Visual pathways and the central neural control of a circadian rhythm in pineal serotonin N-acetyltransferase activity

Effect of sympathetic denervation of the pineal gland on maternal co-ordination of the circadian rhythm of alpha-amylase in parotid gland from young rats

Twenty-five-day-old rats maintained in constant darkness since birth and born from mothers kept in the dark since the 14th day of pregnancy showed a circadian rhythm of alpha-amylase content in parotid glands, which may be explained by a mechanism of maternal co-ordination. Rats in the same conditions, except that their mothers had been submitted to bilateral excision of the superior cervical ganglia 30 days before mating, did not show diurnal variations of alpha-amylase activity in the parotid glands. When ganglionectomized mothers were treated with a daily dose of melatonin (1 mg/kg) from the 14th day of gestation up to the 10th day of lactation, their litters showed significant diurnal variations of amylase in the parotid glands, suggesting a role of the maternal pineal gland in the maternal-fetal and/or maternal-neonatal transfer of photoperiodic information.

Effects of serotonin agonists and melatonin on photic responses of hamster intergeniculate leaflet neurons

Retinal input to the suprachiasmatic nuclei (SCN) and the intergeniculate leaflet (IGL) is involved in photic entrainment of mammalian circadian rhythms. The activating effects of light on firing rates of IGL cells may be regulated by serotonin (5-HT), since the IGL receives a dense serotonergic input from the midbrain raphe. We investigated the effects of 5-HT agonists and melatonin (a derivative of 5-HT) on single-unit discharges of light-sensitive cells in the hamster IGL area, using a microiontophoretic technique. 5-HT and a 5-HT1A-selective agonist, 8-OH-DPAT, potently suppressed both spontaneous and light-induced activity of IGL cells in a dose-related manner. This suppression was unchanged or potentiated by concurrently applied Mg2+, suggesting a direct action. Furthermore, the suppressive effects of both agonists were antagonized by a nonselective 5-HT antagonist, metergoline, and a 5-HT1A-directed antagonist, pindobind-5-HT1A. However, other putative 5-HT1A antagonists were weak (propranolol) or ineffective (pindolol and spiperone) in blocking the effects of 8-OH-DPAT. Neither of two 5-HT2 antagonists tested was able to block the effects of 5-HT. Melatonin generally mimicked the effects of 5-HT agonists on IGL cells, but these effects were not attenuated by 5-HT antagonists. The results indicate that both 5-HT and melatonin exert inhibitory effects on spontaneous activity and photic responses of cells in the hamster IGL, and that these effects are mediated via a 5-HT1A-like receptor and a melatonin receptor, respectively.

Differential responses of identified rat hypothalamic paraventricular neurons to suprachiasmatic nucleus stimulation

The suprachiasmatic nucleus in the anterior hypothalamus contains a circadian oscillator that is responsible for 24-h rhythms in several behavioral, endocrine and autonomic processes. Efferent suprachiasmatic projections are likely to transmit rhythmic information to brain nuclei controlling these functions. The hypothalamic paraventricular nucleus is considered to be a target of the suprachiasmatic nucleus due to its important role in autonomic and endocrine regulation. The present study applied extracellular electrophysiological techniques to intact animals to look for a possible interaction between suprachiasmatic nucleus efferents and identified neurons in the hypothalamic paraventricular nucleus. Results showed that electrical stimulation of the suprachiasmatic nucleus induced an increase in the excitability of 87% of paraventricular neurons that project to the median eminence and are situated in the medial and dorsal parvocellular subnucleus; neurons with similar projections but located in the periventricular subnucleus displayed a reduction in firing rate following suprachiasmatic stimulation. Electrical activation of the suprachiasmatic nucleus provoked a decrease in excitability in 75% of paraventricular neurons in the posterior magnocellular subnucleus that send axons to the posterior pituitary and in 85% of paraventricular neurons, located in the medial parvocellular subnucleus, that project to the dorsal vagus complex in the brainstem. The data imply that functional and selective neural connections exist between suprachiasmatic nucleus efferents and specific cell groups within the hypothalamic paraventricular nucleus. These projections would be able to convey rhythmic information to certain endocrine and autonomic functions. The anatomical and neurochemical characteristics of the underlying pathways remain to be determined.

Circadian rhythm of neuropeptide Y-like immunoreactivity in the iris-ciliary body of the rat

Neuropeptide Y (NPY)-like immunoreactivity (LI) in the iris-ciliary body of rats kept under constant darkness (DD) and in 12 h light-12 h dark cycle (LD) was determined by enzyme immunoassay. NPY-LI contents in the iris-ciliary body were found to oscillate in circadian fashion under DD and LD conditions, with a peak at about circadian time 12 (CT 12) and a trough at around CT 0. Unilateral superior cervical ganglionectomy caused a significant decrease in NPY-LI levels in the sympathectomized eye compare to the contralateral intact eye, independent of lighting phase. These results suggest the presence of an endogenous circadian rhythm in NPY-LI content in the rat iris-ciliary body, and the possible involvement of a sympathetic input from the superior cervical ganglion.

Structures and molecules involved in generation and regulation of biological rhythms in vertebrates and invertebrates

Melatonin from the retina and the pineal gland functions in neuroendocrine hierarchies. Photoreceptors--eyes and extraretinal--detect light. Oscillators--pineal and suprachiasmatic nuclei--act as pacemakers. Driven neuroendocrine rhythms carry temporal hormone signals throughout the body. Light controls melatonin: light sets the phase of the melatonin rhythm and determines the duration of melatonin synthesis. By these means, circadian rhythms (e.g., in locomotor activity and body temperature) and seasonal rhythms (e.g., in reproduction) are controlled.

Vasopressin and vasoactive intestinal peptide infused in the paraventricular nucleus of the hypothalamus elevate plasma melatonin levels

The connection between the suprachiasmatic nucleus (SCN) and the paraventricular nucleus of the hypothalamus (PVN) forms an important component of the melatonin rhythm-generating system. However, the chemical identity of this projection is not known. To test the possible implication of the SCN peptides vasopressin (VP) and vasoactive intestinal peptide (VIP) in this projection, we performed microinfusions in the PVN during the first half of the dark period and subsequently monitored resulting plasma melatonin levels. Infusions for 7 hr of either VP or VIP, but not oxytocin, caused increased plasma melatonin levels in the middle of the dark period. These observations confirm the role of the PVN in the melatonin rhythm-generating pathway and indicate that both VP and VIP released at the level of the PVN, and probably derived from the SCN, are able to influence peripheral plasma melatonin levels.

One-hour exposure to moderate illuminance (500 lux) shifts the human melatonin rhythm

Salivary melatonin levels were measured in 12 healthy volunteers in order to determine whether a moderate light intensity, which suppresses the nocturnal rise of melatonin, was able to shift the melatonin rhythm. The samples were collected at 1-hr intervals under lighting of < 100 lux (experiment 1) or < 10 lux (experiment 2). The control melatonin profiles were determined during the first night. In the second night the subjects were exposed to light of 500 lux for 60 min during the rising phase of melatonin synthesis. The third series of samples was collected during the third night. The mean decrease of melatonin levels by the exposure to light was 56% of the prelight concentrations. The melatonin onset times were delayed significantly (about 30 min) the night after the exposure to light. The melatonin offset times tended to be delayed in experiment 2. The shifts of the melatonin offset correlated positively with the amount of the melatonin suppression. The results suggest that a relatively small and short lasting light-induced interruption of melatonin synthesis may affect the melatonin rhythm in humans.

