Does the intergeniculate leaflet play a role in the integration of the photoperiod by the suprachiasmatic nucleus?

Anxiolytic effects of vestibular stimulation: an update

The vestibular system inhibits both HPA and SAM axis and contributes to the management of anxiety. Both direct and indirect pathways exist in the inhibition of the HPA and SAM axis. In this review article, the authors describe various pathways through which the vestibular system can regulate the HPA and SAM axis activity. Lastly, the authors highlight the need of starting translational research work in this field. Rocking is soothing and this is a universal fact that babies in the swing will calm down and sleep. These soothing effects of vestibular stimulation may be due to the inhibition of cortical and subcortical structures. Vestibular stimulation may be able to manage anxiety through its connections with multiple brain areas. There is a need to undertake translational research in this area to establish strong scientific evidence and recommend implementation of the vestibular stimulation in the management of anxiety.

Seasonal Variation in the Brain μ-Opioid Receptor Availability

Seasonal rhythms influence mood and sociability. The brain μ-opioid receptor (MOR) system modulates a multitude of seasonally varying socioemotional functions, but its seasonal variation remains elusive with no previously reported in vivo evidence. Here, we first conducted a cross-sectional study with previously acquired human [¹¹C]carfentanil PET imaging data (132 male and 72 female healthy subjects) to test whether there is seasonal variation in MOR availability. We then investigated experimentally whether seasonal variation in daylength causally influences brain MOR availability in rats. Rats (six male and three female rats) underwent daylength cycle simulating seasonal changes; control animals (two male and one female rats) were kept under constant daylength. Animals were scanned repeatedly with [¹¹C]carfentanil PET imaging. Seasonally varying daylength had an inverted U-shaped functional relationship with brain MOR availability in humans. Brain regions sensitive to daylength spanned the socioemotional brain circuits, where MOR availability peaked during spring. In rats, MOR availabilities in the brain neocortex, thalamus, and striatum peaked at intermediate daylength. Varying daylength also affected the weight gain and stress hormone levels. We conclude that cerebral MOR availability in humans and rats shows significant seasonal variation, which is predominately associated with seasonal photoperiodic variation. Given the intimate links between MOR signaling and socioemotional behavior, these results suggest that the MOR system might underlie seasonal variation in human mood and social behavior.SIGNIFICANCE STATEMENT Seasonal rhythms influence emotion and sociability. The central μ-opioid receptor (MOR) system modulates numerous seasonally varying socioemotional functions, but its seasonal variation remains elusive. Here we used positron emission tomography to show that MOR levels in both human and rat brains show daylength-dependent seasonal variation. The highest MOR availability was observed at intermediate daylengths. Given the intimate links between MOR signaling and socioemotional behavior, these results suggest that the MOR system might underlie seasonal variation in human mood and social behavior.

Modulation of sleep using electrical vestibular nerve stimulation prior to sleep onset: a pilot study

OBJECTIVES

Electrical stimulation of the vestibular system (VeNS) has been shown to improve Insomnia Severity Index (ISI) when delivered during sleep. We hypothesize that repeated electrical vestibular stimulation, when delivered prior to sleep onset, will improve ISI scores. The primary aim of this study was to assess the effect that VeNS had on ISI scores when delivered prior to sleep onset. A secondary aim was to provide initial data indicating "length of time to effect" that will allow more appropriate design of a larger randomized control trial (RCT).

METHODS

The present study was an experimental study (pre and post without control). The participants acted as self-controls. After recording the baseline values, electrical vestibular nerve stimulation was administered as intervention once in a day for 30 min, 1 h prior to sleep onset using ML1000 device (Neurovalens, UK) for 14 days.

RESULTS

There was significant decrease in the ISI scores followed by the electrical vestibular nerve stimulation. Further, participants reported a significant increase in well-rested sleep post the intervention period.

CONCLUSIONS

This study supports our hypothesis that VeNS has a positive impact on ISI scores when delivered on a regular basis prior to sleep onset.

