No triazolam-induced expression of Fos protein in raphe nuclei of the male Syrian hamster

Distinct feedback actions of behavioural arousal to the master circadian clock in nocturnal and diurnal mammals

The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus provides a temporal pattern of sleep and wake that - like many other behavioural and physiological rhythms - is oppositely phased in nocturnal and diurnal animals. The SCN primarily uses environmental light, perceived through the retina, to synchronize its endogenous circadian rhythms with the exact 24 h light/dark cycle of the outside world. The light responsiveness of the SCN is maximal during the night in both nocturnal and diurnal species. Behavioural arousal during the resting period not only perturbs sleep homeostasis, but also acts as a potent non-photic synchronizing cue. The feedback action of arousal on the SCN is mediated by processes involving several brain nuclei and neurotransmitters, which ultimately change the molecular functions of SCN pacemaker cells. Arousing stimuli during the sleeping period differentially affect the circadian system of nocturnal and diurnal species, as evidenced by the different circadian windows of sensitivity to behavioural arousal. In addition, arousing stimuli reduce and increase light resetting in nocturnal and diurnal species, respectively. It is important to address further question of circadian impairments associated with shift work and trans-meridian travel not only in the standard nocturnal laboratory animals but also in diurnal animal models.

A non-photic gateway to the circadian clock of hamsters

This paper considers the neural mechanisms underlying a particular kind of non-photic phase shifting, that produced by novelty-induced wheel running in the hamster. The projection from the intergeniculate leaflet (IGL) to the suprachiasmatic nucleus (SCN) appears to be an important part of the mechanism mediating such phase shifts. A number of experiments support this view. First, expression of immediate-early genes in the IGL is induced by non-photic phase-shifting stimuli. Second, Fos-like immunoreactivity in the IGL co-localizes with neuropeptide Y (NPY) immunoreactivity. Third, direct application of NPY to the SCN produces phase shifts which do not depend on the hamsters becoming active following the injections. Fourth, blocking the normal actions of NPY at the SCN blocks or greatly attenuates the phase shifting that is normally produced by novelty-induced wheel running. Progress on the physiological basis of phase shifts associated with activity, or a correlate, depends on understanding the behavioural aspects of this phenomenon. The activity-shift response curve is especially useful.

Immediate-early genes and the neural bases of photic and non-photic entrainment

The expression of immediate-early genes (IEGs) within the mammalian suprachiasmatic nucleus (SCN) identifies individual light-responsive cells of the circadian system. Cells immunoreactive for products of IEGs form a neurochemically heterogeneous population, of which a few are VIP (vasoactive intestinal peptide)-immunoreactive or GRP (gastrin-releasing peptide)-immunoreactive, although the phenotypes of most of the others have yet to be determined. Dual-labelling experiments with anatomical tracers reveal that only a minority of efferent projection neurons of the SCN are immunoreactive for IEG products, and it is likely that the majority of the immunoreactive cells are interneurons or glia. Photic induction of IEGs is mediated via NMDA (N-methyl-D-aspartate) and non-NMDA glutamatergic receptors, the SCN expressing a topographically specific complement of subtypes of the NMDA receptor. Non-photic cues (arousal) can shift the clock but this is not associated with expression of IEGs, demonstrating that the proteins encoded by IEGs are probably involved in transducing photic cues, rather than shifting the clock per se. Their induction provides an anatomically explicit marker for circadian phase and photic sensitivity and so is useful in analyses of circadian function, for example, in the tau mutant hamster. Non-photic phase shifts are accompanied by adrenocortical activation, confirming the importance of arousal in shifting of the clock. The phase-shifting effect of arousal can be blocked by treatment with the serotonin receptor antagonist ketanserin, suggesting that ascending serotonergic input to the forebrain, possibly directly to the SCN, is an important mediator of entrainment by arousal.

Alcohol-induced c-Fos expression in the Edinger-Westphal nucleus: pharmacological and signal transduction mechanisms