Substance P in the suprachiasmatic nucleus of the rat: an immunohistochemical and in situ hybridization study

Using a biotin-streptavidin-horseradish peroxidase (HRP) immunohistochemical technique the distribution of substance P-immunoreactive neuronal elements was investigated in the rat suprachiasmatic nucleus (SCN). Substance P-immunoreactive nerve fibres and varicosities were distributed throughout the suprachiasmatic nucleus, with the largest accumulation in its ventral part. Because this location overlaps with the innervation of retinal afferents, the distribution and density of substance P-immunoreactive fibres in bilaterally enucleated rats were compared to normal rats. The density of substance P-immunoreactive fibres and nerve terminals in the ventral part of the suprachiasmatic nuclei was reduced in the rats with bilateral destruction of the optic nerves, whereas the density of fibres and nerve terminals in the dorsal part as well as other retinal target areas in the thalamus and mesencephalon was unaffected. In rats pretreated with an intraventricular injection of colchicine several substance P-immunoreactive perikarya were identified in the suprachiasmatic nucleus. The immunoreactive neurons, measuring 9.7 microns +/- 1.1 microns in diameter, were frequently observed in the central core of the nucleus and to a lesser extent in the dorsomedial and ventrolateral subparts. Using in situ hybridization histochemistry pre-protachykinin-A mRNA was found in the same part of the SCN indicating that synthesis of substance P takes place in SCN neurons. Using a double immunohistochemical approach applying diaminobenzidine and benzidinedihydrochloride as chromagens substance P-, vasoactive intestinal peptide (VIP)-, and vasopressin/neurophysin-immunoreactivities were identified in the same brain section. The substance P-immunoreactive perikarya constituted a separate population of SCN neurons, which were not vasopressin-, neurophysin- or VIP-immunoreactive. Taken together, these observations show that substance P is contained in the retinohypothalamic pathway and within a group of SCN cell bodies, indicating that substance P may play a role in the generation and entrainment of circadian rhythmicity.

Adrenergic stimulation of cyclic GMP formation requires NO-dependent activation of cytosolic guanylate cyclase in rat pinealocytes

Cyclic GMP (cGMP) formation in rat pinealocytes is regulated through a synergistic dual receptor mechanism involving beta- and alpha 1-adrenergic receptors. The effects of NG-monomethyl-L-arginine (NMMA), which inhibits nitric oxide (NO) synthase and NO-mediated activation of cytosolic guanylate cyclase, and methylene blue (MB), which inhibits cytosolic guanylate cyclase, were investigated in an attempt to understand the role of NO in adrenergic cGMP formation. Both NMMA and MB inhibited beta-adrenergic stimulation of cGMP formation as well as alpha 1-adrenergic potentiation of beta-adrenergic stimulation of cGMP formation, whereas they had no effect in unstimulated pinealocytes. The inhibitory action of NMMA was antagonized by addition of L-arginine. On the basis of these findings it can be concluded that the adrenergic stimulation of cGMP formation involves NO synthesis followed by activation of cytosolic guanylate cyclase.

Serotonin and the mammalian circadian system: I. In vitro phase shifts by serotonergic agonists and antagonists

The primary mammalian circadian clock, located in the suprachiasmatic nuclei (SCN), receives a major input from the raphe nuclei. The role of this input is largely unknown, and is the focus of this research. The SCN clock survives in vitro, where it produces a 24-hr rhythm in spontaneous neuronal activity that is sustained for at least three cycles. The sensitivity of the SCN clock to drugs can therefore be tested in vitro by determining whether various compounds alter the phase of this rhythm. We have previously shown that the nonspecific serotonin (5-HT) agonist quipazine resets the SCN clock in vitro, inducing phase advances in the daytime and phase delays at night. These results suggest that the 5-HT-ergic input from the raphe nuclei can modulate the phase of the SCN circadian clock. In this study we began by using autoradiography to determine that the SCN contain abundant 5-HT1A and 5-HT1B receptors, very few 5-HT1C and 5-HT2 receptors, and no 5-HT3 receptors. Next we investigated the ability of 5-HT-ergic agonists and antagonists to reset the clock in vitro, in order to determine what type or types of 5-HT receptor(s) are functionally linked to the SCN clock. We began by providing further evidence of 5-HT-ergic effects in the SCN. We found that 5-HT mimicked the effects of quipazine, whereas the nonspecific 5-HT antagonist metergoline blocked these effects, in both the day and night. Next we found that the 5-HT1A agonist 8-OH-DPAT, and to a lesser extent the 5-HT1A-1B agonist RU 24969, mimicked the effects of quipazine during the subjective daytime, whereas the 5-HT1A antagonist NAN-190 blocked quipazine's effects. None of the other specific agonists or antagonists we tried induced similar effects. This suggests that quipazine acts on 5-HT1A receptors in the daytime to advance the SCN clock. None of the specific agents we tried were able either to mimic or to block the actions of 5-HT or quipazine at circadian time 15. Thus, we were unable to determine the type of 5-HT receptor involved in nighttime phase delays by quipazine or 5-HT. However, since the dose-response curves for quipazine during the day and night are virtually identical, we hypothesize that the nighttime 5-HT receptor is a 5-HT1-like receptor.

Melatonin: a contraceptive for the nineties

The hypothalamic GnRH pulse generator activates the pituitary-gonadal reproductive axis, and contraceptive techniques have advanced to the point where GnRH analogues can block this effect. However, nature has an even finer form of contraception, whereby the GnRH pulse generator is activated or inactivated at different seasons of the year. Darkness affects the retino-pineal nervous pathway to cause the synthesis and release of melatonin from the pineal gland at night. The duration of the night time release of melatonin is longer in winter than in summer; and it is the prolongation in the duration of the night time release of melatonin, with the change of season from summer to winter, which acts as the endocrine signal for inactivating the hypothalamic GnRH pulse generator. Humans are not seasonal breeders, and evidence is presented to indicate that this is due to an impairment of the retino-pineal pathway rather than an impairment of melatonin hypothalamic function. Thus the way is open for utilising melatonin as a human contraceptive, and a melatonin-based contraceptive is at present undergoing phase III clinical trials. The challenge is to develop more refined methods for administering (or releasing) melatonin, so that it has a night time amplitude and duration which mimics that seen in long day breeders.

