Hypothalamic cuts suggest a brain site for the antigonadotrophic action of melatonin in the Syrian hamster

Melatonin and the brain in photoperiodic mammals

The reproductive cycle of photoperiodic species is driven by seasonal changes in daylength. The pineal gland transduces photic information into an endocrine signal. The duration of the nocturnal bout of melatonin secretion is a direct indicator of night-length. The circadian rhythm of melatonin production is driven by a multisynaptic pathway from the suprachiasmatic nuclei (SCN), via the parvocellular portion of the paraventricular nucleus to the preganglionic sympathetic neurons of the thoracic spinal cord. The melatonin signal acts as an interval timer. The cellular basis of the detection of the signal is unknown. The site of detection is possibly within the anterior hypothalamus. The SCN are not essential components of the system that responds to the pineal interval timer. Photoperiod and the pineal melatonin signal have pronounced effects on the function of endogenous opioids, which are probably related to changes in the neuroendocrine mechanisms that regulate gonadotropin release.

Lesions in the bed nucleus of the stria terminalis, but not in the lateral septum, inhibit short-photoperiod-induced testicular regression in Syrian hamsters

The transfer of adult male hamsters from long days (LD) to short days (SD) (i.e. < 12 h of light per day) typically results in marked testicular regression and a decline in plasma testosterone concentrations. To help disclose key brain regions responsible for mediating this photoperiodic response male hamsters received either chemical (i.e. N-methyl-D-aspartate; NMDA) or radiofrequency current lesions in the bed nucleus of the stria terminalis (BNST), and were then exposed to SD for 15 or 12 weeks, respectively. Although body weights were similar between sham-lesioned controls and the NMDA-lesioned hamsters, the latter showed a significant attenuation of testicular regression; additionally, their plasma testosterone concentrations remained at typical LD levels. When radiofrequency current-lesioned hamsters were transferred from LD to SD they also failed to show significant signs of testicular regression, nor a decline in plasma testosterone concentrations, nor a complete arrest of spermatogenesis. In contrast, sham-lesioned controls or hamsters that were lesioned dorsally to the BNST at a site primarily involving the lateral septum all showed the expected degree of testicular regression, a decline in plasma testosterone concentrations, and complete arrest of spermatogenesis; body weights were similar in all of the experimental group. Taken together, these findings suggest that the BNST, a brain area traditionally not associated with reproductive function, may play an important role in mediating photoperiodic information to the neural circuits that control the reproductive axis.

Changes induced by pineal indoles in post-implantation mouse embryos

1. A subcutaneous injection of hydroxyindoleacetic acid (HIAA), melatonin (MEL), methoxytryptophol (MTP) or methoxytryptamine (MTA) (1 mg/25 g body wt/injection) at 8.5 days post-coitum (p.c.) did not produce any effect on the development of embryos in utero at 10.5 days p.c. 2. Two subcutaneous injections (3 mg/25 g body wt/injection) of MTP, but not HIAA, MEL nor MTP, at 7.5 and 8.5 days p.c. brought about a decrease in gravid uterine weight and number of live fetuses and an increased incidence of early resorptions at 17.5 days p.c. 3. MEL at doses 100 and 200 micrograms/ml, MTP at doses 50 and 100 micrograms/ml and MTA at 25, 50 and 100 micrograms/ ml brought about an increase in the number of abnormal embryos cultured in vitro and an increased incidence of abnormal yolk sac circulation, body axis, optic and otic placodes, branchial apparatus, forelimb buds and cranial neural tube. 4. The findings show than melatonin, methoxytryptamine and methoxytryptophol produced an embryotoxic effect on embryos at the early somite stage in vitro while only methoxytryptamine had an adverse effect on embryonic development in vivo.

Effect of photoperiod on testicular histology in golden hamsters and C57 and BALB/C mice

The effects of different photoperiods (14L:10D, 12L:12D, and 8L:16D) on testes and accessory sex organs were examined in mature male golden hamsters, C57 mice, and Balb/c mice. The seminiferous tubules of hamsters exposed to the 14L:10D and 12L:12D photoperiods contained various stages of spermatogenesis, while the testes of hamsters maintained under the 8L:16D photoperiod were atrophic. The seminal vesicles of hamsters kept under the 14L:10D and 12L:12D photoperiods possessed tall columnar epithelial cells and abundant secretion in the lumen. In contrast, the seminal vesicles of hamsters exposed to the 8L:16D photoperiod had an epithelium made up of low columnar or cuboidal cells and contained a reduced amount of secretion in the lumen. The histological appearances of testes and seminal vesicles of C57 and Balb/c mice exposed to the three different photoperiods were similar.

