Luminance coding properties of intergeniculate leaflet neurons in the golden hamster and the effects of chronic clorgyline

Normal behavioral responses to light and darkness and the pupillary light reflex are dependent upon the olivary pretectal nucleus in the diurnal Nile grass rat

The olivary pretectal nucleus (OPT) is a midbrain structure that receives reciprocal bilateral retinal projections, is involved in the pupillary light reflex, and connects reciprocally with the intergeniculate leaflet (IGL), a retinorecipient brain region that mediates behavioral responses to light pulses (i.e., masking) in diurnal Nile grass rats. Here, we lesioned the OPT and evaluated behavioral responses in grass rats to various lighting conditions, as well as their anxiety-like responses to light exposure. While control grass rats remained diurnal, grass rats with OPT lesions exhibited a more night-active pattern under 12h:12h light-dark (LD) conditions. However, when placed in constant darkness, OPT-lesioned grass rats became more active during their subjective day, suggesting that an exaggerated masking response to light may be responsible for the effect of OPT lesions on locomotor activity in LD. To test this hypothesis, we presented dark and light pulses to controls and grass rats with OPT lesions; controls increased their activity in response to light, whereas those with OPT lesions significantly increased activity in response to darkness. Further, when placed in a 7-h ultradian LD cycle, animals with OPT lesions were more active during darkness than controls. OPT lesions also abolished the pupillary light reflex, but did not affect anxiety-like behaviors. Finally, in animals with OPT lesions, light did not induce Fos expression in the ventrolateral geniculate nucleus, as it did in controls. Altogether, these results suggest that masking responses to light and darkness are dependent upon nuclei within the subcortical visual shell in grass rats.

Light-induced responses of slow oscillatory neurons of the rat olivary pretectal nucleus

Background

The olivary pretectal nucleus (OPN) is a small midbrain structure responsible for pupil constriction in response to eye illumination. Previous electrophysiological studies have shown that OPN neurons code light intensity levels and therefore are called luminance detectors. Recently, we described an additional population of OPN neurons, characterized by a slow rhythmic pattern of action potentials in light-on conditions. Rhythmic patterns generated by these cells last for a period of approximately 2 minutes.

Methodology

To answer whether oscillatory OPN cells are light responsive and whether oscillatory activity depends on retinal afferents, we performed in vivo electrophysiology experiments on urethane anaesthetized Wistar rats. Extracellular recordings were combined with changes in light conditions (light-dark-light transitions), brief light stimulations of the contralateral eye (diverse illuminances) or intraocular injections of tetrodotoxin (TTX).

Conclusions

We found that oscillatory neurons were able to fire rhythmically in darkness and were responsive to eye illumination in a manner resembling that of luminance detectors. Their firing rate increased together with the strength of the light stimulation. In addition, during the train of light pulses, we observed two profiles of responses: oscillation-preserving and oscillation-disrupting, which occurred during low- and high-illuminance stimuli presentation respectively. Moreover, we have shown that contralateral retina inactivation eliminated oscillation and significantly reduced the firing rate of oscillatory cells. These results suggest that contralateral retinal innervation is crucial for the generation of an oscillatory pattern in addition to its role in driving responses to visual stimuli.

Targeted mutation of the calbindin D 28k gene selectively alters nonvisual photosensitivity

Light intensity is an important determinant of diverse physiological and behavioral responses within the non-image-forming visual system. Thresholds differ among various photic responses, namely control of circadian rhythms, vigilance state, activity level and pupil constriction, but the mechanisms that regulate photosensitivity are not known. Calbindin D(28k) (CalB) is a calcium-binding protein associated with light processing in the mammalian circadian clock. Loss-of-function studies indicate that CalB-deficient mice (CalB(-/-)) have deficits in their ability to entrain to light-dark cycles. To explore the role of CalB in modulating photosensitivity, thresholds for three behaviors mediated by the non-image-forming visual system (entrainment, masking and pupillary light reflex; PLR) were compared in CalB(-/-) and wildtype mice, and the localization of CalB protein in these circuits was examined in adult and juvenile mice. The results reveal a divergence in how CalB affects thresholds to photic cues among these responses. Entrainment and masking were 40- to 60-fold less sensitive in CalB(-/-) than in wildtype mice. On the other hand, the PLR in CalB(-/-) mice was 80- to 200-fold more sensitive. Though CalB is expressed in the retina and in brain circuits regulating entrainment we found no CalB expression in any component of the PLR pathway, namely the olivary pretectal nucleus, Edinger-Westphal nucleus and ciliary ganglion. The behavioral and anatomical data together suggest that, in normal animals, the retinal response to light is blunted in the presence of CalB, but responsiveness of the higher order processes that transduce afferent retinal input is enhanced.