Melatonin and photoperiodic time measurement: seasonal breeding in the sheep

Well-established circadian physiology supports the view that photoperiodic time measurement utilizes the coincidence between the presence of light and a photosensitive phase of a 'biological clock' to alter reproductive status--the so-called external coincidence model of seasonal breeding. In this review, we examine the mechanism whereby photoperiod interacts with presumed suprachiasmatic nuclei activity to allow endogenous melatonin to normally synchronize reproductive activity to the optimal time of year. The Romney Marsh sheep is particularly explored as an experimental model. It is suggested that the on/off activity of seasonal reproduction may be a robust mechanism able to be predictably manipulated by the judicious use of the light/dark cycle and exogenous melatonin, but firmly based on circadian principles.

Personal light dosimetry in permanent night and day workers

Light exposure was measured in six day and six night watches (working 12-hour shifts five days in a row) during 48 h on work days and 48 h on days off using a photocell with a sensitivity corresponding to photopic vision. The photocell was mounted on a frame of spectacles, thus measuring in viewing direction. Light exposure was low both in night and day watches; however, in night watches exposures were significantly lower: On work days, night watches spent a mean of 13 min above 1,500 lx, day watches 52 min; on days off, night watches spent 3 min above 1,500 lx but day watches 89 min. Unexpectedly, night watches had no higher exposure during days off. We suspect that this is due to a light avoidance tendency in permanent night workers. High negative correlations between the acrophases of subjective state (e.g., alertness and mood) and light exposure in night watches indicate that bright light would probably increase desynchronization between subjective state, sleep, and activity.

A single injection of adrenergic agonists enhances pineal melatonin production in Turkish hamsters

The purpose of this study was to determine whether the pineal gland of Turkish hamsters (Mesocricetus brandti) responds to adrenergic agonists with an increase in melatonin production, and, if it does, whether the sensitivity of the pineal gland to agonists would differ throughout the dark phase. Adult Turkish hamsters weighting 110-210 g received a subcutaneous injection of isoproterenol (ISO, 1 mg/kg B.W.) or norepinephrine (NE, 1 mg/kg B.W.) at different times of night. Animals exposed to LD 16:8 responded to ISO or NE with increased pineal melatonin content only when injected at dawn, when endogenous melatonin is at basal or near-basal levels. When the 8 hr scotophase was entirely replaced with light, the responsiveness to ISO injections at dawn disappeared. In animals exposed to light from 30 min prior to injection to the time of sacrifice, ISO injections increased pineal melatonin content (P < 0.005, three-way ANOVA), which varied, depending on the specific time of injection (effect of time of night, P < 0.05, three-way ANOVA). These results demonstrate that (1) adrenergic agonists enhance the production of pineal melatonin in Turkish hamsters, (2) this stimulatory effect takes place late, but not early in the 8 hr scotophase, and (3) the adrenergic induction of pineal melatonin production in Turkish hamsters requires priming by darkness during the appropriate circadian phase.

Is the vasoactive intestinal peptide-like immunoreactivity in the rat pineal gland present in fibers originating in the superior cervical ganglion?

Serotonin N-acetyltransferase is regulated in the rat pineal gland by the gland's innervation from the superior cervical ganglion. Norepinephrine has been viewed as the sole transmitter involved in this trans-synaptic regulation; however, a possible role for vasoactive intestinal peptide (VIP) has recently emerged. VIP-like immunoreactivity was extracted from rat pineal glands and shown to co-elute on reverse-phase liquid chromatography with authentic VIP. The level of VIP-like immunoreactivity in the gland was unaffected by prior sympathetic denervation, though its level of neuropeptide Y-like immunoreactivity decreased by 85%. The results indicate that VIP and norepinephrine are not colocalized in sympathetic neurons in the pineal gland and raise questions as to the physiological role of VIP in regulation of pineal function in vivo.

Neuropeptide Y-lmmunoreactive Nerve Fibres in the Pineal Gland of the Macaque (Macaca fascicularis)

The distribution of neuropeptide Y (NPY)- and the C-fianking peptide of NPY (CPON)-immunoreactive elements in the pineal gland of the macaque was investigated by means of immunohistochemistry. NPY- and CPON-immunoreactive nerve fibres were located in the precommissural nucleus, around the stria medullaris, and in the posterior commissure. NPY- and CPON-immunoreactive nerve fibres endowed with bulbous varicosities, were traced from the brain via the pineal stalk into the rostral part of the pineal gland. Furthermore, CPON-immunoreactive, and to a lesser extent NPY-immunoreactive nerve fibres, were distributed in the méninges, the choroid plexus and the vasculature related to the pineal organ. Nerve fibres located in the pineal capsule penetrated into the pineal parenchyma, where groups of individual fibres were found most often in an interlobular position. Occasionally, individual nerve fibres dispersed between the pinealocytes were observed. In contrast to the nerve fibres originating from the brain, those originating from the periphery were endowed with smaller immunoreactive nerve terminals. Another apparent difference was that the peripheral nerve fibres innervated only the caudal two-thirds of the gland, whereas the central fibres were found exclusively in the rostral part of the pineal organ. Rarely, positive neuronal-like cells were found in the pineal parenchyma. These results show the presence of a moderate number of NPY- and CPON-immunoreactive nerve fibres within the primate pineal organ and strongly indicate that the primate pineal gland is innervated by NPYergic nerve fibres originating from both a peripheral and a central source.

Plasma melatonin levels in anorexia nervosa

Plasma melatonin levels were measured at three-hourly intervals over 24 hours in 11 women with untreated anorexia nervosa, and in nine healthy women of normal weight. The circadian rhythm was unaltered but the nocturnal secretion of melatonin was significantly greater in anorectics. It is possible that this was related to nocturnal hypoglycaemia.

Pinealectomy decelerates the circadian food intake rhythm of cervically sympathectomized rabbits

Long-term records of the free-running food intake rhythms were obtained from 26 young adult blinded rabbits that were subjected in the course of recording to bilateral superior cervical ganglionectomy (sympathectomy). This always resulted in acceleration of the rhythm (mean delta tau = -0.35 h). Some 4 months afterwards, 12 rabbits were pinealectomized and after another 4 months sham pinealectomized also. In the other 14 animals, these operations were performed in the reverse order. It appeared that suctioning away part of the ventricular walls during sham pinealectomy was followed by a slight reduction in tau (mean = -0.07 h) that could be largely attributed to the spontaneous gradual reduction of tau that occurred throughout all experiments. On the other hand, total pinealectomy in these already sympathectomized blinded rabbits always resulted in a substantial deceleration of the rhythms (mean delta tau = +0.23 h). These observations on blinded rabbits suggest that a sympathetically denervated pineal gland releases appreciable amounts of melatonin or of another hormone with a melatonin-like accelerating effect on the circadian pacemaker (SCN).