Development of pre-implantation mouse embryos under the influence of pineal indoles

The developmental toxicity of pineal hormones on mouse embryos was examined both in vitro and in vivo. Pregnant ICR mice were divided into groups which received at 1.5 days post-coitum (p.c.) and again at 2.5 days p.c. a subcutaneous injection of one of the following pineal indoles: hydroxyindoleacetic acid (HIAA), melatonin (MEL), methoxytryptophol (MTP) or methoxytryptamine (MTA). Mice treated with the injection vehicle served as the control. The animals were sacrificed at 17.5 days p.c. The pineal indole treatment did not cause changes in the gravid uterine weight, numbers of implants, early resorption, late resorption, dead fetuses and live fetuses, fetal weight or fetal crown-rump length, and did not produce embryos with external or visceral defects. However, some mice treated with MTP or MTA produced litters in which all embryos underwent resorption. Cultured embryos at the 4-cell stage were treated with the aforementioned pineal indoles and examined after 24, 48, 72 and 96 hours. It was found that MTA retarded embryonic development at all time points studied. HIAA also produced a slight inhibitory effect on embryonic development. Some embryos underwent degeneration in response to the MTA and HIAA treatments. However, MEL- and MTP-treated embryos were in general developmentally similar to control embryos. When cultured embryos were treated at the 8-cell to compacting stage, it was found that MTA exerted only a slight retarding effect on embryonic development, while other indoles were devoid of any conspicuous effects.

Action of pineal indoleamines on the reproductive systems of the male C57 mouse and golden hamster

Intraperitoneal administration of melatonin, methoxytryptamine and methoxytryptophol in the late afternoon to mice exposed to a 14L:10D photoperiod induced appearance of necrotic cells in seminiferious tubules and an increase in incidence of aspermic tubules. The histological appearances of the seminal vesicles and coagulating glands were, however, unaffected by the treatments. Treatment of mice with methoxyindoleacetic acid did not produce discernible changes in the testes and accessory sex organs. Subcutaneous injections of the aforementioned pineal indoles at a dose of 250 micrograms/injection for 3 months to mature male hamsters kept under a 14L:10D photoperiod did not evoke changes in the testes, seminal vesicles and coagulating glands, suggesting a down-regulation of melatonin receptors by administration of a high dose of pineal indole over a prolonged period.

Extrahypothalamic effects of melatonin administration on serotonin and norepinephrine synthesis in female Syrian hamsters

The effects of daily late afternoon injections of melatonin for 10 weeks on the metabolism of serotonin (5-HT) and norepinephrine (NE) were examined in regional brain extracts of intact and ovariectomized (GX) Syrian hamsters. Accumulation of 5-HT and NE after administration of the monoamine oxidase inhibitor pargyline was used as a measure of the rate of neurotransmitter synthesis-with concentrations determined by HPLC with electrochemical detection. Daytime 5-HT synthesis was significantly decreased in the amygdala of melatonin-treated hamsters that had been GX (to 50% of GX controls). No significant effect on 5-HT synthesis could be detected in the mediobasal hypothalamus (MBH), however, a significant increase was demonstrated in the pontine brain stem of both intact and GX hamsters treated with melatonin. Daytime NE synthesis was decreased to levels not significantly different from zero in the amygdala of GX hamsters treated with melatonin, while in the brain stem, melatonin reduced NE synthesis in both intact and GX animals. The present data demonstrate that these melatonin effects on 5-HT and NE metabolism are not limited to the MBH and are not secondary to melatonin-induced changes in circulating levels of the ovarian steroids.

