Melatonin in the mammalian olfactory bulb

Melatonin is a neurohormone associated with circadian rhythms. A diurnal rhythm in olfactory sensitivity has been previously reported and melatonin receptor mRNAs have been observed in the olfactory bulb, but the effects of melatonin in the olfactory bulb have not been explored. First, we corroborated data from a previous study that identified melatonin receptor messenger RNAs in the olfactory bulb. We then investigated whether melatonin treatment would affect cells in the olfactory bulbs of rats. Using a combination of polymerase chain reaction (PCR), quantitative PCR (qPCR), cell culture, and electrophysiology, we discovered that melatonin receptors and melatonin synthesis enzymes were present in the olfactory bulb and we observed changes in connexin43 protein, GluR1 mRNA, GluR2 mRNA, Per1 mRNA, Cry2 mRNA, and K(+) currents in response to 2-iodomelatonin. Via qPCR, we observed that messenger RNAs encoding melatonin receptors and melatonin biosynthesis enzymes fluctuated in the olfactory bulb across 24h. Together, these data show that melatonin receptors are present in the olfactory bulb and likely affect olfactory function. Additionally, these data suggest that melatonin may be locally synthesized in the olfactory bulb.

Vasopressinergic innervation of the bovine pineal gland: is there a local source for arginine vasopressin?

Although the presence of arginine vasopressin (AVP) in the mammalian pineal has been characterized biochemically, the source of this nonapeptide hormone remains enigmatic. Most earlier data pointed to an extrapineal origin, although some recent evidence suggests intrapineal synthesis of AVP. The present study examined this issue using a combination of immunohistochemistry with antibodies against both the AVP and neurophysin moieties of the AVP precursor polypeptide, together with polymerase chain reaction (PCR) assay for the specific mRNA. Furthermore, the effects of AVP on melatonin production by monolayer cultures of bovine pinealocytes were examined. Bovine pineal glands possessed numerous neurophysin- and AVP-immunopositive nerve fibers, mainly in the distal part of the gland. However, no positively stained perikarya were observed. As a positive control perikarya of AVP cells were easily identifiable in the magnocellular cells of the bovine hypothalamus. Nevertheless, a highly sensitive PCR assay specific for full-length AVP mRNA did indicate the presence of AVP gene transcripts in both bovine and ovine pineal glands, using two different primer combinations. This suggests either that there are AVP perikarya in the pineal whose peptide contents are below the level of detection by immunohistochemistry or that gene transcripts may be present in AVP nerve axons, as in the posterior pituitary. AVP had significant inhibitory effects on noradrenaline-provoked melatonin secretion in vitro. These results indicate that AVP released by nonsympathetic nerve fibers terminating in the bovine pineal gland may act to modulate melatonin production.

Induction of mPer1 expression by GnRH in pituitary gonadotrope cells involves EGR-1

We reported earlier that gonadotropin-releasing hormone (GnRH) activates period1 (mPer1) gene expression in immortalized gonadotropes through protein kinase C and p42/44 mitogen-activated protein kinase pathways. GnRH stimulation also leads to the upregulation of early growth response protein 1 (EGR-1), a critical transcription factor for GnRH-induced luteinizing hormone beta (LHbeta) synthesis. The parallels between the GnRH-LHbeta and the GnRH-mPer1 pathways led us to explore whether EGR-1 is involved in the regulation of mPer1 expression in gonadotropes. Of particular interest was the presence of an EGR-1 binding site in the proximal promoter of the mPer1 gene. Stimulation of LbetaT2 gonadotrope cells with a GnRH agonist caused the rapid induction of Egr-1 mRNA, which was rapidly followed by mPer1 expression. Chromatin immunoprecipitation revealed that the mPer1 promoter can bind EGR-1, while site-directed mutagenesis experiments confirmed the involvement of Egr-1 sequences in maintaining basal and allowing GnRH-stimulated mPer1 transcription. By means of RNA interference experiments, it could also be demonstrated that silencing of Egr-1 expression resulted in markedly lower mPer1 transcript levels. This silencing effect of the Egr-1 siRNA could be rescued by transfecting the cells with an EGR-1 overexpression vector. In summary, these results all point to a role for the EGR-1 protein in transactivating both the LHbeta as well as the mPer1 gene in pituitary gonadotrope cells.

E-box regulation of gonadotropin-releasing hormone (GnRH) receptor expression in immortalized gonadotrope cells

The pituitary gland's ability to respond to the hypothalamic hormone GnRH (gonadotropin-releasing hormone) depends directly on the gonadotrope-specific expression of the GnRH receptor (GnRHR), a G-protein coupled transmembrane protein coded by the GnRHR gene. In the present study, we have investigated the potential regulatory role of seven noncanonical E-box enhancer sequences within the 856bp proximal 5'-flanking region of the mGnRHR gene in regulating transcription. These sequences are known to mediate the action of clock gene proteins on the expression of a diverse array of genes both central and peripheral. In the present studies the expression of all of the cognate clock genes was identified in the alphaT3-1 gonadotrope cell line. Additionally, luteinizing hormone-immunoreactive cells in the adult rodent pituitary gland were also shown to co-express the PERIOD-1 protein. By means of chromatin immunoprecipitation of alphaT3-1 nuclear extracts we were able to capture promoter fragments of the GnRHR and Period-1 genes, indicating that E-boxes in these promoters bind the CLOCK protein. RNA interference experiments with alphaT3-1 cells in which Bmal1 expression was attenuated also confirmed the involvement of E-boxes in transcriptional regulation of the mGnRHR gene. Subsequent luciferase reporter assay experiments with GnRHR constructs possessing intact or mutated E-boxes confirmed the use of these sequences for the regulation of mGnRH-R/luc expression. Transient overexpression of the dominant negative E-box-binding factor CLOCK-Delta19, or the inhibitory clock protein mPER1, markedly reduced CLOCK/BMAL1-driven mGnRH-R/luc expression in a dose-dependent fashion. Our data implicate the clock genes as important factors controlling GnRHR expression in murine gonadotrope cells.