Quantitative Study of Dual Circadian Oscillator Models under Different Skeleton Photoperiods

The daily proportion of light and dark hours (photoperiod) changes annually and plays an important role in the synchronization of seasonal biological phenomena, such as reproduction, hibernation, and migration. In mammals, the first step of photoperiod transduction occurs in the suprachiasmatic nuclei (SCN), the circadian pacemaker that also coordinates 24-h activity rhythms. Thus, in parallel with its role in annual synchronization, photoperiod variation acutely shapes day/night activity patterns, which vary throughout the year. Systematic studies of this behavioral modulation help understand the mechanisms behind its transduction at the SCN level. To explain how entrainment mechanisms could account for daily activity patterns under different photoperiods, Colin Pittendrigh and Serge Daan proposed a conceptual model in which the pacemaker would be composed of 2 coupled, evening (E) and morning (M), oscillators. Although the E-M model has existed for more than 40 years now, its physiological bases are still not fully resolved, and it has not been tested quantitatively under different photoperiods. To better explore the implications of the E-M model, we performed computer simulations of 2 coupled limit-cycle oscillators. Four model configurations were exposed to systematic variation of skeleton photoperiods, and the resulting daily activity patterns were assessed. The criterion for evaluating different model configurations was the successful reproduction of 2 key behavioral phenomena observed experimentally: activity psi-jumps and photoperiod-induced changes in activity phase duration. We compared configurations with either separate light inputs to E and M or the same light inputs to both oscillators. The former replicated experimental results closely, indicating that the configuration with separate E and M light inputs is the mechanism that best reproduces the effects of different skeleton photoperiods on day/night activity patterns. We hope this model can contribute to the search for E and M and their light input organization in the SCN.

The Suprachiasmatic Nucleus of the Dromedary Camel (Camelus dromedarius): Cytoarchitecture and Neurochemical Anatomy

In mammals, biological rhythms are driven by a master circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Recently, we have demonstrated that in the camel, the daily cycle of environmental temperature is able to entrain the master clock. This raises several questions about the structure and function of the SCN in this species. The current work is the first neuroanatomical investigation of the camel SCN. We carried out a cartography and cytoarchitectural study of the nucleus and then studied its cell types and chemical neuroanatomy. Relevant neuropeptides involved in the circadian system were investigated, including arginine-vasopressin (AVP), vasoactive intestinal polypeptide (VIP), met-enkephalin (Met-Enk), neuropeptide Y (NPY), as well as oxytocin (OT). The neurotransmitter serotonin (5-HT) and the enzymes tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) were also studied. The camel SCN is a large and elongated nucleus, extending rostrocaudally for 9.55 ± 0.10 mm. Based on histological and immunofluorescence findings, we subdivided the camel SCN into rostral/preoptic (rSCN), middle/main body (mSCN) and caudal/retrochiasmatic (cSCN) divisions. Among mammals, the rSCN is unusual and appears as an assembly of neurons that protrudes from the main mass of the hypothalamus. The mSCN exhibits the triangular shape described in rodents, while the cSCN is located in the retrochiasmatic area. As expected, VIP-immunoreactive (ir) neurons were observed in the ventral part of mSCN. AVP-ir neurons were located in the rSCN and mSCN. Results also showed the presence of OT-ir and TH-ir neurons which seem to be a peculiarity of the camel SCN. OT-ir neurons were either scattered or gathered in one isolated cluster, while TH-ir neurons constituted two defined populations, dorsal parvicellular and ventral magnocellular neurons, respectively. TH colocalized with VIP in some rSCN neurons. Moreover, a high density of Met-Enk-ir, 5-HT-ir and NPY-ir fibers were observed within the SCN. Both the cytoarchitecture and the distribution of neuropeptides are unusual in the camel SCN as compared to other mammals. The presence of OT and TH in the camel SCN suggests their role in the modulation of circadian rhythms and the adaptation to photic and non-photic cues under desert conditions.

