Glial cells expressing visual cycle genes are vital for photoreceptor survival in the zebrafish pineal gland

Photoreceptors in the vertebrate eye are dependent on the retinal pigmented epithelium for a variety of functions including retinal re-isomerization and waste disposal. The light-sensitive pineal gland of fish, birds, and amphibians is evolutionarily related to the eye but lacks a pigmented epithelium. Thus, it is unclear how these functions are performed. Here, we ask whether a subpopulation of zebrafish pineal cells, which express glial markers and visual cycle genes, is involved in maintaining photoreceptors. Selective ablation of these cells leads to a loss of pineal photoreceptors. Moreover, these cells internalize exorhodopsin that is secreted by pineal rod-like photoreceptors, and in turn release CD63-positive extracellular vesicles (EVs) that are taken up by pdgfrb-positive phagocytic cells in the forebrain meninges. These results identify a subpopulation of glial cells that is critical for pineal photoreceptor survival and indicate the existence of cells in the forebrain meninges that receive EVs released by these pineal cells and potentially function in waste disposal.

Single-cell in vivo imaging of cellular circadian oscillators in zebrafish

The circadian clock is a cell-autonomous time-keeping mechanism established gradually during embryonic development. Here, we generated a transgenic zebrafish line carrying a destabilized fluorescent protein driven by the promoter of a core clock gene, nr1d1, to report in vivo circadian rhythm at the single-cell level. By time-lapse imaging of this fish line and 3D reconstruction, we observed the sequential initiation of the reporter expression starting at photoreceptors in the pineal gland, then spreading to the cells in other brain regions at the single-cell level. Even within the pineal gland, we found heterogeneous onset of nr1d1 expression, in which each cell undergoes circadian oscillation superimposed over a cell type–specific developmental trajectory. Furthermore, we found that single-cell expression of nr1d1 showed synchronous circadian oscillation under a light–dark (LD) cycle. Remarkably, single-cell oscillations were dramatically dampened rather than desynchronized in animals raised under constant darkness, while the developmental trend still persists. It suggests that light exposure in early zebrafish embryos has significant effect on cellular circadian oscillations.

A transgenic zebrafish line, nr1d1-VNP, enables the monitoring of single-cell circadian rhythms in live zebrafish; using this fish line, the authors find that light exposure in early development initializes rather than synchronizes single-cell oscillators.

Circadian regulation and molecular role of the Bsx homeobox gene in the adult pineal gland

The pineal gland is a neuroendocrine organ responsible for production of the nocturnal hormone melatonin. A specific set of homeobox gene-encoded transcription factors govern pineal development, and some are expressed in adulthood. The brain-specific homeobox gene (Bsx) falls into both categories. We here examined regulation and function of Bsx in the mature pineal gland of the rat. We report that Bsx is expressed from prenatal stages into adulthood, where Bsx transcripts are localized in the melatonin-synthesizing pinealocytes, as revealed by RNAscope in situ hybridization. Bsx transcripts were also detected in the adult human pineal gland. In the rat pineal gland, Bsx was found to exhibit a 10-fold circadian rhythm with a peak at night. By combining in vivo adrenergic stimulation and surgical denervation of the gland in the rat with in vitro stimulation and transcriptional inhibition in cultured pinealocytes, we show that rhythmic expression of Bsx is controlled at the transcriptional level by the sympathetic neural input to the gland acting via adrenergic stimulation with cyclic AMP as a second messenger. siRNA-mediated knockdown (>80% reduction) in pinealocyte cultures revealed Bsx to be a negative regulator of other pineal homeobox genes, including paired box 4 (Pax4), but no effect on genes encoding melatonin-synthesizing enzymes was detected. RNA sequencing analysis performed on siRNA-treated pinealocytes further revealed that downstream target genes of Bsx are mainly involved in developmental processes. Thus, rhythmic Bsx expression seems to govern other developmental regulators in the mature pineal gland.

The Lhx4 homeobox transcript in the rat pineal gland: Adrenergic regulation and impact on transcripts encoding melatonin-synthesizing enzymes

Homeobox genes generally encode transcription factors involved in regulating developmental processes. In the pineal gland, a brain structure devoted to nocturnal melatonin synthesis, a number of homeobox genes are also expressed postnatally; among these is the LIM homeobox 4 gene (Lhx4). We here report that Lhx4 is specifically expressed in the postnatal pineal gland of rats and humans. Circadian analyses revealed a fourfold rhythm in Lhx4 expression in the rat pineal gland, with rhythmic expression detectable from postnatal day 10. Pineal Lhx4 expression was confirmed to be positively driven by adrenergic signaling, as evidenced by in vivo modulation of Lhx4 expression by pharmacological (isoprenaline injection) and surgical (superior cervical ganglionectomy) interventions. In cultured pinealocytes, Lhx4 expression was upregulated by cyclic AMP, a second messenger of norepinephrine. By use of RNAscope technology, Lhx4 transcripts were found to be exclusively localized in melatonin-synthesizing pinealocytes. This prompted us to investigate the possible role of Lhx4 in regulation of melatonin-producing enzymes. By use of siRNA technology, we knocked down Lhx4 by 95% in cultured pinealocytes; this caused a reduction in transcripts encoding the melatonin-producing enzyme arylalkylamine N-acetyl transferase (Aanat). Screening the transcriptome of siRNA-treated pinealocytes by RNAseq revealed a significant impact of Lhx4 on the phototransduction pathway and on transcripts involved in development of the nervous system and photoreceptors. These data suggest that rhythmic expression of Lhx4 in the pineal gland is controlled via an adrenergic-cyclic AMP mechanism and that Lhx4 acts to promote nocturnal melatonin synthesis.

Pineal Neurosteroids: Biosynthesis and Physiological Functions

Similar to the adrenal glands, gonads, and placenta, vertebrate brains also produce various steroids, which are known as "neurosteroids." Neurosteroids are mainly synthesized in the hippocampus, hypothalamus, and cerebellum; however, it has recently been discovered that in birds, the pineal gland, a photosensitive region in the brain, produces more neurosteroids than other brain regions. A series of experiments using molecular and biochemical techniques have found that the pineal gland produces various neurosteroids, including sex steroids, de novo from cholesterol. For instance, allopregnanolone and 7α-hydroxypregnenolone are actively produced in the pineal gland, unlike in other brain regions. Pineal 7α-hydroxypregnenolone, an up-regulator of locomotion, enhances locomotor activity in response to light stimuli in birds. Additionally, pineal allopregnanolone acts on Purkinje cells in the cerebellum and prevents neuronal apoptosis within the developing cerebellum in juvenile birds. Furthermore, exposure to light during nighttime hours can cause loss of diurnal variations of pineal allopregnanolone synthesis during early posthatch life, eventually leading to cerebellar Purkinje cell death in juvenile birds. In light of these new findings, this review summarizes the biosynthesis and physiological functions of pineal neurosteroids. Given that the circadian rhythms of individuals in modern societies are constantly interrupted by artificial light exposure, these findings in birds, which are excellent model diurnal animals, may have direct implications for addressing problems regarding the mental health and brain development of humans.

Brain-specific homeobox Bsx specifies identity of pineal gland between serially homologous photoreceptive organs in zebrafish

The pineal gland functioning as a photoreceptive organ in non-mammalian species is a serial homolog of the retina. Here we found that Brain-specific homeobox (Bsx) is a key regulator conferring individuality on the pineal gland between the two serially homologous photoreceptive organs in zebrafish. Bsx knock-down impaired the pineal development with reduced expression of exorh, the pineal-specific gene responsible for the photoreception, whereas it induced ectopic expression of rho, a retina-specific gene, in the pineal gland. Bsx remarkably transactivated the exorh promoter in combination with Otx5, but not with Crx, through its binding to distinct subtypes of PIRE, a DNA cis-element driving Crx/Otx-dependent pineal-specific gene expression. These results demonstrate that the identity of pineal photoreceptive neurons is determined by the combinatorial code of Bsx and Otx5, the former confers the pineal specificity at the tissue level and the latter determines the photoreceptor specificity at the cellular level.

[Pharmacological correction of alterations of apoptosis of the neurons of the hypothalamus suprachiasmatic nucleus and pinealocytes during aging and stress.]