Behavioral and morphological studies of fetal neural transplants into SCN-lesioned rats

We have studied the effects of fetal neuronal grafts on the temporal pattern of drinking behavior of suprachiasmatic nuclei (SCN)-lesioned adult rats. Additionally, in an independent set of animals, the immunohistochemical staining for vasopressin, vasoactive intestinal polypeptide, and neuropeptide Y and the retinal connections to the hypothalamus were studied. The behavioral experiments indicate that anterior hypothalamic transplants induced reorganization of the temporal pattern of drinking behavior when placed in the third ventricle of adult hosts bearing complete SCN lesions, but not when placed in a cavity in the occipital cortex. Such rhythmicity persists only when the animals were recorded under constant darkness but not under constant light, indicating that the restored rhythmicity was generated endogenously but that the oscillator was extremely sensitive to light. Fetal occipital cortex induced reorganization of the temporal pattern of previously arrhythmic hosts, but it disappeared when the animals were recorded under constant light or constant darkness. It is clear that this rhythmicity was exogenous. In contrast to the cortical transplants, the hypothalamic transplants showed a morphological organization similar to that found in the normal hypothalamus regardless of their placement in the host brain. From these observations it is concluded that development of neocortex is more affected by environmental factors than that of the hypothalamus. Both hypothalamic and cortical transplants induced sprouting of retinal fibers into the anterior hypothalamus and the grafted tissue. It is possible that such fibers could be the neuroanatomical substrate by which rhythmicity is induced by cortical tissue.

Blood levels of melatonin, serotonin, cortisol, and prolactin in relation to the circadian rhythm of platelet serotonin uptake

Blood levels of melatonin, serotonin, cortisol, prolactin, and serotonin uptake by platelets were measured at 08:00, 14:00, 20:00, 02:00, and 08:00 hours in 10 healthy men who ranged in age from 27 to 35 years. The Km values of serotonin active transport by platelets were significantly correlated with melatonin blood levels. There were no other significant correlations. The secretion of steroid hormones and prolactin showed an increase at 02:00 hours; levels of prolactin decreased at 08:00 hours, but steroid levels continued to rise. This finding suggests either a direct effect of melatonin on serotonin active transport or the influence of the suprachiasmatic nucleus on serotonin uptake by platelets. It is also possible that there is a simultaneous decrease in serotonin uptake and decline from peak levels of melatonin due to the rise in steroid secretion.

Alpha-amylase circadian rhythm of young rat parotid gland: an endogenous rhythm with maternal coordination

The circadian rhythm of alpha-amylase, E.C. 3.2.1.1. alpha-1,4-glucan-4-glucanohydrolase) in the parotid glands of 25-day-old rats were studied under different experimental designs (fasting, reversed photoperiod, constant lighting conditions and treatment with reserpine and alpha-methyl-p-tyrosine). The rhythm of fasted rats did not change. There were modifications in the rhythm of rats submitted to a reversed photoperiod or treated with reserpine or alpha-methyl-p-tyrosine. The rhythm was present, with changes in the acrophase, in parotids of rats kept during their gestation and postnatal life in constant light or dark. Results suggest that the circadian rhythm of alpha-amylase in parotid gland of young rats is endogenous, synchronized by the photoperiod, and with maternal coordination.

The limbic system and food-anticipatory circadian rhythms in the rat: ablation and dopamine blocking studies

Rats behaviorally anticipate a fixed, daily opportunity to feed by entrainment of circadian oscillators that are physically separate from the light-entrainable circadian pacemaker that has been localized to the suprachiasmatic nucleus. Neural substrates mediating food-entrained rhythms are unknown. A variety of anatomical and functional observations suggest possible involvement of the limbic system and its dopaminergic component in the regulation of these rhythms. To test this hypothesis, the activity rhythms of rats bearing large, combined ablations of the hippocampus and amygdala or nucleus accumbens and medical forebrain anterior to the thalamus were examined under ad-lib feeding, 2 h daily feeding, and total food deprivation conditions. Some hippocampal-ablated rats showed alterations of free-running rhythms under ad-lib feeding, but none of the ablations impaired the rats' ability to anticipate daily feeding, or 'remember' the phase of feeding time during subsequent food deprivation. Additional groups of intact rats were treated with the dopamine antagonist haloperidol (0.3 mg/kg or 2.0 mg/kg) 30 min prior to daily feeding, but this also did not prevent the emergence of food-entrained rhythms. The limbic and dopamine systems do not appear to play a necessary role in the generation or entrainment of food-anticipatory circadian rhythms.

Involvement of cyclic guanosine monophosphosphate (cGMP) and cytosolic guanylate cyclase in the regulation of synaptic ribbon numbers in rat pineal gland

In the rat pineal gland N-acetyltransferase (NAT) activity and synaptic ribbon (SR) numbers display a circadian rhythm. It is well-known that NAT activity is regulated by adrenergic mechanisms involving cyclic adenosine monophosphate (cAMP) as a second messenger. However, the mechanism involved in the regulation of SR numbers has not been established so far. In the present in vitro study, we have investigated the effects of 8-bromo-cyclic guanosine monophosphate (8-bromo-cGMP), a cyclic guanosine monophosphate (cGMP) analog, and stimulation of guanylate cyclase on SR numbers. Incubation with 8-bromo-cGMP increased SR numbers in a dose- and time-dependent manner. Further, stimulation of the cytosolic guanylate cyclase also resulted in increased SR numbers. Adrenergic agonists stimulated cGMP but did not alter SR numbers. These findings suggest that cGMP is involved as a second messenger in the regulation of SR numbers. Since the adrenergically stimulated increase in cGMP did not influence SR numbers, a non-adrenergic cGMP metabolic pathway seems to be involved in the regulation of SR numbers in the rat pineal gland.

Identification and characterization of melatonin binding sites in the gastrointestinal tract of ducks

Utilizing 2-[125I]iodomelatonin as the radioligand, melatonin binding sites were identified and characterized in the jejunum of ducks. These sites were found to be reversible, saturable, specific and exhibited high affinity for melatonin. Scatchard analyses have established the equilibrium dissociation constant (Kd) for tissues collected during mid-photophase to be 40.9 +/- 7 pM and the maximum quantity of binding sites (Bmax) to be 2.0 +/- 0.4 fmol/mg protein while Kd of samples collected during mid-scotophase was found to be 54.1 +/- 10 pM with a corresponding Bmax of 1.5 +/- 0.3 fmol/mg protein. These Kd values are in good proximity to the kinetically derived equilibrium dissociation constant of 47.3 +/- 20 pM. No significant difference in Kd or Bmax was detected between the mid-light and mid-dark samples. Pharmacological profile of these binding sites, developed by their interactions with other indoles and compounds, indicated that these binding sites are highly specific for melatonin. Subcellularly, different densities of binding sites were localized to various fractions in the following order: nuclear greater than microsomal greater than mitochondrial greater than cytosolic. These binding sites in the jejunum might be the receptors accountable for promoting paracrine activities for the locally synthesized gastrointestinal melatonin and/or responsible for eliciting hormonal actions via interactions with melatonin of pineal origin.

Melatonin directly resets the rat suprachiasmatic circadian clock in vitro

The environmental photoperiod regulates the synthesis of melatonin by the pineal gland, which in turn induces daily and seasonal adjustments in behavioral and physiological state. The mechanisms by which melatonin mediates these effects are not known, but accumulating data suggest that melatonin modulates a circadian biological clock, either directly or indirectly via neural inputs. The hypothesis that melatonin acts directly at the level of the suprachiasmatic nucleus (SCN), a central mammalian circadian pacemaker, was tested in a rat brain slice preparation maintained in vitro for 2-3 days. Exposure of the SCN to melatonin for 1 h late in the subjective day or early subjective night induced a significant advance in the SCN electrical activity rhythm; at other times melatonin was without apparent effect. These results demonstrate that melatonin can directly reset this circadian clock during the period surrounding the day-night transition.