Exploratory investigation of the effect of melatonin and caloric restriction on the temporal expression of murine hypothalamic transcripts

Circadian rhythms of behaviour and gene expression are coupled to endogenous neuronal oscillators located in the hypothalamic suprachiasmatic nuclei (SCN), which are synchronised by the environmental light cycle. Besides light, other factors such as the pineal hormone melatonin, temperature and feeding have entraining properties. During senescence, the circadian system becomes weaker and susceptible to desynchronisation. It is unknown to what extent age-related changes are the result of the deterioration of the hypothalamic master clock. Supplementing ageing mice with melatonin as well as maintaining them on a hypocaloric diet extends the life span and delays age-related diseases. By means of DNA microarrays and the quantitative polymerase chain reaction, we have conducted an exploratory study to compare the effect of long-term melatonin substitution (MEL) and caloric restriction (CR) on circadian gene expression in hypothalamic samples, which contained the SCN as well as other important nuclei involved in nutrient balance, reproduction, and so on. Over 4% of the hypothalamic transcripts showed an overt circadian rhythm in expression, and many of these contain E boxes in their promoter regions, suggesting a direct regulation by circadian clock genes. MEL and CR significantly influenced some of these rhythmically expressed transcripts, but often in opposite ways. Importantly, our studies emphasise that the apparent direction of treatment effects (i.e. up-regulation versus down-regulation) depends on the time of day at which the samples are compared.

Expression and regulation of mPer1 in immortalized GnRH neurons

Hypothalamic GnRH (gonadotropin-releasing hormone) neurons play a critical role in the initiation and maintenance of reproduction competence. Using the mouse GnRH neuronal cell line, GT1-7, we have characterized the expression of the gene mPer1, a recognized key element of the mammalian circadian clockwork. Both mPer1 transcripts and the 136 kDa mPER1 gene product could be detected in these cells. Immunocytochemical analysis also confirmed expression of mPER1 both in vitro and in vivo in GnRH neurons. Activation of cyclic AMP signalling pathways in vitro elevated GnRH secretion as well as mPer1 expression and nuclear mPER1 immunoreactivity. As mPER1 is known to feedback on transcriptional activities in many cell models, the data presented here point to a role for mPER1 in the regulation of gene expression in GnRH neurons, and thus in the control of neuroendocrine activities.

The human myometrium as a target for melatonin

The circadian timing of spontaneous human deliveries results in births occurring statistically more often during the nocturnal phase of the 24-h cycle. The neuroendocrine mechanisms underlying this physiological phenomenon are not understood. In an effort to test the hypothesis that melatonin may serve as an endocrine signal for coordinating myometrial events in the human, we determined the mRNA expression of both MT1 and MT2 melatonin receptor isoforms in pregnant as well as nonpregnant myometrial biopsies by means of RT-PCR and in situ hybridization histochemistry. Additionally, we could demonstrate specific, high affinity iodomelatonin binding to myometrial tissues of both pregnant and nonpregnant women. Primary cultures of myocytes responded differentially from melatonin in terms of cAMP signaling depending on the reproductive state. These results imply that melatonin may have the potential to modulate myometrial function in the human, a finding that could open up new possibilities for the development of novel therapeutic agents.

Nocturnal accumulation of cyclic 3',5'-guanosine monophosphate (cGMP) in the chick pineal organ is dependent on activation of guanylyl cyclase-B

The role of cGMP in the avian pineal is not well understood. Although the light-sensitive secretion of melatonin is a well-known output of the circadian oscillator, pharmacologically elevated cGMP levels do not result in altered melatonin secretory amplitude or phase. This suggests that pineal cGMP signalling does not couple the endogenous circadian oscillator to the expression of melatonin rhythms. Nonetheless, the free-running rhythm of cGMP signalling implies a link to the circadian oscillator in chick pinealocytes. As the circadian rhythm of cGMP levels in vitro is not altered by pharmacological inhibition of phosphodiesterase activity, we infer that the synthesis, rather than the degradation of cGMP, is under circadian control. In vitro experiments with the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine as well as with an inhibitor of the NO-sensitive soluble guanylyl cyclase (sGC), showed that the NOS-sGC pathway does not play a major role in the circadian control of cGMP generation. In organ culture experiments, we demonstrated that C-type natriuretic peptide (CNP), but not atrial natriuretic peptide (ANP), elevated daytime levels of cGMP. As CNP acts on the membrane guanylyl cyclase isoform B (GC-B), which is expressed at very high levels in mammalian pineals, we investigated the effect of the membrane GC-specific inhibitor HS-142 on chick pineal cGMP levels. CNP-induced daytime cGMP levels were reduced by HS-142. More importantly, 'spontaneously' high nocturnal levels of cGMP in vitro were reduced to daytime levels by acute addition of HS-142. These data implicate endogenous nocturnal CNP release and subsequent activation of GC-B as the major input driving cGMP synthesis in the chick pineal organ.

Signal transduction in the rodent pineal organ. From the membrane to the nucleus

The rodent pineal organ transduces a photoneural input into a hormonal output. This photoneuroendocrine transduction leads to highly elevated levels of the hormone melatonin at night-time which serves as a message for darkness. The melatonin rhythm depends on transcriptional, translational and posttranslational regulation of the arylalkylamine-N-acetyltransferase, the key enzyme of melatonin biosynthesis. These regulatory mechanisms are fundamentally linked to two second messenger systems, namely the cAMP- and the Ca(2+)-signal transduction pathways. Our data gained by molecular biology, immunohistochemistry and single-cell imaging demonstrate a time- and substance-specific activation of these signaling pathways and provide a framework for the understanding of the complex signal transduction cascades in the rodent pineal gland which in concert not only regulate the basic profile but also fine-tune the circadian rhythm in melatonin synthesis.

Neuropeptide Y (NPY) and NPY receptors in the rat pineal gland

NPY is considered to play an important role in pineal function, because it is co-stored with the dominant pineal transmitter noradrenaline. However, little evidence from the literature suggests that NPY alone is a strong regulator of melatonin synthesis or secretion and it is therefore more likely that NPY modulates noradrenergic neurotransmission in the rat pineal gland. The purpose of the present studies was to determine the nature and origin of NPYergic inputs to, and the type of specific NPY receptor subtypes in, the rat pineal gland. Gel filtration and immunocytochemistry using region-specific antisera revealed that all proNPY present in intrapineal nerve fibres is cleaved to amidated NPY and a C-terminal flanking peptide of NPY (CPON). The vast majority of NPY content in the pineal gland was found to be of sympathetic origin. Receptor autoradiography showed that only a few NPY specific binding sites were present in the superficial pineal gland. A reverse transcriptase polymerase chain reaction detected sequences of only NPY receptor subtype Y1 and not other NPY receptor subtypes in pineal extracts. These results together with the available literature imply that NPY under certain conditions is co-released with noradrenaline and exerts its actions either presynaptically or on the pinealocyte through a Y1 receptor. The available data indicate that NPY has no effect alone, but acts in concert with noradrenaline. A presynaptic action regulating noradrenaline neurotransmission is also possible. NPY has been reported only to act on melatonin secretion in vitro, and it remains to be established what function NPY plays in the pineal gland in vivo. This paper discuss possible modulatory actions of NPY being a predominant sympathetic transmitter.

