted sc with T-filled or empty Silastic capsules 30 mm in length. Ten days later rats were killed, and NPY levels were measured by RIA in microdissected sites. T implants raised serum T levels to the range found in gonad-intact rats and decreased serum LH levels to the basal range. Further, of the six brain sites examined, significant increases in NPY concentrations occurred selectively in the ME, ARC, and VMN of T-implanted rats. In the second experiment, the ability of T to reverse the effect of castration on NPY levels compared to those in intact (sham) rats was assessed. Again, castration decreased NPY levels in the ME, ARC, and VMN only, and replacement of physiological levels of T restored NPY levels approximately 100%, 127%, and 74% in the ARC, VMN, and ME, respectively. In the third experiment, the effect of castration and T implants (30-mm T capsules for 10 days) to 2-week castrated rats on the in vitro release of NPY from medial basal hypothalamus (MBH) was assessed. Basal NPY release was not significantly changed after castration and T replacement. However, in response to a 30-min pulse of KCl (45 mM) NPY release from the MBH of castrated rats was significantly reduced compared to that in intact and T-replaced castrated rats. These studies show that castration decreases and T replacement restores NPY levels selectively in three hypothalamic sites, viz. ME, ARC, and VMN, and KCl-induced NPY release from the MBH in vitro is decreased after castration and restored by T replacement, thereby suggesting that a local subset of androgen-concentrating neurons may regulate NPY levels and release in a site-specific manner. Further, these results are in line with our emerging view that gonadal steroids modulate neurosecretion not only of LHRH, but also of other functionally linked regulatory peptides.     

The role of the thyroid in testicular function in the gecko, Coleonyx variegatus

The effects of thyroidectomy and administration of thyroxine (200 μg/100 g body weight) on the functional integrity of the testis of the variegated ground gecko, Coleonyx variegatus, were studied. Thyroidectomy prevented the occurrence of normal spermatogenesis. Testes were devoid of all spermatogenic activity. Administration of thyroxine to thyroidectomized animals restored the level of testicular function to that seen in intact animals at that time of year. Injections of thyroxine to intact animals resulted in complete cessation of spermatogenic activity.     

Changes of endocrine properties of a transplantable, multihormonal, pituitary tumor (MtT-F4) after hypophysectomy of host rats.

The transplantable rat pituitary tumor MtT-F4 failed to grow in rats hypophysectomized at the time of transplantation, but did grow in thyroxine-treated hypophysectomized rats. In the latter rats, the tumor did not stimulate the adrenals to the same extent as in control rats. When the tumor did not stimulate the adrenals to the same extent as in control rats. When the tumor cells were isolated and incubated in vitro, those from hypophysectomized thyroxine-treated rats released much less ACTH into the incubation medium than the tumor cells from control rats. They also released significantly less growth hormone than tumor cells from intact, intact thyroxine-treated, and thyroidectomized thyroxine-treated rats. Prolactin release by the isolated tumor cells in vitro was the same in all groups studied. The results suggest that the hypophysectomy and thyroxine treatment of the host rat might selectively influence the production of hormones by the MtT-F4 transplantable rat pituitary tumor.     

Serum testosterone after HCG in thyroidectomized or hyperthyroid male rats (author's transl)

Serum testosterone levels were measured by radioimmunoassay in young male rats before and after IM HCG (500 IU) in basal conditions, after thyroidectomy, or during thyroxine administration. Basal testosterone levels decreased significantly with time in control group (p less than 0.025) and in thyroidectomized rats (p less than 0.025). After HCG, testosterone increased in the 3 groups. There was no difference between control and hypothyroid groups but a lower response (p less than 0.005) in hyperthyroid rats.