Serum LH levels in intact and castrated golden hamsters

Ultrastructure of Hypothalamic Neurons and of the Median Eminence

Our light, and electron microscopic (EM) findings within the hypothalamic supraoptic (SO) and paraventricular (PV) nuclei of the normal female rabbit are in agreement with those reported earlier by other investigators for the same nuclei of the dog and rat. The neurons of these nuclei are the hypothalamic synthesis sites of the neurohypophyseal hormones.

With the exception of the arcuate nucleus, none of the hypothalamic nuclei associated with the control of adenohypohpyseal function have been studied extensively with the electron microscope. On the basis of our EM findings within the female rabbit hypothalamus, all neurons observed within the preoptic (PO) and suprachiasmatic (SCH) nuclei of the non-mated control animal were morphologically identical to the conventional neuron as described by Peters, Palay and Webster (1970). However, following coitus, castration and laparotomy, many neurons of these nuclei showed subcellular changes that have been repeatedly associated with enhanced protein synthesis. These large ‘neurosecretory’ neurons were usually located near capillaries and characterized by their well developed Rough endoplasmic reticulum (RER) and Golgi profiles, dense populations of mitochondria and lysosomes and by the presence of a homogeneous population of densecore vesicles (DCV) showing a peak distribution of 120-140 nm. Since similar neurons were not observed within the PO and SCH of the normal control rabbit it is suggested that we were observing functional states of the same type of neuron and that these ultrastructural changes occur in response to endocrine manipulation.

Two types of neurons described as ‘pale’ and ‘dark’ were observed within the arcuate nucleus of both the control and experimental female rabbit. Ultrastructurally, these neuron types were identical to those described by other investigators for the rat. It has been suggested that the ‘pale’ and ‘dark’ neurons of this hypothalamic nucleus represent functional states of the same type of cell. However, increases in the ratio of ‘dark’ to ‘pale’ neurons as observed within the arcuate nucleus of the rat following castration, were not seen in the rabbit. Similar findings were also not evident within the arcuate nucleus of the female rabbit following coitus.

As far as could be determined, all neurons of the ventromedial (VMN) nuclei of both the control and experimental rabbit were morphologically identical to the smaller, conventional type neuron. Certainly, ultrastructural changes similar to those observed within the PO and SCH nuclei of the female rabbit following coitus, castration or laparotomy, were never observed.

The basic zonation and subcellular organization of the female rabbit Median Eminence (ME) is similar to that described for other mammalian species. Our EM findings within the external layer of the rabbit ME, however, are not entirely in agreement with the earlier study of Duffy and Menefeef 1965). These investigators reported only one population of DCV within the axon terminals of the rabbit ME external layer. We feel that we have ultrastructural evidence for the presence of at least two distinct populations of DCV within this layer of the rabbit ME. Furthermore, since these vesicle populations occurred within separate axon profiles and terminals, differences in their content and origin are suggested.

Certainly, the relationship between releasing factors (RF) and the various populations of DCV observed within the external layer of the mammalian ME is not well established. The smaller (90 nm - 100 nm) DCV we have observed probably contain the catecholamines, while those of larger (120 nm - 140 nm) diameters may well represent the carriers of the RF associated with gonadotropic activity. The latter view is based primarily on our finding or numerous ‘vesicle ghosts’ within the axon terminals abutting the perivascular space (PVS) of portal capillaries of rabbits sacrificed at 10 minutes post-coitus. The mean diameters of 137±14 nm obtained for these ghosts strongly supports the suggested depletion of only the larger of the two DCV populations. Similar changes were not apparent within the axon terminals containing homogenous populations of only the smaller DCV.

Unquestionably, the precise hypothalamic synthesis sites for the RF associated with control of adenohypophyseal function, continues to provoke comment. From the results obtained from countless studies that have employed a variety of neuroendocrinilogical techniques, two main hypothalamic centers of RF synthesis have been suggested: a) the medial basal hypothalamus (MBH) or hypophysiotropic area (HTA) and b) the anterior hypothalamus. The ultrastructural studies carried out to date within this laboratoiy are in favour of the latter for the following reasons:

1) — the presence of large DCV and ‘vesicle ghosts’ within the external layer of the rabbit ME with diameters similar to those of the large (120-150 nm) DCV synthesized within the PO and SCH nuclei of the same animal in response to coitus, castration and laparotomy.