On the intrinsic regulation of neuropeptide Y release in the mammalian suprachiasmatic nucleus circadian clock

Timing of the circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic and non-photic inputs. Of these, neuropeptide Y (NPY) signaling from the intergeniculate leaflet (IGL) to the SCN plays a prominent role. Although NPY is critical to clock regulation, neither the mechanisms modulating IGL NPY neuronal activity nor the nature of regulatory NPY signaling in the SCN clock are understood, as NPY release in the SCN has never been measured. Here, microdialysis procedures for in vivo measurement of NPY were used in complementary experiments to address these questions. First, neuronal release of NPY in the hamster SCN was rhythmic under a 14L : 10D photocycle, with the acrophase soon after lights-on and the nadir at midday. No rhythmic fluctuation in NPY occurred under constant darkness. Second, a behavioral phase-resetting stimulus (wheel-running at midday that induces IGL serotonin release) acutely stimulated SCN NPY release. Third, bilateral IGL microinjection of the serotonin agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) (another non-photic phase-resetting stimulant), at midday enhanced SCN NPY release. Conversely, similar application of the serotonin antagonist, metergoline, abolished wheel-running-induced SCN NPY release. IGL microinjection of the GABA agonist, muscimol, suppressed SCN NPY release. These results support an intra-IGL mechanism whereby behavior-induced serotonergic activity suppresses inhibitory GABAergic transmission, promoting NPY activity and subsequent phase resetting. Collectively, these results confirm IGL-mediated NPY release in the SCN and verify that its daily rhythm of release is dependent upon the 14L : 10D photocycle, and that it is modulated by appropriately-timed phase-resetting behavior, probably mediated by serotonergic activation of NPY units in the IGL.

Juxtacellular recording/labeling analysis of physiological and anatomical characteristics of rat intergeniculate leaflet neurons

The thalamic intergeniculate leaflet (IGL) is involved in mediating effects of both photic and nonphotic stimuli on mammalian circadian rhythms. IGL neurons containing neuropeptide Y (NPY) have been implicated in mediating nonphotic effects, but little is known about those involved in photic entrainment. We used juxtacellular recording/labeling in rats to characterize both photic responses and neurochemical phenotypes of neurons in the lateral geniculate area, focusing on the IGL and ventral lateral geniculate (VLG). Single neurons were recorded to characterize photic responsiveness and were labeled with Neurobiotin (Nb); tissue was stained for Nb, NPY, and in some cases for orexin A. Three classes of neurons were identified in the IGL/VLG. Type I neurons lacked NPY and showed sustained activations during retinal illumination and moderate firing rates in darkness. Type II neurons contained large amounts of NPY throughout the soma and showed varied responses to illumination: suppression, complex responses, or no response. Type III neurons had patches of NPY both on the external soma surface and within the soma, apparently representing internalization of NPY. Type III neurons resembled type I cells in their sustained activation by illumination but were virtually silent during the intervening dark period. These neurons appear to receive NPY input, presumably from other IGL cells, which may suppress their activity during darkness. These results demonstrate the presence of several classes of neurons in the IGL defined by their functional and anatomical features and reinforce the role of the IGL/VLG complex in integrating photic and nonphotic inputs to the circadian system.

In vitro extracellular recording of spontaneous activity of the intergeniculate leaflet neurons

In the view of importance of intergeniculate leaflet in circadian rhythms processes and lack of information about electrophysiological properties of isolated intergeniculate leaflet (IGL) neurons, we carried out extracellular recordings of the spontaneous activity of rat IGL cells in vitro. Unlike other structures of visual thalamus, IGL neurons have the ability to generate a robust spontaneous neuronal activity when maintained in vitro. We have observed that in a standard incubation fluid IGL neurons display at least three distinct firing patterns: continuously irregular-with a wide variety of firing rates, tonic-with a very stable level of activity, and phasic (slow bursts)-with intermittent silent periods. Our study is the first electrophysiological demonstration of IGL neuronal activity in vitro.