Guanylyl cyclase-B represents the predominant natriuretic peptide receptor expressed at exceptionally high levels in the pineal gland

The generation and function(s) of the signalling molecule cyclic GMP (cGMP) in brain are still poorly understood. One mechanism to raise intracellular cGMP levels is binding of C-type natriuretic peptide (CNP) to a membrane guanylyl cyclase (GC), termed GC-B. Here, we demonstrate an exceptionally strong expression of GC-B in the pineal gland. Crosslinking experiments performed with 125I-Tyr(0)-CNP and membranes from various rat tissues identified the receptor as a 130-kDa protein, expressed at highest levels in pineal membranes. Receptor autoradiography on brain sections confirmed a striking density of CNP binding sites in pineal tissue, whereas binding sites for the related atrial natriuretic peptide (ANP) predominate in other regions of the brain. Incubations of freshly dissected whole pineal glands in either the absence or presence of natriuretic peptides followed by immunohistochemical analyses of cGMP revealed strong accumulations of cGMP in response to CNP but not to ANP in the majority of pinealocytes. Stimulation of soluble GC (sGC) activity by use of sodium nitroprusside (SNP) resulted in a very similar pattern of cGMP immunostaining, indicating a co-expression at high levels of particulate and soluble forms of GC. These findings point to a major role of cGMP signalling in pinealocytes and suggest an important regulatory function for CNP.

CREB phosphorylation and melatonin biosynthesis in the rat pineal gland: involvement of cyclic AMP dependent protein kinase type II

Phosphorylation of cyclic AMP response element binding protein (CREB) at amino acid serine 133 appears as an important link between the norepinephrine (NE)-induced activation of second messenger systems and the stimulation of melatonin biosynthesis. Here we investigated in the rat pineal gland: 1) the type of protein kinase that mediates CREB phosphorylation: and 2) its impact on melatonin biosynthesis. Immunochemical or immunocytochemical demonstration of serine133-phosphorylated cyclic AMP regulated element binding protein (pCREB) and radioimmunological detection of melatonin revealed that only cyclic AMP-dependent protein kinase (PKA) inhibitors suppressed NE-induced CREB phosphorylation and stimulation of melatonin biosynthesis, whereas inhibitors of cyclic GMP-dependent protein kinase (PKG), mitogen-activated protein kinase kinase, protein kinase C, or calcium-calmodulin-dependent protein kinase (CaMK) were ineffective. Investigations with cyclic AMP-agonist pairs that selectively activate either PKA type I or II link NE-induced CREB phosphorylation and stimulation of melatonin biosynthesis to the activation of PKA type II. Our data suggest that PKA type II plays an important role in the transcriptional control of melatonin biosynthesis in the rat pineal organ.

Transcription factors in neuroendocrine regulation: rhythmic changes in pCREB and ICER levels frame melatonin synthesis

Neurotransmitter-driven activation of transcription factors is important for control of neuronal and neuroendocrine functions. We show with an in vivo approach that the norepinephrine cAMP-dependent rhythmic hormone production in rat pineal gland is accompanied by a temporally regulated switch in the ratio of a transcriptional activator, phosphorylated cAMP-responsive element-binding protein (pCREB), and a transcriptional inhibitor, inducible cAMP early repressor (ICER). pCREB accumulates endogenously at the beginning of the dark period and declines during the second half of the night. Concomitant with this decline, the amount of ICER rises. The changing ratio between pCREB and ICER shapes the in vivo dynamics in mRNA and, thus, protein levels of arylalkylamine-N-acetyltransferase, the rate-limiting enzyme of melatonin synthesis. Consequently, a silenced ICER expression in pinealocytes leads to a disinhibited arylalkylamine-N-acetyltransferase transcription and a primarily enhanced melatonin synthesis.

Pineal rhythms are synchronized to light-dark cycles in congenitally anophthalmic mutant rats

Genetically mutant anophthalmic rats lacking a complete visual system due to the absence of eyeballs and optic nerves up to the optic chiasma were used as a model to study photo-regulated physiological activities. The photoreception in these mutant rats was determined by measuring the neuroendocrine response of the pineal gland-melatonin levels in the serum, and synaptic ribbon complexes (SRCs) in the pinealocytes. These parameters were studied in both normal and anophthalmic rats maintained under light-dark (LD 12:12), continuous dark (DD) and light (LL) conditions. Both normal and mutant anophthalmic animals showed nocturnal increases in serum melatonin levels and in the number and diameter of SRC and their vesicles in the pinealocytes in LD. The daily rhythms persisted even upon transfer to DD both in normal and mutant rats, whereas in LL, the nocturnal elevation of both the parameters disappeared. These observations suggested that congenitally blind rats can perceive light. The studies of these parameters in both normal and mutant rats in reversed-LD conditions confirmed that pineal rhythms can be entrained by light-dark cycles in congenitally anophthalmic mutant rats through a nonvisual system for light perception.

A semiquantitative image-analytical method for the recording of dose-response curves in immunocytochemical preparations

Knowledge about intracellular signal transduction cascades is largely based on investigations of cultured cells whose responses to different stimuli are typically quantified via RIA, ELISA, or immunoblots. These techniques, which require relatively large amounts of biological material, are performed with homogenized cells and therefore do not allow localization of the molecules under investigation. We describe a protocol for recording dose-response curves directly from immunocytochemical preparations using rat pinealocytes as a model system. The cells were exposed to beta-adrenergic stimuli inducing the phosphorylation of the transcription factor CREB (mediated by PKA), an increase in ICER protein levels, and synthesis and release of melatonin. Melatonin concentrations were determined by ELISA. cPKA, phosphorylated CREB, and ICER were demonstrated by immunocytochemistry and immunoblots. Dose-response curves were recorded by measuring the integrated density of the immunoreactive sites with an image analysis program. Dose-response curves from immunoblots and immunocytochemical preparations showed almost identical dynamics, validating the immunocytochemical approach, which minimizes the amount of biological material needed for such studies, allows combined quantification and localization of biomolecules, and may even be more sensitive than immunoblotting.