As is known, the pineal gland plays an important role in adaptogenesis, and the hypothalamus is one of the main links of the stress-reactive system and is involved in the regulation of the involution of the whole organism. So, the study of changes in these organs during stress and aging is very interesting. The aim of the work is to study the mechanisms of apoptosis of pinealocytes and neurosecretory cells of the suprachiasmatic nucleus of the hypothalamus during aging, stress, and under the conditions of pharmacological correction of involutional processes and stress response (antioxidant alpha-tocopherol acetate, immunomodulator cycloferon). We used Wistar rats as model, young (2-4 months) and old (30 months). Age-related features of the apoptosis dynamics of pinealocytes and neurosecretory cells of the hypothalamic suprachiasmatic nucleus were studied using TUNEL and immunohistochemistry, and the possibilities of pharmacological correction of apoptotic processes are determined. An age-dependent increase of apoptosis level of cells of suprachiasmatic nucleus and epiphysis in rats was revealed. The stress effect (immobilization) led to the intensification of cell death, more significant in older animals. The pineal gland and suprachiasmatic nucleus, traditionally regarded as regulators of circadian rhythms, are at the same time actively involved in general adaptation processes. The studied drugs (α-tocopherol-acetate, cycloferon, and their combination) have a pronounced anti-apoptotic, cytoprotective effect under physiological conditions during aging, as well as during non-specific emotional stress (immobilization) in young and old animals. The regulatory effect is accomplished by activating the expression of the anti-apoptotic protein Bcl-2 in the neurosecretory cells of the suprachiasmatic nucleus and pinealocytes.

Single-cell RNA sequencing of the mammalian pineal gland identifies two pinealocyte subtypes and cell type-specific daily patterns of gene expression

The vertebrate pineal gland is dedicated to the production of the hormone melatonin, which increases at night to influence circadian and seasonal rhythms. This increase is associated with dramatic changes in the pineal transcriptome. Here, single-cell analysis of the rat pineal transcriptome was approached by sequencing mRNA from ~17,000 individual pineal cells, with the goals of profiling the cells that comprise the pineal gland and examining the proposal that there are two distinct populations of pinealocytes differentiated by the expression of Asmt, which encodes the enzyme that converts N-acetylserotonin to melatonin. In addition, this analysis provides evidence of cell-specific time-of-day dependent changes in gene expression. Nine transcriptomically distinct cell types were identified: ~90% were classified as melatonin-producing α- and β-pinealocytes (1:19 ratio). Non-pinealocytes included three astrocyte subtypes, two microglia subtypes, vascular and leptomeningeal cells, and endothelial cells. α-Pinealocytes were distinguished from β-pinealocytes by ~3-fold higher levels of Asmt transcripts. In addition, α-pinealocytes have transcriptomic differences that likely enhance melatonin formation by increasing the availability of the Asmt cofactor S-adenosylmethionine, resulting from increased production of a precursor of S-adenosylmethionine, ATP. These transcriptomic differences include ~2-fold higher levels of the ATP-generating oxidative phosphorylation transcriptome and ~8-fold lower levels of the ribosome transcriptome, which is expected to reduce the consumption of ATP by protein synthesis. These findings suggest that α-pinealocytes have a specialized role in the pineal gland: efficiently O-methylating the N-acetylserotonin produced and released by β-pinealocytes, thereby improving the overall efficiency of melatonin synthesis. We have also identified transcriptomic changes that occur between night and day in seven cell types, the majority of which occur in β-pinealocytes and to a lesser degree in α-pinealocytes; many of these changes were mimicked by adrenergic stimulation with isoproterenol. The cellular heterogeneity of the pineal gland as revealed by this study provides a new framework for understanding pineal cell biology at single-cell resolution.

The environmental neurotoxin β-N-methylamino-L-alanine inhibits melatonin synthesis in primary pinealocytes and a rat model

The environmental neurotoxin β-N-methylamino-L-alanine (BMAA) is a glutamate receptor agonist that can induce oxidative stress and has been implicated as a possible risk factor for neurodegenerative disease. Detection of BMAA in mussels, crustaceans, and fish illustrates that the sources of human exposure to this toxin are more abundant than previously anticipated. The aim of this study was to determine uptake of BMAA in the pineal gland and subsequent effects on melatonin production in primary pinealocyte cultures and a rat model. Autoradiographic imaging of 10-day-old male rats revealed a high and selective uptake in the pineal gland at 30 minutes to 24 hours after ¹⁴ C-L-BMAA administration (0.68 mg/kg). Primary pinealocyte cultures exposed to 0.05-3 mmol/L BMAA showed a 57%-93% decrease in melatonin synthesis in vitro. Both the metabotropic glutamate receptor 3 (mGluR3) antagonist Ly341495 and the protein kinase C (PKC) activator phorbol-12-myristate-13-acetate prevented the decrease in melatonin secretion, suggesting that BMAA inhibits melatonin synthesis by mGluR3 activation and PKC inhibition. Serum analysis revealed a 45% decrease in melatonin concentration in neonatal rats assessed 2 weeks after BMAA administration (460 mg/kg) and confirmed an inhibition of melatonin synthesis in vivo. Given that melatonin is a most important neuroprotective molecule in the brain, the etiology of BMAA-induced neurodegeneration may include mechanisms beyond direct excitotoxicity and oxidative stress.

[Mast cells of the human pineal gland.]

The purpose of this study was to assess the possibilities of identifying mast cells using different histochemical and immunohistochemical methods and elucidating the features of their localization in the human pineal gland. The undertaken study showed that mast cells are an essential component of the human pineal gland, regardless of age. The data obtained indicate an increase in the number of mast cells in the pineal gland with age. Mast cells are mostly located in the pineal stroma and their preferred location has not been related to concrements, cysts or melanin accumulations. Mast cells in the pineal gland are predominantly non-degranulating, which indicates their inactive state. The detectability of mast cells in the pineal gland depended significantly on the applied method of staining of the preparations. The largest number of mast cells was revealed by tryptase immunohistochemistry, which should be used to accurately determine the population of mast cells of the pineal gland.

Novel hypophysiotropic AgRP2 neurons and pineal cells revealed by BAC transgenesis in zebrafish

The neuropeptide agouti-related protein (AgRP) is expressed in the arcuate nucleus of the mammalian hypothalamus and plays a key role in regulating food consumption and energy homeostasis. Fish express two agrp genes in the brain: agrp1, considered functionally homologous with the mammalian AgRP, and agrp2. The role of agrp2 and its relationship to agrp1 are not fully understood. Utilizing BAC transgenesis, we generated transgenic zebrafish in which agrp1- and agrp2-expressing cells can be visualized and manipulated. By characterizing these transgenic lines, we showed that agrp1-expressing neurons are located in the ventral periventricular hypothalamus (the equivalent of the mammalian arcuate nucleus), projecting throughout the hypothalamus and towards the preoptic area. The agrp2 gene was expressed in the pineal gland in a previously uncharacterized subgroup of cells. Additionally, agrp2 was expressed in a small group of neurons in the preoptic area that project directly towards the pituitary and form an interface with the pituitary vasculature, suggesting that preoptic AgRP2 neurons are hypophysiotropic. We showed that direct synaptic connection can exist between AgRP1 and AgRP2 neurons in the hypothalamus, suggesting communication and coordination between AgRP1 and AgRP2 neurons and, therefore, probably also between the processes they regulate.

Cellular Basis of Pineal Gland Development: Emerging Role of Microglia as Phenotype Regulator

The adult pineal gland is composed of pinealocytes, astrocytes, microglia, and other interstitial cells that have been described in detail. However, factors that contribute to pineal development have not been fully elucidated, nor have pineal cell lineages been well characterized. We applied systematic double, triple and quadruple labeling of cell-specific markers on prenatal, postnatal and mature rat pineal gland tissue combined with confocal microscopy to provide a comprehensive view of the cellular dynamics and cell lineages that contribute to pineal gland development. The pineal gland begins as an evagination of neuroepithelium in the roof of the third ventricle. The pineal primordium initially consists of radially aligned Pax6+ precursor cells that express vimentin and divide at the ventricular lumen. After the tubular neuroepithelium fuses, the distribution of Pax6+ cells transitions to include rosette-like structures and later, dispersed cells. In the developing gland all dividing cells express Pax6, indicating that Pax6+ precursor cells generate pinealocytes and some interstitial cells. The density of Pax6+ cells decreases across pineal development as a result of cellular differentiation and microglial phagocytosis, but Pax6+ cells remain in the adult gland as a distinct population. Microglial colonization begins after pineal recess formation. Microglial phagocytosis of Pax6+ cells is not common at early stages but increases as microglia colonize the gland. In the postnatal gland microglia affiliate with Tuj1+ nerve fibers, IB4+ blood vessels, and Pax6+ cells. We demonstrate that microglia engulf Pax6+ cells, nerve fibers, and blood vessel-related elements, but not pinealocytes. We conclude that microglia play a role in pineal gland formation and homeostasis by regulating the precursor cell population, remodeling blood vessels and pruning sympathetic nerve fibers.

