Potentiating effects of serotonin and vasoactive intestinal peptide on the action of glutamate on suprachiasmatic neurons in brain slices

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Nociceptin/orphanin FQ (N/OFQ) inhibits excitatory and inhibitory synaptic signaling in the suprachiasmatic nucleus (SCN)

Environmental synchronization of the endogenous mammalian circadian rhythm involves glutamatergic and GABAergic neurotransmission within the hypothalamic suprachiasmatic nucleus (SCN). The neuropeptide nociceptin/orphanin FQ (N/OFQ) inhibits light-induced phase shifts, evokes K(+)-currents and reduces the intracellular Ca(2+) concentration in SCN neurons. Since these effects are consistent with a modulatory role for N/OFQ on synaptic transmission in the SCN, we examined the effects of N/OFQ on evoked and spontaneous excitatory and inhibitory currents in the SCN. N/OFQ produced a consistent concentration-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSC) evoked by optic nerve stimulation. N/OFQ did not alter the amplitude of currents induced by application of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-d-aspartate (NMDA) nor the amplitude of miniature EPSC (mEPSC) consistent with a lack of N/OFQ effect on postsynaptic AMPA or NMDA receptors. N/OFQ significantly reduced the mEPSC frequency. The inhibitory actions of N/OFQ were blocked by omega-conotoxin GVIA, an N-type Ca(2+)channel antagonist and partially blocked by omega-agatoxin TK, a P/Q type Ca(2+) channel blocker. These data indicate that N/OFQ reduces evoked EPSC, in part, by inhibiting the activity of N- and P/Q-type Ca(2+) channels. In addition, N/OFQ produced a consistent reduction in baseline Ca(2+) levels in presynaptic retinohypothalamic tract terminals. N/OFQ also inhibited evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSC) in a concentration dependent manner. However, N/OFQ had no effect on currents activated by muscimol application or on the amplitude of miniature IPSC (mIPSC) and significantly reduced the mIPSC frequency consistent with an inhibition of GABA release downstream from Ca(2+) entry. Finally, N/OFQ inhibited the paired-pulse depression observed in SCN GABAergic synapses consistent with a presynaptic mechanism of action. Together these results suggest a widespread modulatory role for N/OFQ on the synaptic transmission in the SCN.

Involvement of the retinohypothalamic tract in the photic-like effects of the serotonin agonist quipazine in the rat

Light is the major synchronizer of the mammalian circadian pacemaker located in the suprachiasmatic nucleus. Photic information is perceived by the retina and conveyed to the suprachiasmatic nucleus either directly by the retinohypothalamic tract or indirectly by the intergeniculate leaflet and the geniculohypothalamic tract. In addition, serotonin has been shown to affect the suprachiasmatic nucleus by both direct and indirect serotonin projections from the raphe nuclei. Indeed, systemic as well as local administrations of the serotonin agonist quipazine in the region of the suprachiasmatic nucleus mimic the effects of light on the circadian system of rats, i.e. they induce phase-advances of the locomotor activity rhythm as well as c-FOS expression in the suprachiasmatic nucleus during late subjective night. The aim of this study was to localize the site(s) of action mediating those effects. Phase shifts of the locomotor activity rhythm as well as c-FOS expression in the suprachiasmatic nucleus after s.c. injection of quipazine (10 mg/kg) were assessed in Lewis rats, which had received either radio-frequency lesions of the intergeniculate leaflet or infusions of the serotonin neurotoxin 5,7-dihydroxytryptamine into the suprachiasmatic nucleus (25 microg) or bilateral enucleation. Lesions of intergeniculate leaflet and serotonin afferents to the suprachiasmatic nucleus did not reduce the photic-like effects of quipazine, whereas bilateral enucleation and the subsequent degeneration of the retinohypothalamic tract abolished both the phase-shifting and the FOS-inducing effects of quipazine. The results indicate that photic-like effects of quipazine are mediated via the retinohypothalamic tract.

Pathophysiology of cerebral ischemia and brain trauma: similarities and differences

Current knowledge regarding the pathophysiology of cerebral ischemia and brain trauma indicates that similar mechanisms contribute to loss of cellular integrity and tissue destruction. Mechanisms of cell damage include excitotoxicity, oxidative stress, free radical production, apoptosis and inflammation. Genetic and gender factors have also been shown to be important mediators of pathomechanisms present in both injury settings. However, the fact that these injuries arise from different types of primary insults leads to diverse cellular vulnerability patterns as well as a spectrum of injury processes. Blunt head trauma produces shear forces that result in primary membrane damage to neuronal cell bodies, white matter structures and vascular beds as well as secondary injury mechanisms. Severe cerebral ischemic insults lead to metabolic stress, ionic perturbations, and a complex cascade of biochemical and molecular events ultimately causing neuronal death. Similarities in the pathogenesis of these cerebral injuries may indicate that therapeutic strategies protective following ischemia may also be beneficial after trauma. This review summarizes and contrasts injury mechanisms after ischemia and trauma and discusses neuroprotective strategies that target both types of injuries.