Long-Term Dynamic in vitro System for Investigating Rat Pineal Melatonin Secretion

Abstract Several details regarding the control of melatonin (MT) secretion from the pinealocytes are still to be clarified. To obtain more data on the mechanism and kinetics of MT secretion and on the interactions between bioactive materials affecting MT production, a perifusion system has been developed in our laboratory. In this dynamic in vitro system the surviving pinealocytes maintain their full responsiveness for at least 5 days. In order to determine the MT contents of large numbers of samples collected in the perifusion system, a sensitive MT radioimmunoassay was set up utilizing our specific MT antibody. In our perifusion system the basal MT release does not change significantly during the 5-day experiments. Norepinephrine (NE) was used at 1 muM concentration for 30 min as a marker of the responsiveness of pinealocytes, given at the beginning and at the end of the same experiments. No significant difference was found in the MT responses to NE stimulation over 5 days. The kinetics of MT response and the dose-response relationship were investigated after NE exposure at various concentrations (100 nM to 10 mM). NE at 100 nM was found to be ineffective. Between 1 muM and 1 mM concentrations NE increased the MT release in a dose-dependent manner. No significant difference was found between the responses above 1 mM concentration. NE seems to be a specific stimulator of pineal MT production. The MT production reached the maximum value after a relatively long lag (2 to 3 h) when NE application had been stopped, and returned to basal values after 5 to 6 h. This prolonged time-course of MT secretion, in contrast with the fast responses of pituitary cells to releasing hormones, suggests that NE stimulated the synthesis of MT rather than the release of stored hormone. The modulatory effects of light-dark cycle on basal and stimulated MT release of perifused pineals was also investigated: Neither basal nor NE-stimulated (100 nM to 10muM) MT release was influenced significantly by light-dark conditions showing that the light-dark cycle does not have a direct modulatory effect on MT secretion under in vitro circumstances. Based on our observations, this perifusion system should be a useful tool for investigating: 1) hormone interactions on the regulation of pineal MT release, and 2) accurate kinetics of MT response.

Labeling of human retinohypothalamic tract with the carbocyanine dye, DiI

The carbocyanine dye, DiI, was used to demonstrate human retinohypothalamic tract (RHT) projections in 6 normal human postmortem brains. In 5 of 6 brains, labeling was seen extending from the site of implantation in the distal optic nerve to both the ipsilateral and contralateral suprachiasmatic nuclei. This study confirms the presence of RHT projections in humans, and demonstrates the usefulness of DiI for neuronal tracing in human postmortem tissue.

No difference in day-night serum melatonin concentration after pineal grafting into the third cerebral ventricle of pinealectomized rats

Serum melatonin concentration and its day-night difference in pinealectomized, stereotaxically grafted rats with pineal transplants was examined. The nighttime serum melatonin concentration increased significantly only in pinealectomized rats that received two pineal transplants. In neither pinealectomized rats receiving two pineal glands and one cotransplant of superior cervical sympathetic ganglion nor pinealectomized-ganglionectomized rats receiving two pineal transplants was there a demonstrable increased in serum melatonin concentration. Although some pineal transplanted hosts demonstrated increases in serum melatonin concentration, there was no day-night variation in serum melatonin concentration. This may be due to a number of mitigating factors. Pineal grafts may not receive the appropriate functional reinnervation from the host brain due to the location of the transplantation in the cerebral ventricle or due to lack of sufficient time for the growth of invading host neurites.

Human plasma melatonin and urinary 6-sulphatoxy melatonin: studies in natural annual photoperiod and in extended darkness

OBJECTIVES

The aims of the study were (1) to examine the human plasma melatonin rhythm at the equinoxes and the solstices in the natural photoperiod (at 35 degrees S); (2) to examine melatonin rhythms in the same subjects under extended darkness conditions to expose any suppressive (gating) effects of light at any time of the year; (3) to undertake a rigorous examination of the relationship between plasma melatonin and the urinary metabolite 6-sulphatoxy melatonin at varying times of the year.

DESIGN

At the equinoxes and solstices, unrestricted subjects had hourly urine collections followed by venous blood sampling taken under natural light conditions for 24 hours. Following a 24 hour interval, a similar collection regime was performed with subjects held under conditions of extended darkness (5 hours darkness prior to natural sunset and following natural sunrise) for a further 24 hours.

SUBJECTS

Groups of four (minimum) to six female volunteers (age range 18-35 years) were studied, who had a normal lifestyle, no history of depression, and were not taking any medication or recently engaged in shiftwork.

MEASUREMENTS

The plasma was assayed for melatonin and the urine samples for 6-sulphatoxy melatonin by radioimmunoassay.

RESULTS

The onset of natural melatonin secretion was delayed until after sunset at all seasons but was earlier in summer, and not different from the time of sunset in extended darkness. The offset of melatonin secretion under natural conditions occurred at sunrise in autumn and winter but was delayed until after sunrise during spring and summer, particularly in extended darkness. No significant changes in the duration of melatonin secretion were observed between seasons nor between the duration of melatonin secretion under natural photoperiod or extended darkness. The measurement of 6-sulphatoxy melatonin proved to be a close indicator of the phase and amplitude of secretion of plasma melatonin. Both onset and offset times of 6-sulphatoxy melatonin were delayed compared to the times when plasma melatonin was detectable/undetectable. A good correlation exists between the total plasma melatonin secretion and that of 6-sulphatoxy melatonin.

CONCLUSIONS

The results suggest evidence for a suppressive (gating) effect of light at dawn only during summer which was associated with a phase advance of the onset of melatonin secretion at this time of year. The lack of a major gating effect of environment light on melatonin secretion, and the unchanging duration of secretion through the year in the normally entrained human, highlight differences between the human and those photoperiodic animal species which breed seasonally. Urinary 6-sulphatoxy melatonin proved to be a good indicator of plasma melatonin levels under rigorous examination and is confirmed as a useful clinical measure.