Visual-ocular motor activity in the macaque pregeniculate complex

The anatomical connections of the pregeniculate complex (PrGC) with components of the visual-ocular motor system suggested its contribution to ocular motor behavior. Subsequent studies reported saccade-related activity in the primate PrGC. To determine its contribution, we characterized pregeniculate units (n = 128) in alert macaques during ocular motor tasks and visual stimulation. We found that 36/109 saccade-related units exhibited postsaccadic bursts or pauses in tonic discharge for saccades of any amplitude or direction. In contrast to previous results, 46/109 responses preceded or coincided with the saccade, while 47/109 responses were directionally tuned. Pregeniculate units were modulated not only in association with saccades (109/128) but also with smooth eye movements and visual motion (20/128) or eye position (23/128). Multiple ocular motor signals were recorded from 19% of the units, indicating signal convergence on individual neurons. Visual responses were demonstrated in 51% of PrGC units: visual field illumination modulated the resting discharge of 33 units; the responses of 37 saccade-related units and all 23 position-dependent units were modulated by visual stimulation. Early saccadic activity in the PrGC suggests that it contributes more to gaze than postsaccadic modulation of visual or ocular motor activity. The patterns of saccadic responses and the modulation of PrGC activity in association with a variety of visual-ocular motor behaviors suggest its potential role as a relay between the parietal cortex and elements of the brain stem ocular motor pathways, such as the superior colliculus and pretectal nucleus of the optic tract.

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.

Afferent projections to the hamster intergeniculate leaflet demonstrated by retrograde and anterograde tracing

The intergeniculate leaflet (IGL) is considered involved in nonphotic shifting of the circadian clock through a direct connection, the geniculo-hypothalamic tract. The brain areas mediating nonphotic arousal to the hamster IGL have not been thoroughly investigated by both retrograde and anterograde tracing. We, therefore, reinvestigated the IGL afferent connections with the retrograde tracer Cholera toxin B and subsequently verified the results with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We also defined a subset of neurons projecting to the IGL that were activated by arousal using c-Fos immunocytochemistry. Apart from a dense afferent projection from the retina- and the contralateral leaflet, there were ipsilateral projections from other structures: layer V and VI of the prefrontal cortex, the zona incerta, the magnocellular part of the subparafascicular nucleus, the dorsal raphe nucleus, the locus coeruleus, and the cuneiform nucleus. Dense bilateral projections to the leaflet from the pretectal nuclei were found. Hypothalamic afferents were observed dorsal to the suprachiasmatic nuclei, in the retrochiasmatic area (RCh) and in the ventromedial hypothalamic nuclei. All of these projections were confirmed by anterograde tracing. Furthermore, arousal (wheel-running) induced c-Fos in neurons projecting to the IGL (prefrontal cortex, RCh, pretectum). Taken together, the data strengthen the view that the IGL integrates photic and nonphotic information.

Gradual changes in environmental light intensity and entrainment of circadian rhythms

Environmental light-dark alternation is the most potent zeitgeber of circadian rhythm in most organisms. Responses of the circadian system to brief light pulses and rectangular light-dark cycles have been extensively studied in many species. Under natural conditions, however, light intensity changes gradually throughout the day, and light-responsive neurons in the mammalian circadian system have response characteristics suitable for detection of gradual changes in light intensity during twilight. Several researchers have examined rhythm-entraining properties of artificial twilight and fluctuating light intensity cycles, and have stressed the importance of gradual transition between light and darkness for entrainment of circadian rhythms. But many questions about photic entrainment still remain to be answered. Further studies on entrainment of circadian rhythms will be useful for prevention and treatment of circadian rhythm-related disorders.