Melatonin Synthesis: Acetylserotonin O-Methyltransferase (ASMT) Is Strongly Expressed in a Subpopulation of Pinealocytes in the Male Rat Pineal Gland

The rat pineal gland has been extensively used in studies of melatonin synthesis. However, the cellular localization of melatonin synthesis in this species has not been investigated. Here we focus on the localization of melatonin synthesis using immunohistochemical methods to detect the last enzyme in melatonin synthesis, acetylserotonin O-methyltransferase (ASMT), and in situ hybridization techniques to study transcripts encoding ASMT and two other enzymes in melatonin synthesis, tryptophan hydroxylase (TPH)-1 and aralkylamine N-acetyltransferase. In sections of the rat pineal gland, marked cell-to-cell differences were found in ASMT immunostaining intensity and in the abundance of Tph1, Aanat, and Asmt transcripts. ASMT immunoreactivity was localized to the cytoplasm in pinealocytes in the parenchyma of the superficial pineal gland, and immunopositive pinealocytes were also detected in the pineal stalk and in the deep pineal gland. ASMT was found to inconsistently colocalize with S-antigen, a widely used pinealocyte marker; this colocalization was seen in cells throughout the pineal complex and also in displaced pinealocyte-like cells of the medial habenular nucleus. Inconsistent colocalization between ASMT and TPH protein was also detected in the pineal gland. ASMT protein was not detected in extraepithalamic parts of the central nervous system or in peripheral tissues. The findings in this report are of special interest because they provide reason to suspect that melatonin synthesis varies significantly among individual pinealocytes.

In the Heat of the Night: Thermo-TRPV Channels in the Salmonid Pineal Photoreceptors and Modulation of Melatonin Secretion

Photoperiod plays an essential role in the synchronization of metabolism, physiology, and behavior to the cyclic variations of the environment. In vertebrates, information is relayed by the pineal cells and translated into the nocturnal production of melatonin. The duration of this signal corresponds to the duration of the night. In fish, the pinealocytes are true photoreceptors in which the amplitude of the nocturnal surge is modulated by temperature in a species-dependent manner. Thus, the daily and annual variations in the amplitude and duration of the nocturnal melatonin signal provide information on daily and calendar time. Both light and temperature act on the activity of the penultimate enzyme in the melatonin biosynthesis pathway, the arylalkylamine N-acetyltransferase (serotonin → N-acetylserotonin). Although the mechanisms of the light/dark regulation of melatonin secretion are quite well understood, those of temperature remain unelucidated. More generally, the mechanisms of thermoreception are unknown in ectotherms. Here we provide the first evidence that two thermotransient receptor potential (TRP) channels, TRPV1 and TRPV4, are expressed in the pineal photoreceptor cells of a teleost fish, in which they modulate melatonin secretion in vitro. The effects are temperature dependent, at least for TRPV1. Our data support the idea that the pineal of fish is involved in thermoregulation and that the pineal photoreceptors are also thermoreceptors. In other nervous and nonnervous tissues, TRPV1 and TRPV4 display a ubiquitous but quantitatively variable distribution. These results are a fundamental step in the elucidation of the mechanisms of temperature transduction in fish.

The adrenergic-regulated CRTC1 and CRTC2 phosphorylation and cellular distribution is independent of endogenous SIK1 in the male rat pinealocyte

Salt inducible kinase 1 (SIK1) has been reported to repress cAMP-response element binding protein (CREB)-mediated gene transcription by causing the nuclear export of CREB-regulated transcription coactivators (CRTCs) through phosphorylation. Although the repressor role of SIK1 in suppressing the expression of arylalkylamine N-acetyltransferase, the enzyme that controls the daily rhythm in melatonin production in the rat pineal gland, has been established, whether SIK1 regulates the phosphorylation and localization of CRTC1 and CRTC2 in this tissue remains unclear. The present study found that overexpressing SIK1 in NE-stimulated rat pinealocytes could increase the phosphorylation of CRTC1 and CRTC2, reduced selectively the nuclear level of CRTC2 (but not that of CRTC1), and elevated the cytosolic levels of both CRTC1 and CRTC2. In contrast, transient knockdown of endogenous SIK1 had no effect on the phosphorylation or distribution of CRTC1 and CRTC2 in norepinephrine (NE)-stimulated pinealocytes. Our results also showed that adrenergic blockade during NE stimulation led to a rapid rephosphorylation and decline in the nucleus levels of CRTC1 and CRTC2; however SIK1 knockdown had no effect on this rapid rephosphorylation. Moreover, studies with kinase inhibitors revealed that kinase(s) sensitive to KT5823 appeared to be involved in this rapid rephosphorylation. Together, these results indicate that although overexpressing SIK1 can phosphorylate CRTC1 and CRTC2 in the NE-stimulated pinealocyte, the endogenous SIK1, in spite of its induction by NE, does not appear to be the main regulator of the phosphorylation and intracellular localization of these two coactivators.

THE INFLUENCE OF CARBON MONOXIDE ON THE SECRETION OF MELATONIN BY PINEALOCYTES MEASURED IN VITRO

Photoperiod is considered the most important factor entraining the circannual physiological rhythms through changing circadian patterns of melatonin (MEL) secretion from the pineal gland. The pineal gland of mammals does not respond directly to light but is controlled by light via neuronal phototransduction originating in the retina. In accordance with humoral phototransduction hypothesis, the aim of this study was to determine whether an increased concentration of CO, as a carrier of a light signal in pineal cell culture, affects the synthesis of melatonin. This study demonstrates that a commonly used carbon monoxide donor (CORM-2) markedly stimulated melatonin release from pineal cells incubated in vitro in a time-dependent manner, but the mechanism whereby CO modulates MEL release needs to be further explored.

The pineal complex in the cichlid Cichlasoma dimerus: effect of different photoperiods on its cell morphology

This research describes the pineal complex histology in juvenile and adult Cichlasoma dimerus, and the effect of different photoperiods on its cell morphology. In both juveniles and adults, the pineal complex of C. dimerus has three components: the pineal organ, consisting of a pineal vesicle (PV) and a pineal stalk, the parapineal organ and the dorsal sac. Although a strong morphological resemblance exists between the two stages, different synthesis patterns of cone and rod opsins were detected in the two life stages. An effect of the photoperiod length was observed on putative pinealocytes' activity from the PV, measured indirectly through nuclear area morphometry. Individuals exposed to a natural photoperiod (14L:10D) had smaller nuclear areas (mean ± s.e. = 13·82 ± 1·52 µm(2) ) than those exposed to a short photoperiod (8:16) (21·45 ± 2·67 µm(2) ; P < 0·001). Eventually, the nuclear area of pinealocytes could be used as a putative indicator of melatonin synthesis in fishes where it is difficult to obtain plasma samples, e.g. due to its small size or age. This work constitutes one of the few comparative descriptions of the pineal complex of juvenile and adult teleost and suggests potential approaches for the study of melatonin synthesis in fish larvae or small adult fishes.

Pleiotropic effects of Sox2 during the development of the zebrafish epithalamus

The zebrafish epithalamus is part of the diencephalon and encompasses three major components: the pineal, the parapineal and the habenular nuclei. Using sox2 knockdown, we show here that this key transcriptional regulator has pleiotropic effects during the development of these structures. Sox2 negatively regulates pineal neurogenesis. Also, Sox2 is identified as the unknown factor responsible for pineal photoreceptor prepatterning and performs this function independently of the BMP signaling. The correct levels of sox2 are critical for the functionally important asymmetrical positioning of the parapineal organ and for the migration of parapineal cells as a coherent structure. Deviations from this strict control result in defects associated with abnormal habenular laterality, which we have documented and quantified in sox2 morphants.

