The effects of lesions of the thalamic intergeniculate leaflet on the pineal metabolism

Retinorecipient areas in the common marmoset (Callithrix jacchus): An image-forming and non-image forming circuitry

The mammalian retina captures a multitude of diverse features from the external environment and conveys them via the optic nerve to a myriad of retinorecipient nuclei. Understanding how retinal signals act in distinct brain functions is one of the most central and established goals of neuroscience. Using the common marmoset (Callithrix jacchus), a monkey from Northeastern Brazil, as an animal model for parsing how retinal innervation works in the brain, started decades ago due to their marmoset's small bodies, rapid reproduction rate, and brain features. In the course of that research, a large amount of new and sophisticated neuroanatomical techniques was developed and employed to explain retinal connectivity. As a consequence, image and non-image-forming regions, functions, and pathways, as well as retinal cell types were described. Image-forming circuits give rise directly to vision, while the non-image-forming territories support circadian physiological processes, although part of their functional significance is uncertain. Here, we reviewed the current state of knowledge concerning retinal circuitry in marmosets from neuroanatomical investigations. We have also highlighted the aspects of marmoset retinal circuitry that remain obscure, in addition, to identify what further research is needed to better understand the connections and functions of retinorecipient structures.

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

Daily corticosterone rhythm modulates pineal function through NFκB-related gene transcriptional program

Melatonin and glucocorticoids are key hormones in determining daily rhythmicity and modulating defense responses. In nocturnal animals, corticosterone peaks at light/dark transition,while melatonin peaks at the middle of the night in both nocturnal and diurnal animals. The crosstalk between adrenal and pineal glands under inflammatory conditions indicates that corticosterone potentiates nocturnal melatonin synthesis by reducing the activity of NFκB. This transcription factor, which modulates the expression of a key enzyme in melatonin synthesis, is sharply reduced at the entrance of darkness in the rat pineal gland. In this study, we established the basis for understanding the crosstalk between adrenal and pineal glands in physiological conditions. Here we show that the expression of 70 out of 84 genes implied in defense responses exhibit a sharp reduction exactly at the entrance of darkness. Mifepristone impair the changes of 13 out of 84 genes, suggesting that the rhythm of corticosterone modulates pineal phenotype, as mifepristone also reduces the expression of Aanat and the nocturnal synthesis of melatonin. Therefore, darkness-induced synthesis of the pineal hormone, besides being controlled by the central clock located in the hypothalamus, is also influencedby glucocorticoids through the regulation of NFκB transcriptional program.

Acute effects of light on the brain and behavior of diurnal Arvicanthis niloticus and nocturnal Mus musculus

Photic cues influence daily patterns of activity via two complementary mechanisms: (1) entraining the internal circadian clock and (2) directly increasing or decreasing activity, a phenomenon referred to as "masking". The direction of this masking response is dependent on the temporal niche an organism occupies, as nocturnal animals often decrease activity when exposed to light, while the opposite response is more likely to be seen in diurnal animals. Little is known about the neural mechanisms underlying these differences. Here, we examined the masking effects of light on behavior and the activation of several brain regions by that light, in diurnal Arvicanthis niloticus (Nile grass rats) and nocturnal Mus musculus (mice). Each species displayed the expected behavioral response to a 1h pulse of light presented 2h after lights-off, with the diurnal grass rats and nocturnal mice increasing and decreasing their activity, respectively. In grass rats light induced an increase in cFOS in all retinorecipient areas examined, which included the suprachiasmatic nucleus (SCN), the ventral subparaventricular zone (vSPZ), intergeniculate leaflet (IGL), lateral habenula (LH), olivary pretectal nucleus (OPT) and the dorsal lateral geniculate (DLG). In mice, light led to an increase in cFOS in one of these regions (SCN), no change in others (vSPZ, IGL and LH) and a decrease in two (OPT and DLG). In addition, light increased cFOS expression in three arousal-related brain regions (the lateral hypothalamus, dorsal raphe, and locus coeruleus) and in one sleep-promoting region (the ventrolateral preoptic area) in grass rats. In mice, light had no effect on cFOS in these four regions. Taken together, these results highlight several brain regions whose responses to light suggest that they may play a role in masking, and that the possibility that they contribute to species-specific patterns of behavioral responses to light should be explored in future.