2) — the absence of evidence for the storage of these large DCV within the somata of PO and SCH nuclei, suggesting their immediate transport toward the ME.

3) — the absence of any ultrastructural changes within neuron somata of the rabbit arcuate nuclei which might reflect enhanced neurosecretory activity in response to coitus and/or castration.

These ultrastructural findings within the rabbit hypothalamus may, therefore, provide the first evidence of a morphological nature for the actual release of RF from their ME storage sites, as well as their synthesis within certain neurons of the anterior hypothalamus.

The circadian timing system and reproduction in mammals

Circadian systems in a wide variety of organisms all appear to include three basic components: 1) biological oscillators that maintain a self-sustained circadian periodicity in the absence of environmental time cues; 2) input pathways that convey environmental information, especially light cues, that can entrain the circadian oscillations to local time; and 3) output pathways that drive overt circadian rhythms, such as the rhythms of locomotor activity and a variety of endocrine rhythms. In mammals, the circadian system is employed in the regulation of reproductive physiology and behavior in two very important ways. 1) In some species, there is a strong circadian component in the timing of ovulation and reproductive behavior, ensuring that these events will occur at a time when the animal is most likely to encounter a potential mate. 2) Many mammals exhibit seasonal reproductive rhythms that are largely under photoperiod regulation; in these species, the circadian system and the pineal gland are crucial components of the mechanism that is used to measure day length. The rhythm of pineal melatonin secretion is driven by a neural pathway that includes the circadian oscillator(s) in the suprachiasmatic nuclei. Melatonin is secreted at night in all mammals, and the duration of each nocturnal episode of melatonin secretion is inversely related to day length. The pineal melatonin rhythm appears to serve as an internal signal that represents day length and that is capable of regulating a variety of seasonal variations in physiology and behavior.

Diurnal sensitivity of the neuroendocrine-reproductive axis to the antigonadotrophic influence of melatonin in male Syrian hamsters with experimentally altered cortisol rhythms

Two different experimental models were used to test if a temporal relationship exists between the rhythm of adrenal steroid secretion and the vulnerability of the hamster reproductive system to short photoperiod exposure or to the daily afternoon injection of melatonin. In the first experiment adrenalectomized hamsters were implanted with a cortisol pellet to provide a sustained, rather than rhythmic, level of the hormone. The animals were either placed in short photoperiod or given a daily afternoon melatonin injection. In both cases the gonads underwent atrophy. In the second experiment adrenalectomized hamsters were given a cortisol injection either in the morning (approx. 8 hr before the subsequent afternoon injection of melatonin) or in the afternoon (approx. 1 hr before the subsequent melatonin injection). Measurements of testicular and accessory organ weights 7 weeks later indicated regression of the reproductive system in both the groups when compared with their appropriate controls. Depressed levels of plasma LH, PRL, testosterone and thyroxine (T4) in these animals confirmed the melatonin induced gonadal collapse. The results suggest that apparently there is no temporal correlation between the rhythm of secretion of the adrenal steroids and the responsiveness of the reproductive system to late afternoon injection of melatonin. Interestingly, all the adrenalectomized cortisol injected control animals (not receiving melatonin) had depressed plasma LH and PRL while the testicular weights and plasma testosterone titers remain unaffected.