A fusca gene homologue in mammalian tissues: Developmental regulation in the rat testes

By differential screening of a rat pineal cDNA library we identified earlier a novel transcript having a 57% nucleotide homology and a 45% amino acid identity with a plant fusca-gene (fus6) to which a corresponding human sequence (gps1) has recently been reported. Expression of this mammalian fusca homologue (mfh) was seen in a variety of mammalian tissues, including kidney, pineal and retina, but it was particularly strong in the testes. Northern blot analysis demonstrated that the rat testicular mfh message increases markedly from day 28 onwards. Additionally, by in situ hybridization, mfh was localized primarily to the seminiferous tubules with a stage-dependent distribution pattern, a result which was confirmed by immunohistochemistry with antibodies raised against a synthetic MFH oligopeptide. Western blotting also revealed strong signals of the expected molecular weight in testicular extracts from several species. In view of its homology to fus6, a plant gene known to be involved in repressing photomorphogenesis in darkness, the conservation of mfh in mammals suggests a potential function for MFH in signaling pathways involved in the regulation of mammalian differentiation and development.

Pituitary adenylate cyclase-activating peptide and vasoactive intestinal peptide receptor expression in immortalized LHRH neurons

The regulation of LHRH secretion is extraordinarily multifarious. To no small extent, this insight has been gained through studies using the immortalized hypothalamic LHRH neuronal line, GT1-7. In the present study, we examined these cells for potential expression of the receptors for the related peptides PACAP and VIP. By means of reverse transcription-polymerase chain reaction (RT-PCR) with PACAP receptor-specific primers, in combination with restriction enzyme analysis and cDNA sequencing, we were able to identify all PACAP-specific receptor splice variant forms with variable degrees of expression. Of the two nonselective VIP/PACAP receptors (i.e. VIP-R type I and II) only the latter isoform was detected by RT-PCR. In view of these results, we sought to establish whether PACAP and VIP receptors are functional in GT1-7 cells. Cyclic AMP (cAMP) accumulation after addition of PACAP-38 (or PACAP-27) was dose-dependent with maximal 3-fold increases. VIP also elevated cAMP with a similar potency. Phosphatidylinositol (PI) turnover was unaffected by either PACAP or VIP. Acute LHRH secretion was stimulated equally by nanomolar concentrations of both PACAP and VIP. These results point to PACAP and VIP having direct actions via the VIP2R on cAMP signalling and LHRH release, in addition to the known effects of these peptides on pituitary functions.

Synthesis of C-type natriuretic peptide (CNP) by immortalized LHRH cells

Former studies have indicated an influence of natriuretic peptides on LHRH secretion. In this report we demonstrate local synthesis of CNP in immortalized LHRH neurons (GT1-7 cells). Using reverse transcription-polymerase chain reaction and RNase protection assays a transcript for the CNP precursor was identified in these cells. Immunocytochemical data revealed the presence of the peptide CNP in GT1 cells, using a specific polyclonal antiserum against CNP. Electron microscopic immunohistochemical investigations also showed the strongest CNP-immunoreactivity in some small vesicles, providing initial evidence for the potential secretion of this peptide by immortalized LHRH neurons. Subsequent experiments demonstrated also that CNP elevates LHRH production in static cultures of GT1 cells. These data show for the first time the co-production of the functionally relevant natriuretic peptide, CNP, by immortalized LHRH neurons. Together with the recent demonstration of CNP receptor expression by these cells, we suggest that CNP may represent a novel autocrine regulator of LHRH neuronal activity. It remains to be elucidated, however, to what extent CNP expression in immortalized LHRH neurons reflects a co-localization in situ of CNP and LHRH peptides.

Melatonin synthesis in the bovine pineal gland is regulated by type II cyclic AMP-dependent protein kinase

We investigated the expression of regulatory (R) and catalytic (C) subunits of cyclic AMP-dependent protein kinase (cAK; ATP:protein phosphotransferase; EC 2.7.1.37) in the bovine pineal gland. In total RNA extracts of bovine pineal glands moderate levels of RI alpha/RII beta and high levels of C alpha and C beta mRNA were found. We were able to detect a strong signal for RII and C subunit at the protein level, whereas RI was apparently absent. Probing sections of the intact bovine pineal gland with RI and RII antibodies stained only RII in pinealocytes. Pairs of cyclic AMP analogues complementing each other in activation of type II cAK, but not cAKI-directed analogue pairs, showed synergistic stimulation of melatonin synthesis. Moreover, melatonin synthesis stimulated by the physiological activator norepinephrine in pineal cell cultures was inhibited by cAK antagonists. Taken together these results show the presence of RII regulatory and both C alpha and C beta catalytic subunits and thus cAKII holoenzyme in the bovine pineal gland. The almost complete inhibition of norepinephrine-mediated melatonin synthesis by the cAK antagonists emphasizes the dominant role of cyclic AMP as the second messenger and cAK as the transducer in bovine pineal signal transduction.

Expression of C-type natriuretic peptide in the bovine pineal gland

Recent studies have pointed to membrane-bound guanylyl cyclases (GCs) type A and type B in the rat pineal gland, which augment levels of cyclic GMP (cGMP) in response to atrial natriuretic peptide (ANP), brain-type natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). The present report demonstrates for the first time the expression of CNP in the bovine pineal gland. The CNP prohormone transcript (unlike pre-pro-ANP) was found by reverse transcriptase (RT)-PCR in bovine pineal extracts. CNP immunoreactivity (ir) was revealed in a subpopulation of pinealocytes in situ and in nearly all pinealocytes in culture. Electron microscopic immunohistochemical investigations showed the presence of CNP-ir in cytoplasmic vesicles, providing evidence for the potential secretion of this peptide by pineal cells. Furthermore, the CNP receptor (GC-B) and GC-A (receptor for ANP and BNP) were identified by RT-PCR. Although melatonin secretion was unaffected, natriuretic peptides were able to elevate markedly cGMP production in cultured bovine pinealocytes with a rank order of potency of CNP > BNP = ANP. These findings describe a tissue CNP system in the bovine pineal gland and suggest that CNP may be a local auto- or paracrine modulator of pineal function.

Neuropeptide Y: distribution of immunoreactivity and quantitative analysis in diencephalic structures and cerebral cortex of dwarf hamsters under different photoperiods

The distribution of neuropeptide Y-like immunoreactivity (NPY-LI) was investigated by immunohistochemistry and radioimmunoassay (RIA) in the brain of the Djungarian hamster (Phodopus sungorus) held under either long or short photoperiods. In the diencephalic and telencephalic structures studied, distinct patterns of NPY-LI were basically consistent in male and female animals of both groups. NPY levels detected by RIA from tissue samples taken at six time points throughout the 24-hour cycle were in the range of 15-60 pmol/mg protein in the diencephalon or below 5 pmol/mg protein in cerebral cortex. In the diencephalon, immunoreactive structures were seen in the preoptic, peri- and paraventricular, supraoptic, anterior, lateral, dorso- and ventromedial hypothalamic nuclei and in the median eminence. The suprachiasmatic nuclei exhibited a dense innervation by NPY-LI terminals mainly in its ventrolateral subdivision. NPY levels in the suprachiasmatic nucleus were nocturnally augmented under long-day, but not under short-day conditions. The quantification of NPY in the paraventricular nucleus revealed a decrease at night in long-day animals and a small nocturnal augmentation in short-day hamsters. In the pineal gland and habenular nuclei, varicose fibers were observed which appeared mainly perivascular in location (pineal) or formed a dense plexus (habenular nuclei). Pineal NPY contents fell during the night in long-day animals and were relatively constant in short-day hamsters. NPY-LI structures were also observed in the metathalamic intergeniculate leaflet and in a variety of telencephalic structures including the cerebral cortex, caudate nucleus putamen, lateral septal nucleus, bed nucleus of the stria terminalis and amygdala.