Serotoninergic afferents preferentially innervate distinct subclasses of peptidergic interneurons in the rat visual cortex

Although it is well documented that the non-pyramidal neurons of the cerebral cortex are under the influence of the vast serotoninergic input, the ultrastructural substrate for such functional interactions appears largely obscure. We sought to address this issue by dual immunoelectron microscopy, combining antibodies against serotonin (5-HT) and three neurochemical markers for peptidergic interneurons, namely somatostatin (SRIF), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP). The gold-substituted silver-peroxidase method was employed to intensify and differentiate the end-product of the peptide-immunoreaction from the non-intensified 5-HT fibers. Mainly the SRIF but also the NPY neurons were encountered among the postsynaptic targets of the 5-HT boutons. Recipients of synapses were perikarya and proximal dendrites of SRIF and NPY cells but also distal dendrites of the SRIF neurons. Neither synaptic relationships nor close appositions were ever identified between 5-HT boutons and VIP-immunoreactive elements. This remarkable synaptic preference/avoidance of 5-HT afferents for specific peptidergic subpopulations reveals a 'wired' component of cortical serotonin neurotransmission, which should be carefully interpreted within the frame of the available literature for extrasynaptic serotonin release.

MK-801 administration blocks the effects of a 5-HT(2A/2C) agonist on melatonin rhythmicity and c-fos induction in the suprachiasmatic nucleus

Both excitatory amino acids and serotonin have been implicated in the photic control of rhythms, but they have rarely been considered to interact. This study investigated the effects of the NMDA receptor antagonist, MK-801 on the phase shift of the melatonin rhythm and the induction of c-fos in the rat suprachiasmatic nucleus (SCN) provoked by the administration of the serotonin agonist DOI ((+/-)-1-(4-Iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride). The urinary excretion rate rhythm of the melatonin metabolite, 6-sulphatoxymelatonin was delayed by administration of DOI (0.5 mg/kg) at CT18 (6 h after subjective darkness onset) as previously reported by our group. Administration of MK-801 (3 mg/kg) 30 min before DOI blocked the shift in the onset of excretion of the melatonin metabolite on the following nights. Pre-treatment with MK-801 also inhibited by approximately 90% the induction of c-fos in the SCN by DOI at ZT18 (6 h after actual darkness onset) as determined by immunohistochemistry. These results provide evidence for a role of excitatory amino acids in the photomimetic effects of serotonin 5-HT(2C) agonists in the rat.

Levels of monoamine and amino acid neurotransmitters in the developing male mouse hypothalamus and in histotypic hypothalamic cultures

The variation in the levels of the monoamine and amino acid neurotransmitters was studied during the period of neurogenesis in male mouse hypothalamus, from embryonic day 15 until the age of young adult. The results shown in this study demonstrate that the monoamines appear early in the fetal brain and that the maximum expression of the catecholaminergic system, particularly that of dopamine, occurs during the late neonatal period or mouse infancy, when the role played by the catecholamines on the maturation of the neurosecretory systems is more significant. In relation to the amino acid neurotransmitters, glutamate and taurine seem to be the principal transmitters of the hypothalamus since their concentrations were about five-tenfold higher than the levels of glycine and GABA. Both amino acids had the same pattern of variation during development, showing elevated values during the prenatal, late neonatal and early pubertal period. Increased concentrations of the inhibitory neurotransmitter GABA were observed on the day before birth, at the end of the neonatal period and throughout the prepubertal period, suggesting that the influence of GABA on hypothalamic neurogenesis increases by the time when the hypothalamic nuclei have reached maturity and the local circuits have been established. To determine the intrinsic neurotransmitter production, primary hypothalamic histotypic cultures prepared from mice at postnatal days 8-10 were analyzed for their content of neurotransmitters. The in vitro analysis revealed that the hypothalamic neurons intrinsically produce dopamine, glutamate, taurine and glycine in homologable amounts with those of young adult animals. The comparative analysis also showed that about 50% of the GABA content and less than 5% of the hypothalamic epinephrine level are locally produced, while serotonin comes mainly from extrinsically located neurons.

Mutation of the prion protein gene at codon 208 in familial Creutzfeldt-Jakob disease

Four point mutations and one insertion within the prion protein (PrP) gene have been tightly linked to the development of inherited prion disease. We developed a denaturing gradient gel electrophoresis system that allowed us to screen the entire open reading frame of the PrP gene. Using this system, we found a new mutation of the PrP gene in a patient with pathologically confirmed Creutzfeldt-Jakob disease and a negative family history for dementia. DNA sequencing revealed an adenine substitution for guanine at the second position of codon 208, which results in the nonconservative substitution of histidine for arginine. The same PrP mutation was identified in another younger member of the pedigree but was not present in more than 200 alleles tested. Such findings suggest that the frequency of inherited prion disease might be higher than ascertained by clinical history alone.