Range of entrainment of rat circadian rhythms to sinusoidal light-intensity cycles

The range of entrainment of the circadian behavioral rhythm was compared between two groups of Sprague-Dawley rats (each n = 10) exposed to daily cycles of rectangular light-dark alternation (LD) and sinusoidal fluctuations of light intensity (SINE), respectively. The maximum illuminance (20 lx), the minimum illuminance (0.01 lx), and the total amount of light exposure per cycle were the same under the two lighting conditions. The periods (Ts) of both lighting cycles were lengthened stepwise from 24 through 25, 26, 26.5, 27, 27. 5, and 28 h to 28.5 h in experiment 1 and were shortened stepwise from 24 through 23.5, 23, and 22.5 h to 22 h in experiment 2. Each T cycle lasted for 30 cycles. In experiment 1, 60% of rats under the LD condition entrained up to T = 28.5 h, whereas 50% of rats under the SINE condition entrained up to T = 28.5 h. In experiment 2, no animal under the LD condition entrained to T < 23.5 h, whereas 40% of rats under the SINE condition entrained down to T = 23 h and 20% of rats remained to entrain down to T = 22 h cycles. The phase angle of entrainment was systematically changed, depending on T under both conditions. These results suggest that the lower limit of entrainment is expanded under the SINE condition compared with the LD condition.

Involvement of glial fibrillary acidic protein (GFAP) expressed in astroglial cells in circadian rhythm under constant lighting conditions in mice

To clarify the role of glial fibrillary acidic protein (GFAP)-expressed glial cells in the circadian clock, we examined GFAP expression in the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) under various lighting conditions in mice. We demonstrated that GFAP expression did not show daily change in the SCN under a light-dark cycle; however, long-term housing under constant lighting conditions led to dramatic changes in GFAP expression, i.e., a decrease in the SCN and an increase in the IGL. Furthermore, mice that had a targeted deletion in the GFAP gene (GFAP mutant mice) showed longer and more arrhythmic circadian activity rhythms in constant lighting conditions than wild-type mice, while GFAP mutant mice exhibited stable circadian rhythms both in a light-dark cycle and constant darkness, and showed normal entrainment to environmental light stimuli. These results suggest that the GFAP-expressed astroglial cells in the SCN and the IGL may have some role in circadian oscillation under constant lighting conditions.

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

The circadian clock located in the suprachiasmatic nuclei (SCN) is influenced by the photoperiod. After the transfer from a long (LP 14:10) to a short photoperiod (SP 10:14), the adjustment of the light sensitivity of the SCN, in terms of Fos expression, takes 25 nights. To examine the contribution of the thalamic intergeniculate leaflet (IGL) and its NPY-immunoreactive projection in the extension of the duration of the photosensitive phase of the SCN, male Syrian hamsters received electrolytic lesions of the IGL. We showed a lower number of Fos-ir cells in the SCN of IGLx hamsters following a light pulse applied 13 h after dark onset, 25 nights after the transfer from LP to SP compared to sham operated hamsters. The present study shows that the integrity of the IGL is necessary to have a complete integration of photoperiodic changes by the SCN. This demonstrates the involvement of the IGL in the integration of photoperiodic information by the SCN.

Transgenic ablation of rod photoreceptors alters the circadian phenotype of mice

The impact of photoreceptor loss on the circadian system was examined by utilizing a transgenic mouse model (rdta) in which rod photoreceptors were specifically ablated. These mice were able to phase-shift their circadian locomotor behaviour in response to light, but features of this circadian behaviour were markedly altered. The amplitude of circadian responses to light were approximately 2.5 greater, the circadian period (tau) was reduced (c. 20 min) and the total duration of activity (alpha) was increased (c. 50 min) when compared to wild type (+/+) and rd/rd mice (retinal degeneration, mice which also lack rod photoreceptors) of the same genetic background. The pattern of Fos expression in the suprachiasmatic nuclei (the site of the primary circadian clock in mammals) was indistinguishable between +/+ and rdta mice. However, Fos expression in the retina suggested that rod loss in rdta mice resulted in a functional reorganization of the retina and the constitutive activation of a population of retinal ganglion cells. Although it has been known for several years that the entraining photoreceptors of mammals are ocular, and that rod photoreceptors are not required for light regulation of the clock, these are the first data to show that features of the circadian phenotype (amplitude of the phase response curve, alpha, tau) can be influenced by photoreceptor ablation. These data support the hypothesis that the circadian phenotype of mammals is the product of an interaction between the suprachiasmatic nuclei and the retina. Thus, mammals which show an altered circadian behaviour can no longer be assumed to have defects associated only with specific clock genes; genes that affect photoreceptor survival may also modify circadian behaviour.

