Transduction of light in the suprachiasmatic nucleus: evidence for two different neurochemical cascades regulating the levels of Per1 mRNA and pineal melatonin

Antidepressants and Circadian Rhythm: Exploring Their Bidirectional Interaction for the Treatment of Depression

Scientific evidence that circadian rhythms affect pharmacokinetics and pharmacodynamics has highlighted the importance of drug dosing-time. Circadian oscillations alter drug absorption, distribution, metabolism, and excretion (ADME) as well as intracellular signaling systems, target molecules (e.g., receptors, transporters, and enzymes), and gene transcription. Although several antidepressant drugs are clinically available, less than 50% of depressed patients respond to first-line pharmacological treatments. Chronotherapeutic approaches to enhance the effectiveness of antidepressants are not completely known. Even so, experimental results found until this day suggest a positive influence of drug dosing-time on the efficacy of depression therapy. On the other hand, antidepressants have also demonstrated to modulate circadian rhythmicity and sleep-wake cycles. This review aims to evidence the potential of chronotherapy to improve the efficacy and/or safety of antidepressants. It includes pre-clinical and clinical studies that demonstrate the relevance of determining the most appropriate time of administration for antidepressant drugs. In parallel, their positive influence on the resynchronization of disrupted circadian rhythms is also herein discussed. It is expected that this review will promote the investigation of chronotherapy for the treatment of depression, contribute to a better understanding of the relationship between antidepressants and circadian rhythms, and consequently promote the development of new therapeutics.

Baseline insomnia as a predictor of antidepressant efficacy to repeated intravenous ketamine for unipolar and bipolar depression: A preliminary study

OBJECTIVE

Ketamine has been demonstrated to have robust and rapid antidepressant effects, and few studies have focused on the relationship between insomnia and the efficacy of ketamine. The objective of this study was to examine whether baseline insomnia predicted the antidepressant efficacy of repeated intravenous ketamine infusions for unipolar and bipolar depression.

METHOD

Patients with high insomnia (n = 64) or low insomnia (n = 68) received six intravenous infusions of ketamine (0.5 mg/kg over 40 min) over 12 days (Monday-Wednesday-Friday). The Montgomery-Asberg Depression Rating Scale (MADRS) without sleep item was used to assess depressive symptoms. Response was defined as a MADRS total score ≥ 50%, and remission was defined as a MADRS total score ≤ 10.

RESULT

There were no differences in response or remission rates between patients with high and low insomnia. However, the logistic regression model showed that high insomnia predicted an increased likelihood of response and remission. Cox proportional hazards models showed a reduced latency to respond and remit in patients with high insomnia. A linear mixed model showed that the high insomnia subgroup had greater improvement than the low insomnia subgroup (all p < 0.05).

LIMITATION

The major limitation of this study is the open-label design.

CONCLUSION

When given six ketamine infusions, patients with high insomnia were more likely to respond and remit than those with low insomnia. Patients with high insomnia showed not only a shorter latency to respond and remit, but also greater improvement than those with low insomnia.

Functional decoupling of melatonin suppression and circadian phase resetting in humans

KEY POINTS

There is assumed to be a monotonic association between melatonin suppression and circadian phase resetting induced by light exposure. We tested the association between melatonin suppression and phase resetting in humans. Sixteen young healthy participants received nocturnal bright light (∼9500 lux) exposure of continuous or intermittent patterns, and different durations ranging from 12 min to 6.5 h. Intermittent exposure patterns showed significant phase shifts with disproportionately less melatonin suppression. Each and every bright light stimulus in an intermittent exposure pattern induced a similar degree of melatonin suppression, but did not appear to cause an equal magnitude of phase shift. These results suggest that phase shifts and melatonin suppression are functionally independent such that one cannot be used as a proxy measure of the other.