[Interrelations of pineal gland morpho-functional indices and immune system organs in rats exposed to natural illumination regime and continuous illumination]

The objective of this investigation was to determine the complex response of the pineal gland (PG) and of the organs of the immune system in Wistar rats in response to a violation of the illumination regime in the experiment. Animals were kept under natural light regime and continuous illumination for 14 days. After that rats were sacrificed and the mass of the body, PG, gonads, thymus and spleen was measured. Thymus and spleen cell subpopulations were determined by flow cytometry. The lipofuscin content in PG was determined by measuring an autofluorescence intensity in frozen tissue sections in the wavelength range of 505-545 nm using a confocal laser scanning microscope LSM 510 META (Carl Zeiss). The correlation analysis showed an increase in the amount and the change of sign and direction of relations between the indices of the state of PG and the immune system. This indicates the up-regulation of the intensity of inter-system relationships and the change of migration and differentiation vector of immunocompetent cells.

Sustained adrenergic stimulation is required for the nuclear retention of TORC1 in male rat pinealocytes

The process involved in relocation of the coactivator, transducer of regulated cAMP-regulated element-binding protein (TORC) to the cytoplasm, unlike its activation, is not well understood. Using cultured pineal cells prepared from male rats, we found that although both α- and β-adrenergic stimulation could cause TORC1 dephosphorylation, only α-adrenergic stimulation was effective in the norepinephrine (NE)-mediated translocation of TORC1 into the nucleus. In contrast, blockade of either the α- or the β-adrenergic receptor after NE stimulation was effective in causing the rephosphorylation and rapid relocation of TORC1 into the cytoplasm. Studies with phosphoprotein phosphatase (PP) inhibitors indicated that although both PP2A and PP2B could dephosphorylate TORC1, only PP2B could cause translocation into the nucleus. However, after NE stimulation, treatment with either PP2A or PP2B inhibitors could cause the rephosphorylation and cytoplasmic relocation of TORC1. These results indicate a requirement of continuous activation of both α- and β-adrenergic receptors as well as PP2A and PP2B activities for the nuclear retention of TORC1 during NE stimulation. Knockdown of salt-inducible kinase 1 (SIK1) had no effect on the phosphorylation or localization of TORC1. Although overexpressing SIK1 could induce TORC1 phosphorylation in the nucleus, it did not reduce TORC1 level in the nucleus, indicating that SIK1-mediated TORC1 phosphorylation may not be sufficient for its relocation into the cytoplasm. Together, these results demonstrate that, in the rat pineal gland, different mechanisms are involved in regulating the nuclear entry and exit of TORC1 and that the SIK1-mediated phosphorylation of TORC1 may not lead to its nuclear exit.

Different signaling mechanisms are involved in the norepinephrine-stimulated TORC1 and TORC2 nuclear translocation in rat pinealocytes

The distribution of transducers of regulated cAMP-response element-binding protein activity (TORC) between the cytoplasm and the nucleus is tightly regulated and represents one of the main mechanisms whereby the cAMP response element activation activities of TORC are controlled. Whereas both cAMP and Ca(2+) pathways can cause translocation of TORC, the relative importance of these two pathways in regulating different TORC within the same cell is unclear. In this study, we determined the mechanism that regulated TORC1 translocation and compared it with that of TORC2 in rat pinealocytes. Stimulation of pinealocytes with norepinephrine (NE), although having no effect on Torc1 transcription, caused rapid dephosphorylation of TORC1. Although NE also caused rapid dephosphorylation of TORC2, pharmacological studies revealed that TORC1 dephosphorylation could be induced by both β-adrenoceptor/cAMP and α-adrenoceptor/intracellular Ca(2+) pathways contrasting with TORC2 dephosphorylation being induced mainly through the β-adrenoceptor/cAMP pathway. PhosTag gel indicated a different pattern of TORC1 desphosphorylation resulting from the selective activation of α- or β-adrenoceptors. Interestingly, only the α-adrenoceptor/intracellular Ca(2+)-mediated dephosphorylation could translocate TORC1 to the nucleus, whereas the β-adrenoceptor/cAMP-mediated dephosphorylation of TORC1 was ineffective. In comparison, translocation of TORC2 was induced predominantly by the β-adrenoceptor/cAMP pathway. Studies with different protein phosphatase (PP) inhibitors indicated that the NE-mediated translocation of TORC1 was blocked by cyclosporine A, a PP2B inhibitor, but that of TORC2 was blocked by okadaic acid, a PP2A inhibitor. Together these results highlight different intracellular signaling pathways that are involved in the NE-stimulated dephosphorylation and translocation of TORC1 and TORC2 in rat pinealocytes.

Effect of short peptides on expression of signaling molecules in organotypic pineal cell culture

We demonstrated the influence of short peptides on the expression of signaling molecules in organotypic culture of the pineal gland from 3-month-old rats. Peptides Ala-Glu-Asp-Gly and Lys-Glu-Asp stimulate the expression of proliferative protein Ki-67 in pineal gland culture. These peptides as well as Glu-Asp-Arg and Lys-Glu do not affect the expression of apoptosis marker AIF. The synthesis of transcription factor CGRP by pinealocytes was stimulated only by Ala-Glu-Asp-Gly. Thus, peptide Ala-Glu-Asp-Gly tissue-specifically stimulates proliferative and secretory activities of pinealocytes, which can be used for recovery of pineal gland functions at the molecular level.

Pineal photoreceptor cells are required for maintaining the circadian rhythms of behavioral visual sensitivity in zebrafish

In non-mammalian vertebrates, the pineal gland functions as the central pacemaker that regulates the circadian rhythms of animal behavior and physiology. We generated a transgenic zebrafish line [Tg(Gnat2:gal4-VP16/UAS:nfsB-mCherry)] in which the E. coli nitroreductase is expressed in pineal photoreceptor cells. In developing embryos and young adults, the transgene is expressed in both retinal and pineal photoreceptor cells. During aging, the expression of the transgene in retinal photoreceptor cells gradually diminishes. By 8 months of age, the Gnat2 promoter-driven nitroreductase is no longer expressed in retinal photoreceptor cells, but its expression in pineal photoreceptor cells persists. This provides a tool for selective ablation of pineal photoreceptor cells, i.e., by treatments with metronidazole. In the absence of pineal photoreceptor cells, the behavioral visual sensitivity of the fish remains unchanged; however, the circadian rhythms of rod and cone sensitivity are diminished. Brief light exposures restore the circadian rhythms of behavioral visual sensitivity. Together, the data suggest that retinal photoreceptor cells respond to environmental cues and are capable of entraining the circadian rhythms of visual sensitivity; however, they are insufficient for maintaining the rhythms. Cellular signals from the pineal photoreceptor cells may be required for maintaining the circadian rhythms of visual sensitivity.

A hyphenated optical trap capillary electrophoresis laser induced native fluorescence system for single-cell chemical analysis

Single-cell measurements allow a unique glimpse into cell-to-cell heterogeneity; even small changes in selected cells can have a profound impact on an organism's physiology. Here an integrated approach to single-cell chemical sampling and assay are described. Capillary electrophoresis (CE) with laser-induced native fluorescence (LINF) has the sensitivity to characterize natively fluorescent indoles and catechols within individual cells. While the separation and detection approaches are well established, the sampling and injection of individually selected cells requires new approaches. We describe an optimized system that interfaces a single-beam optical trap with CE and multichannel LINF detection. A cell is localized within the trap and then the capillary inlet is positioned near the cell using a computer-controlled micromanipulator. Hydrodynamic injection allows cell lysis to occur within the capillary inlet, followed by the CE separation and LINF detection. The use of multiple emission wavelengths allows improved analyte identification based on differences in analyte fluorescence emission profiles and migration time. The system enables injections of individual rat pinealocytes and quantification of their endogenous indoles, including serotonin, N-acetyl-serotonin, 5-hydroxyindole-3-acetic acid, tryptophol and others. The amounts detected in individual cells incubated in 5-hydroxytryptophan ranged from 10(-14) mol to 10(-16) mol, an order of magnitude higher than observed in untreated pinealocytes.