Serotonin and the regulation of mammalian circadian rhythmicity

The suprachiasmatic nucleus (SCN), the site of the primary mammalian circadian clock, contains one of the densest serotonergic terminal plexes in the brain. Although this fact has been appreciated for some time, only in the last decade has there been substantial approach toward the understanding of the function of serotonin in the circadian rhythm system. The intergeniculate leaflet, which projects to the SCN via the geniculohypothalamic tract, receives serotonergic innervation from the dorsal raphe nucleus, and the SCN receives its serotonergic input from the median raphe nucleus. This separation of serotonergic origins provides the opportunity to investigate the function of the two projections. Loss of serotonergic neurones of the median raphe yields earlier onset and later offset of the nocturnal activity phase, longer duration of the activity phase, and increased sensitivity of circadian rhythm response to light. Despite the simplicity of the origins of serotonergic anatomy with respect to the circadian rhythm system, the actual involvement of serotonin in rhythm modulation is not so obvious. A variety of pharmacological studies have clearly implicated serotonin as a direct regulator of circadian rhythm phase, but others employing different methods suggest that simple elevation of SCN serotonin concentrations does not modify rhythm phase. The most convincing role of serotonin is its apparent ability to modulate sensitivity of the circadian rhythm to light. The putative method for such modulation is via a presynaptic 5-HT1B receptor on the retinohypothalamic tract, the activation of which attenuates photic input to the SCN thereby reducing phase response to light. Serotonin may modulate phase response to benzodiazepines, but does not appear to modify such response to environmentally induced locomotor activity. Current interest in serotonergic modulation of circadian rhythmicity is strong and the research is vigorous. There is an abundance of information about serotonin and circadian rhythm function that lacks a satisfactory framework for its interpretation. The next decade is likely to see the gradual evolution of this framework as the role of serotonin in circadian rhythm regulation is further elucidated.

Roles of suprachiasmatic nuclei and intergeniculate leaflets in mediating the phase-shifting effects of a serotonergic agonist and their photic modulation during subjective day

Serotonin (5-HT) has been implicated in the phase adjustment of the circadian system during the subjective day in response to nonphotic stimuli. Two components of the circadian system, the suprachiasmatic nucleus (SCN) (site of the circadian clock) and the intergeniculate leaflet (IGL), receive serotonergic projections from the median raphe nucleus and the dorsal raphe nucleus, respectively. Experiment 1, performed in golden hamsters housed in constant darkness, compared the effects of bilateral microinjections of the 5-HT1A/7 receptor agonist, 8-hydroxydipropylaminotetralin (8-OH-DPAT; 0.5 microgram in 0.2 microliter saline per side), into the IGL or the SCN during the mid-subjective day. Bilateral 8-OH-DPAT injections into either the SCN or the IGL led to significant phase advances of the circadian rhythm of wheel-running activity (p < .001). The phase advances following 8-OH-DPAT injections in the IGL were dose department (p < .001). Because a light pulse administered during the middle of the subjective day can attenuate the phase-resetting effect of a systemic injection of 8-OH-DPAT, Experiment 2 was designed to determine whether light could modulate 5-HT agonist activity at the level of the SCN and/or the IGL. Serotonergic receptor activation within the SCN, followed by a pulse of light (300 lux of white light lasting 30 min), still induced phase advances. In contrast, the effect of serotonergic stimulation within the IGL was blocked by a light pulse. These results indicate that the respective 5-HT projections to the SCN and IGL subserve different functions in the circadian responses to photic and nonphotic stimuli.

Retinal projections in mice with inherited retinal degeneration: implications for circadian photoentrainment

The availability of naturally occurring and transgenic retinal mutants has made the mouse an attractive experimental model to address questions regarding photoentrainment of circadian rhythms. However, very little is known about the retinal cells and the retinal projections to the nuclei of the murine circadian timing system. Furthermore, the effect of inherited retinal degeneration on these projections is not understood. In this report, we have used pseudorabies virus as a neuroanatomical tract tracer in mice to address a series of questions: Which retinal cells mediate circadian responses to light? What is the nature of the retinohypothalamic projection? What is the impact of the inherited retinal disorder, retinal degenerate (rd/rd), on the structures of the photoentrainment pathway? Our results show that a class ofretinal ganglion cell, morphologically similar to the type III ganglion cells of the rat, appears to project to central circadian structures of the mouse. They are few in number and sparsely distributed throughout the retina. The low number and broad distribution of these specialized retinal ganglion cells may be an adaptive mechanism to integrate environmental irradiance without compromising the spatial resolution required for vision. In addition, viral infection of conelike and rodlike photoreceptors and amacrinelike cells suggest that these cells may mediate or contribute to circadian responses to light. Inherited retinal degeneration has no obvious effect on the anatomy of the retinal cells or their projections to the circadian axis. These anatomical findings are consistent with our previous findings showing that aged rd/rd mice are capable of regulating their circadian rhythms by light with unattenuated sensitivity.