Asymmetric Control of Short Day Response in European Hamsters

The European hamster ( Cricetus cricetus) is a circannual species in which the synchronization of the circannual cycle to the natural year occurs during 2 annual phases of sensitivity. Around the summer solstice, the animals are sensitive to a shortening of photoperiod. During this sensitive phase, pronounced changes in circadian output parameters are observed, indicating a different functional state of the circadian system. This special state is assumed to be necessary to develop the extreme sensitivity to short day length in European hamsters during this phase. In natural conditions, the animals are able to recognize the shortening of photoperiod already in mid-July, when the photoperiod is reduced only by 30 min. To investigate the short-day response in sensitive European hamsters on the basis of the 2-coupled oscillator model of Pittendrigh and Daan (1976), daily activity and the reproductive state of European hamsters were recorded after an asymmetrical reduction of photoperiod from long (LD 16:08) to short (LD 08:16) photoperiods. The activity pattern of the animals showed an immediate response to the short photoperiod at the day of transfer when the night was extended only into the evening, but there was a significant delay in the response time when the night was extended into the morning. Thus, the evening oscillator E is more important in inducing the photoperiodic response than the morning oscillator M. Moreover, the broad intragroup variation in the latter conditions strongly suggests that the changes in the activity pattern were endogenously induced and that the animals were not able to recognize a lengthening of the night into the morning. Gonadal regression started in both groups 3 weeks after the change in the activity pattern, indicating that this process is initiated when the circadian system has received the short-day signal either through changes in photoperiod or through the circannual clock.

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

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

Neuropeptide Y-deficient mice show altered circadian response to simulated natural photoperiod

Circadian rhythms, endogenously generated by the suprachiasmatic nucleus (SCN), can be synchronized to a variety of photic and non-photic environmental stimuli. Neuropeptide Y (NPY) is produced in the intergeniculate leaflet (IGL) and known to mediate both photic and non-photic influences on the SCN. We recently found that npy-/- mice were slower to shift their locomotor activity onset to the new time of light offset when photoperiod was abruptly changed from light/dark (LD) cycle 18:6 to LD 6:18. In the present study, we measured the locomotor response of npy-/- mice to gradual changes in photoperiod (4 min a day) for 141 days (LD 16:8 changing to LD 8:16), mimicking external LD cycles in nature. When the photoperiod approached LD 8:16, npy-/- mice showed a significantly delayed onset of activity compared to wild-type mice. Activity patterns disintegrated into multiple bouts and intensity of activity decreased as the photoperiod changed and these changes were more pronounced in npy-/- mice. Our results lend further support to the idea that NPY is involved in circadian entrainment responses to seasonal photoperiod changes.

Behavioral and neurochemical sources of variability of circadian period and phase: studies of circadian rhythms of npy-/- mice

The cycle length or period of the free-running rhythm is a key characteristic of circadian rhythms. In this study we verify prior reports that locomotor activity patterns and running wheel access can alter the circadian period, and we report that these treatments also increase variability of the circadian period between animals. We demonstrate that the loss of a neurochemical, neuropeptide Y (NPY), abolishes these influences and reduces the interindividual variability in clock period. These behavioral and environmental influences, from daily distribution of peak locomotor activity and from access to a running wheel, both act to push the mean circadian period to a value < 24 h. Magnitude of light-induced resetting is altered as well. When photoperiod was abruptly changed from a 18:6-h light-dark cycle (LD18:6) to LD6:18, mice deficient in NPY were slower to respond to the change in photoperiod by redistribution of their activity within the prolonged dark and eventually adopted a delayed phase angle of entrainment compared with controls. These results support the hypothesis that nonphotic influences on circadian period serve a useful function when animals must respond to abruptly changing photoperiods and point to the NPYergic pathway from the intergeniculate leaflet innervating the suprachiasmatic nucleus as a circuit mediating these effects.