Effects of diets supplemented with branched-chain amino acids on the performance and fatigue mechanisms of rats submitted to prolonged physical exercise

This study aimed to determine the effects of diets chronically supplemented with branched-chain amino acids (BCAA) on the fatigue mechanisms of trained rats. Thirty-six adult Wistar rats were trained for six weeks. The training protocol consisted of bouts of swimming exercise (one hour a day, five times a week, for six weeks). The animals received a control diet (C) (n = 12), a diet supplemented with 3.57% BCAA (S1) (n = 12), or a diet supplemented with 4.76% BCAA (S2) (n = 12). On the last day of the training protocol, half the animals in each group were sacrificed after one hour of swimming (1H), and the other half after a swimming exhaustion test (EX). Swimming time until exhaustion was increased by 37% in group S1 and reduced by 43% in group S2 compared to group C. Results indicate that the S1 diet had a beneficial effect on performance by sparing glycogen in the soleus muscle (p < 0.05) and by inducing a lower concentration of plasma ammonia, whereas the S2 diet had a negative effect on performance due to hyperammonemia (p < 0.05). The hypothalamic concentration of serotonin was not significantly different between the 1H and EX conditions. In conclusion, chronic BCAA supplementation led to increased performance in rats subjected to a swimming test to exhaustion. However, this is a dose-dependent effect, since chronic ingestion of elevated quantities of BCAA led to a reduction in performance.

Neurophysiology of circadian rhythm sleep disorders of children with neurodevelopmental disabilities

This article reviews circadian rhythm sleep disorders (CRSDs) of children with neurodevelopmental disabilities. These sleep disturbances frequently occur in this population but they are misunderstood and under diagnosed. The causes and features of CRSD in children with brain disorders differ in many ways from those seen in typically developing children. It is the brain, including the eyes, which regulates sleep and circadian rhythmicity by modulating pineal melatonin production/secretion and when there is significant brain damage, the sleep/wake patterns may be modified. In most instances CRSD are not disorders of the suprachiasmatic nuclei because these small hypothalamic structures only adjust their functions to the changing photic and non-photic modulatory influences. Each form of CRSD is accompanied by characteristic changes in serum melatonin levels and clinical features. When nocturnal melatonin production/secretion is inappropriately timed or impaired in relation to the environment, timed melatonin replacement therapy will often be beneficial. In this review an attempt is made to clarify the neurophysiological mechanisms underlying the various forms of CRSD because without understanding the photic and non-photic influences on sleep, these sleep disorders can not be fully characterized, defined or even appropriately treated. In the future, the existing definitions for the different forms of CRSD should be modified by experts in pediatric sleep medicine in order to include children with neurodevelopmental disabilities.

Dim nighttime illumination alters photoperiodic responses of hamsters through the intergeniculate leaflet and other photic pathways

In mammals, light entrains the central pacemaker within the suprachiasmatic nucleus (SCN) through both a direct neuronal projection from the retina and an indirect projection from the intergeniculate leaflet (IGL) of the thalamus. Although light comparable in intensity to moonlight is minimally effective at resetting the phase of the circadian clock, dimly lit and completely dark nights are nevertheless perceived differentially by the circadian system, even when nighttime illumination is below putative thresholds for phase resetting. Under a variety of experimental paradigms, dim nighttime illumination exerts effects that may be characterized as enhancing the plasticity of circadian entrainment. For example, relative to completely dark nights, dimly lit nights accelerate development of photoperiodic responses of Siberian hamsters transferred from summer to winter day lengths. Here we assess the neural pathways underlying this response by testing whether IGL lesions eliminate the effects of dim nighttime illumination under short day lengths. Consistent with previous work, dimly lit nights facilitated the expansion of activity duration under short day lengths. Ablation of the IGL, moreover, did not influence photoperiodic responses in animals held under completely dark nights. However, among animals that were provided dimly lit nights, IGL lesions prevented the short-day typical expansion of activity duration as well as the seasonally appropriate gonadal regression and reduction in body weight. Thus, the present data indicate that the IGL plays a central role in mediating the facilitative effects of dim nighttime illumination under short day lengths, but in the absence of the IGL, dim light at night influences photoperiodic responses through residual photic pathways.