Glia-pinealocyte network: the paracrine modulation of melatonin synthesis by tumor necrosis factor (TNF)

The pineal gland, a circumventricular organ, plays an integrative role in defense responses. The injury-induced suppression of the pineal gland hormone, melatonin, which is triggered by darkness, allows the mounting of innate immune responses. We have previously shown that cultured pineal glands, which express toll-like receptor 4 (TLR4) and tumor necrosis factor receptor 1 (TNFR1), produce TNF when challenged with lipopolysaccharide (LPS). Here our aim was to evaluate which cells present in the pineal gland, astrocytes, microglia or pinealocytes produced TNF, in order to understand the interaction between pineal activity, melatonin production and immune function. Cultured pineal glands or pinealocytes were stimulated with LPS. TNF content was measured using an enzyme-linked immunosorbent assay. TLR4 and TNFR1 expression were analyzed by confocal microscopy. Microglial morphology was analyzed by immunohistochemistry. In the present study, we show that although the main cell types of the pineal gland (pinealocytes, astrocytes and microglia) express TLR4, the production of TNF induced by LPS is mediated by microglia. This effect is due to activation of the nuclear factor kappa B (NF-kB) pathway. In addition, we observed that LPS activates microglia and modulates the expression of TNFR1 in pinealocytes. As TNF has been shown to amplify and prolong inflammatory responses, its production by pineal microglia suggests a glia-pinealocyte network that regulates melatonin output. The current study demonstrates the molecular and cellular basis for understanding how melatonin synthesis is regulated during an innate immune response, thus our results reinforce the role of the pineal gland as sensor of immune status.

Norepinephrine causes a biphasic change in mammalian pinealocye membrane potential: role of alpha1B-adrenoreceptors, phospholipase C, and Ca2+

Perforated patch clamp recording was used to study the control of membrane potential (V(m)) and spontaneous electrical activity in the rat pinealocyte by norepinephrine. Norepinephrine did not alter spiking frequency. However, it was found to act through α(1B)-adrenoreceptors in a concentration-dependent manner (0.1-10 μM) to produce a biphasic change in V(m). The initial response was a hyperpolarization (∼13 mV from a resting potential of -46 mV) due to a transient (∼5 sec) outward K(+) current (∼50 pA). This current appears to be triggered by Ca(2+) released from intracellular stores, based on the observation that it was also seen in cells bathed in Ca(2+)-deficient medium. In addition, pharmacological studies indicate that this current was dependent on phospholipase C (PLC) activation and was in part mediated by bicuculline methiodide and apamin-sensitive Ca(2+)-controlled K(+) channels. The initial transient hyperpolarization was followed by a sustained depolarization (∼4 mV) due to an inward current (∼10 pA). This response was dependent on PLC-dependent activation of Na(+)/Ca(2+) influx but did not involve nifedipine-sensitive voltage-gated Ca(2+) channels. Together, these results indicate for the first time that activation of α(1B)-adrenoreceptors initiates a PLC-dependent biphasic change in pinealocyte V(m) characterized by an initial transient hyperpolarization mediated by a mixture of Ca(2+)-activated K(+) channels followed by a sustained depolarization mediated by a Ca(2+)-conducting nonselective cation channel. These observations indicate that both continuous elevation of intracellular Ca(2+) and sustained depolarization at approximately -40 mV are associated with and are likely to be required for activation of the pinealocyte.

Adrenergic regulation of the distribution of transducer of regulated cAMP-response element-binding protein (TORC2) in rat pinealocytes

Transducers of regulated cAMP-response element-binding protein (CREB) activity (TORC) are coactivators that can increase CREB transcriptional activity, suggesting that TORC may regulate the transcription of Aanat, a CREB-target gene. In the present study, we focused on the regulation of TORC2 and its role in Aanat transcription in the rat pineal gland. Although there was no endogenous Torc2 mRNA rhythm in the rat pineal gland and treatment of cultured pinealocytes with norepinephrine (NE) had no effect on the mRNA level of Torc2, the phosphorylation state and intracellular distribution of TORC2 protein were regulated by NE. Immunoblot analysis combined with cytosolic/nuclear fractionation or phosphatase treatment showed that TORC2 protein was rapidly dephosphorylated and translocated to the nucleus after NE stimulation in rat pinealocytes. Similar dephosphorylation of TORC2 also occurred nocturnally in the rat pineal gland. The NE-mediated TORC2 dephosphorylation was blocked by cotreatment with propranolol (a β-adrenergic antagonist) but not prazosin (an α(1)-adrenergic antagonist) and mimicked by dibutyryl cAMP, indicating the participation of the β-adrenergic receptor/cAMP pathway. Studies with protein phosphatase inhibitors showed that only okadaic acid and calyculin A were effective in blocking the NE-mediated TORC2 dephosphorylation, suggesting the involvement of protein phosphatase 2A in this dephosphorylation. Moreover, TORC2 overexpression had an enhancing effect on NE-stimulated Aanat transcription. Together, these results indicate that NE stimulation causes nuclear translocation of TORC2 by dephosphorylating the protein through a β-adrenoceptor/cAMP mechanism and that nuclear localization of TORC2 appears to regulate Aanat transcription by NE in the rat pineal gland.

Histone modifications on the adrenergic induction of type II deiodinase in rat pinealocytes

Histone modifications have been shown to play an important role in regulating gene expression. In this study, we investigated the impact of histone modifications on the adrenergic-regulated transcription of type 2 deiodinase (Dio2), a CREB-target gene in the rat pinealocyte. Treatment of pinealocytes with inhibitors of aurora C, a histone kinase, resulted in an inhibitory effect on the adrenergic-stimulated histone H3 Ser10 phosphorylation and Dio2 transcription. Given the established link between histone phosphorylation and acetylation, the role of histone acetylation on the adrenergic-induced Dio2 transcription was investigated. Treatment of pinealocytes with histone deacetylase inhibitors inhibited the adrenergic-induced Dio2 transcription. Chromatin immunoprecipitation with antibodies against acetylated Lys14 of H3 showed an increase in DNA recovery of the promoter region of Dio2 following treatment with trichostatin A. Together, our results indicate that, beside activation of CREB, epigenetic factors such as histone modifications also play an important role in regulating Dio2 transcription.

TLR4 and CD14 receptors expressed in rat pineal gland trigger NFKB pathway

Nuclear factor-kappa B (NFKB), a pivotal player in inflammatory responses, is constitutively expressed in the pineal gland. Corticosterone inhibits pineal NFKB leading to an enhancement of melatonin production, while tumor necrosis factor (TNF) leads to inhibition of Aa-nat transcription and the production of N-acetylserotonin in cultured glands. The reduction in nocturnal melatonin surge favors the mounting of the inflammatory response. Despite these data, there is no clear evidence of the ability of the pineal gland to recognize molecules that signal infection. This study investigated whether the rat pineal gland expresses receptors for lipopolysaccharide (LPS), the endotoxin from the membranes of Gram-negative bacteria, and to establish the mechanism of action of LPS. Here, we show that pineal glands possess both CD14 and toll-like receptor 4 (TLR4), membrane proteins that bind LPS and trigger the NFKB pathway. LPS induced the nuclear translocation of p50/p50 and p50/RELA dimers and the synthesis of TNF. The maximal expression of TNF in cultured glands coincides with an increase in the expression of TNF receptor 1 (TNFR1) in isolated pinealocytes. In addition, LPS inhibited the synthesis of N-acetylserotonin and melatonin. Therefore, the pineal gland transduces Gram-negative endotoxin stimulation by producing TNF and inhibiting melatonin synthesis. Here, we provide evidence to reinforce the idea of an immune-pineal axis, showing that the pineal gland is a constitutive player in the innate immune response.

Thyroid hormone and adrenergic signaling interact to control pineal expression of the dopamine receptor D4 gene (Drd4)

Dopamine plays diverse and important roles in vertebrate biology, impacting behavior and physiology through actions mediated by specific G-protein-coupled receptors, one of which is the dopamine receptor D4 (Drd4). Here we present studies on the >100-fold daily rhythm in rat pineal Drd4 expression. Our studies indicate that Drd4 is the dominant dopamine receptor gene expressed in the pineal gland. The gene is expressed in pinealocytes at levels which are approximately 100-fold greater than in other tissues, except the retina, in which transcript levels are similar. Pineal Drd4 expression is circadian in nature and under photoneural control. Whereas most rhythmically expressed genes in the pineal are controlled by adrenergic/cAMP signaling, Drd4 expression also requires thyroid hormone. This advance raises the questions of whether Drd4 expression is regulated by this mechanism in other systems and whether thyroid hormone controls expression of other genes in the pineal gland.

