Dynamic organization of endocrine networks. The neuroendocrine-reproductive axis and the pineal gland as examples

Influence of sex on sleep regulatory mechanisms

The ability of biological sex and sex-driven characteristics to alter sleep states may contribute to gender disparities in sleep disorders. Sex influences sleep-wake amount, the daily timing of the sleep-wake cycle, and the ability to restore sleep after extended wakefulness. Several lines of evidence suggest that in mammals, reproductive hormones are responsible for the effects of sex on sleep and may have organizational and activational influences on sleep regulatory mechanisms. In humans, exogenously administered estrogens and progestins generally enhance sleep amount and continuity, whereas androgens appear to have a positive impact on rapid eye movement (REM) sleep but disrupt sleep consolidation. In rodent studies, however, female reproductive hormones appear to enhance wakefulness, and male gonadal hormones reinforce sleep. Rodent studies have also revealed that neonatal exposure to reproductive hormones organizes adult sleep-wake architecture. This paper reviews how sex and reproductive hormones interact with circadian and homeostatic sleep regulatory mechanisms in humans and animal models. We examine the organizational and activational nature of these interactions and also review how these interactions change with advancing age. Finally, we discuss the potential for genetic sex to influence sleep states. It is our hope that a better understanding of the mechanisms through which sex influences sleep-wake states will lead to improvements in the design of studies that examine gender disparities in sleep-wake disorders.

Daily rhythm of glucose-induced insulin secretion by isolated islets from intact and pinealectomized rat

It is well known that pinealectomy induces in rats a diminished glucose tolerance, insulin resistance, a reduction in GLUT4 content in adipose and muscular tissues, a decrease in hepatic and muscular glycogenesis, impairment of glucagon action and an increase in blood pyruvate concentration. In addition, it has been shown that melatonin suppresses insulin secretion in several experimental conditions. The objective of the present study was to investigate the daily rhythm of glucose-induced insulin secretion and glucose oxidation by isolated pancreatic islets and to investigate the effect of chronic absence of melatonin (30 days of pinealectomy) on this rhythmic process. The data obtained confirmed the presence of a strong 24-hr rhythm of insulin secretion by isolated pancreatic islets. In addition, it was demonstrated that the glucose-metabolizing ability of the B-cell follows a daily rhythm phase locked to insulin secretion rhythm. Most interesting, however, was the demonstration that the daily rhythmic processes of insulin secretion and B-cell -[U-14C]-glucose oxidation by isolated pancreatic islets is completely modified by the chronic absence of the pineal gland. Thus, pinealectomy induced in all groups an increase in 24-hr mean glucose-stimulated insulin secretion and [U-14C]-glucose oxidation, in addition to some alterations in the rhythmic amplitude and a remarkable phase-advancing of the daily curves for 8.3 mm glucose (a condition similar to that observed in fed animals and where the B-cells are supposedly more active). These observations strongly suggest that the presence of the pineal gland may be necessary for the proper synchronization of these metabolic rhythms with other circadian rhythms like activity-rest and feeding.

Possible involvement of beta-adrenergic receptors in the enhancement of nocturnal pineal N-acetyltransferase activity due to parathion administration

The purpose of the present study was to examine the effects of administration of sublethal doses of O,O-diethyl-O-p-nitrophenyl phosphorothioate (parathion) on serum epinephrine (EPI) and norepinephrine (NE), as well as on night-time rat pineal melatonin synthesis, both in the presence and absence of propranolol, a beta-adrenergic receptor antagonist. In the first experiment, two groups of adult albino rats were administered parathion orally (1.08 and 2.17 mg/kg/day; the total received by each animal was 6.5 and 13.0 mg/kg body weight over 6 days); another two groups received corn oil only. Animals were killed at 23:00 and 01:00 h by decapitation. Serum EPI was augmented at 01:00 h, but NE was increased at 01:00 and 23:00 h due to administration of the high dose of parathion (13 mg/kg). In the second experiment, two groups of adult male albino rats were administered parathion orally (13 mg/kg); another two groups received an intraperitoneal injection of propranolol (20 mg/kg body weight, 1 h before the lights were turned off). In addition, two groups were given a saline injection. Four hours after darkness onset, pineal N-acetyltransferase (NAT) activity as well as pineal and serum melatonin levels were measured. Parathion by itself significantly augmented nocturnal pineal NAT activity and serum melatonin levels in otherwise untreated rats; the insecticide was ineffective in reference to this enzyme when it was given in conjunction with the beta-adrenergic receptor antagonist propranolol. The augmentation of NAT activity by parathion also caused significant reduction in pineal serotonin (5-HT); again, this response was blocked by propranolol treatment. The results are consistent with the idea that parathion influences pineal 5-HT metabolism either at the level of the beta-adrenergic receptor or via the sympathetic innervation to the pineal gland.

Pulsatile adrenocorticotropic hormone: an overview

Adrenocorticotropic hormone (ACTH) is secreted by corticotrophic cells in pulsatile bursts. This paper reviews the extant literature on the phenomenon of pulsatile ACTH after addressing basic issues of hormone pulsatility in neuroendocrine systems. The following themes emerged from reviewing 51 studies measuring plasma ACTH at intervals of 20 min or less: marked inter-individual variability in the pattern of ACTH, the dependence of pulse identification on sampling frequency, the similarity in ACTH pulse amplitude and frequency across species, and the predominance of amplitude over frequency changes in ACTH pulses in altered physiological states. As the hypothalamic-pituitary-adrenocortical (HPA) axis plays a critical role in orchestrating adaptation and survival, the ability to modulate the shape of ACTH signals may prove to be an important means of transmitting complex information to ACTH responsive cells. The clinical and neurobiological significance of temporal alterations in ACTH secretion represents an area for future investigation.