Intergeniculate leaflet: contributions to photic and non-photic responsiveness of the hamster circadian system

The circadian visual system is able to integrate light energy over time, enabling phase response and Fos induction in the suprachiasmatic nucleus to increase in proportion to the total energy of the photic stimulus. In the present studies, the contribution of the intergeniculate leaflet to light energy integration by the hamster circadian rhythm system was evaluated. Fos protein is induced in intergeniculate leaflet neurons at much lower irradiance levels than seen in suprachiasmatic nucleus neurons. Bilateral N-methyl-d-aspartate lesions of the intergeniculate leaflet decreased phase response of the circadian locomotor rhythm to high irradiance and, in animals exposed to long duration light stimuli, reduced Fos induction in the suprachiasmatic nucleus. Normal photon integration, as indicated by attenuated rhythm phase shifts and Fos induction in suprachiasmatic nucleus cells in response to the energy in light stimuli, does not occur in the absence of the intergeniculate leaflet and is likely to be a property of the circadian rhythm system, rather than solely of the suprachiasmatic nucleus. Anatomical analysis showed that virtually no intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus contain Fos induced by either light or locomotion in a novel wheel. However, cells projecting to the pretectum were found to contain novel-wheel induced Fos. The intergeniculate leaflet is implicated in the normal assessment of light by the circadian rhythm system, but the circuitry by which either photic or non-photic information gains access to the suprachiasmatic nucleus may be more complex than previously thought.

Pinealectomy reduces optic nerve but not intergeniculate leaflet input to the suprachiasmatic nucleus at night

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates circadian rhythms in mammals. It receives, among others, direct inputs from the retina and from the thalamic intergeniculate leaflet (IGL). The former sends photic signals to the SCN, whereas the latter probably integrates photic and nonphotic information. To characterise these inputs in vivo, extracellular single-unit recordings were made from the SCN of rats under urethane anaesthesia during electrical stimulation of the optic nerve (OptN) or the IGL region. Cell responses were evaluated by creating peri-stimulus time histograms. Because humoral signals such as melatonin might modulate the activity of the SCN in addition to neural inputs, recordings were also made using pinealectomised (Px) rats to test for a possible role of this hormone in regulating inputs to the SCN. A significantly greater number of cells responded to IGL (60 of 90, 67%) than to OptN (35 of 75, 47%) stimulation in intact animals (chi(2) = 5.905, P = 0.015). The same was true when Px animals were tested (IGL, 82 of 131, 63%; OptN, 31 of 111, 28%; chi(2) = 27.637, P < 0.001). In intact animals, the proportion of cells responsive to IGL stimulation during the day and during the night was not significantly different from the proportion responsive in Px animals. The same was true for OptN stimulation during the day. However, during the night, the proportion of cells responsive to OptN stimulation in intact animals was significantly greater than the proportion responsive in Px animals (chi(2) = 7.127, P = 0.008). Our findings suggest that a lack of melatonin modulates OptN but not IGL inputs to the SCN.

Conflicting effects of exercise on the establishment of a short-photoperiod phenotype in Syrian hamster

In the Syrian hamster, winter seasonal inhibition of reproduction occurs in response to decreasing day length. This inhibitory response is modulated by nonphotic cues. In particular, access to a running wheel has been shown to produce incomplete gonadal regression. The present study sought to determine whether this occurs as a consequence of wheel effect on adaptation of the circadian system to short days or whether downstream physiological responses are involved. Short-day adaptation of the circadian clock, which is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, was tested by lengthening the photosensitive phase of the SCN (assayed by light-induced c-Fos expression in the SCN) as a parameter. We found that wheel-running activity does not inhibit the integration of the photoperiodic change by the SCN even if complete testicular regression is prevented. Moreover, this exercise was even capable of accelerating the lengthening of the photosensitive phase after the transfer to short day length. Thus, although wheel-running activity inhibits the short photoperiod-induced gonadal regression, it acts on the SCN to accelerate the integration of the photoperiodic change by the biological clock.