The effect of NO-donors in bovine and rat pineal cells: stimulation of cGMP and cGMP-independent inhibition of melatonin synthesis

The presence of soluble guanylate cyclase in the pineal and its regulation by adrenergic pathways has been well documented. Recent evidence points to adrenergically stimulated nitric oxide generation as a mechanism for coupling this pathway. To what extent nitric oxide (NO) signalling can influence adrenergically stimulated melatonin synthesis has not been investigated. Cyclic guanosine 3',5'-monophospate (cGMP) signal transduction in the bovine pineal has also received little attention. We describe in the present report: 1) a dose-dependent elevation of cGMP in response to the nitrovasodilators, sodium nitroprusside (SNP) and 3-morpholino-sydnonimine (SIN-1), 2) a dose-dependent inhibition of melatonin synthesis by SNP and SIN-1, but not by 8-Br-cGMP in both bovine and rat pineal cell cultures, which is not due to cytotoxicity as judged by two different approaches, and 3) immunohistochemical evidence for the presence of nitric oxide synthase (NOS) (EC 1.14.23.-) in the intact bovine pineal gland and in cultured bovine pinealocytes. These data support the view that NOS is a component of the cGMP-generating system in mammalian pinealocytes. Although NO-donor molecules are also potent activators of cGMP accumulation, they may have other important actions in the pineal, namely the inhibition of adrenergic-stimulated melatonin synthesis. As SNP and SIN-1 exerted this inhibitory effect on cells regardless of whether they were stimulated by isoproterenol, forskolin or 8-Br-cAMP it would appear that NO-donors can act 'downstream' from the receptor/adenylate cyclase level.

The mammalian pineal gland and reproduction. Controversies and strategies for future research

Evidence for a role of the pineal gland and its major hormonal product, melatonin, in mammalian reproduction has accumulated for over three decades. In sorting through the masses of data pertaining to this issue, certain facts are becoming established. For example, the pineal gland is clearly involved in seasonal reproductive cycles via transduction of daylength (more properly nightlength) information in the form of plasma melatonin rhythms. Specific melatonin receptors are found in the hypothalamus and pituitary pars tuberalis of most of the mammalian species examined thus far. Melatonin's mode of action on the reproductive axis is quite variable but may, in many cases, involve modulation of gonadotropin-releasing hormone secretion from the median eminence. Clinical evidence continues to support the idea that melatonin may play a role in the timing of puberty.

Pituitary adenylate cyclase activating peptide-38 (PACAP-38), PACAP-27, and PACAP related peptide (PRP) in the rat median eminence and pituitary

Pituitary adenylate cyclase activating peptide (PACAP) is a member of the vasoactive intestinal peptide-like peptide family. It is found in the hypothalamus, where the PACAP precursor is processed to form PACAP-38, the C-terminal truncated PACAP-27 and PACAP related peptide (PRP). Both PACAPs are potent stimulators of anterior pituitary adenylate cyclase activity, but the physiologically relevant anatomical sources of PACAP and possible importance of PRP in this regard are poorly understood. Using immunocytochemistry with epitope-specific antisera, we now show that PACAP38-, PACAP27- and PRP-positive nerve fibres are all present in the rat median eminence. The major immunoreactive species present was PACAP38. Numerous PACAP38-immunoreactive nerve fibres were observed in the internal layer and a few were present in the posterior pituitary lobe. The external layer of the median eminence contained a few PACAP-38-immunoreactive fibres and PACAP-38-positive nerve terminals were rarely seen in the perivascular portal spaces. Surprisingly, delicate PACAP-38-positive nerve fibres were identified in the anterior pituitary lobe intermingled between the pituitary cells although none of the secretory pituitary cells contained immunoreactive PACAP38, PACAP27 or PRP and preproPACAP mRNA was not detected in the gland by Northern blotting or in situ hybridization. PACAP-27- and PRP-immunoreactive nerve fibres and terminals were found in the same locations as PACAP-38 although generally in lower numbers. Specific radioimmunoassays and HPLC revealed that PACAP-38 accounts for the vast majority of the adenohypophyseal PACAP-immunoreactivity, whereas PACAP-27 and PRP were found in low to undetectable concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

C-type natriuretic peptide (CNP) in the pituitary: is CNP an autocrine regulator of gonadotropes?

Natriuretic peptides act via receptors with intrinsic guanylate cyclase activity to stimulate cGMP production and are thought to be important regulators of neuroendocrine systems. C-Type natriuretic peptide (CNP) is of particular interest in this regard because the highest tissue concentrations of CNP occur in the anterior pituitary, where it is a highly potent stimulator of cGMP production. Here we show that pituitaries of rats and mice contain abundant CNP prohormone messenger RNA (mRNA), but no atrial natriuretic peptide or B-type natriuretic peptide prohormone mRNAs. Using reverse transcriptase-polymerase chain reaction, both A- and B-type natriuretic peptide receptor (GC-A and GC-B, respectively) transcripts were detected in rat and mouse pituitaries, although only the GC-B mRNA was measurable by Northern blotting. Immunohistochemistry revealed CNP-positive cells in the anterior, but not posterior, pituitaries of rats, and the vast majority of these cells were identified as gonadotropes by colocalization of CNP and LH immunoreactivities. Targeted toxicity using GnRH conjugated to the ricin-A chain was used to test whether gonadotropes are also direct targets for GnRH action. The conjugate dose dependently inhibited the proliferation of alpha T3-1 cells (gonadotrope-derived cells with GnRH receptors), but had no such effect on GH3 cells (which do not have GnRH receptors). Culture of rat pituitary cells with the conjugate caused comparable reductions in CNP-stimulated cGMP production, GnRH-stimulated LH release, and CA2+ ionophore (A23187)-stimulated LH release, but did not measurably alter cAMP production in response to pituitary adenylate cyclase-activating polypeptide. We conclude that CNP is synthesized in the pituitary, where it is located predominantly in gonadotropes, and GC-B receptors expressed in the pituitary mediate the direct effects of CNP in gonadotropes. Together with the recent demonstration of CNP synthesis and action in alpha T3-1 cells, the data suggest CNP to be a novel autocrine regulator of gonadotropes.