Static and extremely low frequency electromagnetic field exposure: reported effects on the circadian production of melatonin

The circadian rhythm of melatonin production (high melatonin levels at night and low during the day) in the mammalian pineal gland is modified by visible portions of the electromagnetic spectrum, i.e., light, and reportedly by extremely low frequency (ELF) electromagnetic fields as well as by static magnetic field exposure. Both light and non-visible electromagnetic field exposure at night depress the conversion of serotonin (5HT) to melatonin within the pineal gland. Several reports over the last decade showed that the chronic exposure of rats to a 60 Hz electric field, over a range of field strengths, severely attenuated the nighttime rise in pineal melatonin production; however, more recent studies have not confirmed this initial observation. Sinusoidal magnetic field exposure also has been shown to interfere with the nocturnal melatonin forming ability of the pineal gland although the number of studies using these field exposures is small. On the other hand, static magnetic fields have been repeatedly shown to perturb the circadian melatonin rhythm. The field strengths in these studies were almost always in the geomagnetic range (0.2 to 0.7 Gauss or 20 to 70 mu tesla) and most often the experimental animals were subjected either to a partial rotation or to a total inversion of the horizontal component of the geomagnetic field. These experiments showed that several parameters in the indole cascade in the pineal gland are modified by these field exposures; thus, pineal cyclic AMP levels, N-acetyltransferase (NAT) activity (the rate limiting enzyme in pineal melatonin production), hydroxyindole-O-methyltransferase (HIOMT) activity (the melatonin forming enzyme), and pineal and blood melatonin concentrations were depressed in various studies. Likewise, increases in pineal levels of 5HT and 5-hydroxyindole acetic acid (5HIAA) were also seen in these glands; these increases are consistent with a depressed melatonin synthesis. The mechanisms whereby non-visible electromagnetic fields influence the melatonin forming ability of the pineal gland remain unknown; however, the retinas in particular have been theorized to serve as magnetoreceptors with the altered melatonin cycle being a consequence of a disturbance in the neural biological clock, i.e., the suprachiasmatic nuclei (SCN) of the hypothalamus, which generates the circadian melatonin rhythm. The disturbances in pineal melatonin production induced by either light exposure or non-visible electromagnetic field exposure at night appear to be the same but whether the underlying mechanisms are similar remains unknown.

Alterations of the circadian melatonin rhythm by the electromagnetic spectrum: a study in environmental toxicology

The nightly production and secretion of melatonin by the pineal gland, an endocrine organ near the anatomical center of the brain, provides important time-of-day and time-of-year information to the remainder of the body. In mammals, the circadian rhythm of melatonin (low levels during the day and high levels at night) is synchronized by the prevailing light:dark environment with the retinas of the eyes doing the photoreception required for the induction of this rhythm. The advent of artificial light sources has allowed animals or humans to be exposed to light at unusual times, i.e., during the night. Light falling on the retinas at night leads to a rapid depression in the production and secretion of melatonin by the pineal gland. The magnitude of the drop in circulating melatonin due to light exposure at night is related to the brightness (intensity) as well as the wavelength (color) of light to which humans are exposed. The lowered melatonin values following unusual light exposure at night provide erroneous information to a number of organs that respond to the melatonin message since the signal implies it is day when, in fact, it is still night. Besides visible light, certain ultraviolet wavelengths as well as extremely low frequency electric and magnetic fields may also disturb the melatonin rhythm. These nonvisible wavelengths may influence the circadian melatonin rhythm by mechanisms similar to those by which light causes disturbances of melatonin production and release.

Serotonin content and melatonin production in the pineal gland of the male Djungarian hamster (Phodopus sungorus)

Diurnal variations in serum melatonin levels and pineal concentrations of serotonin (5-HT), N-acetylserotonin (NAS), 5-hydroxyindole acetic acid (5-HIAA), and melatonin were estimated in adult male Djungarian hamsters kept under long-day (LD 16:8) or short-day (LD 8:16) photoperiods. The nocturnal increase in melatonin production was accompanied by a marked drop in pineal serotonin concentrations. Serotonin levels, however, decreased approximately 4 hr before pineal melatonin increased. Correlations of the mean values for pineal serotonin and pineal melatonin indicated significant correlations at both LD 16:8 (r = -0.92, P less than 0.001) and LD 8:16 (r = -0.85, P less than 0.001). The mean levels of pineal serotonin and serum melatonin were correlated as well (LD 16:8, r = -0.91, P less than 0.001; LD 8:16, r = -0.81, P less than 0.01). The levels of pineal serotonin declined at approximately the same time as serum melatonin levels increased. These data suggest that the drop in pineal serotonin is primarily a consequence of melatonin production (as reflected by increasing serum concentrations). Consequently, pineal concentrations of melatonin may not be the best estimate of actual melatonin production, but a measure of how much melatonin is accumulated within the pineal due to high synthesis rates while the release of the hormone from the gland is limited.