Inhibition of sexual maturation in male rats by melatonin: evidence linking the mechanism of action to changes in the regulation of hypothalamic neuropeptide y

Activation of gonadotrophin-releasing hormone (GnRHJ pathways is a pivotal event in the process of sexual maturation, however the regulatory influences that precipitate this change and lead to the onset of puberty remain poorly understood. Recent studies indicate that neuropeptide Y (NPY) may participate in the regulation of luteinizing hormone secretion by modulating the pattern of GnRH secretion and by directly altering the pituitary responsiveness to GnRH stimulation. To determine whether NPY plays a role in puberty-associated changes in hypothalamic function, levels of NPY-like immunoreactivity (NPY-IR) were measured in a fragment of the hypothalamus encompassing the median eminence and medial portion of the arcuate nucleus (ME-AN), and also in the remainder of the hypothalamus from male rats of different ages. To identify changes in hypothaiamic NPY linked to the process of sexual development, the effect of delaying sexual maturation by daily afternoon administration of 100 μg melatonin (MT) from 20 to 40 days was investigated. In the hypothalamus and ME-AN, total NPY content increased progressively with age. Expressed as a concentration (fmol/μg extracted protein), peak values for the ME-AN (55.4 ± 7.0) were observed at 30 days of age followed by a decline to lower levels (30.2 ± 1.9) at 40 days. Daily afternoon administration of MT from 20 days of age resulted in significant increases (P<0.01) in the levels of NPY-IR in the ME-AN compared to control values at 30 and 40 days of age. MT was without effect on NPY-IR levels in the remainder of the hypothalamus. When MT was administered in the early morning, a procedure which does not delay sexual maturation, NPY-IR values for the ME-AN region were not different from control rats indicating that the MT-induced changes in NPY were related to the effects on sexual maturation. Using pituitary luteinizing hormone content and seminal vesicle weight as indices of sexual development, significant inverse correlation coefficients (P<0.001) between these parameters and the NPY concentration in the ME-AN were observed (r =-0.79 and -0.70, respectively). From published data it is not possible to conclude whether the main effects of NPY are exerted at the hypothalamic or pituitary level. However, the changes in the NPY content of the ME-AN observed during the onset of puberty, and the influence of MT on these changes, support assertions that NPY is involved in the regulation of sexual maturation.

Localization and characterization of melatonin binding sites in the brain of the rabbit (Oryctolagus cuniculus) by autoradiography and in vitro ligand-receptor binding

The distribution and the properties of the melatonin binding sites were characterized in the brain of the rabbit by combined use of autoradiography and in vitro ligand-receptor binding. Autoradiography revealed widespread specific binding in the brain. The pars tuberalis of the pituitary gland, suprachiasmatic nuclei, ventromedial hypothalamic nuclei, tapetum, hippocampus, indusium griseum, cingulate gyrus, cortex and the choroid plexus were intensely labelled. Diffuse specific binding was recorded in the olfactory bulb and the anterior hypothalamus. Series of in vitro ligand-receptor binding experiments, using the anterior hypothalamus, confirmed that the binding was of high affinity and specificity. Coincubation with a non-hydrolyzable GTP analogue provoked a shift in the binding affinity, the numerical values of the Kd increasing from 20-30 pM to 280-300 pM. Apparently the melatonin receptor in the rabbit brain is linked to its second messenger via a G protein, similarly to what has been described for the brain of other vertebrates.

Diurnal variations in melatonin binding sites in the hamster brain: impact of melatonin

The distribution of 125I-melatonin binding sites in the male Syrian hamster brain was recorded at 3 times over a 24 h period. The binding in the hypothalamus, hippocampus, medulla-pons and midbrain of the hamsters varied significantly over the 24 h period with different patterns and phases. No such variations were observed in the parietal cortex. Daily morning (10.00 h) or late afternoon (18.00 h) injections of melatonin for 28 days markedly increased the serum concentrations of melatonin at all times recorded. Serum concentrations of testosterone were significantly lower in animals injected with melatonin in the late afternoon than in the untreated controls; no such decrease was observed in animals injected in the morning despite the continuously elevated levels of circulating melatonin. The daily melatonin injections did not significantly affect 125I-melatonin binding in the hypothalamus, parietal cortex and medulla-pons. In the midbrain, 125I-melatonin binding decreased regardless of the time of injection. In the hippocampus, morning melatonin injections caused a marked decrease in 125I-melatonin binding at all times recorded whereas melatonin injected in the late afternoon led to a decrease in 125I-melatonin binding at 10.00 h only. These results indicate diurnal variations in 125I-melatonin binding sites in discrete brain areas of the golden hamster, persisting despite prolonged duration of elevated levels of circulating melatonin. The differential effects of timed melatonin injections on the hippocampal 125I-melatonin binding sites are positively correlated with the counter-antigonadal response produced by morning melatonin injections.