Natriuretic peptides elevate cyclic 3',5'-guanosine monophosphate levels in cultured rat pinealocytes: evidence for guanylate cyclase-linked membrane receptors

Cyclic GMP formation in the rat pinealocyte has generally been thought to involve guanylate cyclases (GC) which are activated via GTP-regulatory proteins following beta 1-adrenergic receptor stimulation. Recent studies have also pointed to a cytosolic GC in these cells whose activity can be elevated by nitric oxide donors. Little attention has been paid to the possibility that pinealocytes might express membrane-bound GC in the form of natriuretic peptide receptors. The present report demonstrates functional membrane GC in rat pinealocytes by (1) cross-linking analyses with radiolabelled atrial natriuretic peptide (ANP); (2) reverse transcriptase polymerase chain reaction (RT-PCR) and DNA blot hybridization with probes for both the GC-A and GC-B forms of the natriuretic receptor; and (3) monolayer cell cultures of pinealocytes, which accumulate cGMP in response to ANP and its related peptides. As the role for cGMP in the rat pineal gland does not appear to be directly coupled to the synthesis of melatonin, the natriuretic peptides may have other regulatory functions in this neuroendocrine tissue.

Extracellular serotonin promotes melatonin release from cultured rat pinealocytes: evidence for an S2-type receptor-mediated autocrine feedback

Recent evidence points to the secretion of serotonin from the rat pineal gland both in vivo and in vitro. In view of the fact that adrenergic stimulation of cultured pinealocytes leads to rapid serotonin release well in advance of melatonin production, the question arises as to the physiological significance of serotonin secretion. The present studies examined the impact of ketanserine, a specific serotonin type 2 receptor antagonist, on the cyclic AMP and melatonin response of cultured rat pinealocytes to adrenergic stimuli. Whereas the beta-adrenergic agonist isoproterenol (1 microM) stimulated cAMP accumulation as well as melatonin release significantly, the presence of ketanserine inhibited these responses in a dose dependent manner. Since the same inhibitory effect of ketanserine was seen in cells stimulated by dibutyryl-cAMP (which acts post-adrenergic receptor to elevate melatonin synthesis), the mechanism for serotonin's feedback actions does not appear to involve changes in adrenergic receptor/cAMP coupling. These results indicate that extracellular serotonin may be important for the full activation of melatonin secretion following adrenergic stimulation.

Natriuretic peptides stimulate cyclic GMP production in an immortalized LHRH neuronal cell line

The role of cyclic 3',5'-guanosine monophosphate (cGMP) as a second messenger in LHRH neurons is not well understood. Recent studies involving nitric oxide, a direct activator of soluble guanylate cyclase (GC), have implicated cGMP in the regulation of LHRH secretion both in vivo and in vitro. Evidence for the membrane-bound form of GC in LHRH neurons has thus far not been reported. In polymerase chain reaction screening of various cell lines for the natriuretic peptide receptors--which represent GCs--we identified both GC-A and GC-B cDNAs by southern blot hybridization in reverse transcribed and amplified extracts of the GT1-7 cell line, an immortalized LHRH neuronal cell line. Subsequent experiments demonstrated that all of the natriuretic peptides elevated cGMP production with a rank order of potency: CNP > ANP > BNP. Time course studies revealed a rapid intracellular accumulation of cGMP following exposure to CNP with a peak at 2.5 min. CNP was some 200-fold more potent than the NO donor, sodium nitroprusside, in stimulating cGMP accumulation in these cells. These data show for the first time the presence of functional mGCs on LHRH cells, and suggest that the natriuretic peptides may also participate in the regulation of LHRH activity.

Sex steroids do not alter melatonin secretion in the human

It is unclear whether sex steroids influence melatonin secretion in the human. In an attempt to find an answer to this important question 36 women within an age range of 19 to 40 years were studied within a 3-month period under the following conditions: natural menstrual cycle, ovulation induction with gonadotrophins, early pregnancy, and intake of monophasic or triphasic oral contraceptives. Except in the case of pregnancy, repeated measurements in the same individual were done because of the well-known large inter-individual variations in melatonin secretion. Melatonin concentration was measured in plasma samples obtained at 4-hourly intervals in a 24 h period and < 200 lux for all subjects studied. No consistent change in melatonin blood concentrations was demonstrated in response to the varying endogenous or exogenous concentrations of sex steroids. These observations suggest that circadian melatonin secretion is not significantly modulated by sex steroids.

Effects of acute tryptophan depletion on nocturnal melatonin secretion in humans

Measurement of melatonin secretion throughout the night provides an index of net noradrenergic activity mediated by postsynaptic beta-adrenergic receptors in the pineal gland. Reduced melatonin secretion in some patients with depression might be related to reduced net noradrenergic function. However, a dysregulation in serotonin function has also been implicated in the pathophysiology of depression. The essential amino acid tryptophan is the precursor for both serotonin and melatonin production. To determine the effects of serotonin function on nocturnal melatonin secretion, eight healthy volunteers underwent active and sham tryptophan depletion in a randomized, double-blind fashion. Blood samples for melatonin and free and total tryptophan were obtained before and after the depletion. Acute tryptophan depletion decreased free and total plasma tryptophan levels to less than 20% of baseline levels. Melatonin secretion, expressed as area under the curve, was decreased in all eight subjects after tryptophan depletion when compared to sham depletion. These results suggest that reduced plasma tryptophan levels, and presumably brain serotonin concentrations, decrease nocturnal melatonin secretion in humans. Additional studies investigating the relationship between serotonin metabolism and pineal function in humans appear warranted.

Urinary 6-hydroxymelatonin sulfate as a measure of melatonin secretion during acute tryptophan depletion

The essential amino acid tryptophan (TRP) is the precursor for serotonin (5-HT), which is in turn an intermediary product in the synthesis of melatonin. Acute TRP depletion has recently been shown to decrease nocturnal plasma levels of melatonin in humans. Melatonin is metabolized to 6-hydroxymelatonin sulfate (6-SM), a highly stable end-product which is excreted into urine. To determine the effects of TRP bioavailability on 6-SM, 11 healthy volunteers underwent active and sham TRP depletion in a randomized, double-blind fashion. Samples of plasma free and total TRP, plasma melatonin, and urinary 6-SM were obtained before and after the depletion. Acute TRP depletion decreased free and total plasma tryptophan levels by more than 80% from baseline levels. Nocturnal 6-SM excretion was significantly decreased and highly correlated with decreases in plasma melatonin. These results suggest that nocturnal urinary 6-SM excretion is a valid measure of melatonin secretion under conditions of decreased 5-HT function. Collection of urine for 6-SM is considerably easier than nocturnal plasma sampling for melatonin. Further studies are needed to clarify the relationship between 6-SM excretion and other measures of 5-HT function in neuropsychiatric disorders.