The thalamic intergeniculate leaflet modulates photoperiod responsiveness in Siberian hamsters

Siberian hamsters are seasonal breeders that use changes in day length to synchronize their reproductive effort with those times of the year most favorable for successful reproduction. The ability of Siberian hamsters to measure and respond to changes in day length depends upon accurate photoentrainment of the circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. Two pathways have been characterized through which entraining stimuli reach the SCN: the retinohypothalamic tract (RHT), which transmits light information from the retinae, and the geniculohypothalamic tract (GHT) from the intergeniculate leaflet of the thalamus (IGL), which is involved in transmitting both photic and nonphotic cues. Ablating the IGL/GHT results in only modest alterations in entrainment to static day lengths and fails to interfere with seasonal responses induced by transfer from static long day to static short day lengths. Because several studies suggest that the IGL may be involved in tracking the time of dusk and dawn, we sought to determine whether an intact IGL is necessary for hamsters to respond to a simulated natural photoperiod (SNP) in which the time of dusk and dawn gradually changes in a pattern approximating the rate of change in day length that occurs during autumn at the latitude this species inhabits in nature. The results indicate that neurochemical lesions of the IGL alter both the pattern of circadian entrainment and photoperiodic responsiveness of Siberian hamsters to an SNP. Both intact and IGL-lesioned hamsters exhibited testicular regression in shortening day lengths, but only IGL-intact hamsters exhibited seasonal pelage molt.

Inhibition of hibernation by exercise is not affected by intergeniculate leaflets lesion in hamsters

The circadian clock of mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, has been demonstrated to integrate day length change from long (LP) to short photoperiod (SP). This photoperiodic change induces in Syrian hamsters a testicular regression through melatonin action, a phenomenon that is inhibited when hamsters have free access to a wheel. The intergeniculate leaflets (IGL), which modulate the integration of photoperiod by the SCN, are a key structure in the circadian system, conveying nonphotic information such as those induced by novelty-induced wheel running activity. We tested in hamsters transferred from LP to a cold SP the effects of wheel running activity on a photoperiod-dependent behavior, hibernation. Lesions of the IGL were done to test the role of this structure in the inhibition induced by exercise of photoperiod integration by the clock. We show that wheel running activity actually inhibits hibernation not only in sham-operated animals, but also in hamsters with a bilateral IGL lesion (IGLX). In contrast, IGL-X hamsters without a wheel integrate slower to the SP but hibernate earlier compared with sham-operated animals. Moreover, some hibernation characteristics are affected by IGL lesion. Throughout the experiment at 7 degrees C, IGL-X hamsters were in hypothermia during 18% of the experiment vs. 32% for sham-operated hamsters. Taken together, these data show that the IGL play a modulatory role in the integration of photoperiodic cues and modulate hibernation, but they are not implicated in the inhibition of hibernation induced by wheel running activity.

Loss of circadian rhythmicity in aging mPer1-/-mCry2-/- mutant mice

The mPer1, mPer2, mCry1, and mCry2 genes play a central role in the molecular mechanism driving the central pacemaker of the mammalian circadian clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus. In vitro studies suggest a close interaction of all mPER and mCRY proteins. We investigated mPER and mCRY interactions in vivo by generating different combinations of mPer/mCry double-mutant mice. We previously showed that mCry2 acts as a nonallelic suppressor of mPer2 in the core clock mechanism. Here, we focus on the circadian phenotypes of mPer1/mCry double-mutant animals and find a decay of the clock with age in mPer1-/- mCry2-/- mice at the behavioral and the molecular levels. Our findings indicate that complexes consisting of different combinations of mPER and mCRY proteins are not redundant in vivo and have different potentials in transcriptional regulation in the system of autoregulatory feedback loops driving the circadian clock.