Effects of the blockade of high voltage-activated calcium channels on in vitro pineal melatonin synthesis

The presence of high voltage-activated calcium channels in the rat pineal gland is well known. However, their role in pineal metabolism is not completely understood and is even controversial. Better to understand this matter, we investigated the effects of L-, N- or P/Q-type calcium channel blockers (nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, respectively) on melatonin content and arylalkylamine-N-acetyltransferase activity of denervated rat pineal glands kept for 48 h in culture and stimulated with norepinephrine. Melatonin was measured by high performance liquid chromatography with electrochemical detection and arylalkylamine-N-acetyltransferase activity was quantified by radiometric assay. Pre-incubation with any of these high voltage-activated calcium channel blockers reduced the melatonin production induced by norepinephrine although arylalkylamine-N-acetyltransferase activity was reduced only by the N-type calcium channel antagonist, omega-conotoxin GVIA. The results indicate that calcium influx through L-, N- or P/Q-type of high voltage-activated calcium channels is necessary for the full expression of the metabolic process leading to melatonin synthesis in the rat pineal glands. However, the mechanisms involved in this process are different for the L- or P/Q- and N-type calcium channels.

Paradoxical effects of NPY in the suprachiasmatic nucleus

The circadian clock in the suprachiasmatic nucleus (SCN) is synchronized by the 24 h, light : dark cycle, and is reset by photic and non-photic cues. The acute effects of light in the SCN include the increase of mRNA levels of the circadian clock gene Per1 and a dramatic reduction of pineal melatonin. Neuropeptide Y (NPY), which appears to mediate the phase-resetting effects of non-photic stimuli, prevents the ability of light, and stimuli that mimic light, to phase shift the circadian clock when injected into the SCN. The purpose of the present study was to determine if NPY inhibits the ability of light to suppress pineal melatonin. Surprisingly, NPY injected into the SCN of hamsters mimicked the effects of light by suppressing pineal melatonin levels. To confirm that NPY inhibited the effects of light on the induction of Per1 mRNA levels, Per1 mRNA levels in the SCN were measured in these same animals. NPY significantly reduced Per1 mRNA levels induced by the light pulse. The suppression of melatonin by NPY appears to be mediated by the same subtype of NPY receptors in the SCN that mediate the modulation of phase shifts. Injection of Y5 receptor agonists mimicked the effects of NPY on pineal melatonin, while injection of a Y2 agonist did not. Thus, these data are the first to demonstrate the paradoxical effects of NPY within the SCN. NPY mimics the effects of light on pineal melatonin and inhibits the effects of light on the induction of Per1 mRNA.

The profile of melatonin production in tumour-bearing rats

The pineal gland is involved in the regulation of tumour growth through the anticancer activity of melatonin, which presents immunomodulatory, anti-proliferative and anti-oxidant effects. In this study we measured melatonin content directly in the pineal gland, in an attempt to clarify the modulation of pineal melatonin secretory activity during tumour growth. Different groups of Walker 256 carcinosarcoma bearing rats were sacrificed at 12 different time points during 24h (12h:12h light/dark cycle) on different days during the tumour development (on the first, seventh and fourteenth day after tumour inoculation). Melatonin content in the pineal gland was determined by high-performance liquid chromatography with electrochemical detection. During tumour development the amount of melatonin secreted increased from 310.9 ng/mg of protein per day from control animals, to 918.1 ng/mg of protein per day 14 days after tumour implantation, and there were changes in the pineal production profile of melatonin. Cultured pineal glands obtained from tumour-bearing rats turned out to be less responsive to noradrenaline, suggesting the existence, in vivo, of putative factor(s) modulating pineal melatonin production. The results demonstrated that during tumour development there is a modification of pineal melatonin production daily profile, possibly contributing to cachexia, associated to changes in pineal gland response to noradrenaline stimulation.