Rat pineal gland phosducin: cDNA isolation, nucleotide sequence, and chromosomal assignment in the mouse

The pineal gland contains a soluble phosphoprotein, phosducin, which is homologous to that of retinal photoreceptors. Phosducin has been shown to bind the beta, gamma subunits of the retinal G-protein transducin. Retinal phosducin has been cloned and now we report a rat pineal cDNA encoding phosducin. A 1217-nucleotide cDNA was isolated from a rat pineal library by DNA-DNA hybridization with a polymerase chain reaction-amplified cDNA of bovine retina mRNA for phosducin. Northern blot analysis demonstrates that the mRNA for phosducin is approximately 1.3 kb in both rat pineal and rat retina. The translated mRNA from rat pineal encodes a protein with 246 amino acids, compared to the 245 amino acids of bovine retina phosducin. The predicted molecular weight of rat pineal phosducin is 28,201. Immunoblot analysis with affinity-purified antibodies against bovine retina phosducin identify a single immunoreactive protein of approximately 33 kDa in both rat retina and rat pineal. The amino acid sequence of rat pineal phosducin is homologous to that of bovine retina phosducin, revealing 89% identity and another 5.7% similarity. Both rat pineal and bovine retina phosducins are acidic proteins with pIs of 4.3 and 4.5, respectively. The translated protein lacks hydrophobic domains that would suggest an integral membrane protein. Rat pineal phosducin has a single consensus phosphorylation domain for protein kinase A that is nearly identical to that of retinal phosducin, which is phosphorylated by protein kinase A in situ. Rat phosducin also contains three potential phosphorylation domains for protein kinase C and nine for casein kinase II as well as a predicted site for N-glycosylation. The cDNA encoding phosducin was used to localize the gene within a linkage group to a large segment of mouse chromosome 1 in a conserved region with the long arm of human chromosome 1 with a panel of DNA samples from an interspecific cross. In keeping with a proposed role of retinal phosducin in down-regulation of the photo-transduction cascade, a modulatory role in signal transduction is proposed for pineal phosducin.

Circadian locomotor rhythms, but not photoperiodic responses, survive surgical isolation of the SCN in hamsters

Surgical isolation of the suprachiasmatic nuclei (SCN) within a hypothalamic island is reported to produce loss of circadian rhythmicity. The results have been interpreted to indicate that SCN efferents are necessary for the expression of circadian rhythms. It is not clear, however, whether the loss of circadian rhythms in behavioral responses following SCN isolation is attributable to transection of efferents, to loss of cells within the island, or to gliosis produced by the knife cut. To explore this issue, we examined locomotor activity and gonadal state of male golden hamsters housed in constant darkness (DD, with a dim red light for maintenance) for at least 10 weeks following isolation of the SCN from the rest of the brain by cuts by means of a Halasz wire microknife. Brain sections were immunocytochemically stained for the peptides vasoactive intestinal polypeptide (VIP), vasopressin (VP) or neurophysin II (NP II), and neuropeptide Y (NPY) to localize the SCN and to assess its viability, and for glial fibrillary acidic protein (GFAP) to delimit the border of the knife cut. Experimental animals with VIP and VP/NP II immunoreactivity in the SCN within the island retained free-running locomotor rhythms following transection of SCN efferents. Animals with cuts that failed to sever SCN efferents, and sham-operated animals (in which the Halasz knife was lowered but not rotated), also maintained circadian rhythmicity. Hamsters sustaining severe damage to the SCN showed disrupted locomotor activity. In those hamsters that retained circadian locomotor rhythmicity following SCN isolation, gonads failed to regress in DD, demonstrating the absence of an appropriate photoperiodic response. The results suggest a multiplicity of SCN coupling mechanisms in the control of circadian rhythms.

Beta-adrenergic stimulation increases cAMP and melatonin production in ovine pinealocyte cultures

An ovine pinealocyte suspension culture method, using glands from young sheep as a source of cells, has been developed to study mechanisms of pineal regulation in this species. Cell suspensions are obtained by enzymatic digestion (preliminary trypsinization followed by collagenase/pronase treatment) of pineal glands and these cells may be usefully retained in culture for up to 48 hr. Initial characterization of cyclic adenosine monophosphate and melatonin production responses to adrenergic stimuli (norepinephrine, phenylephrine, isoproterenol) in the absence and presence of antagonists (propranolol, prazosin) by pinealocytes in suspension culture indicate that a beta-adrenergic receptor-mediated mechanism is primarily involved in transduction of the adrenergic signal to melatonin synthesis. This is in agreement with data from earlier work on the sheep using short-term tissue incubations but contrasts with in vivo evidence suggesting a predominant alpha 1-adrenergic receptor-mediated mechanism.

Neuroendocrine rhythms

Hormones are secreted with circhoral, circadian and seasonal periodicities. Circhoral pulsatility is a temporal code, many chronic and acute changes in neuroendocrine status being mediated by changes in the frequency of circhoral release. The identity of the neuronal circuits controlling circhoral release is not known. Circadian release of hormones occurs with a precise temporal order entrained to the light-dark cycle, synchronized to the activity/rest rhythm and generated by circadian oscillators, of which the suprachiasmatic nuclei are the most important. Seasonal rhythms are driven either by an endogenous circannual clock mechanism or by a process of photoperiodic time measurement which is dependent upon the duration of the nocturnal peak of the pineal hormone melatonin.

GABA-immunoreactive intrinsic and -immunonegative secondary neurons in the cat pineal organ

The pineal organ of the cat was studied by postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. Two polyclonal rabbit GABA antisera were used with light microscopic peroxidase and electron microscopic immunogold techniques. A considerable number of intrinsic neurons are scattered in the proximal portion of the pineal organ. Some of the nerve cells were GABA-immunoreactive; other neurons as well as pinealocytes and glial/ependymal cells were immunonegative. A few GABA-immunoreactive neurons behave like CSF-contacting neurons by penetrating the ependymal lining of the pineal recess. GABA-immunoreactive neurons were more frequently found in the subependymal region. Small bundles of thin immunoreactive unmyelinated and thick immunoreactive myelinated nerve fibers occurred in the proximal pineal, especially near the habenular commissure. There were synapses of various types between GABA-immunoreactive and -immunonegative fibers. Myelinated immunoreactive axons seemed to loose their sheaths after entering the organ. Axon-like processes of pinealocytes terminated on dendrites of immunonegative neurons present near the posterior and habenular commissures. The axons of these neurons were found to join the commissural fibers and may represent a pinealofugal pathway conducting information originating from pinealocytes. The pinealocytic axons forming ribbon-containing synapses on dendrites of secondary neurons speak in favor of the sensory-cell nature of the pinealocytes. The pinealopetal myelinated GABA-immunoreactive axons and the intrinsic "GABA-ergic" neurons are proposed to inhibit the action of intrapineal neurons on which the pinealocytic axons terminate.

Neurophysiological responses to melatonin in the SCN of short-day sensitive and refractory hamsters

The pineal hormone melatonin plays a central role in the regulation of seasonal reproductive cycles in mammals and several studies have implicated the suprachiasmatic nucleus (SCN) as a target on which melatonin acts. The Syrian hamster is a long-day breeder which exhibits gonadal regression when housed in short (less than 12.5 h) daily photoperiods or injected daily with melatonin in long photoperiods. In the present paper we address the question whether melatonin affects firing rates of SCN neurones and whether these effects change as the animals become refractory to short photoperiods. In long-day (LD14:10) hamsters SCN neurones were suppressed (31%), activated (15%) or unaffected (54%) by melatonin. In contrast, there was an increased proportion of melatonin insensitive cells (88%) in short-day (refractory) hamsters. Melatonin-responsive cells were found primarily during the late projected day and early projected night in both long-day and short-day animals. This reduced responsiveness of SCN neurones to melatonin in hamsters refractory to short-day exposure may represent part of the mechanism underlying the development of gonadal refractoriness and the onset of gonadal growth in anticipation of long spring photoperiods.