Calcium concrements in the pineal gland of the Arctic fox (Vulpes lagopus) and their relationship to pinealocytes, glial cells and type I and III collagen fibers

The aim of the present study was to analyze the presence and morphology of the pineal concretions in the Arctic fox and their relationship to pinealocytes, glial cells and collagen fibers. Pineals collected from 7-8 month-old and 3-4 year-old foxes (6 in each age-group) were investigated. Sections of the glands were stained with HE, Mallory's method and alizarin red S as well as subjected to a combined procedure involving immunofluorescent staining with antibodies against antigen S, glial fibril acid protein (GFAP), type I and III collagen and histochemical staining with alizarin red S. The pineal concretions were found in 2 of 6 investigated Arctic foxes aged 3 years and they were not observed in animals aged 7-8 months. The acervuli were present in the parenchyma and the connective tissue septa. They were more numerous in the distal part than in the proximal part of the gland. The acervuli stained with alizarin red S revealed an intensive red fluorescence, what enabled the use of this compound in a combined histochemical-immunofluorescent procedure. A majority of cells in the fox pineal showed positive staining with antibodies against antigen S, a marker of pinealocytes. GFAP-positive cells were especially numerous in the proximal part of the gland. Both antigen S- and GFAP-positive cells were frequently observed close to the concrements. Collagen fibers of type I and III were found in the capsule, connective tissue septa and vessels. Immunoreactive fibers did not form any capsules or basket-like structures surrounding the concrements.

Salt-inducible kinase 1 in the rat pinealocyte: adrenergic regulation and role in arylalkylamine N-acetyltransferase gene transcription

The recognition of the basic leucine zipper domain in the regulation of transcriptional activity of cAMP response element-binding protein by salt-inducible kinase (SIK) prompted our investigation of the regulatory role of this kinase in the induction of Aa-nat and other cAMP-regulated genes in the rat pineal gland. Here we report Sik1 expression was induced by norepinephrine (NE) in rat pinealocytes primarily through activation of beta-adrenergic receptors, with a minor contribution from activation of alpha-adrenergic receptors. Treatments with dibutyryl cAMP, and to a lesser extent, agents that elevate intracellular Ca(2+) mimicked the effect of NE on Sik1 expression. In parallel to the results of the pineal cell culture studies, a marked nocturnal induction of Sik1 transcription was found in whole-animal studies. Knockdown of Sik1 by short hairpin RNA amplified the NE-stimulated Aa-nat transcription and other adrenergic-regulated genes, including Mapk phosphatase 1, inducible cAMP repressor, and type 2 iodothyronine deiodinase in a time-dependent manner. In contrast, overexpressing Sik1 had an inhibitory effect on the NE induction of Aa-nat and other adrenergic-regulated genes. Together, our results indicate that the adrenergic induction of Sik1 in the rat pineal gland is primarily through the beta-adrenergic receptor --> protein kinase A pathway. SIK1 appears to function as part of an endogenous repressive mechanism that regulates the peak and indirectly the duration of expression of Aa-nat and other cAMP-regulated genes. These findings support a role for SIK1 in framing the temporal expression profile of Aa-nat and other adrenergic-regulated genes in the rat pineal gland.

Opposite Effects of Proteasome Inhibitors in the Adrenergic Induction of ArylalkylamineN‐acetyltransferase in Rat Pinealocytes

In the rat pineal gland, the steady-state level of arylalkylamine N-acetyltransferase (AANAT) protein is controlled by transcriptional and translational mechanisms as well as by proteasome-mediated degradation. Studies with proteasome inhibitors, MG132 and clasto-lactacystin beta-lactone (c-lact), show two opposite effects of proteasomal inhibition on norepinephrine (NE)-induction of Aanat. Addition of MG132 or c-lact following NE stimulation causes an increase in AANAT protein level and enzyme activity without affecting the level of Aanat mRNA. In contrast, addition of inhibitors prior to NE stimulation reduces the NE-stimulated Aanat mRNA, AANAT protein, and enzyme activity. The inhibitory effect of proteasomal inhibition on adrenergic-induced Aanat transcription appears specific for Aanat because it has no effect on the adrenergic induction of mitogen-activated protein kinase phosphatase-1 (mkp-1). The effects of the proteasome inhibitors on NE-stimulated Aanat induction appear to be mediated by accumulation of a protein repressor.

Pineal melatonin acts as a circadian zeitgeber and growth factor in chick astrocytes

Melatonin is rhythmically synthesized and released by the avian pineal gland and retina during the night, targeting an array of tissues and affecting a variety of physiological and behavioral processes. Among these targets, astrocytes express two melatonin receptor subtypes in vitro, the Mel(1A) and Mel(1C) receptors, which play a role in regulating metabolic activity and calcium homeostasis in these cells. Molecular characterization of chick astrocytes has revealed the expression of orthologs of the mammalian clock genes including clock, cry1, cry2, per2, and per3. To test the hypothesis that pineal melatonin entrains molecular clockworks in downstream cells, we asked whether coculturing astrocytes with pinealocytes or administration of exogenous melatonin cycles would entrain metabolic rhythms of 2-deoxy [14C]-glucose (2DG] uptake and/or clock gene expression in cultured astrocytes. Rhythmic secretion of melatonin from light-entrained pinealocytes in coculture as well as cyclic administration of exogenous melatonin entrained rhythms of 2DG uptake and expression of Gallus per2 (gper2) and/or gper3, but not of gcry1 mRNA. Surprisingly, melatonin also caused a dose-dependent increase in mitotic activity of astrocytes, both in coculture and when administered exogenously. The observation that melatonin stimulates mitotic activity in diencephalic astrocytes suggests a trophic role of the hormone in brain development. The data suggest a dual role for melatonin in avian astrocytes: synchronization of rhythmic processes in these cells and regulation of growth and differentiation. These two processes may or may not be mutually exclusive.

Local corticosterone infusion enhances nocturnal pineal melatonin production in vivo

Melatonin, an important marker of the endogenous rhythmicity in mammals, also plays a role in the body defence against pathogens and injuries. In vitro experiments have shown that either pro- or anti-inflammatory agents, acting directly in the organ, are able to change noradrenaline-induced pineal indoleamine production. Whereas corticosterone potentiates melatonin production, incubation of the gland with tumour necrosis factor-alpha decreases pineal hormonal production. In the present study, we show that nocturnal melatonin production measured by intra-pineal microdialysis is enhanced in pineals perfused with corticosterone at concentrations similar to those measured in inflamed animals. In vitro experiments suggest that this enhancement may be due to an increase in the activity of the two enzymes that convert serotonin to N-acetylserotonin (NAS) and NAS to melatonin. The present results support the hypothesis that the pineal gland is a sensor of inflammation mediators and that it plays a central role in the control of the inflammatory response.

Tryptophan hydroxylase is modulated by L-type calcium channels in the rat pineal gland

Calcium is an important second messenger in the rat pineal gland, as well as cAMP. They both contribute to melatonin synthesis mediated by the three main enzymes of the melatonin synthesis pathway: tryptophan hydroxylase, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase. The cytosolic calcium is elevated in pinealocytes following alpha(1)-adrenergic stimulation, through IP(3)-and membrane calcium channels activation. Nifedipine, an L-type calcium channel blocker, reduces melatonin synthesis in rat pineal glands in vitro. With the purpose of investigating the mechanisms involved in melatonin synthesis regulation by the L-type calcium channel, we studied the effects of nifedipine on noradrenergic stimulated cultured rat pineal glands. Tryptophan hydroxylase, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase activities were quantified by radiometric assays and 5-hydroxytryptophan, serotonin, N-acetylserotonin and melatonin contents were quantified by HPLC with electrochemical detection. The data showed that calcium influx blockaded by nifedipine caused a decrease in tryptophan hydroxylase activity, but did not change either arylalkylamine N-acetyltransferase or hydroxyindole-O-methyltransferase activities. Moreover, there was a reduction of 5-hydroxytryptophan, serotonin, N-acetylserotonin and melatonin intracellular content, as well as a reduction of serotonin and melatonin secretion. Thus, it seems that the calcium influx through L-type high voltage-activated calcium channels is essential for the full activation of tryptophan hydroxylase leading to melatonin synthesis in the pineal gland.