Tryptophan consumption and indoleamines production by peritoneal cavity macrophages

Melatonin has been shown to regulate several immune functions, and some authors showed that leukocytes are also able to produce the indolamine. In fact, it seems to take part in some immunoregulatory axis, including that related to interferon (IFN) production. So, we evaluated the rate of tryptophan consumption and melatonin and serotonin production in peritoneal cavity-isolated macrophages and the effect of IFN-alpha and -gamma, lipopolysaccharide (LPS), and phorbol myristate acetate (PMA) on such parameters. Our results indicate that macrophages obtained from the peritoneal cavity of normal rats when incubated with tryptophan show an increase in arylalkylamine N-acetyltransferase activity that corresponds to an increased melatonin production, as determined in the incubation medium. This process is regulated by IFN-alpha and -gamma, PMA, LPS, and the serum from tumor-bearing rats, opening the possibility of speculation about different immunoregulatory loops acting through the balance of melatonin/serotonin production by such cells.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Neural basis and biological function of masking by light in mammals: suppression of melatonin and locomotor activity

Light influences mammalian circadian rhythms in two different ways: (1) It entrains endogenous oscillators (clocks), which regulate physiology and behavior; and (2) it affects directly and often immediately physiology and behavior (these effects are also referred to as masking). Masking effects of light on pineal melatonin, locomotor activity, and the sleep-wake cycle in mammals and man are reviewed. They seem to represent a universal response in this group. The review reveals that the mechanism of photic inhibition of melatonin is fairly well understood, whereas only little is known about the influence of light on other circadian rhythm outputs, such as locomotor activity.

A 5-HT(1B) receptor agonist inhibits light-induced suppression of pineal melatonin production

Serotonin (5-HT) modulates the phase adjusting effects of light on the mammalian circadian clock through the activation of presynaptic 5-HT(1B) receptors located on retinal terminals in the suprachiasmatic nucleus (SCN). The current study was conducted to determine whether activation of 5-HT(1B) receptors also alters photic regulation of nocturnal pineal melatonin production. Systemic administration of the 5-HT(1B) receptor agonist TFMPP attenuated the inhibitory effect of light on pineal melatonin synthesis in a dose-related manner with an apparent ED(50) value of 0.9 mg/kg. The effect of TFMPP on light-induced melatonin suppression was blocked by the 5-HT(1) receptor antagonist, methiothepin, but not by the 5-HT(1A) antagonist, WAY 100,635, consistent with the involvement of 5-HT(1B) receptors. The results are consistent with the interpretation that activation of presynaptic 5-HT(1B) receptors on retinal terminals in the SCN attenuates the effect of light on pineal melatonin production, as well as on circadian phase.

Suprachiasmatic nucleus and intergeniculate leaflet in the diurnal rodent Octodon degus: retinal projections and immunocytochemical characterization