Differential expression of mRNA and protein encoding retinal and pineal S-antigen during the light/dark cycle

S-Antigen is a soluble cell protein unique to the retina and pineal gland. In the former, it is a well-characterized molecule that participates in light-induced signal transduction in photoreceptor cells. In the latter, the functional role is presently not known. The expression of S-antigen and its mRNA was examined in the rat retina and pineal gland throughout the diurnal cycle and with light interruption of the dark cycle. A cDNA for rat S-antigen was isolated from a pineal gland library to examine the mRNAs. A 1.7-kb mRNA for S-antigen was observed in both the pineal gland and the retina. Retinal S-antigen mRNA was expressed throughout the diurnal cycle and increased with light interruption of the dark cycle. In contrast, pineal gland S-antigen mRNA levels were detectable only during the dark and were absent preceding and during light. The phenotypic expression of immunoreactive S-antigen, identified with two S-antigen monoclonal antibodies (MAbs), MAb A9C6 and MAb C10C10, was analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel (PAGE) and isoelectric focusing (IEF) electrophoresis. Immunoblot analysis of gels after SDS-PAGE revealed a single 46-kDa protein in retina. In contrast, two bands of approximately 43 and 46 kDa were identified in the pineal gland. Immunoblots of the retinal extracts separated by IEF electrophoresis revealed five S-antigen isomers, which vary quantitatively throughout the diurnal cycle and when light interrupted the dark cycle. Immunoblots of the pineal gland samples separated by IEF electrophoresis indicated that the pineal gland possesses four pineal gland-specific forms of S-antigen in addition to the five forms present in the retina. The differences observed in the mRNA and protein analyses suggest tissue-specific structural components for S-antigen in the retina and pineal gland that are not regulated in the same manner.

Location of the suprachiasmatic nucleus grafts in rats which restored circadian rhythmicity after transplantation

In Wistar male rats whose circadian wheel running activity rhythms were disrupted by bilateral suprachiasmatic nuclei (SCN) lesions, transplantation of neonatal rat SCN into the 3rd ventricle was performed. Out of 49 rats from which adequate wheel running activity records were obtained, 15 rats showed restoration of circadian rhythm starting 2 to 13 weeks (average 1 month) after transplantation. The existence of active SCN neurons in the graft was shown by immunocytochemical reactivity to vasopressin- and VIP-like substances. In all rats, effective grafts were found in the diencephalon, mostly on the wall of the 3rd ventricle.

N-acetylaspartylglutamate: a transmitter candidate for the retinohypothalamic tract

The retinohypothalamic tract is the neural pathway mediating the photic entrainment of circadian rhythms in mammals. Important targets for these retinal fibers are the suprachiasmatic nuclei (SCN) of the hypothalamus, which are thought to be primary sites for the biological clock. The neurotransmitters that operate in this projection system have not yet been determined. Immunohistochemistry and radioimmunoassay performed with affinity-purified antibodies to N-acetylaspartylglutamate (NAAG) demonstrate that this neuron-specific dipeptide, which may act as an excitatory neurotransmitter, is localized extensively in the retinohypothalamic tract and its target zones, including the SCN. Optic nerve transections resulted in significant reductions in NAAG immunoreactivity in the optic chiasm and SCN. Analysis of NAAG concentrations in micropunches of SCN, by means of radioimmunoassay, showed approximately 50% reductions in NAAG levels. These results suggest that this peptide may act as one of the neurotransmitters involved in retinohypothalamic communication and circadian rhythm entrainment.

Daily variation in the concentration of melatonin and 5-methoxytryptophol in the human pineal gland: effect of age and Alzheimer's disease

Melatonin and 5-methoxytryptophol (ML) were measured in human pineals (38 controls, 16 subjects with Alzheimer's disease). Time of death had a major influence on the indole concentrations with significantly higher melatonin levels occurring at night (22.00-10.00 h) and significantly higher ML levels occurring during the day (10.00-22.00 h). This daily variation disappeared in both the older subjects (55-92 years) and in the Alzheimer patients (55-89 years).

Projections from the lateral geniculate nucleus to the hypothalamus of the Mongolian gerbil (Meriones unguiculatus): an anterograde and retrograde tracing study

The lateral geniculate nucleus of the thalamus sends efferents to the hypothalamic suprachiasmatic nucleus, which is involved in generation and entrainment of several circadian rhythms. It seems reasonable to believe that the lateral geniculate conveys visual information about the length of the photoperiod to the circadian oscillator. In order to study in more detail the topographical relationship between the lateral geniculate and the suprachiasmatic nucleus, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheatgerm agglutinin coupled to horseradish peroxidase (WGA-HRP) were performed in the gerbil. After iontophoretic injections of PHA-L in the lateral geniculate, a large number of PHA-L-immunoreactive fibers and nerve terminals were observed in the ventrolateral part of the suprachiasmatic nucleus. Nerve fibers were also present in the ventromedial and dorsolateral portions, particularly in the caudal half of the nucleus. PHA-L-immunoreactive nerve fibers continued outside the borders of the suprachiasmatic nucleus to the adjacent anterior hypothalamic, the periventricular, and the subparaventricular areas. A moderate number of fibers entered the lateral hypothalamic area and the tuber cinerum via the optic tract and chiasm. Moreover, the paraventricular nucleus, the supraoptic nucleus, the medial preoptic area, the lateral preoptic area, and the supramammillary nucleus contained a few labeled fibers. In all parts of the hypothalamus receiving an input from the lateral geniculate, fine beaded immunoreactive fibers with varicosities and nerve terminals were observed, some of which were found in close apposition to hypothalamic neurons. Only after labeling of neurons in the intergeniculate leaflet of the lateral geniculate nucleus, fibers were found in the hypothalamus. This topographical organization of the geniculohypothalamic pathway was supported by retrograde tracing after injections of WGA-HRP in the suprachiasmatic area. In these experiments, retrograde labeled neurons were observed in the intergeniculate leaflet and, in agreement with the anterograde studies, most of labeling was observed in the ipsilateral side. These results confirm that the suprachiasmatic nucleus receives a substantial input from the intergeniculate leaflet of the lateral geniculate. Moreover, the present data demonstrate that the suprachiasmatic nucleus is not the only nucleus that receives a direct visual input. Thus other hypothalamic areas might be influenced by a direct rhythmic neuronal input as well.

A neuronal projection from the lateral geniculate nucleus to the lateral hypothalamus of the rat demonstrated with Phaseolus vulgaris leucoagglutinin tracing

Iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L), were placed in different portions of the rat lateral geniculate nucleus, and the tracer was localized by immunohistochemistry. The efferent projections of the lateral geniculate to limbic areas were analysed with special reference to projections of the lateral hypothalamic area. Only in animals, in which neurons in the intergeniculate leaflet were labeled were PHA-L-immunoreactive axons in the lateral hypothalamic area observed. The nerve fibers were mostly located in the ventral and anterior parts of the lateral hypothalamic area. This tracing study suggests that the optic system probably mediating information about the photoperiod influences not only the suprachiasmatic nucleus, but also the lateral hypothalamic area.

