Effects of long-term treatment with melatonin or melatonin plus ectopic pituitary transplants on testicular LH/hCG and prolactin receptors in juvenile Syrian hamsters (Mesocricetus auratus) (original) (raw)
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Annales de Biologie Animale Biochimie Biophysique, 1979
Adult female Syrian hamsters (64-82 g) were ovariectomized and thereafter maintained under light : dark cycles of 14 : 10. After 56 days of treatment the animals were killed and pituitary and plasma levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) were measured by radioimmunoassay. Ovariectomy caused marked rises in both the content and concentration of LH and FSH within the pituitary gland while pituitary PRL levels fell. Plasma levels of LH and FSH also rose while PRL titers remained unchanged. The daily afternoon injection of melatonin (25 ¡ L g subcutaneously) further increased the accumulation of LH and FSH within the pituitary. These responses to daily melatonin injections were prevented if animals additionally received biweekly subcutaneous melatonin-beeswax pellets (1 mg melatonin in 24 mg wax) which presumably released melatonin continually.
Biology of Reproduction, 1993
The aims of this study were to validate the use of a timed infusion protocol for restoring physiologically appropriate rhythms of melatonin in the circulation of pinealectomized hamsters and, using such infusions, to compare the relative importance of the parameters of the nocturnal melatonin signal-frequency, phase, and duration of the interval between signals-in the photoperiodic control of testicular function in male Syrian hamsters. Hamsters were pinealectomized and fitted with a chronic s.c. cannula enabling them to receive timed infusions of melatonin (50 ng/h) or saline vehicle (50 jl/h). In experiment 1, RIA of serum samples confirmed that s.c. infusions produced a pattern of melatonin in the blood equivalent in amplitude and duration to that observed previously in pineal-intact animals exposed to a short photoperiod. In experiment 2, we investigated the relative importance of the frequency of the melatonin signal and the duration of the interval between signals. Pinealectomized animals that received infusions of saline for 6 wk had large testes and high concentrations of LH in the serum. Animals that received a series of short-day-like infusions of melatonin of 14-h duration, separated by an interval of either 10 h or 6 h, underwent gonadal regression and had low serum concentrations of LH. Animals that received infusions of melatonin of 8-h duration, separated by intervals of 12 h, also exhibited full gonadal collapse. However, animals that received the same 8-h infusions separated by intervals of 8 h (i.e., once every 16 h) did not undergo gonadal regression and their circulating levels of LH remained high. These results demonstrate that the frequency at which melatonin signals are received, rather than the duration of the interval between them, is critical to interpretation of photoperiodic information. To confirm that the continuous duration of a melatonin signal is also important, another group of animals received two daily infusions of melatonin of 5-h duration, separated by a break of 4 h, giving an overall signal of 14 h. These infusions were ineffective, there being no difference in testicular size or serum LH titers between animals receiving melatonin and those receiving saline. In experiment 3, we tested whether the phase at which a signal is presented is important in order for animals to recognize a series of long signals and engage a short-day-like response. Controls receiving infusions of melatonin of 8-h duration at the same phase every day (infusions terminated 1 h before lights-off) for 6 wk exhibited gonadal regression. Experimental animals received the same number of infusions, but these were delivered at one of three phases of the light:dark (L:D) cycle, in an irregular order such that no infusion was predicted by the phase of its predecessor. After 6 wk, these animals exhibited significant gonadal atrophy, demonstrating that the phase of the L:D cycle at which a melatonin signal is encountered does not matter and that a series of signals need not be encountered in an ordered fashion for a photoperiodic response to occur.
Reproduction, 1983
Adult male golden hamsters were given ectopic pituitary transplants at the time of transfer from a long (stimulatory) photoperiod (14 h L:10 h D) to a short photoperiod (5L:19D) or at various intervals thereafter. Grafts given at the time of transfer to 5L:19D failed to maintain plasma FSH levels significantly above those observed in sham-operated controls or to prevent regression of the testes. Grafts given after 4 weeks in 5L :19D increased plasma FSH levels and significantly diminished the decrease in testicular volume. Grafts given 14 weeks after transfer to 5L :19D caused a modest increase in plasma FSH levels and did not significantly alter the time course of testicular redevelopment. Similarly, grafts given 18 weeks after transfer to 5L :19D had no effect on testicular weight measured 2\m=.\5weeks later. These results indicate that the ability of prolactin (secreted by the grafted pituitary) to increase testicular size and the responsiveness of the hypothalamic\p=n-\pituitarysystem to prolactin gradually increase during short photoperiod-induced testicular regression, are maximal when the testes are fully regressed and decrease again during spontaneous redevelopment of the testes. We suggest that the effects of prolactin on the testes in golden hamsters are partly mediated by prolactin-induced stimulation of FSH release from the in-situ pituitary.