Acute effects of a single afternoon melatonin injection on LH, prolactin, and pineal melatonin titers in the Syrian hamster

Young adult male Syrian hamsters maintained in an LD 14:10 photo-period (lights out at 8 p.m.) were given a single injection of 25 micrograms melatonin or ethanolic-saline at 4 p.m. Animals from each group were sacrificed at 5 p.m., 4 a.m., and 10 a.m. following the injections. Pituitary and plasma LH and prolactin as well as pineal melatonin levels were determined in all animals by RIA. Pineal melatonin levels were significantly (p greater than 0.025) reduced at 10 a.m. in the melatonin treated group. Melatonin administration also suppressed pituitary LH levels at 4 a.m. and plasma LH levels at 10 a.m. Plasma prolactin concentrations at 4 a.m. and pituitary prolactin levels at both 5 p.m. and 10 a.m. were significantly elevated in melatonin treated hamsters compared with those given ethanolic-saline. These results indicate that exogenous melatonin has immediate effects on several hormones important in the regulation of reproductive events in this species. This acute injection model may therefore aid in the elucidation of melatonin's mechanism of action in the Syrian hamster.

Follicular growth and intraovarian and extraovarian oocyte release after daily injections of melatonin and 6-chloro-melatonin in the Syrian hamster

Groups of adult female Syrian hamsters (Mesocricetus auratus) were injected daily at 17:00 hr with 2.5, 15, or 25 micrograms of melatonin (Mel) or 6-chloro-melatonin (Cl-Mel) for 12 weeks. An ovary from each animal was completely serially sectioned for light microscopic investigation. Judging from the presence of corpora lutea, there were some animals in each group that continued to cycle, although the postestrous, white mucous discharge had disappeared. Noncycling animals were most often found in the 25-micrograms group of Cl-Mel. Only uterine weights of noncycling animals treated with either 25 or 15 micrograms of Mel or Cl-Mel were statistically significantly depressed versus controls. Cl-Mel (25 micrograms) significantly suppressed the total number and size of antral follicles (P less than 0.05). Follicular ruptures with incomplete or complete release of the oocyte out of the follicular compartment were observed. The oocyte release occurred either into the ovary ("intraovarian oocyte release: IOR") or outside of the ovary ("extraovarian oocyte release: EOR"). Compared with controls, the total number of IOR was increased in all experimental groups with the exception of the 2.5-micrograms group of Cl-Mel. IOR appeared in both preantral and antral follicles, and often IOR was complete. In controls, only preantral follicles were involved in IOR; these were primarily incomplete ones. IOR was seen in cycling and noncycling animals. By contrast, EOR was exclusively observed in noncycling hamsters. It is concluded that the cessation of postestrous, white mucous discharge is not necessarily an index for a halt in cyclic ovarian function. Injections of 25 micrograms of Cl-Mel are more effective than 25 micrograms of Mel in suppressing ovarian function. Both Mel and Cl-Mel increase the frequency of IOR. Finally, noncycling hamsters show EOR that is regarded as an abnormal ovulation.

Natural and synthetic analogues of melatonin and related compounds. I. Effects on the reproductive system of the male Syrian hamster

The potential antigonadotrophic properties of a number of synthetic and natural melatonin analogues were examined. Adult male hamsters received daily subcutaneous injections (25 micrograms/animal/day) each afternoon for 7 or 10 weeks. The only analogue to possess antigonadotrophic activity similar to melatonin wa 6-chloromelatonin; melatonin and 6-chloromelatonin produced a significant reduction in both testicular and accessory sex organ weights, plasma LH and pituitary prolactin. These results suggest that, in the Syrian hamster, 6-chloromelatonin acts as a melatonin agonist and may be of use to elucidate further the physiological role of melatonin.

Arginine vasotocin alters hamster pituitary LH and prolactin secretion in vitro

1. Arginine vasotocin (AVT) stimulated the release of both luteinizing hormone (LH) and prolactin (PRL) from normal hamster (Mesocricetus auratus) anterior pituitary glands in a dose-dependent manner. 2. AVT significantly augmented the depressed level of PRL release from pituitaries of blind-pinealectomized animals. 3. LH release from the pituitaries of blinded hamsters was inhibited by AVT while FSH levels were unaffected.