Testicular response to melatonin or suprachiasmatic nuclei ablation in the spotted skunk

Testes of the Western spotted skunk enlarge and regress seasonally. The pineal hormone, melatonin, may be important in timing this seasonal reproductive activity. Likewise, the suprachiasmatic nuclei (SCN) have been implicated as possible neural regulators of pineal and reproductive events. These experiments were conducted to determine whether ablation of the SCN or constant administration of melatonin alters timing of the seasonal pattern of testicular regression and recrudescence. Male skunks (n = 24) were treated as follows: six received two empty Silastic capsules, six received two melatonin-filled Silastic capsules, six received sham lesions to the SCN, and six received lesions to the SCN (SCNx). All skunks were exposed to a natural photoperiod and had regressed testes at the onset of the experiment. Four of six males from the SCNx group had an average of 94 +/- 11.3% of these nuclei destroyed. Sham SCNx, animals with less than 40% of the SCN ablated, and males with empty capsules did not have fully enlarged testes until October. SCNx and melatonin-treated skunks exhibited a hastening of testicular recrudescence with maximal testis size being reached in June. Skunks with lesions to the SCN maintained enlarged testes for 5 months while all other groups exhibited rapid regression after attaining maximal testis size. Testicular regression occurred from July through October in animals receiving continuous melatonin, while controls exhibited recrudescence during this same period. Our data suggest that the SCN, melatonin, and thus the pineal gland, play a role in regulating the seasonal testicular cycle of the spotted skunk.

Noradrenergic control of the synthesis of two rat pineal proteins

Pineal physiology is controlled by norepinephrine released from sympathetic nerves terminating in the gland. In the present study, the effect of norepinephrine on the labelling of specific proteins was investigated by incubating glands with [35S]methionine and then resolving the proteins by two-dimensional polyacrylamide gel electrophoresis; the patterns were analyzed by computer-assisted image analysis. The most prominent effects of norepinephrine were distinct and consistent increases in the labelling of two proteins (37 kDa, pI = 6.0, 50 kDa, pI = 6.0), designated adrenergically induced protein (AIP 37/6 and AIP 50/6). In both cases, norepinephrine was effective at low concentrations (EC50 = 10 nM). Pharmacological studies indicated that the effects of norepinephrine on both proteins involved a beta-adrenergic receptor, and that cyclic AMP was the second messenger. Pulse-chase labelling experiments revealed that these effects of norepinephrine did not involve post-translational modification of previously labelled precursor proteins, but depended upon de novo synthesis of protein. An inhibitor of mRNA synthesis, actinomycin-D, was found to block the effect of norepinephrine on AIP 50/6 but not on AIP 37/6, suggesting that norepinephrine acted on AIP 50/6 via a transcriptional mechanism and on AIP 37/6 via a translational mechanism. These in vitro studies were extended into in vivo investigations by measuring silver-stained AIP 37/6 in the two-dimensional gels. Changes in the amount of AIP 37/6 in pineal glands were studied in response to treatments which block the adrenergic stimulation of the gland, including exposure to constant lighting or removal of the superior cervical ganglia. Both treatments reduced AIP 37/6 by 50-75% in 8 weeks. These observations, together with those from in vitro studies, suggest that the amount of AIP 37/6 in the pineal gland is regulated by norepinephrine; and further, that norepinephrine acts through a beta-adrenergic-cyclic AMP mechanism to control AIP 37/6 synthesis at a translational level.

Diurnal variations in brain serotonin are driven by the photic cycle and are not circadian in nature

In an effort to determine the driving force of the diurnal variation of serotonin (5-HT) in the brain, over 500 Syrian hamsters were exposed to long photoperiods (LD17:7; LD), short photoperiods (LD8:16; SD), constant dark (DD) or constant light (LL) for 12 weeks. Hamsters exposed to LD or SD were sacrificed at 3 h intervals; those in constant conditions were sacrificed around the clock. The circadian time (CT) of tissue collection, in the animals in constant conditions, was determined from the onset of locomotor activity (defined as CT12; the beginning of the subjective night). Serotonin content was determined by HPLC with electrochemical detection in the medial basal hypothalamus (MBH); anterior hypothalamus (AH) and olfactory bulbs (OB). In LD and SD, 5-HT levels displayed significant diurnal variation in the MBH, AH and OB (ANOVA; P less than 0.001). The sine waves of the 5-HT rhythm in these conditions were similar in amplitude and phase with relation to lights on (i.e. high 5-HT content during the day and low content at night, with a sharp rise occurring just after lights on). This variation, however, was not apparent in animals exposed to DD or LL; 5-HT content did not display a significant diurnal oscillation. Since 5-HT failed to oscillate in the absence of environmental time cues, the rhythm is likely driven by the environment and not an internal circadian clock.

The role of the hypothalamic paraventricular nuclei for the regulation of pineal melatonin synthesis: new aspects derived from the vasopressin-deficient Brattleboro rat

There is evidence for an involvement of the hypothalamic paraventricular nuclei (PVN) in the regulation of pineal melatonin synthesis in rats. Since electrical stimulation of the PVN or the systemic administration of arginine-vasopressin (AVP) result in a depression of the nocturnal melatonin surge, this neuropeptide appears to be pivotal for the transduction of PVN-efferent, pinealopetal signals. We therefore used an AVP-deficient animal model, the Brattleboro rat, to further investigate the mechanisms responsible for pineal regulation. Anesthetized adult male animals received 2 min of bilateral electrical stimulation of the PVN either during the day or at night. Thirty min later, pineal glands were removed and pineal N-acetyltransferase (NAT) activities and melatonin contents were determined. Stimulation resulted neither during the day nor at night in any significant alterations of pineal NAT activity or melatonin content when compared to control or sham-stimulated animals. These data further support the proposed modulatory role of AVP for the regulation of melatonin synthesis in the Epiphysis cerebri of genetically intact rats.

Photoperiodic responsiveness of hamsters with lesions of the lateral geniculate nucleus is related to hippocampal damage

We tested the hypothesis that the geniculohypothalamic tract is important for hamster photoperiodism. Adult male hamsters, maintained in a long photoperiod (LD 14:10), received either large bilateral neurotoxic lesions of the lateral geniculate nucleus (LGN) or sham lesions. One week later, half of the animals from each group were transferred to a short photoperiod (LD 8:16) where they were maintained for 15 weeks. Most lesions effectively destroyed the intergeniculate leaflet (IGL) and much of the lateral geniculate complex. They also caused substantial damage to the overlying hippocampus. The lesions had no effect on long-day animals, but significantly reduced the extent of testicular regression during short photoperiod exposure. This effect, however, appeared to be the result of hippocampal, rather than geniculate, damage. Four individuals with complete IGL lesions, but minimal hippocampal damage, underwent a pattern of regression that was indistinguishable from controls. Body weight was increased by lesions in short-day, but not long-day, animals. This effect was not related to the extent of hippocampal damage. We conclude that geniculate input to the suprachiasmatic nuclei is not essential for hamster photoperiodism and that hippocampal damage may modify the effect of short daylengths on testes size.