ABSTRACT

Continuous experimental light exposures show that, in general, the conditions that produce greater melatonin suppression also produce greater phase shift, leading to the assumption that one can be used as a proxy for the other. We tested this association in 16 healthy individuals who participated in a 9-day inpatient protocol by assessing melatonin suppression and phase resetting in response to a nocturnal light exposure (LE) of different patterns: (i) dim-light control (<3 lux; n = 6) or (ii) two 12-min intermittent bright light pulses (IBL) separated by 36 min of darkness (∼9500 lux; n = 10). We compared these results with historical data from additional LE patterns: (i) dim-light control (<3 lux; n = 11); (ii) single continuous bright light exposure of 12 min (n = 9), 1.0 h (n = 10) or 6.5 h (n = 6); or (iii) an IBL light pattern consisting of six 15-min pulses with 1.0 h dim-light recovery intervals between them during a total of 6.5 h (n = 7). All light exposure groups had significantly greater phase-delay shifts than the dim-light control condition (P < 0.0001). While a monotonic association between melatonin suppression and circadian phase shift was observed, intermittent exposure patterns showed significant phase shifts with disproportionately less melatonin suppression. Each and every IBL stimulus induced a similar degree of melatonin suppression, but did not appear to cause an equal magnitude of phase shift. These results suggest unique specificities in how light-induced phase shifts and melatonin suppression are mediated such that one cannot be used as a proxy measure of the other.

Increased photic sensitivity for phase resetting but not melatonin suppression in Siberian hamsters under short photoperiods

Light regulates a variety of behavioral and physiological processes, including activity rhythms and hormone secretory patterns. Seasonal changes in the proportion of light in a day (photoperiod) further modulate those functions. Recently, short (SP) versus long days (LP) were found to markedly increase light sensitivity for phase shifting in Syrian hamsters. To our knowledge, photoperiod effects on light sensitivity have not been studied in other rodents, nor is it known if they generalize to other circadian responses. We tested whether photic phase shifting and melatonin suppression vary in Siberian hamsters maintained under LP or SP. Select irradiances of light were administered, and shifts in activity were determined. Photic sensitivity for melatonin suppression was examined in a separate group of animals via pulses of light across a 4 log-unit photon density range, with post-pulse plasma melatonin levels determined via RIA. Phase shifting and melatonin suppression were greater at higher irradiances for both LP and SP. The lower irradiance condition was below threshold for phase shifts in LP but not SP. Melatonin suppression did not vary by photoperiod, and the half saturation constant for fitted sigmoid curves was similar under LP and SP. Thus, the photoperiodic modulation of light sensitivity for phase shifting is conserved across two hamster genera. The dissociation of photoperiod effects on photic phase shifting and melatonin suppression suggests that the modulation of sensitivity occurs downstream of the common retinal input pathway. Understanding the mechanistic basis for this plasticity may yield therapeutic targets for optimizing light therapy practices.

Rapid-acting antidepressant strategies: mechanisms of action

Current antidepressants are ineffective in many depressed patients. Thus there is an urgent need to develop treatment strategies which have significantly faster response, can be sustained and have minimal side-effects. This paper reviews clinical data, potential biomarkers, mechanisms of action and future research directions for two proven strategies that produce marked improvement in severe depressive symptoms within 48 h, ketamine and sleep deprivation therapy (SDT). These treatments provide unequivocal evidence that the depressive process can be rapidly reversed in a subgroup of patients. Seventeen ketamine studies in over 150 patients showed a rapid response. Low-dose intravenous ketamine produced mild psychotomimetic effects but response has not been effectively sustained. SDT has been investigated in over 60 studies with a 40-60% response rate within 48 h. Although SDT is often used in Europe to initiate a rapid response, it is less utilized within the USA, in part, because it has a short duration when administered alone. We review data concerning chronotherapeutic strategies of bright-light therapy (BLT) and sleep-phase advance (SPA) which successfully sustain the antidepressant efficacy of SDT. Evidence is further discussed that a significant group of mood disorders have abnormal circadian rhythms which are known to be controlled by clock genes. It is hypothesized that chronotherapeutic manipulations can reset clock genes and thus, abnormalities in circadian rhythms. Further findings are reviewed that ketamine, in addition to its role as an NMDA antagonist, can also alter circadian rhythms. Thus, ketamine may share a critical mechanism with SDT.