Testosterone-dependent and -independent mechanisms involved in the photoperiodic control of neuropeptide levels in the brain of the jerboa (Jaculus orientalis)

Vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) content in the suprachiasmatic nucleus have been shown to exhibit seasonal changes with an increase in late summer, the period of sexual quiescence in the jerboa (Jaculus orientalis). In this study, VIP content in the SCN and NPY and enkephaline (ENK) content in the geniculo-suprachiasmatic system have been assayed in wild-caught male jerboas (Jaculus orientalis) in order to determine whether these neuropeptides are controlled directly by photoperiod changes or indirectly by short photoperiod induced changes in circulating sex hormones levels. In agreement with previous studies seasonal variations occur in the VIP and NPY content in the SCN. Variations also occur in NPY content in the IGL with an increase in the period of sexual quiescence. In contrast, no seasonal changes are observed in Enk content in the IGL or the SCN. In short photoperiod conditions increases are observed in both VIP and NPY content in the SCN as well as NPY content in the IGL. Castration during the period of sexual activity (spring) or under long photoperiod which drastically reduces testosterone, also induced an increase in the levels of these neuropeptides. Testosterone implants which reproduce the sex hormonal status of the sexual activity period failed to prevent the short photoperiod-induced increase of VIP and NPY in the SCN and of NPY in the IGL. These results clearly show that the photoperiod modulates VIP and NPY in the geniculo-suprachiasmatic system both by testosterone-linked and testosterone-independent mechanisms.

A switch from diurnal to nocturnal activity in S. ehrenbergi is accompanied by an uncoupling of light input and the circadian clock

The subterranean mole rat Spalax ehrenbergi superspecies represents an extreme example of adaptive visual and neuronal reorganization. Despite its total visual blindness, its daily activity rhythm is entrainable to light-dark cycles, indicating that it can confer light information to the clock. Although most individuals are active during the light phase under laboratory conditions (diurnal animals), some individuals switch their activity period to the night (nocturnal animals). Similar to other rodents, the Spalax circadian clock is driven by a set of clock genes, including the period (sPer) genes. In this work, we show that diurnal mole rats express the Per genes sPer1 and sPer2 with a peak during the light period. Light can synchronize sPer gene expression to an altered light-dark cycle and thereby reset the clock. In contrast, nocturnal Spalax express sPer2 in the dark period and sPer1 in a biphasic manner, with a light-dependent maximum during the day and a second light-independent maximum during the night. Although sPer1 expression remains light inducible, this is not sufficient to reset the molecular clockwork. Hence, the strict coupling of light, Per expression, and the circadian clock is lost. This indicates that Spalax can dissociate the light-driven resetting pathway from the central clock oscillator.

The circadian clock as a molecular calendar

There are two dominant environmental oscillators shaping the living conditions of our world: the day-night cycle and the succession of the seasons. Organisms have adapted to these by evolving internal clocks to anticipate these variations. An orchestra of finely tuned peripheral clocks slaved to the master pacemaker of the suprachiasmatic nuclei (SCN) synchronizes the body to the daily 24h cycle. However, this circadian clockwork closely interacts with the seasonal time-teller. Recent experiments indeed show that photoperiod--the dominant Zeitgeber of the circannual clock--might be deciphered by the organism using the tools of the circadian clock itself. From the SCN, the photoperiodic signal is transferred to the pineal where it is decoded as a varying secretion of melatonin. Different models have been proposed to explain the mechanism by which the circadian clock measures day-length. Recent work using mutant mice suggests a set of two molecular oscillators tracking dusk and dawn, respectively, thereby translating day-length to the body. However, not every aspect of photoperiodism is covered by this theory and majoradjustments will need tobe made toestablish a widely acceptable uniform model of circadian/circannual timekeeping.