Evidence that short photoperiod-induced gonadal regression in the Mongolian gerbil is mediated by the action of melatonin in the medial hypothalamus

The present study was undertaken to examine the effects of exposure to short photoperiod (SD) and treatment with subcutaneous (s.c.) or intrahypothalamic melatonin-containing beeswax implants on reproduction in the Mongolian gerbil Meriones unguiculatus. Exposure of adult female gerbils to SD (8 h light: 16 h dark) caused a significant decrease in weight of the reproductive tract (ovaries, oviducts, uterus and vagina; RTW) compared to animals maintained under stimulatory photoperiod (12 h light: 12 h dark; 81.4 +/- 8.0 mg vs 151.8 +/- 16.0 mg, respectively (P less than 0.01]. Treatment with two large s.c. implants, each containing 3 mg melatonin, mimicked the antigonadal effect of SD (68.7 +/- 3.9 mg vs 118.1 +/- 19.5 mg for the blank (melatonin-free) implant controls; P less than 0.05). Gerbils with a small melatonin-beeswax pellet (containing 0.2 mg melatonin) in the anterior hypothalamus (AH) underwent a significant reduction in RTW compared to gerbils with a blank intrahypothalamic implant (73.0 +/- 5.7 mg vs 134.6 +/- 14.5 mg, respectively; P less than 0.01). Melatonin pellets in the mediobasal hypothalamus (MBH) also induced gonadal regression, but to a lesser degree (88.7 +/- 12.6 mg; P less than 0.05 vs blank controls). Small melatonin pellets placed elsewhere in the hypothalamus or s.c. had little effect on RTW (143.4 +/- 20.9 mg and 129.2 +/- 19.7 mg, respectively). Only 25% of the gerbils with melatonin pellet in the AH or MBH had a corpus luteum compared to 73% of the blank controls (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Paraventricular nucleus projections mediating pineal melatonin and gonadal responses to photoperiod in the hamster

Knife cuts were placed around the paraventricular nucleus of the hypothalamus (PVN) in order to identify the pathways mediating photoperiodism and pineal melatonin production in male golden hamsters. Cuts in the coronal plane caudal to the PVN, have no effect on photoperiodic control of the testes unless they actually damage the PVN. Bilateral parasagittal cuts at the medial border of the lateral hypothalamus block short photoperiod-induced gonadal regression. Nighttime levels of pineal melatonin are reduced by these cuts, but unaffected by caudal cuts. Projections from the lateral PVN region descending towards the spinal cord appear to be critical for the control of pineal melatonin production and the control of the testicular function by short photoperiod.

Melatonin and pituitary-gonadal function in disorders of eating behavior

In order to study the possible relationships between melatonin secretion and pituitary-gonadal function, the circadian rhythm of plasma melatonin, the basal levels of estradiol-17beta and testosterone and the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) response to luteinizing hormone-releasing hormone (LH-RH) stimulation were evaluated in normally cycling healthy women and in two groups of women with menstrual dysfunctions related to eating disorders (19 patients with anorexia nervosa and 16 with primary obesity). The circadian rhythm of plasma melatonin reached statistical significance in anorectic patients but not in obese patients. The mean 24 h melatonin level was significantly higher in anorectic than in obese patients and in control subjects. However, both groups of patients shared some abnormalities of melatonin circadian pattern, such as increased ratio between day and night melatonin levels, abnormal secretory peaks during the light hours and great interindividual variability for timing, amplitude, and duration of melatonin nocturnal peak. A selective impairment of LH secretion was observed in both anorectic and obese patients. By considering together the two groups of patients and controls, a linear inverse correlation between the circadian mesor of plasma melatonin and the basal and LH-RH stimulated LH levels was found. The persistence of a certain melatonin secretion during the light hours in both anorectic and obese patients could play an inhibitory role on the pituitary gonadal function in these subjects.