Ketamine influences CLOCK:BMAL1 function leading to altered circadian gene expression

Major mood disorders have been linked to abnormalities in circadian rhythms, leading to disturbances in sleep, mood, temperature, and hormonal levels. We provide evidence that ketamine, a drug with rapid antidepressant effects, influences the function of the circadian molecular machinery. Ketamine modulates CLOCK:BMAL1-mediated transcriptional activation when these regulators are ectopically expressed in NG108-15 neuronal cells. Inhibition occurs in a dose-dependent manner and is attenuated after treatment with the GSK3β antagonist SB21673. We analyzed the effect of ketamine on circadian gene expression and observed a dose-dependent reduction in the amplitude of circadian transcription of the Bmal1, Per2, and Cry1 genes. Finally, chromatin-immunoprecipitation analyses revealed that ketamine altered the recruitment of the CLOCK:BMAL1 complex on circadian promoters in a time-dependent manner. Our results reveal a yet unsuspected molecular mode of action of ketamine and thereby may suggest possible pharmacological antidepressant strategies.

Activation of AMPA receptors in the suprachiasmatic nucleus phase-shifts the mouse circadian clock in vivo and in vitro

The glutamatergic neurotransmission in the suprachiasmatic nucleus (SCN) plays a central role in the entrainment of the circadian rhythms to environmental light-dark cycles. Although the glutamatergic effect operating via NMDAR (N-methyl D-aspartate receptor) is well elucidated, much less is known about a role of AMPAR (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor) in circadian entrainment. Here we show that, in the mouse SCN, GluR2 and GluR4 AMPAR subtypes are abundantly expressed in the retinorecipient area. In vivo microinjection of AMPA in the SCN during the early subjective night phase-delays the behavioral rhythm. In the organotypic SCN slice culture, AMPA application induces phase-dependent phase-shifts of core-clock gene transcription rhythms. These data demonstrate that activation of AMPAR is capable of phase-shifting the circadian clock both in vivo and in vitro, and are consistent with the hypothesis that activation of AMPA receptors is a critical step in the transmission of photic information to the SCN.

Stoichiometry of N-methyl-D-aspartate receptors within the suprachiasmatic nucleus

The circadian pacemaker within the suprachiasmatic nucleus (SCN) confers daily rhythms to bodily functions. In nature, the circadian clock will adopt a 24-h period by synchronizing to the solar light/dark cycle. This light entrainment process is mediated, in part, at glutamatergic synapses formed between retinal ganglion afferents and SCN neurons. N-methyl-D-aspartate receptors (NMDARs) located on SCN neurons gate light-induced phase resetting. Despite their importance in circadian physiology, little is known about their functional stoichiometry. We investigated the NR2-subunit composition with whole cell recordings of SCN neurons within the murine hypothalamic brain slice using a combination of subtype-selective NMDAR antagonists and voltage-clamp protocols. We found that extracellular magnesium ([Mg](o)) strongly blocks SCN NMDARs exhibiting affinities and voltage sensitivities associated with NR2A and NR2B subunits. These NMDAR currents were inhibited strongly by NR2B-selective antagonists, Ro 25-6981 (3.5 microM, 55.0 +/- 9.0% block; mean +/- SE) and ifenprodil (10 microM, 55.8 +/- 3.0% block). The current remaining showed decreased [Mg](o) affinities reminiscent of NR2C and NR2D subunits but was highly sensitive to [Zn](o), a potent NR2A blocker, showing a approximately 44.2 +/- 1.1% maximal inhibition at saturating concentrations with an IC(50) of 7.8 +/- 1.1 nM. Considering the selectivity, efficacy, and potency of the drugs used in combination with [Mg](o)-block characteristics of the NMDAR, our data show that both diheteromeric NR2B NMDARs and triheteromeric NR2A NMDARs (paired with an NR2C or NR2D subunits) account for the vast majority of the NMDAR current within the SCN.

The role of retinal photoreceptors in the regulation of circadian rhythms

The circadian clock is an evolutionarily, highly conserved feature of most organisms. This internal timing mechanism coordinates biochemical, physiological and behavioral processes to maintain synchrony with the environmental cycles of light, temperature and nutrients. Several studies have shown that light is the most potent cue used by most organisms (humans included) to synchronize daily activities. In mammals, light perception occurs only in the retina; three different types of photoreceptors are present within this tissue: cones, rods and the newly discovered intrinsically photosensitive retinal ganglion cells (ipRGCs). Researchers believe that the classical photoreceptors (e.g., the rods and the cones) are responsible for the image-forming vision, whereas the ipRGCs play a key role in the non-image forming vision. This non-image-forming photoreceptive system communicates not only with the master circadian pacemaker located in the suprachiasmatic nuclei of the hypothalamus, but also with many other brain areas that are known to be involved in the regulation of several functions; thus, this non-image forming system may also affect several aspects of mammalian health independently from the circadian system.