Effects of natriuretic peptides and dopamine on single-unit activity of dorsomedial arcuate neurons in rat brain slices

The effects of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) on single-unit activity of dorsomedial arcuate (DM-ARC) neurons were reported. The modulatory effect of CNP on dopamine's (DA) action was also studied. ANP alone in 0.05-0.5 nmol doses induced 26% inhibition and 14% excitation of 37 DM-ARC neurons; the majority (60%) were not responsive. CNP, however, inhibited 46% and excited 4% of 74 DM-ARC neurons. Dose-dependent inhibitory effects of CNP were also observed. In 71 neurons tested with both CNP and DA, more neurons were inhibited by DA (66%) than those by CNP (46%). About one-third (34%) of them were inhibited by both. Furthermore, in 41 neurons inhibited by DA, more than half (54%) of their responses were potentiated by co-administration of CNP. In conclusion, CNP by itself exhibited a predominantly inhibitory action on DM-ARC neurons; and it also potentiated the inhibitory effect of DA on these neurons.

Visible light induced changes in the immune response through an eye-brain mechanism (photoneuroimmunology)

The immune system is susceptible to a variety of stresses. Recent work in neuroimmunology has begun to define how mood alteration, stress, the seasons, and daily rhythms can have a profound effect on immune response through hormonal modifications. Central to these factors may be light through an eye-brain hormonal modulation. In adult primates, only visible light (400-700 nm) is received by the retina. This photic energy is then transduced and delivered to the visual cortex and by an alternative pathway to the suprachiasmatic nucleus (SCN). The SCN is a part of the hypothalamic region in the brain believed to direct circadian rhythm. Visible light exposure also modulates the pituitary and pineal gland which leads to neuroendocrine changes. Melatonin, norepinephrine and acetylcholine decrease with light activation, while cortisol, serotonin, gaba and dopamine levels increase. The synthesis of vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and neuropeptide Y (NPY) in rat SCN has been shown to be modified by light. These induced neuroendocrine changes can lead to alterations in mood and circadian rhythm. All of these neuroendocrine changes can lead to immune modulation. An alternative pathway for immune modulation by light is through the skin. Visible light (400-700 nm) can penetrate epidermal and dermal layers of the skin and may directly interact with circulating lymphocytes to modulate immune function. However, even in the presence of phototoxic agents such as eosin and rose bengal, visible light did not produce suppression of contact hypersensitivity with suppresser cells. In contrast to visible light, in vivo exposure to UV-B (280-320 nm) and UV-A (320-400 nm) radiation can only alter normal human immune function by a skin mediated response. Each UV subgroup (B, A) induces an immunosuppressive response but by differing mechanisms involving the regulation of differing interleukins and growth factors. Some effects observed in humans are: inhibition of allergic contact dermatitis; inhibition of delayed hypersensitivity to an injected antigen; prolongation of skin-graft survival and induction of a tumor-susceptible state. The following article will review much of the progress in this field and explore possible areas of future research.

Differential effects of angiotensin II on the activities of suprachiasmatic neurons in rat brain slices

The effects of angiotensin II (AII) on the firing rates of suprachiasmatic neurons were determined in rat brain slices. AII in pmol ranges stimulated 25% and inhibited another 25% of 52 irregular firing neurons, while it stimulated 23% and inhibited 4% of 30 regular firing neurons. Three "oscillating" neurons whose firing rates oscillated with rather constant amplitudes and periods were recorded. AII induced the occurrence of oscillation in one unit and modulated the oscillation amplitude of the other two. Pretreatment with saralasin, an AII antagonist, effectively blocked (100%) the actions of AII (n = 5). The present findings suggest that AII may act as an important mediator in the suprachiasmatic nucleus and its mode of action may be variable in different neurons.

Neuroprotective activity of HU-211, a novel NMDA antagonist, in global ischemia in gerbils

HU-211, a nonpsychotropic cannabinoid and a noncompetitive NMDA antagonist, was tested in a global ischemia model in the Mongolian gerbil. Male Mongolian gerbils underwent a 10-min bilateral common carotid artery occlusion. HU-211, administered i.v. at 4 mg/kg, 30 min postischemia, induced statistically significant neuroprotection of the CA1 subfield of the hippocampus. A dose-response study demonstrated an inverted U curve in which the 4 mg/kg dose induced the best neuroprotection in the CA1 subfield of the hippocampus (p < 0.05 ANOVA followed by Duncan's post-hoc test). The therapeutic window was then investigated, and in another study, HU-211 4 mg/kg were administered i.v. at 30, 60, 120, and 180 min postinsult. A statistically significant neuroprotection was detected at 30 and 60 min administration postinsult.