Ovulation, ovum transport and implantation in the adult golden hamster

The time of ovulation in various hamster colonies and its relationship with the period of light as reported in the literature was compared with the observations made in this study. Ovulation and transport of ova to the oviduct was studied during days 4 and 1 of the estrous cycle. Microscopical examination of the various oviducal segments for the presence of ova resulted in a detailed description of physiological ovum transport for the adult golden hamster. Simultaneously the developmental stages of ova were checked together with the occurrence of unfertilized ova and the percentage of ova implanted in the uterus. Finally, the contractility of the oviduct in vitro was observed for groups of animals at different times in the pre- and postovulatory period. The presented data indicate that the golden hamster is useful for the study of ovum transport processes. This is due to the regularity of its estrous cycle including multiple ovulations, and because ovum transport can easily be monitored microscopically with a high degree of reliability.

Melatonin prevents decrease in plasma PRL and LH levels in male hamsters exposed to a short photoperiod

Treatment of male golden hamsters with melatonin injections can cause a decrease in plasma gonadotropin and Prl levels and induce testicular regression. However, administration of melatonin via subcutaneously implanted Silastic capsules can prevent short photoperiod from causing the testes to regress. In order to explain this effect of melatonin capsules, we examined plasma Prl, LH and FSH levels in pinealectomized and sham-operated hamsters in which empty or melatonin-filled Silastic capsules had been implanted. After implantation, the hamsters were transferred to a short photoperiod (5 h L:19 h D) for 9 weeks. Both pinealectomy and melatonin completely prevented the decline in plasma Prl and LH levels and in the weight of the testes and the seminal vesicles. Moreover, testicular weight and plasma Prl and LH levels were higher in pinealectomized animals given melatonin capsules than in pinealectomized animals given empty capsules. In order to obtain some indication whether melatonin may affect testicular function directly, we have examined the influenced of melatonin on the production of testosterone by hamster testes in response to hCG in vitro. Addition of 0.2, 10 or 500 ng of melatonin per ml of incubation medium had no effect on testosterone production in this system. It is concluded that melatonin capsules prevent regression of the male reproductive system in short photoperiod, most likely by preventing the decline in plasma Prl and LH levels and that these effects of melatonin are not mediated through or dependent on the pineal.

Depression of nocturnal pineal serotonin N-acetyltransferase activity in castrate male rats

Pineal serotonin N-acetyltransferase (NAT) activity was examined in intact rats, castrated rats, and in rats that had been castrated and had received testosterone proprionate. Castration resulted in significantly depressing nocturnal levels of pineal NAT (p less than 0.05) when compared to enzyme activity in intact rats. Testosterone proprionate administration restored plasma LH levels to normal values in castrate rats but did not induce nocturnal pineal enzyme activity to levels seen in the pineal glands of intact rats. The data substantiate the existence of a feedback control of pineal biosynthetic activity by the hypophyseal-gonadal system, but the identity of the hormone(s) responsible for regulation of pineal NAT activity is not known.

Melatonin induction of gonadal quiescence in pinealectomized Syrian hamsters

Pinealectomized Syrian hamsters were injected thrice daily with 25 micrograms of melatonin per injection. The injections were administered at 3-hour intervals either during the day or during the night of a photoperiodic cycle of 14 hours of light and 10 hours of darkness. After 6 weeks of treatment with melatonin during the night, both pinealectomized and intact hamsters had reduced testis weight, and pinealectomized hamsters showed decreased levels of serum gonadotropins. Injection of melatonin during the day for 7 weeks either once (75 micrograms) a day or thrice (25 micrograms per injection) daily caused a reduction in testis weight in pinealectomized hamsters. Both pinealectomized and intact females injected with melatonin thrice daily during the day became anovulatory by week 7 of treatment. These results are similar to those observed when hamsters are exposed to a short photoperiod, suggesting that melatonin may be acting as a hormone in mediating the effects of photoperiod on the reproductive system of the Syrian hamster.