Light Entrainment of the Mammalian Circadian Clock by a PRKCA-Dependent Posttranslational Mechanism

Light is the most potent stimulus for synchronizing endogenous circadian rhythms with external time. Photic clock resetting in mammals involves cAMP-responsive element binding protein (CREB)-mediated transcriptional activation of Period clock genes in the suprachiasmatic nuclei (SCN). Here we provide evidence for an additional photic input pathway to the mammalian circadian clock based on Protein Kinase C alpha (PRKCA). We found that Prkca-deficient mice show an impairment of light-mediated clock resetting. In the SCN of wild-type mice, light exposure evokes a transient interaction between PRKCA and PERIOD 2 (PER2) proteins that affects PER2 stability and nucleocytoplasmic distribution. These posttranslational events, together with CREB-mediated transcriptional regulation, are key factors in the molecular mechanism of photic clock resetting.

Light and GABAAreceptor activation alterPeriodmRNA levels in the SCN of diurnal Nile grass rats

We examined Period (Per) mRNA rhythms in the suprachiasmatic nucleus (SCN) of a diurnal rodent and assessed how phase-shifting stimuli acutely affect SCN Per mRNA using semiquantitative in situ hybridization. First, Per1 and Per2 varied rhythmically in the SCN over the course of one circadian cycle in constant darkness: Per1 mRNA was highest in the early to mid-subjective day, while Per2 mRNA levels peaked in the late subjective day. Second, acute light exposure in the early subjective night significantly increased both Per1 and Per2 mRNA. Third, Per2 but not Per1 levels decreased 1 and 2 h after injection of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol into the SCN during the subjective day. Fourth, muscimol also reduced the light-induced Per2 in the early subjective night, but Per1 induction by light was not significantly affected. Consistent with previous studies, these data demonstrate that diurnal and nocturnal animals show very similar daily patterns of Per mRNA and light-induced Per increases in the SCN. As with light, muscimol alters circadian phase, and daytime phase alterations induced by muscimol are associated with significant decreases in Per2 mRNA. In diurnal animals, muscimol-induced decreases in Per are associated with phase delays rather than advances. The direction of the daytime phase shift may be determined by the relative suppression of Per1 vs. Per2 in SCN cells. As in nocturnal animals, changes in Per1 and Per2 mRNA by photic and non-photic stimuli appear to be associated with circadian phase alteration.

Paradoxical effects of NPY in the suprachiasmatic nucleus

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

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Effect of long-term exposure to constant dim light on the circadian system of rats

The newly discovered multi-oscillatory nature of the mammalian circadian clock system and the cloning of the genes involved in the molecular mechanism that generates circadian rhythmicity have opened new approaches for understanding how mammals are temporally organized and how the mammalian circadian system reacts to the lack of normal synchronization cues. In the present study we investigated the effects of long-term exposure to constant red dim light on the pattern of the expression of Period 1 in the suprachiasmatic nuclei of the hypothalamus and of Arylalkylamine N-acetyltransferase(Aa-nat) in the retina and pineal gland. Our data demonstrate that Period 1 mRNA expression in the suprachiasmatic nuclei of the hypothalamus was not affected by exposure to constant red dim light for 60 days, whereas Aa-nat mRNA expression in the retina and in the pineal gland was significantly affected, since in some animals (20-30%) Aa-nat mRNA levels were found to be higher during the subjective day. A circadian rhythm of serum melatonin and locomotor activity was present in all the animals tested. In 4 animals serum melatonin levels were high during the subjective day. Our data suggest that long-term exposure to constant red dim light may induce desynchronization between the circadian rhythm of locomotor activity and serum melatonin levels.