Acetylation of histone H3 and adrenergic-regulated gene transcription in rat pinealocytes

In this study we investigated the effect of histone acetylation on the transcription of adrenergic-induced genes in rat pinealocytes. We found that treatment of pinealocytes with trichostatin A (TSA), a histone deacetylase inhibitor, caused hyperacetylation of histone H3 (H3) Lys14 at nanomolar concentrations. Hyperacetylation of H3 was also observed after treatment with scriptaid, a structurally unrelated histone deacetylase inhibitor. The effects of TSA and scriptaid were inhibitory on the adrenergic induction of arylalkylamine-n-acetyltransferase (aa-nat) mRNA, protein, and enzyme activity, and on melatonin production. TSA at higher concentrations also inhibited the adrenergic induction of mapk phosphatase-1 (mkp-1) and inducible cAMP early repressor mRNAs. In contrast, the effect of TSA on the norepinephrine induction of the c-fos mRNA was stimulatory. Moreover, the effect of TSA on adrenergic-induced gene transcription was dependent on the time of its addition; its effect was only observed during the active phase of transcription. Chromatin immunoprecipitation with antibodies against acetylated Lys14 of H3 showed an increase in DNA recovery of the promoter regions of aa-nat, mkp-1, and c-fos after treatment with TSA. Together, our results demonstrate that histone acetylation differentially influences the transcription of adrenergic-induced genes, an enhancing effect for c-fos but inhibitory for aa-nat, mkp-1, and inducible cAMP early repressor. Moreover, both inhibitory and enhancing effects appear to be mediated through specific modification of promoter-bound histones during gene transcription.

Neural adrenergic/cyclic AMP regulation of the immunoglobulin E receptor alpha-subunit expression in the mammalian pinealocyte: a neuroendocrine/immune response link?

The high affinity immunoglobulin E receptor (FcepsilonRI) complex is dedicated to immunoglobulin E-mediated allergic responses. Expression of the FcepsilonRI receptor is thought to be relatively stable and limited to mast cells, basophils, eosinophils, monocytes, Langerhans cells, platelets, and neutrophils. We now report that the FcepsilonRIalpha and FcepsilonRIgamma polypeptides are expressed in the pinealocyte, the melatonin-secreting cell of the pineal gland. Moreover, Fcer1a mRNA levels increased approximately 100-fold at night to levels that were higher than in other tissues examined. Pineal FcepsilonRIalpha protein also increased markedly at night from nearly undetectable daytime levels. Our studies indicate that pineal Fcer1a mRNA levels are controlled by a well described neural pathway that controls pineal function. This pathway includes the master circadian oscillator in the suprachiasmatic nucleus and passes through central and peripheral structures. The circadian expression of FcepsilonRIalpha in the pineal gland is driven by this neural circuit via an adrenergic/cyclic AMP mechanism. Pineal FcepsilonRIalpha and FcepsilonRIgamma may represent a previously unrealized molecular link between the neuroendocrine and immune systems.

Pineal gland expression of the transcription factor Egr-1 is restricted to a population of glia that are distinct from nestin-immunoreactive cells

Egr-1 is a plasticity-related transcription factor that has been implicated in circadian regulation of the pineal gland. In the present study we have investigated the cellular expression pattern of Egr-1 in the adult rat pineal. Egr-1 protein is restricted to the nucleus of a sub-population of cells. These cells were characterised using a new transgenic rat model (egr-1-d2EGFP) in which green fluorescent protein is driven by the egr-1 promoter. Cellular filling by GFP revealed that Egr-1-positive cells exhibited processes, indicating a glial cell-type morphology. This was confirmed by co-localizing the GFP-filled processes with vimentin and S-100beta. However, GFP/Egr-1 is expressed in only a tiny minority of the previously identified Id-1/vimentin-positive glial cells and therefore represents a novel sub-set of this (GFAP-negative) glial population. We have also demonstrated for the first time an extensive network of nestin-positive cells throughout the adult pineal gland, however these cells do not co-express Egr-1. Our studies have therefore broadened our understanding of the cell populations that constitute the adult pineal. Cellular localization of Egr-1 has revealed that this factor does not appear to be directly involved in pinealocyte production of melatonin but is required in a sub-set of pineal glia.

A median third eye: pineal gland retraces evolution of vertebrate photoreceptive organs

In many vertebrates, the pineal gland serves as a photoreceptive neuroendocrine organ. Morphological and functional similarities between the pineal and retinal photoreceptor cells indicate their close evolutionary relationship, and hence the comparative studies on the pineal gland and the retina are the keys to deciphering the evolutionary traces of the vertebrate photoreceptive organs. Several studies have suggested common genetic and molecular mechanisms responsible for their similarities, but largely unknown are those underlying pineal-specific development and physiological functions. Recent studies have identified several cis-acting DNA elements that participate in transcriptional control of the pineal-specific genes. Genetic approaches in the zebrafish have also contributed to elucidating the genetic network regulating the pineal development and neurogenesis. These efforts toward elucidating the molecular instrumentation intrinsic to the pineal gland, back to back with those to the retina, should lead to a comprehensive understanding of the evolutionary history of the vertebrate photoreceptive structures. This article summarizes the current status of research on these topics.

The role of repressor proteins in the adrenergic induction of type II iodothyronine deiodinase in rat pinealocytes

In this study, we investigated the transcriptional regulation of the adrenergic induction of type II iodothyronine deiodinase (Dio2) in rat pinealocytes. Treatment of pinealocytes with norepinephrine (NE) caused an increase in the mRNA level of Dio2 that peaked around 2 h and declined over the next 5 h. Both beta- and alpha1-adrenergic receptors contributed to the NE induction of Dio2 expression through a cAMP/protein kinase A mechanism. In pinealocytes that had been stimulated by NE, inhibition of transcription by actinomycin had no discernible effect on Dio2 expression. In contrast, inhibition of protein synthesis by cycloheximide enhanced the NE induction of Dio2 expression, suggesting the involvement of a repressor protein. Transient transfection of pinealocytes with adenovirus expressing small interfering RNA against Fos-related antigen 2 (Fra2) enhanced the NE induction of Dio2 expression, whereas the effect of overexpression of the full-length transcript of Fra2 was inhibitory. Time-course study indicated that preventing the NE induction of Fra2 enhanced the NE induction of Dio2 after 3 h, and the enhancement persisted beyond 6 h after NE stimulation. In comparison, transient transfection of pinealocytes with small interfering RNA against inducible cAMP early repressor (Icer) had no effect on the NE induction of Dio2 expression, whereas overexpression of the full-length transcript of Icer caused a small reduction of the NE-stimulated Dio2 expression. Together, our results support Fra-2 as an important transcriptional repressor that helps shape the time profile of the adrenergic induction of Dio2 expression in the rat pineal gland.

The effects of environmental illumination on the in vitro melatonin secretion from the embryonic and adult chicken pineal gland

The aim of this study was to monitor the changes in the pattern of the in vitro melatonin (MT) secretion under reversed illumination cycles with low intensity of light during photo phase. Although light is known to be one of the major synchronizing factors of the circadian MT rhythm in birds, there are no data available on the limits of direct light sensitivity of the avian pinealocytes. In our experiments, MT secretion from adult or from embryonic chicken pineal glands was monitored in a perifusion system for 4 days. The glands were repeatedly exposed to light with various intensities and duration. Reversed daily illumination inverted the in vitro MT rhythm even if the intensity of light was only 10lux at the surface of the perifusion columns. MT release of embryonic pineal glands was also found to be sensitive to dim light. Our results showed that the chicken pineal gland is directly sensitive to light of low intensity. However, the various oscillator units in the gland may have different sensitivity to dim light. Light perception mechanism in the chicken pinealocytes is already fully developed on the 17th embryonic day.