Comparison of the effects of short photoperiod exposure and melatonin treatment in ovariectomized LSH/SsLaK hamsters

The object of this study was to compare the effects of short photoperiod (SP) and melatonin (MEL) treatment on the reproductive axis in ovariectomized LSH/SsLak hamsters. Animals acclimatized in long photoperiods (LP) (14L:10D) and showing regular estrous cycles were ovariectomized. Half of the operated hamsters received Silastic capsules containing 17-beta estradiol (E2). On the following day the animals were further subdivided into three groups: the animals in one group received daily afternoon injects of melatonin (MEL), those in a second group were given the vehicle, and animals in the third group were transferred from LP to SP (8L:16D). All animals were killed after 30 days. In hamsters without E2 replacement, MEL or SP exposure significantly suppressed serum and pituitary FSH levels, although MEL was more effective in this regard. On the other hand, SP exposure did not change serum FSH levels in animals with E2 implants, whereas MEL effectively suppressed them. SP or MEL reduced serum LH levels to a similar extent in the absence of E2 replacement, yet in animals with E2 implants only MEL significantly lowered LH levels below LP E2-treated controls. This was in contrast to effects on the pituitary where both treatments were equally effective in the depression of LH content. Serum PRL levels were similarly suppressed by MEL or SP exposure in E2-treated hamsters. On the other hand, pituitary PRL levels were not affected by either treatment in animals with E2-containing capsules, whereas SP or MEL treatment both significantly depressed pituitary PRL contents in hamsters without E2 replacement. SP treatment lowered MBH LHRH contents in animals with E2-containing capsules; no other significant changes in hypothalamic LHRH were noted. The data suggest that daily treatment with 25 micrograms of MEL is generally more effective in the suppression of gonadotropin levels than SP exposure. It is suspected that the mode of administration of MEL, and its quantity, may interact with estrogen differently than SP in the induction of physiological changes and regulation of the LHRH system.

Intra-hypothalamic melatonin blocks photoperiodic responsiveness in the male Syrian hamster

Exposure of male Syrian hamsters to a short daylength of 8L:16D leads to gonadal regression. This effect of photoperiod was prevented by pinealectomy or chronic exposure of the brain to exogenous melatonin delivered from in-dwelling cannulae. However, the effect of melatonin was dependent on the neural site of application. Melatonin delivered into the mid-brain, lateral hypothalamus or amygdala was ineffective. In contrast, bilateral administration of melatonin to the medial or amygdala was ineffective. In contrast, bilateral administration of melatonin to the medial hypothalamus prevented testicular regression and maintained high circulating levels of luteinizing hormone and prolactin. These findings suggest that the medial hypothalamus contains target sites for melatonin involved in pineal-mediated photoperiodic responses.

Inhibitory actions of pineal indoles on steroidogenesis in isolated rat Leydig cells

The inhibition elicited by pineal indoles on testosterone production by isolated rat Leydig cells could not be overcome by a maximally active dose of luteinizing hormone (LH), and dibutyryl-cAMP-induced steroidogenesis was also suppressed, suggesting that the indoles did not exert their effect through an interaction with LH receptors on Leydig cells. Pregnenolone-induced progesterone secretion was unaffected, indicating that the activity of 3 beta-dehydrogenase was not altered. Methoxytryptamine (MTN) at a dose of 1 mM decreased progesterone-induced 17 alpha-hydroxyprogesterone secretion by 50%, suggesting that the enzyme 17 alpha-hydroxylase was inhibited. The inhibition caused by other pineal indoles was either very slight or absent. MTN reduced 17 alpha-hydroxyprogesterone-induced androstenedione production by 65%, methoxytryptophol (MTOL) and melatonin (MEL) by 35%, and methoxyindoleacetic acid (MIAA) and hydroxyindoleacetic acid (HIAA) by 10%, revealing an inhibition of 17-20 desmolase. The reduction of androstenedione-induced testosterone production by MTN infers inhibition of 17-ketoreductase activity. However, testosterone production induced by either dehydroepiandrosterone or androstenedione was unaffected by other indoles. The data suggest that MTN inhibited 17 alpha-hydroxylase, 17-20 desmolase, and 17-ketoreductase while MEL, MTOL, MIAA, and HIAA inhibited only 17-20 desmolase. The highest potency of MTN in inhibiting enzymes on the testosterone biosynthestic pathway was reflected in its greatest inhibition of testosterone production. On the other hand, MIAA and HIAA had the lowest potency in inhibiting the enzymes and testosterone production while MEL and MTOL had intermediate potencies.