AMPA/kainate receptor antagonist DNQX blocks the acute increase of Per2 mRNA levels in most but not all areas of the SCN

The daily light:dark cycle synchronizes the circadian timing system by resetting the phase of the circadian pacemaker on a daily basis. Light acutely increases mRNA levels of the clock genes Per1 and Per2 in the suprachiasmatic nucleus (SCN), the site of the primary circadian pacemaker in mammals. Light is conveyed to the SCN through the retinohypothalamic tract (RHT), an efferent projection from retinal ganglion cells that releases the excitatory amino acid (EAA) neurotransmitter glutamate in the SCN. EAA receptor activation in the SCN is critical for the ability of light to phase-shift the circadian pacemaker. In a previous study, we demonstrated that EAA receptor activation is necessary and sufficient for light to acutely increase Per1 mRNA levels in the SCN. In the current study, we determined whether EAA receptor activation in the SCN is necessary for the ability of light to increase Per2 mRNA levels in the SCN in Syrian hamsters. The NMDA receptor antagonist AP5 and the AMPA/kainate receptor antagonist DNQX inhibited the ability of light and NMDA to acutely increase Per2 mRNA levels in the SCN. In hamsters injected with DNQX, Per1 and Per2 mRNA levels remained slightly elevated in the ventrolateral SCN, suggesting that AMPA/kainate receptor activation in this region is not critical for the effects of light on the circadian pacemaker.

The suprachiasmatic nucleus is a functionally heterogeneous timekeeping organ

Ever since the locus of the brain clock in the suprachiasmatic nucleus (SCN) was first described, methods available have both enabled and encumbered our understanding of its nature at the level of the cell, the tissue, and the animal. A combination of in vitro and in vivo approaches has shown that the SCN is a complex heterogeneous neuronal network. The nucleus is composed of cells that are retinorecipient and reset by photic input; those that are reset by nonphotic inputs; slave oscillators that are rhythmic only in the presence of the retinohypothalamic tract; endogenously rhythmic cells, with diverse period, phase, and amplitude responses; and cells that do not oscillate, at least on some measures. Network aspects of SCN organization are currently being revealed, but mapping these properties onto cellular characteristics of electrical responses and patterns of gene expression are in early stages. While previous mathematical models focused on properties of uniform coupled oscillators, newer models of the SCN as a brain clock now incorporate oscillator and gated, nonoscillator elements.

Biological Rhythms, Medication Safety, and Women's Health

Biological processes and functions in women are well organized in time, as evidenced by the expression of ultradian (high frequency), circadian ( approximately 24-hour), circamensual ( approximately monthly), and circannual ( approximately yearly) rhythms and by the changes that occur with menarche, reproduction, and menopause. Attributes of women's circamensual structure have been explored in depth, particularly with regard to fertility/infertility and birth control. However, the role of 24-hour and other rhythms in health, disease, and treatment has been little studied. The symptom intensity of a variety of chronic medical conditions is rhythmic, as is the risk of severe events, such as stroke and myocardial infarct (MI). Improving the safety, efficacy, and preventive qualities of medications requires the understanding of how rhythms impact drug pharmacokinetics and pharmacodynamics. The therapeutic and adverse effects of prescription and nonprescription medications widely used by women can vary markedly with the (circadian) time of administration. Circadian rhythm-dependent differences in the safety of medications are particularly relevant to pregnant women; laboratory animal studies show that the fetal toxicity of various treatments varies not only with developmental stage but also with circadian time. Rhythm-dependent differences in the actions of medications are also of great importance to perimenopausal and postmenopausal women, who are advised to ingest prescribed pharmacotherapy for osteopenia and osteoporosis in the morning to minimize the risk of adverse effects and, as a consequence, may elect to take other medications at times not recommended in the instructions for their use. Medication trials must be comprehensive and representative of women and men of different life stages, ethnicities, and likely times (morning vs. evening) of drug use.