Possible prolactin-mediated effects of melatonin on gonadotropin secretion in the rat
Pharmacology Biochemistry and Behavior, 1989
Melatonin administration to pituitary-grafted male and female rats resulted in a marked decrease of previously high plasma prolactin levels, while an increase in prolactin levels was observed in sham-operated controls. The latter effect was significant only in males. Treatment with melatonin did not modify basal LH hormone levels or LH responses to luteinizing hormone-releasing hormone (LHRH) in sham-operated rats of either sex. However, in pituitary-grafted females, melatonin increased both basal and LHRH-stimulated LH levels towards values recorded in sham-operated controls. No effects on basal LH levels were detected in grafted males under melatonin treatment, but the response of LH to LHRH was markedly increased and no longer differed from the values measured in sham-operated control animals pretreated with saline. Melatonin did not affect follicle stimulating hormone (FSH) levels except for an increase in FSH response to LHRH in grafted females. These findings suggest the existence of sex-dependent effects of melatonin on LH and FSH secretions. These effects of melatonin may be mediated by the different plasma prolactin levels in pituitary-grafted and sham-operated rats.
The Journal of Steroid Biochemistry and Molecular Biology, 2000
The purpose of this study was to investigate the eects of prenatal melatonin administration on the sensitivity of the androgens negative feedback eect on gonadotropin and prolactin secretion in male ospring. Male ospring of control (control-ospring) and melatonin treated (MEL-treated) (150 mg/100 g BW) mother rats during pregnancy (MEL-ospring), at infantile, prepubertal, and pubertal periods were studied. LH secretion in response to testosterone propionate (TP) in controlospring showed the classical negative feedback eect at all ages studied. In MEL-ospring a negative response after TP was also observed in all ages studied although the magnitude of this response was altered in this group as compared to controls. FSH values were signi®cantly lower at most ages and time points studied in MEL-ospring than in control-ospring. FSH secretion in MEL-ospring showed a delayed negative feedback action of TP injection as compared to control-ospring. This response was observed at 21 days of age in control-ospring and delayed until day 30 of life in MEL-ospring. Parallely it remain at later age in MEL-ospring than in control-ospring. Prolactin secretion in control-ospring showed increased values after TP injections from infantile to pubertal periods. This increase was blunted in MEL-ospring at 17 and 35 days of age showing signi®cantly reduced p`0X01; p`0X05 plasma prolactin levels. During pubertal period a prolactin positive response to TP administration was observed in MEL-ospring but with signi®cantly lower magnitude than in control-ospring. These results indicate that prenatal melatonin exposure induced changes in the sensitivity of gonadotropin and prolactin feedback response to testosterone, indicating a delayed sexual maturation of the neuroendocrine-reproductive axis in male ospring.
Journal of Pineal Research, 1984
The combination of pituitary grafts and daily injections of gonadotrophin-releasing hormone (GnRH) completely prevents gonadal atrophy in blind (BL) and melatonin (Mel)-treated male hamsters. In order to avoid potential problems associated with the use of pituitary grafts and further define the interactions between prolactin (PRL) and GnRH in preventing reproductive regression, we injected various doses of each hormone either alone or in combination into BL or Mel-treated male hamsters. In another experiment, BL hamsters received either weekly beeswax implants of estradiol benzoate (EB) (1 mg) alone or EB implants in combination with daily injections of GnRH (2 micrograms). In each experiment the hamsters were BL and/or treated with hormones for 10 wk. Either GnRH (8 micrograms) or PRL (5 micrograms) per d partially prevented gonadal atrophy in BL hamsters. However, increasing doses of GnRH plus PRL were more effective than either hormone alone in preventing gonadal atrophy. The combined effect of these hormones was greater than the algebraic sum of their individual effects. Injections of either GnRH or PRL alone resulted in a significant maintenance of gonadal and accessory organ size in Mel-treated animals. The combination of GnRH and PRL resulted in virtually complete maintenance of testicular weight while the accessory sex organs remained atrophic. The combined effect of these hormones was equivalent to the algebraic sum of their individual effects. The treatment of BL animals with EB implants with or without GnRH did not prevent reproductive regression in spite of elevated serum PRL levels. In conclusion, the effects of GnRH and PRL were synergistic in BL hamsters and additive in Mel-treated animals. The data suggest that there is a differential responsiveness of BL v. Mel-treated hamsters to the individual as well as the combined actions of GnRH and PRL.