Mitogen-activated protein kinase phosphatase-1 (MKP-1) preferentially dephosphorylates p42/44MAPK but not p38MAPK in rat pinealocytes

We recently reported a diurnal and norepinephrine (NE) -induced expression of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) in the rat pineal gland and postulated that this MKP-1 expression might impact adrenergic-regulated arylalkylamine-N-acetyltransferase (AA-NAT) activity via modulation of MAPKs. In this study, we investigated the effect of depletion of MKP-1 expression by using doxorubicin, a topoisomerase inhibitor that suppresses the expression of MKP-1 in other cell types and small interfering RNA targeted against Mkp1 in NE-stimulated pinealocytes. We found that both treatments were effective in inhibiting NE induction of MKP-1 expression. Moreover, both treatments also resulted in a prolonged activation of p42/44MAPK and an increase in AA-NAT induction by NE. In contrast, treatment of pinealocytes with PD98059, an inhibitor of MAPK kinase, reduced NE-stimulated AA-NAT activity. Interestingly, suppressing MKP-1 expression had no effect on the time profile of NE-stimulated p38MAPK activation. These results indicate that MKP-1 modulates the profile of AA-NAT activity by selectively shaping the activation profile of p42/44MAPK but not that of p38MAPK.

The mammalian pineal gland: known facts, unknown facets

In the mammalian pineal gland, information on environmental lighting conditions that is neuronally encoded by the retina is converted into nocturnally elevated synthesis of the hormone melatonin. Evolutionary pressure has changed the morphology of vertebrate pinealocytes, eliminating direct photoreception and the endogenous clock function. Despite these changes, nocturnally elevated melatonin synthesis has remained a reliable indicator of time throughout evolution. In the photo-insensitive mammalian pineal gland this message of darkness depends on the master circadian pacemaker in the hypothalamic suprachiasmatic nuclei. The dramatic change in vertebrate pinealocytes has received little attention; here, we therefore link the known evolutionary morphodynamics and well-investigated biochemical details responsible for rhythmic synthesis of melatonin with recently characterized patterns of gene expression in the pineal gland. We also address the enigmatic function of clockwork molecules in mammalian pinealocytes.

Enzymatic and cellular study of a serotonin N-acetyltransferase phosphopantetheine-based prodrug

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) regulates the daily rhythm in the production of melatonin and is therefore an attractive target for pharmacologic modulation of the synthesis of this hormone. Previously prepared bisubstrate analogs show potent inhibition of AANAT but have unfavorable pharmacokinetic properties due to the presence of phosphate groups which prevents transfer across the plasma membrane. Here, we examine a bis-pivaloyloxymethylene (POM)-tryptamine-phosphopantetheine prodrug (2) and its biotransformations in vitro by homogenates and pineal cells. Compound 2 is an efficient porcine liver esterase substrate for POM cleavage in vitro although cyclization of the phosphate moiety is a potential side product. Tryptamine phosphopantetheine (3) is converted to tryptamine-coenzyme A (CoA) bisubstrate analog (1) by human phosphoribosyl pyrophosphate amidotransferase (PPAT) and dephosphocoenzyme A kinase (DPCK) in vitro. Compound 2 was found to inhibit melatonin production in rat pineal cell culture. It was also found that the POM groups are readily removed to generate 3; however, further processing to tryptamine-CoA (1) is much slower in pineal extracts or cell culture. Implications for CoA prodrug development based on the strategy used here are discussed.

The role of inducible repressor proteins in the adrenergic induction of arylalkylamine-N-acetyltransferase and mitogen-activated protein kinase phosphatase-1 in rat pinealocytes

In this study, we investigated the role of two inducible repressor proteins, inducible cAMP early repressor (ICER) and Fos-related antigen 2 (Fra-2) in the adrenergic induction of MAPK phosphatase-1 (MKP-1) as compared with their roles in the induction of arylalkylamine-N-acetyltransferase (AA-NAT) in rat pinealocytes. Treatment of pinealocytes with norepinephrine (NE) caused an increase in the mRNA and protein levels of MKP-1 and AA-NAT, as well as in the AA-NAT activity and melatonin production. NE stimulation also caused a simultaneous increase in the mRNA and protein levels of ICER and Fra-2. Transient knockdown of icer using adenovirus expressing small interfering RNA (siRNA) abolished the NE induction of icer expression but had little effect on the NE induction of mkp-1 or aa-nat expression. In contrast, pretreatment with adenovirus overexpressing icer was effective in reducing the NE induction of mkp-1 and aa-nat. The inhibitory effect of overexpressing icer was reversed by cotreatment with siRNA against icer. siRNA against fra-2 also abolished the NE-stimulated expression of fra-2 but had little effect on the NE induction of mkp-1 and aa-nat expression. Proteasomal inhibition, which reduced the NE-stimulated induction of aa-nat, caused a reduction of ICER and Fra-2. Together, these results indicate that whereas overexpression of ICER can suppress the NE induction of aa-nat and mkp-1, the amount of the repressors, ICER and Fra-2, present during NE induction appears insufficient to exert a significant effect in controlling the expression of these genes.

Expression of the G protein gammaT1 subunit during zebrafish development

Here, we report the identification and expression analysis of the zebrafish G protein gammaT1 subunit gene (gngT1) during development. Similar to its human and mouse homologs, we confirm zebrafish gngT1 is expressed in the developing retina, where its transcription overlaps with the photoreceptor cell-specific marker, rhodopsin (rho). Surprisingly, we also show zebrafish gngT1 is expressed in the dorsal diencephalon, where its transcription overlaps with the pineal specific markers, arylalkylamine N-acetyltransferase-2 (annat-2) and extra-ocular rhodopsin (exorh). Analysis of the proximal promoter sequence of the zebrafish gngT1 gene identifies several conserved binding sites for the cone-rod homeobox/orthodenticle (Crx/Otx) homeodomain family of transcription factors. Using a morpholino anti-sense approach in zebrafish, we show that targeted knockdown of otx5 potently suppresses gngT1 expression in the pineal gland, whereas knockdown of crx markedly reduces gngT1 expression in the retina. Taken together, these data indicate that pineal- and retinal-specific expression of the gngT1 gene are controlled by different transcription factors and exogenous signals.

The role of protein turnover in regulating MKP-1 levels in rat pinealocytes

We have previously shown that mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) is induced at night under the control of a photoneural system in the rat pineal gland. Because of the established roles of MAPKs, glucocorticoids and proteasome activity in regulating MKP-1 expression in other cell types, their relative contributions to MKP-1 regulation were investigated in rat pinealocytes. We found that neither inhibition of MAPKs nor treatment with dexamethasone affected norepinephrine-stimulated MKP-1 expression. In contrast, treatment with proteasome inhibitors increased norepinephrine-stimulated MKP-1 protein levels and abolished the decline in norepinephrine-stimulated MKP-1 protein levels caused by inhibition of transcription or translation, or blockade of alpha-adrenergic receptors. Taken together, our results indicate that in rat pinealocytes, the continuous and rapid turnover of MKP-1 protein allows for its rapid induction but is not sufficient to generate the sustained increase in MKP-1 expression post-adrenergic stimulation.

Differential enhancement of neural and photoreceptor cell differentiation of cultured pineal cells by FGF-1, IGF-1, and EGF

There are several common features between the pineal organ and the lateral eye in their developmental and evolutionary aspects. The avian pineal is a photoendocrine organ that originates from the diencephalon roof and represents a transitional type between the photosensory organ of lower vertebrates and the endocrine gland of mammals. Previous cell culture studies have shown that embryonic avian pineal cells retain a wide spectrum of differentiative capacities, although little is known about the mechanisms involved in their fate determination. In the present study, we investigated the effects of various cell growth factors on the differentiation of photoreceptor and neural cell types using pineal cell cultures from quail embryos. The results show that IGF-1 promotes differentiation of rhodopsin-immunoreactive cells, but had no effect on neural cell differentiation. Simultaneous administration of EGF and IGF-1 further enhanced differentiation of rhodopsin-immunoreactive cells, although the mechanism of the synergistic effect is unknown. FGF-1 did not stimulate proliferation of neural progenitor cells, but intensively promoted and maintained expression of a neural cell phenotype. FGF-1 appeared to lead to the conversion from an epithelial (endocrinal) to a neuronal type. It also enhanced phenotypic expression of retinal ganglion cell markers but rather suppressed expression of an amacrine cell marker. These results indicate that growth factors are important regulatory cues for pineal cell differentiation and suggest that they play roles in determining the fate of the pineal organ and the eye. It can be speculated that the differences in environmental cues between the retina and pineal may result in the transition of the pineal primordium from a potentially ocular (retinal) organ to a photoendocrine organ.