Melatonin concentration in the cerebral vascular sinuses of sheep and evidence for its episodic release

Blood was collected from the cerebral sinuses and from the jugular vein of 5 ewes during both the day and night. Cerebral sinus samples were collected by means of a permanently indwelling cannula (roughly every 5 min) while jugular vein samples were collected by venipuncture (roughly every 10 min). In each of the 5 animals mean nighttime melatonin concentrations were greater at night than during the day. In 2 animals, cerebral sinus plasma melatonin concentrations were greater than in the jugular vein; in 2 animals the sinus and jugular plasma had similar melatonin levels; in 1 ewe jugular vein blood melatonin levels exceeded those in the cerebral sinus plasma. These differences among animals are presumably due to slight positional differences in the cerebral venous cannula placement. In several animals episodic release of melatonin was apparent. Whereas the episodes were most obvious in the cerebral venous blood at night, they were also apparent in 1 case in the jugular vein plasma and in 1 animal during the day. When episodes appeared they occurred about every 15-20 min.

Structure and function in circadian timing systems: evidence for multiple coupled circadian oscillators

Considerable progress has been made in elucidating the mechanisms underlying the generation of circadian rhythmicity. This review describes several distinct lines of evidence which converge on the general hypothesis that circadian timing systems consist of multiple circadian oscillators, coordinated by both hierarchical and non-hierarchical coupling relationships. Such a view is supported by the complex phenomenology of circadian systems, as well as by physiological considerations. We have attempted wherever possible to integrate these two sources of evidence, in order to define the current "state of the art" in bridging the gap between structure and function in the analysis of circadian timing systems. While we concentrate mainly on the mammalian, and particularly the rodent, circadian system, we also incorporate comparative evidence obtained from a variety of vertebrate and invertebrate species.

Annual reproductive rhythms in mammals: mechanisms of light synchronization

Animals restrict the time of birth of offspring to the most advantageous time of year, usually spring or summer. This is achieved by controlling the preceding period of fertility and, in some cases, by delaying implantation of the zygote. Seasonal changes in daylength, the principal, though not the only cue, regulate pulsatile release of hypothalamic releasing factors that in turn activates the pituitary-gonadal axis. The role of the neuroendocrine system is therefore to translate the photoperiodic stimulus into an endocrine signal (Figure 12). The measurement of day length is a function of the circadian system, environmental light being sampled on a 24-hour basis. Experimental manipulations of the photoperiodic response have revealed the existence of a rhythm of sensitivity to the presence of light that is entrained by the prevailing photoperiod. Light falling within the period of maximal sensitivity results in an LD type response. It is important to note that although different species measure day length in a similar manner, the gonadal response to a given photoperiod will vary between species depending upon the nature of their seasonal reproductive strategy. Photic information is conveyed from the retina to the pineal gland by way of the suprachiasmatic nuclei of the hypothalamus and the cervical sympathetic trunk. The central connections between these structures are poorly understood. The pineal is an essential mediator of the photoperiodic response. The effects of pinealectomy vary between species, but in all cases the responses to changes in day length are blocked. The gland is neither anti- nor progonadotrophic; it merely provides a signal. This signal is probably the nocturnal release of melatonin. Studies on in vivo melatonin production and the responses of photoperiodic species to timed administration of exogenous melatonin have suggested that the duration of nocturnal melatonin production by the pineal is read by the CNS as an indicator of the length of darkness. This model for PTM provides a physiological basis to the observed rhythm of sensitivity to light. This period of sensitivity is probably a parallel to the nocturnal rhythm of melatonin production. Light falling in this phase blocks melatonin production, truncates the pineal signal, and hence produces an LD response by the CNS. The site of the signal detector is not known, although the anterior hypothalamus may be involved. How the pineal signal triggers changes in the hypothalamic LHRH pulse generator is not known. The endogenous opioids, however, especially beta-END may have a major role in exercising photoperiodic control over pituitary action.