Disruption of the activity-rest cycle by MAOI treatment: dependence on light and a secondary visual pathway to the circadian pacemaker

The disruptive effects on the activity-rest cycle of the monoamine oxidase inhibitor (MAOI) clorgyline and of continuous light were examined in Syrian hamsters. When administered in dim and moderate light intensities, clorgyline delayed the daily onset of wheel-running. When administered in bright light, it dissociated the circadian rhythm of wheel-running. This dissociation was prevented by lesions of the intergeniculate leaflet of the ventral lateral geniculate nucleus. Constant darkness restored the circadian rhythm of wheel-running in hamsters with disrupted circadian rhythms. The phase of the restored rhythm of wheel-running was shifted 6-12 h later than the phase of wheel-running prior to dissociation. Our results suggest that MAOI treatment weakens the coupling between oscillators that comprise the circadian pacemaker, and augments the disruptive effects of continuous light acting via the intergeniculate leaflet region of the ventral lateral geniculate nucleus. These effects on the circadian pacemaker may be responsible for disruptions of the sleep-wake cycle that occur as side effects when MAOIs are used clinically to treat depression and might play a role in the induction of mania and rapid cycling by antidepressants.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Circadian phase-response curves for simulated dawn and dusk twilights in hamsters

Phase shifts of the circadian rhythm of wheel-running activity were compared in Syrian hamsters maintained in constant darkness and exposed to 1-h naturalistic dawn or dusk twilight ramps (0.003-10 lx), or to 1-h rectangular light pulses (1 lx) providing equal photon exposure. The phase-response curves (PRCs) for dusk and rectangular pulses were virtually identical and resembled the PRC for dawn pulses, except that the mean phase advance caused by dawn pulses at circadian time 19 (CT 19) was approximately 1 h smaller. This difference could not be accounted for by differences in the amount of wheel-running observed during light pulse exposure, because the animals ran more during dusk pulses than during either of the other two pulse types. In a second experiment, 15-min rectangular light pulses (1 lx) immediately preceded by a 47-min dawn ramp caused smaller phase delays at CT 13 than rectangular pulses alone, despite a 40% increase in total photon exposure, but phase advances at CT 19 did not differ between the two light treatments. These results indicate that phase shifts of the circadian pacemaker in hamsters are determined primarily, though not entirely, by total photon exposure. They also indicate that dawn pulses may be less effective than dusk or rectangular pulses at certain circadian phases, possibly due to light adaptation during the early portion of the dawn twilight.

Constant light induces persistent Fos expression in rat intergeniculate leaflet

Fos protein expression in retinorecipient suprachiasmatic nucleus (SCN) neurons is a marker of photic entrainment of circadian rhythms. Light-induced Fos in neurons of the intergeniculate leaflet (IGL) is not well-characterized. We compared Fos immunoreactivity (Fos-IR) in SCN and IGL neurons of rats housed under various lighting conditions and sacrificed at different phases of the circadian period. IGL neurons of rats that received 1 h-3 weeks of light exposure prior to sacrifice displayed Fos-IR, whereas the IGL of animals exposed only to darkness displayed little if any staining. In contrast with light-induced Fos in SCN neurons, Fos-IR was observed in the IGL regardless of circadian time. This work supports the idea that the IGL is involved in transmission of photic information to the SCN in rats.