Bovine pinealocytes in monolayer culture: studies on the adrenergic regulation of melatonin secretion

Most of what is known regarding adrenergic control of mammalian pineal functions has been derived from investigations with rodents. In an effort to provide a new animal model for pineal research, the bovine pineal gland was selected for practical and theoretical reasons. Bovine pinealocytes in monolayer culture were first characterized in terms of optimal culture conditions, i.e. medium requirements, cell density, preincubation duration and stimulation times. In Dulbecco's minimum essential medium/Ham's F12 medium, 2 x 10(5) cells per well preincubated for 6 days and stimulated for 8 h resulted in large increases in melatonin (mel) release (15- to 17-fold) after adrenergic stimulation. Likewise, cAMP accumulation within 10 min of stimulation increased 4- to 8-fold. (Bu)2cAMP (10(-5)-10(-3) M) produced a significant elevation in MEL secretion. Neither adrenergic dose-response studies, adrenergic antagonist studies nor experiments in which alpha- and beta-agonists were added together provided any evidence for a positive alpha/beta synergism on either MEL release or cAMP accumulation by bovine pinealocytes. This culture system should prove useful for identifying species differences in the signal transduction mechanisms underlying the activation of MEL secretion in the mammalian pineal.

Neuropeptide Y: an endogenous inhibitor of norepinephrine-stimulated melatonin secretion in the rat pineal gland

The biosynthesis of the hormone melatonin (MEL) by the mammalian pineal gland has been thought to be regulated strictly by stimulatory factors, most predominantly norepinephrine (NE), released from the sympathetic nerve fibers which heavily innervate the gland. Evidence from many investigators suggests that sympathetic fibers may colocalize other neuroactive factors in addition to NE. One of these factors is neuropeptide Y (NPY), which has been found in the nerve fibers of the pineal gland. The present study sought to explore potential interactions between NE and NPY in the regulation of pineal MEL secretion. Specific, saturable, and reversible binding of 125I-NPY to intact cultured pinealocytes was measured with an affinity constant of 1 nM and an NPY binding site density of 0.04 pmol/mg of protein. In addition, cell culture studies revealed that NPY represents a potent (IC50 of 0.4 nM) endogenous inhibitor of NE-stimulated MEL secretion. However, this inhibition is accompanied by only a modest reduction (35%) of cyclic AMP accumulation. These findings reinforce the view that the mammalian pineal gland, which appears to integrate both inhibitory as well as stimulatory signals, is an important model of autonomic function, particularly in the context of biological rhythmicity.

Comparative studies on the retinal dopamine response to altered magnetic fields in rodents

Previously it was demonstrated that experimental alteration of the ambient magnetic field at night significantly reduced catecholamine levels in the retinae of Sprague-Dawley rats. As this effect appeared to depend on intact photoreceptors, it seemed of interest to examine whether the dopamine response to altered magnetic fields (MFs) differed between rod-dominant retinae and cone-dominant retinae. Furthermore, the effect of MFs on daytime dopamine content was explored. As in previous nocturnal investigations, dopamine levels in light-adapted (i.e. daytime) retinae from albino rats were significantly reduced by MFs. In the cone-dominant retina of the pigment ground squirrel, a similar MF effect was observed. However, in the rod-dominant retina of the golden hamster, dopamine levels increased significantly following daytime MF exposure. These results indicate that the retinal dopaminergic system is differentially responsive to MFs in various rodent species. Hence, the retina may play an important role in the perception of MFs by mammals.

Characterization of serotonin N-acetyltransferase activity in the retina of the Mongolian gerbil, Meriones unguiculates

In order to better understand the nature of melatonin synthesis in the mammalian retina, characterization of the enzyme serotonin N-acetyltransferase (NAT; EC 2.3.1.87) was undertaken in the Mongolian gerbil. Retinal and pineal homogenates were assayed for NAT activity by means of the radiometric method of Deguchi and Axelrod (1972). The activities of the retinal and pineal enzymes were similar with regard to pH optima and reaction rate. However, enzyme product formation by retinal NAT under varying substrate concentrations (including no substrate) remained virtually constant, hence preventing the determination of retinal NAT kinetics. In contrast, the pineal enzyme was determined to have a maximal velocity of 1 pmol/min and an apparent Michaelis constant of 100.7 microM, which agrees well with earlier reports in the rat. A daily rhythm of retinal NAT activity was observed with low levels during the daytime, early nighttime and very late nighttime hours. Peak levels were observed at 01.00 h in darkness. Acute light exposure reduced nocturnal NAT activity significantly within 10 min. This is the first demonstration of such an effect on retinal NAT activity in a mammalian species.

Responses of the mammalian retina to experimental alteration of the ambient magnetic field

The detection of earth strength magnetic fields by rodents has been demonstrated previously by numerous physiological and behavioral techniques. This phenomenon appears to require input from the eyes. In an effort to better understand this phenomenon retinal melatonin synthesis and catecholamine contents were assayed in rats exposed at night to an alteration of the ambient magnetic field. In normal animals both dopamine and norepinephrine levels in the retina were reduced by this stimulus, while retinal melatonin synthesis was unaffected. Animals that had lost their intact photoreceptors as a result of 8 weeks of previous constant light exposure did not show a catecholamine response to the magnetic stimulus. These results support the view that the mammalian retina participates in the relaying of magnetic information into the central nervous system.

Geomagnetic field detection in rodents

In addition to behavioral evidence for the detection of "earth-strength" magnetic fields (MF) by rodents, recent investigations have revealed that electrophysiological and biochemical responses to MF occur in the pineal organ and retina of rodents. In addition, ferrimagnetic deposits have been identified in the ethmoidal regions of the rodent skull. These findings point to a new sensory phenomenon, which interfaces with many fields of biology, including neuroscience, psychophysics, behavioral ecology, chronobiology and sensory physiology.