The neural connections and neurotransmitter content of the suprachiasmatic nucleus and intergeniculate leaflet have been characterized thoroughly in only a few mammalian species, primarily nocturnal rodents. Few data are available about the neural circadian timing system in diurnal mammals, particularly those for which the formal characteristics of circadian rhythms have been investigated. This paper describes the circadian timing system in the diurnal rodent Octodon degus, a species that manifests robust circadian responses to photic and non-photic (social) zeitgebers. Specifically, this report details: (i) the distribution of six neurotransmitters commonly found in the suprachiasmatic nucleus and intergeniculate leaflet; (ii) the retinohypothalamic tract; (iii) the geniculohypothalamic tract; and (iv) retinogeniculate projections in O. degus. Using immunocytochemistry, neuropeptide Y-immunoreactive, serotonin-immunoreactive and [Met]enkephalin-immunoreactive fibers and terminals were detected in and around the suprachiasmatic nucleus; vasopressin-immunoreactive cell bodies were found in the dorsomedial and ventral suprachiasmatic nucleus; vasoactive intestinal polypeptide-immunoreactive cell bodies were located in the ventral suprachiasmatic nucleus; [Met]enkephalin-immunoreactive cells were located sparsely throughout the suprachiasmatic nucleus; and substance P-immunoreactive fibers and terminals were detected in the rostral suprachiasmatic nucleus and surrounding the nucleus throughout its rostrocaudal dimension. Neuropeptide Y-immunoreactive and [Met]enkephalin-immunoreactive cells were identified in the intergeniculate leaflet and ventral lateral geniculate nucleus, as were neuropeptide Y-immunoreactive, [Met]enkephalin-immunoreactive, serotonin-immunoreactive and substance P-immunoreactive fibers and terminals. The retinohypothalamic tract innervated both suprachiasmatic nuclei equally; in contrast, retinal innervation to the lateral geniculate nucleus, including the intergeniculate leaflet, was almost exclusively contralateral. Bilateral electrolytic lesions that destroyed the intergeniculate leaflet depleted the suprachiasmatic nucleus of virtually all neuropeptide Y- and [Met]enkephalin-stained fibers and terminals, whereas unilateral lesions reduced fiber and terminal staining by approximately half. Thus, [Met]enkephalin-immunoreactive and neuropeptide Y-immunoreactive cells project equally and bilaterally from the intergeniculate leaflet to the suprachiasmatic nucleus via the geniculohypothalamic tract in degus. This is the first report examining the neural circadian system in a diurnal rodent for which formal circadian properties have been described. The data indicate that the neural organization of the circadian timing system in degus resembles that of the most commonly studied nocturnal rodents, golden hamsters and rats. Armed with such data, one can ascertain differences in the functional organization of the circadian system between diurnal and nocturnal mammals.

Persistent elevation of nocturnal activity in rodents following apparent recovery from lithium/pilocarpine-induced limbic seizures

The nocturnal and day-time activity of rats was monitored continuously after the induction of seizures by a single systemic injection of lithium followed 4 hr. later by the muscarinic agent pilocarpine. Although there was a transient increase in activity during the day cycle shortly after the seizure induction, this behavior returned to baseline levels; however, nocturnal activity increased and remained elevated two or three times above the baseline levels. Implications for the enhancement of nocturnal melatonin levels within the brains of these chronically epileptic rats are discussed.

Pineal metabolic reaction to retinal photostimulation in ganglionectomized rats

The aim of the present work was to test the pineal gland metabolic reactivity to nocturnal retinal short term photic stimulation in superior cervical ganglionectomized rats. The experimental support for this work is the appearance of a transitory post synaptic hyperactivity in the pineal gland, during the anterograde degenerating process of the conarii sympathetic nerve fibers after surgical removal of the cell body. In this situation the pineal gland is deafferented from the peripheral sympathetic nervous system keeping intact, however, the direct central connections to the deep pineal/lamina intercalaris region (DP). The results show a blockade of the pineal noradrenergic stimulatory process due to the retinal photostimulation. The inactivation of N-acetyltransferase led to a true metabolic shift to the oxidative pathway resulting in a decrease of the amount of N-acetylserotonin and an increase of the amount of serotonin, 5-hydroxyindoleacetic acid and 5-hydroxytryptophan. This inhibitory process brought into action by retinal illumination is dependent on the direct central neural connections to the pineal gland, since rats that were lesioned in the DP, previously to ganglionectomy, did not show any alteration on the indolic content of the pineal gland when subjected to nocturnal retinal photostimulation.