Fos expression in rat visual cortex induced by ocular input of ultraviolet light

We used immunostaining for the cellular transcription factor Fos to assess patterns of neuronal activation in rat visual cortex during exposure to ultraviolet light. Exposure to monochromatic ultraviolet light (lambda max 360 nm: half-bandwidth 8.8 nm, 10 microW/cm2 at eye level) induced strong expression of Fos immunoreactivity in the primary visual cortex and associated cortical visual areas of dark-adapted rats. The stimulatory effect of ultraviolet light on Fos expression was related to exposure duration, was independent of stimulus novelty or phase of the circadian cycle in which exposure occurred, and it was mediated by a mechanism located in the eye. These results demonstrate that ocular input of ultraviolet light is capable of altering neuronal activity in cortical structures involved in visual processing and are consistent with the hypothesis that rodents may use ultraviolet light for vision.

A serotonin neurotoxin attenuates the phase-shifting effects of triazolam on the circadian clock in hamsters

Several lines of evidence suggest the potential involvement of serotonergic pathways in mediating the effects of activity-inducing stimuli on the circadian clock in rodents. The aim of the present 3 experiments was to examine the effects of the serotonergic neurotoxin, p-chloroamphetamine (PCA, 10 mg/kg) on: (1) the monoamine levels of the hypothalamus, frontal cortex and hippocampus in the hamster; (2) the phase shifts in the circadian rhythm of locomotor activity of hamsters in response to treatment with the short-acting benzodiazepine, triazolam (7.5 mg/kg); and (3) the magnitude of the acute increase in locomotor activity associated with triazolam administration in this species. The administration of PCA to hamsters caused changes of specific monoaminergic systems in the hypothalamus, that were limited to a selective decrease in serotonin levels 7 days post-treatment. The phase shifts of the circadian clock in response to triazolam treatment at CT 6 were considerably attenuated following the administration of the 5-HT neurotoxin. The total amount and the profiles of triazolam-induced wheel-running and general cage activity between CT 6 and CT 12 were not significantly affected by the PCA treatment. The finding that a 5-HT neurotoxin can attenuate the phase-shifting effects of triazolam in hamsters, without interfering with its activity-inducing properties, suggests that serotonergic afferents might be involved in the mechanism for non-photic phase-shifting of the circadian system.

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Development of the hamster serotoninergic system: cell groups and diencephalic projections

Nuclei of the circadian visual system are extensively innervated by serotoninergic neurons and rhythmicity is modulated by the serotoninergic system. This study investigated the temporal relationships between prenatal origins of serotoninergic cell groups and perinatal innervation of structures in the hamster circadian visual system as well as in the remaining diencephalon. Serotonin-immunoreactive (5-HT-IR) neurons of the B4-B9 complex were first seen on embryonic day 8 (E8). The number of neurons increases sharply by E10 when the first 5-HT-IR cells are evident in the medulla (B1-B3 complex). The distribution of serotoninergic neurons in the hamster brainstem is generally adult-like by E14. Thick 5-HT-IR fibers arch around the mesencephalic flexure at E10 and reach more rostral mesencephalic areas at E11. A branch of the medial forebrain bundle (MFB) projects ventrally toward the retrochiasmatic area; a second branch ascends along the fasciculus retroflexus. Fibers cross the midline in the supraoptic commissure by E12, other arrive in the lateral geniculate region, and a branch of the MFB extends toward the mammillary area. At E13, a periventricular medial thalamic branch of the MFB is seen, axons appear in the supramammillary commissure, and a fine fasciculus between the medial thalamus and intergeniculate leaflet is visible. Lateral, paraventricular, and retrochiasmatic hypothalamic areas and centro- and dorsomedial thalamus are densely innervated at E14. The mammillary area and lateral geniculate body are moderately innervated, and the first fibers appear in the deep laminae of the superior colliculus. The innervation of the suprachiasmatic nuclei, periventricular hypothalamus, and superficial layers of the superior colliculus occurs postnatally. The results are consistent with serotoninergic system development in other species.