Encoding le quattro stagioni within the mammalian brain: photoperiodic orchestration through the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) is a pacemaker that not only drives circadian rhythmicity but also directs the circadian organization of photoperiodic (seasonal) timekeeping. Recent evidence using electrophysiological, molecular, and genetic tools now strongly supports this conclusion. Important questions remain regarding the SCN's precise role(s) in the brain's photoperiodic circuits, especially among different species, and the cellular and molecular mechanisms for its photoperiodic "memory." New data suggesting that SCN "clock" genes may also function as "calendar" genes are a first step toward understanding how a photoperiodic clock is built from cycling molecules.

Intergeniculate leaflets lesion delays but does not prevent the integration of photoperiodic change by the suprachiasmatic nuclei

The duration of the photosensitive phase of the suprachiasmatic nuclei (SCN), as revealed by light-induced Fos protein expression, depends on the photoperiod and is tied to the length of the night. We show here in Syrian hamsters that after a transfer from long to short photoperiod, lengthening of the photosensitive phase of the SCN is significantly delayed but not abolished when the intergeniculate leaflets (IGL) are lesioned. Thus IGL modulate the integration by the SCN of a photoperiodic change.

Orexin A-like immunoreactivity in the hypothalamus and thalamus of the Syrian hamster (Mesocricetus auratus) and Siberian hamster (Phodopus sungorus), with special reference to circadian structures

The orexins are recently discovered neuropeptides that reportedly play a role in energy homeostasis, in addition to various other physiological processes. The synthesis of orexin A undergoes diurnal variation in certain areas of the brain, while the mutation of the orexin receptor 2 gene has been implicated in canine narcolepsy. Since the circadian pacemaker in the suprachiasmatic nucleus modulates the sleep/wake cycle, there is a putative role for orexins in the mammalian circadian system. In this study, immunohistochemical techniques were used to determine the distribution of orexin A in the structures of the hypothalamus and thalamus of Syrian and Siberian hamsters. In both species, the pattern of immunoreactivity was similar. Cells immunoreactive for orexin A were noted in the lateral hypothalamic area. Immunoreactive varicose orexin A fibres were found throughout the hypothalamus. The suprachiasmatic nucleus possessed little or no immunoreactive orexin A fibres in its core, but had fibres at its periphery. The thalamus of both species contained comparatively few immunoreactive fibres, which were mainly localised around the midline. The thalamic intergeniculate leaflet contained a plexus of immunoreactive orexin A fibres throughout its rostro-caudal extent. Three areas of the brainstem, the dorsal and median raphe nuclei and the locus coeruleus, were also investigated owing to their relevance to the circadian system and all were found to contain immunoreactive orexin A fibres. The presence of orexin A-immunoreactive fibres in the neural architecture of the mammalian circadian system suggests an important role for orexin A in circadian timekeeping processes.

Neuropeptide Y phase advances the in vitro hamster circadian clock during the subjective day with no effect on phase during the subjective night

The mammalian daily (circadian) clock is located in the suprachiasmatic nuclei of the hypothalamus. Clock function can be detected by the measurement of the circadian change in spontaneous firing rate of suprachiasmatic nuclei cells in a brain slice preparation in vitro. We investigated the effects of neuropeptide Y on this rhythm of firing rate in hamster suprachiasmatic nuclei neurons. Slices were prepared using standard techniques. On the 1st day in vitro, neuropeptide Y (200 ng/200 nL; 47 pmol) was applied as a microdrop to the suprachiasmatic nuclei region at various times. Spontaneous single-unit firing was measured for 6-12 h on the 2nd day in vitro. Peak firing rate in treated slices was compared with that of untreated control slices to measure phase shifts induced by the peptide. Neuropeptide Y induced phase advances of circa-3h when applied during the subjective day (ZT 2-10) but did not significantly alter phase when applied during the subjective night. The phase shifts to neuropeptide Y in the hamster tissue in vitro are similar in phase dependency and magnitude to shifts measured in vivo.Key words: circadian, neuropeptide Y, rhythm, suprachiasmatic.