Neuroendocrine regulation of seasonal reproductive activity in the male golden hamster

Golden (Syrian) hamsters are seasonal breeders. Under natural photoperiodic conditions, their reproductive systems are functional during spring and summer and atrophic during the fall and winter. This reproductive cycle can be duplicated in the laboratory by exposing the animals to artificially-created photoperiods. The endocrine correlates of photoperiod-induced changes in reproductive activity of the male hamster are fairly well characterized, but the neural control of seasonal reproductive activity has not been as extensively studied. Recent studies indicate that short day (less than 12.5 hr light/day) exposure leads to complex changes in central neurotransmitter metabolism, as well as neurotransmitter and hormonal receptor content, which, in turn, are reversed by exposure to long days or during the period of spontaneous testicular recrudescence. Many of these endocrine and neuroendocrine changes are dependent on the presence of the pineal gland, but photoperiod-induced changes in neurotransmitter metabolism have also been described in pinealectomized hamsters. Further studies of the neuroendocrine transduction of photoperiodic signals will not only provide a better understanding of seasonal reproductive and metabolic activities, but will increase our basic understanding of the neural control of the endocrine system.

Defective trophic response of brown adipose tissue of myopathic hamsters

Hamsters with muscular dystrophy (BIO 14.6) have a smaller than normal amount of brown adipose tissue. Two stimuli that promote growth of brown adipose tissue in normal hamsters, short photoperiod and eating a high-fat diet, are here shown to be without effect on brown adipose tissue of myopathic hamsters. Cold-induced growth of brown adipose tissue occurs normally [Am. J. Physiol. 239 (Cell Physiol. 8): C18-C22, 1980]. There is a normal rate of turnover of norepinephrine in brown adipose tissue of the myopathic hamster but a failure of the tissue to hypertrophy in response to norepinephrine is unlikely since norepinephrine does not appear to mediate the trophic response [Am. J. Physiol. 247 (Endocrinol. Metab. 10): E793-E799, 1984]. Denervation results in a marked reduction in size (protein content) of brown adipose tissue of normal hamsters but has very little effect on the size of brown adipose tissue of myopathic hamsters. A central, possibly hypothalamic, defect in the myopathic hamster is postulated to underlie its abnormal control of brown adipose tissue hypertrophy.

Nonshivering thermogenesis

Nonshivering thermogenesis was originally defined as a cold-induced increase in heat production not associated with the muscle activity of shivering. Recent research shows it to be a metabolic process located primarily in brown adipose tissue and controlled by the activity of the sympathetic nervous supply of this tissue. Another stimulus to sympathetic nervous activity, the ingestion of food, promotes diet-induced thermogenesis in brown adipose tissue. Brown adipose tissue grows and regresses in accordance with the extent to which it is stimulated, either by cold or by diet, and the capacity of the animal for cold-induced nonshivering thermogenesis and diet-induced thermogenesis increases or decreases accordingly. In certain hibernators another stimulus, photoperiod, promotes growth or regression of brown adipose tissue. The neural regulation of thermogenesis in brown adipose tissue is thus not only part of the central control mechanisms involved in thermoregulation but also part of those involved in the regulation of energy balance. In hibernators , such as the hamster, the neural regulation of thermogenesis in brown adipose tissue includes, in addition, central components that control the function of brown adipose tissue during entry into and arousal from hibernation and pineal or melatonin-related components that control its growth in response to photoperiod. In animals which become intermittently torpid, such as the mouse, the regulation includes in addition central components that control the function of brown adipose tissue during entry into and arousal from torpor. The central neural components involved in control of thermoregulation are better understood than are those involved in the regulation of energy balance. Studies of animal with hypothalamic obesity indicate that the control of diet-induced thermogenesis in brown adipose tissue requires the participation of the ventromedial region of the hypothalamus whereas the control of cold-induced nonshivering thermogenesis does not. The importance of comparative studies in different species is emphasized since any neural model for the control of brown adipose tissue thermogenesis is likely to apply in detail only to the species for which it was developed.

Effects of daily melatonin injections on the photoperiodic gonadal response of the female frog Rana ridibunda

The effects of daily melatonin injections on female gonads in Rana ridibunda were examined. Melatonin-treated R. ridibunda maintained on a long photoperiod (18L:6D) had significantly smaller gonads than control animals. Thus, frogs injected daily with melatonin exhibited a substantial decrease in ovary weight when compared to saline-injected animals. Our findings suggest that melatonin exerts antigonadal effects in female Rana ridibunda.