Processing of photic information within the intergeniculate leaflet of the lateral geniculate body: assessed by neuropeptide Y immunoreactivity in the suprachiasmatic nucleus of rats

Entrainment of the circadian pacemaker in the suprachiasmatic nucleus is accomplished by two neural pathways, the retinohypothalamic and geniculohypothalamic tracts. The geniculohypothalamic tract, which originates from the intergeniculate leaflet and a portion of the ventral lateral geniculate nucleus, is composed of fibers immunoreactive to neuropeptide Y. To assess the processing of photic information by the geniculohypothalamic tract, neuropeptide Y immunoreactivity in the suprachiasmatic nucleus of rats kept under various external lighting conditions was determined by enzyme immunoassay of micropunched tissues. Neuropeptide Y levels in the suprachiasmatic nucleus steadily increased when rats were exposed to continuous light and reached a peak in 2 h before returning to basal level. The amount of increase did not depend on intensity and duration of light exposure. A light pulse as short as 5 min elicited a similar rise in neuropeptide Y, indicating that the response is due to the sudden transition from dark to light. This response, however, was only observed when the dark to light transition occurred at circadian time 0 (subjective dawn) of the pacemaker. A light pulse at circadian time 0, which effectively induces the increase in neuropeptide Y level, does not significantly shift the phase of the circadian rhythm. This observation indicates that the photic pathway utilizing neuropeptide Y may be functional only when the endogenous circadian rhythm is synchronized to external light and dark cycles. Administration of an excitatory amino acid antagonist (MK-801) blocked the increase of neuropeptide Y by light, while an agonist (N-methyl-D-aspartate) induced similar facilitatory effects to that of light on the neuropeptide Y level in the rat suprachiasmatic nucleus. These results suggest that the geniculohypothalamic tract processes photic information so as to facilitate distinction of the transition between light and darkness that occurs either at subjective dawn or dusk.

Monoamine depletion alters the entrainment and the response to light of the circadian activity rhythm in hamsters

Reduced amplitude, shorter free-running periods and desynchronization among a number of circadian rhythms are associated with advanced age in rodents. The response of the hamster circadian system to photic stimuli is also altered during senescence. Decreased monoamine levels, receptor sites and neuronal populations are commonly observed in the aging brain. The objective of the present study was to determine if monoamine depletion with reserpine in young hamsters induces changes in the circadian rhythm of locomotor activity similar to those that occur spontaneously with aging. Wheel-running activity of 12 young hamsters under a 14 h-light/10 h-dark cycle was continuously monitored. The total activity level, the times of activity onset, peak and offset and the duration of activity were determined during a 1-week period after vehicle treatment and for three 1-week periods after reserpine treatment (4 mg/kg). A second group of eight reserpine-treated and six vehicle-treated animals was kept in constant darkness (DD). The period of the circadian activity rhythm in DD and the phase-shifts after short light pulses at circadian time 19 (CT19) were determined in the control and reserpine-treated groups. Brain monoamines in the hypothalamus, striatum and pons/medulla after reserpine and vehicle treatment were determined by high-pressure liquid chromatography. The data were analyzed with x2 periodogram and one-way ANOVA followed by Duncan's post hoc test. Reserpine treatment significantly reduced total wheel-running activity and the monoamine levels in the hypothalamus, striatum and pons/medulla.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin and the mammalian circadian system: II. Phase-shifting rat behavioral rhythms with serotonergic agonists

The suprachiasmatic nuclei (SCN) receive primary afferents from the median and dorsal raphe, but the role of these projections in circadian timekeeping is poorly understood. Studies of the SCN in vitro suggest that quipazine, a general serotonin (5-HT) receptor agonist, can produce circadian time-dependent phase advances and phase delays in circadian rhythms of neuronal activity. The present study addresses whether quipazine and the selective 5-HT1A receptor agonist 8-OH-DPAT are similarly effective in vivo. Drinking and wheel-running patterns of male Wistar rats individually housed in constant darkness were monitored before and after subcutaneous administration of quipazine (5-10 mg/kg) at either circadian time (CT) 6 or CT 18, with and without running wheels available. Dose-dependent phase advances (20-180 min) were produced at CT 6. Significant phase shifts were not observed at CT 18. CT 6 quipazine-treated animals also showed a sustained and significant shortening of rhythm period (tau) following treatment (-0.28 hr; p < 0.002). tau shortening was inconsistently observed in CT 18 quipazine-treated rats. Neither quipazine-induced phase shifts nor tau effects were dependent on wheel-running activity per se. 8-OH-DPAT delivered via intracerebral ventricular treatment into the third ventricle (5 microliters at 100 microM in saline) produced slightly smaller phase advances (20-90 min) at CT 6, but did not produce phase delays at CT 18 or changes in tau. These findings support in vitro evidence that 5-HT-ergic agonists can phase-shift the circadian pacemaker.