Subcutaneous melatonin implants inhibit reproductive atrophy in male hamsters induced by daily melatonin injections

Daily afternoon (at 7 p.m.) injections of melatonin (25 microng in oil) into adult male hamsters for 50 days led to atrophy of the testes and accessory sex organs (seminal vesicles and coagulating glands) and in a significant depression in pituitary LH and prolactin content and concentration. These actions of melatonin were prevented if the animals had been pinealectomized before the daily melatonin injections were begun. Likewise, if hamsters received a weekly subcutaneous implant of melatonin in beeswax (1 mg melatonin in 24 mg beeswax) the daily melatonin injections failed to inhibit the growth of the reproductive organs and to depress pituitary LH and prolactin levels. Beeswax by itself had no such effect.

Melatonin induction of testicular recrudescence in hamsters and its subsequent inhibitory action on the antigonadotrophic influence of darkness on the pituitary-gonadal axis

Exposure of adult male hamsters to light-dark (LD) cycles of 0:24 (in hours) for 12 weeks induced atrophy of the testes and accessory sex organs and a drop in immunoreactive pituitary and plasma prolactin levels. The subsequent exposure of these hamsters to LD 14:10 cycles caused regeneration of the sexual organs and an increase in pituitary LH and prolactin levels. Likewise, the weekly treatment of dark-exposed hamsters with subcutaneous melatonin-beeswax pellets led to regeneration of the peripheral reproductive organs and an increase in pituitary LH and prolactin levels. After gonadal regeneration, complete darkness failed to induce atrophy of the reproductive system indicating that either long photoperiods or melatonin treatment cause the pituitary-gonadal axis to be refractory to the inhibitory influence of darkness.

Exogenous and endogenous control of the annual reproductive cycle in the male golden hamster: participation of the pineal gland

Testes and accessory sex organs (seminal vesicles and coagulating glands) of hamsters exposed to natural lighting (NL) conditions beginning September 22 underwent complete degeneration by October 31. The following February the testes began to regrow with the regeneration being complete by mid to late March. Associated with the atrophic response of the testes during the winter months was consistent depression in pituitary prolactin and an inconsistent decrease in pituitary luteinizing hormone levels. If hamsters are pineal lectomized prior to their exposure to NL, the sexual organs do not atrophy and the pituitary hormone levels do not drop. Moving hamsters from NL to the long daily photoperiods (light:dark cycles of 14 hrs light and 10 hrs darknessLD 14:10) of the laboratory near mid winter is followed by regrowth of the gonads and accessory glands. Regeneration of the reproductive system in the spring is not a function of increasing photoperiodic length since if animals are completely deprived of light (by blinding) in February, the gonads still regenerate. When hamsters are exposed to LD 14:10 cycles during the subsequent summer, the return to NL on September 22 is followed by a second involution of the reproductive system. However, if the period of LD 14:10 (the simulated summer) is shortened by ten weeks, the second return to NL does not initiate involution of the reproductive system. During the simulated summer complete light deprivation by blinding is incapable of forcing atrophy of the sexual organs.

A study of indoles which inhibit pineal antigonadotrophic activity in male hamsters

Blinding adult male golden hamsters led to involution of the testes and accessory sex organs (seminal vesicles and coagulating glands) and to a regression in pituitary prolactin levels within 8 weeks. The subcutaneous implantation of either melatonin or 6-hydroxymelatonin (1 mg/wk in beeswax) prevented the atrophy of the reproductive organs and the decrease in the stores of pituitary prolactin. Two other indoles, N-acetylserotonin and 5-hydroxytryptophol, failed to counteract the reproductive effects of blinding. Both melatonin and 6-hydroxymelatonin significantly elevated plasma LH titers.

Ovulation induced by synthetic luteinizing hormone-releasing hormone in the hamster

A synthetic decapeptide, corresponding to the chemical structure of luteinizing hormone-releasing hormone from porcine hypothalami, was tested for the induction of ovulation in golden hamsters that had previously been treated with phenobarbital to prevent spontaneous ovulation. Subcutaneous injection of 0.089 to 0.357 nanomole of this synthetic luteinizing hormone-releasing hormone stimulated release of luteinizing hormone and induced ovulation.