Central neural control of pineal melatonin synthesis in the rat

To investigate a possible central neural influence on nocturnal pineal metabolic activity in rats, frontal transsections of the stria medullaris thalami were conducted. Enzymes involved in melatonin synthesis, i.e. N-acetyltransferase and hydroxyindole-O-methyl-transferase, exhibited reduced activities in operated animals when compared to controls. These results indicate a modulatory role of central structures on nocturnal pineal indole metabolism.

Electrical stimulation of the hypothalamic nucleus paraventricularis mimics the effects of light on pineal melatonin synthesis

In an attempt to clarify further the role of the hypothalamic paraventricular nuclei (PVN) in the control of pineal function, the effects of 2 min electrical stimulation of these nuclei were investigated in acutely blinded, adult, male Sprague-Dawley rats. Pineal serotonin-N-acetyltransferase (NAT) activity, melatonin content and catecholamine levels were measured by means of radio-enzymatic, radioimmunoassay and high-performance liquid-chromatography methods, respectively. All three pineal parameters underwent significant declines following brief PVN stimulation during the night time. These observations lend credence to the view that the neural pathways transmitting light information to the sympathetic innervation controlling pineal melatonin synthesis.

Magnetic field effects on pineal gland melatonin synthesis: comparative studies on albino and pigmented rodents

Previous investigations have shown that the inhibitory effects of an earth-strength magnetic field on albino rat pineal melatonin synthesis is dependent on optic input. The possibility that ocular pigmentation might play a role in mammalian magnetosensitivity was explored in the present study by comparing hooded rat and golden hamsters with albino rats. Pineal melatonin synthesis, i.e. N-acetyl-transferase activity and melatonin content, was utilized as a parameter for assessing magnetosensitivity. In both rat strains nocturnal pineal melatonin synthesis was markedly inhibited following a single 30-min magnetic field stimulus consisting of a 50 degree rotation of the earth's field horizontal component. However, golden hamsters did not respond to the same magnetic stimulus, indicating a species-specific magnetosensitivity that is apparently independent of ocular pigmentation. Possible reasons for these differences are discussed.

Magnetic field effects on the rat pineal gland: role of retinal activation by light

In view of the reported involvement of the retinae in mediating magnetic field effects on pineal function in rats, the present study sought to test the hypothesis - based on theoretical calculations - that dim light activation of photoreceptors is necessary for magnetoreception by the retinae. Adult male rats were exposed to a single nocturnal inversion of the earth's magnetic field in the presence or absence of dim red light. Pineal gland N-acetyltransferase and hydroxyindole-O-methyltransferase activities were measured as indices of magnetosensitivity. In animals exposed to dim red light, pineal enzyme activities were inhibited significantly by the magnetic stimulus in comparison to controls (dim red light only). In contrast, the pineal response to a magnetic stimulus was absent in total darkness. These results support the notion that photoreceptor stimulation by dim light is necessary for the perception of weak magnetic fields.

Electrophysiological and endocrinological aspects of aging in the rat pineal gland

Previous morphological and biochemical studies point to an age-dependent loss of pineal gland function. In order to investigate the possible alterations of electrical activity of aged rat pineal glands, electrophysiological recordings in 3- and 18-month-old male Sprague-Dawley rats were undertaken at both daytime and nighttime. Extracellularly recorded action potentials were characterized according to discharge frequency and serum melatonin levels were determined concomitantly. Significant differences in the frequency distributions between young adult and old rats were found to occur at nighttime, with a shift towards lower frequencies in old animals. Serum melatonin levels were reduced significantly in old rats and the characteristic nocturnal peak was attenuated. These results demonstrate a distinct decrease of pineal electrical and secretory activity with advancing age, which may contribute to an age-dependent decline of neuroendocrine capacity.

Electrical stimulation of the hypothalamic paraventricular nuclei inhibits pineal melatonin synthesis in male rats

Recent findings have shown that lesions of the hypothalamic paraventricular nuclei (PVN) disrupt the synthesis of melatonin in the rat pineal gland. In order to further clarify the role of the PVN in the control of pineal function, the effects of electrical stimulation of these nuclei were investigated in acutely blinded adult male Sprague-Dawley rats. Following electrical stimulation, pineal serotonin-N-acetyltransferase (NAT) activity and pineal melatonin content were measured by means of radioenzymatic and radioimmunoassay methods, respectively. Stimulation had no significant effect on pineal melatonin synthesis throughout the early part of the dark phase, but caused a significant reduction in NAT activity during the light phase and the latter part of the dark phase. The pineal melatonin content appeared reduced, but due to large individual variations this reduction was not statistically significant. Stimulation duration experiments reveal that reduction of NAT activity is time dependent, with significant inhibition occurring after 30 min of stimulation. These observations further support the involvement of the PVN in the melatonin rhythm generating pathway and suggest that electrical activation of fibers in the PVN is similar to the effects of light on pineal melatonin synthesis.

Evidence for the involvement of the visual system in mediating magnetic field effects on pineal melatonin synthesis in the rat

In order to elucidate whether magnetic field effects on mammalian pineal function are direct, or instead indirect via retinal disturbances, acutely blinded and intact male rats were subjected to a single nocturnal magnetic stimulus. Then pineal N-acetyltransferase activity and melatonin content were assayed. Only in intact animals did the magnetic stimulus significantly reduce pineal activity, i.e. no effects were detected in blinded animals. These data point to a retinal magnetosensitivity which may serve to modulate pineal gland function.

Photoperiod effects on carbohydrate metabolites in the goldfish, Carassius auratus: role of the pineal and retinal pathways

The effects of exposure to various photoregimes or to constant darkness on liver glycogen and plasma glucose levels in the goldfish were examined during different seasons. In addition, pinealectomized fish and optic nerve sectioned fish were exposed to the photoregimes to assess the potential role of the pineal organ and retinal pathways in mediating photoperiod effects. Neither photoperiod manipulation nor exposure to constant darkness affected liver glycogen levels in experiments conducted during October and January. Photoperiod alteration and exposure to constant darkness resulted in changes of liver glycogen and plasma glucose levels during May, suggesting that response to photoperiod varies with season. Likewise, optic nerve sectioning altered liver glycogen levels during May, but not during October or January, indicating that retinal pathways may be involved in mediating photoperiod effects. During both October and May pinealectomy resulted in a lowering of liver glycogen levels independent of photoregime ; the extent to which hepatic glycogen levels were decreased by pinealectomy was affected by lighting conditions. Plasma glucose levels were elevated in pinealectomized fish regardless of photoregime during May ; the degree of this increase varied with lighting conditions. Our data suggest that the pineal organ does not mediate photoperiod effects on plasma glucose and hepatic glycogen levels, but that there is a functional relationship between this organ and carbohydrate metabolism.