DNA binding, but not interaction with Bmal1, is responsible for DEC1-mediated transcription regulation of the circadian gene mPer1

DEC1 (differentially expressed in chondrocytes 1) and DEC2 are E-box-binding transcription factors and exhibit a circadian expression pattern. Recently, both proteins were found to repress the Clock/Bmal1-activated E-box promoters (e.g. mPer1). Yeast two-hybrid assay detected interactions between Bmal1 and DECs. It was hypothesized that DEC-mediated repression on the mPer1 promoter is achieved by binding to E-box elements and interacting with Bmal1. In the present study, we report that E-box binding rather than Bmal1 interaction is responsible for the observed repression. In the absence of Clock/Bmal1, both DEC1 and DEC2 markedly repressed the mPer1 promoter reporter; however, DNA-binding mutants showed no repressive activity. Similarly, DEC1, but not its DNA-binding mutants, repressed the Clock/Bmal1-induced activation. In addition, DEC1(R58P), a DNA-binding mutant with Bmal1 interactivity, repressed neither the mPer1 reporter directly nor the Clock/Bmal1-induced activation, providing direct evidence that DNA binding, rather than Bmal1 interactions, is responsible for the repression on the mPer1 promoter. Furthermore, disruption of the Sp1 site in the proximal promoter of mPer1 increased the repression of DEC1 proteins. Previous studies with mouse DEC2 showed that this factor interacts with Sp1. These findings suggest that DEC proteins regulate the expression of mPer1 through E-box binding and Sp1 interaction. Alterations on circadian systems are increasingly recognized as important risk factors for disease initiation and progression, and the expression of Dec genes is rapidly induced by environmental stimuli and is highly increased in tumour tissues. Therefore de-regulated expression of DEC genes probably alters normal circadian rhythms and contributes significantly to the pathogenesis of many diseases including cancer.

Tetrodotoxin administration in the suprachiasmatic nucleus prevents NMDA-induced reductions in pineal melatonin without influencing Per1 and Per2 mRNA levels

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains a light-entrainable circadian pacemaker. Neurons in the SCN are part of a circuit that conveys light information from retinal efferents to the pineal gland. Light presented during the night acutely increases mRNA levels of the circadian clock genes Per1 and Per2 in the SCN, and acutely suppresses melatonin levels in the pineal gland. The present study investigated whether the ability of light to increase Per1 and Per2 mRNA levels and suppress pineal melatonin levels requires sodium-dependent action potentials in the SCN. Per1 and Per2 mRNA levels in the SCN and pineal melatonin levels were measured in Syrian hamsters injected with tetrodotoxin (TTX) prior to light exposure or injection of N-methyl-D-aspartate (NMDA). TTX inhibited the ability of light to increase Per1 and Per2 mRNA levels and suppress pineal melatonin levels. TTX did not, however, influence the ability of NMDA to increase Per1 and Per2 mRNA levels, though it did inhibit the ability of NMDA to suppress pineal melatonin levels. These results demonstrate that action potentials in the SCN are not necessary for NMDA receptor activation to increase Per1 and Per2 mRNA levels, but are necessary for NMDA receptor activation to decrease pineal melatonin levels. Taken together, these data support the hypothesis that the mechanism through which light information is conveyed to the pacemaker in the SCN is separate from and independent of the mechanism through which light information is conveyed to the SCN cells whose efferents suppress pineal melatonin levels.

Neuropeptide y and n-methyl-d-aspartic acid interact within the suprachiasmatic nuclei to alter circadian phase

Circadian rhythms are reset by exposure to photic stimuli and nonphotic stimuli. Glutamate appears to be the primary neurotransmitter that communicates photic stimuli to the circadian clock located in the suprachiasmatic nucleus. There is also substantial evidence that neuropeptide Y (NPY) mediates the effects of at least some nonphotic stimuli on the circadian clock. The purpose of this study was to investigate how NPY and glutamate receptor activation interact to reset the phase of the circadian clock. Microinjection of the glutamate agonist N-methyl-D-aspartic acid (NMDA) during the subjective day significantly decreased NPY-induced phase advances. During the late subjective night, NMDA induced light-like phase advances, which were significantly reduced by microinjection of NPY. Microinjection of NPY inhibited NMDA-induced phase advances during the late subjective night, even when sodium-dependent action potentials were inhibited by tetrodotoxin. These data support the hypothesis that, during the subjective night, NPY and NMDA act on the same clock cells or on cells that communicate with clock cells by mechanisms not requiring action potentials. Although NPY and NMDA appear to be mutually inhibitory during both the day and the night, the mechanisms of this inhibition appear to be different during the day versus the night.