Mapping inducible transcription factors has shown that the Edinger-Westphal nucleus is preferentially sensitive to alcohol intoxication. Herein, we characterize the pharmacological and signal transduction mechanisms related to alcohol-induced c-Fos expression in Edinger-Westphal neurons. Using immunohistochemistry, we show that pretreatment with gamma-aminobutyric acid (GABA)-ergic antagonists (4 mg/kg bicuculline and 45 mg/kg pentylenetetrazole) attenuates induction of c-Fos expression by alcohol (2.4 g/kg, intraperitoneal). In addition, 10 mg/kg 2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)4,5-dihydro-1H-imidazole (RX 821002), an alpha(2A/D)-adrenoceptor antagonist, and 20 mg/kg haloperidol, a dopamine antagonist, also block alcohol-induced c-Fos expression in Edinger-Westphal neurons. No effects were seen in alcohol-induced c-Fos after the pretreatment of 20 mg/kg propranolol (beta-adrenoceptor antagonist), 10 mg/kg 2-(2-(4-(2-methoxyphenyl)piperazin-1-yl) ethy)-4,4-dimethyl-1,3-(2H,4H)-isoquinolindione dihydrochloride (ARC 239) (alpha(2B/C)-adrenoceptor antagonist), or 30 mg/kg naltrexone (opioid antagonist). Although positive modulators for the GABA(A) receptor (20 mg/kg 3alpha-hydroxy-5alpha-pregnan-20-one and 10-30 mg/kg chlordiazepoxide) and opioid receptor (10 mg/kg morphine) produced significant elevations, agonists for alpha(2)-adrenoceptors (clonidine) and dopamine receptors (apomorphine) had no effect on Edinger-Westphal c-Fos expression. These findings suggest that alcohol-induced c-Fos expression in Edinger-Westphal results from direct interactions with GABA(A) receptors, which are modified by alpha(2A/D)-adrenoceptors and dopamine receptors. Also using immunohistochemistry to identify potential intracellular mechanisms associated with alcohol-induced c-Fos expression in Edinger-Westphal, we show time-dependent increases in serine 727 phospho-signal transducer and activator of transcription 3 (Stat3) but no changes in phospho-cAMP response element-binding protein and phospho-Elk1. Time-dependent increases in phospho-extracellular signal-regulated kinase (ERK) 1/2 were found to occur simultaneously with increases in serine 727 phospho-Stat3. Finally, blockade of ERK 1/2 phosphorylation with the mitogen-activated protein kinase (MEK) 1/2 inhibitor SL327 blocked alcohol-induced c-Fos expression, suggesting that alcohol induces c-Fos in Edinger-Westphal neurons through activation of the MEK1/2-ERK1/2-Stat3 pathway.

The hamster circadian rhythm system includes nuclei of the subcortical visual shell

The clock regulating mammalian circadian rhythmicity resides in the suprachiasmatic nucleus. The intergeniculate leaflet, a major component of the subcortical visual system, has been shown to be essential for certain aspects of circadian rhythm regulation. We now report that midbrain visual nuclei afferent to the intergeniculate leaflet are also components of the hamster circadian rhythm system. Loss of connections between the intergeniculate leaflet and visual midbrain or neurotoxic lesions of pretectum or deep superior colliculus (but not of the superficial superior colliculus) blocked phase shifts of the circadian activity rhythm in response to a benzodiazepine injection during the subjective day. Such damage did not disturb phase response to a novel wheel stimulus. The amount of wheel running or open field locomotion were equivalent in lesioned and control groups after benzodiazepine treatment. Electrical stimulation of the deep superior colliculus, without its own effect on circadian rhythm phase, greatly attenuated light-induced phase shifts. Such stimulation was associated with increased FOS protein immunoreactivity in the suprachiasmatic nucleus. The results show that the circadian rhythm system includes the visual midbrain and distinguishes between mechanisms necessary for phase response to benzodiazepine and those for phase response to locomotion in a novel wheel. The results also refute the idea that benzodiazepine-induced phase shifts are the consequence of induced locomotion. Finally, the data provide the first indication that the visual midbrain can modulate circadian rhythm response to light. A variety of environmental stimuli may gain access to the circadian clock mechanism through subcortical nuclei projecting to the intergeniculate leaflet and, via the final common path of the geniculohypothalamic tract, from the leaflet to the suprachiasmatic nucleus.

Role of the thalamic intergeniculate leaflet and its 5-HT afferences in the chronobiological properties of 8-OH-DPAT and triazolam in syrian hamster

The 5-HT(1A/7) receptor agonist 8-hydroxy-2-[di-n-propylamino]-tetralin (8-OH-DPAT) has chronobiological effects on the circadian system and, in the Syrian hamster, it is known that serotonergic (5-HT) projections connecting the median raphe nucleus to the suprachiasmatic nuclei (SCN) of the hypothalamus are a prerequisite for the expression of 8-OH-DPAT-induced phase advance of locomotor activity rhythm. We examined the possible involvement of the thalamic intergeniculate leaflet (IGL) in the phase-shifting properties of 8-OH-DPAT injections at CT7. Bilateral electrolytic lesions of the IGL blocked phase-shift responses to 8-OH-DPAT of the activity rhythm. Phase changes induced by injections of 8-OH-DPAT at CT7 and triazolam (Tz), a short-acting benzodiazepine, at CT6 were also studied after bilateral chemical lesion of the 5-HT fibres connecting the dorsal raphe nucleus (DR) to IGL. Destruction of 5-HT fibres within the IGL blocked the phase-shift response to Tz, but not the phase-shift response to 8-OH-DPAT. In conclusion, (a) IGL is essential for the phase-shifting effect of peripheral 8-OH-DPAT injections; (b) 5-HT fibres connecting DR to IGL are necessary for the expression of the phase-shifting effect of Tz but not of 8-OH-DPAT.

Expression of Fos in the circadian system following nonphotic stimulation

Syrian hamsters, Mesocricetus auratus, were confined to novel running wheels for a 3-h period, starting at approximately circadian time (CT) 4.5 (i.e., approaching the middle of their subjective day). It can be reliably predicted from the amount of running in this situation whether or not there will be a subsequent phase-shift. Expression of the immediate early genes c-fos and fosB was examined by immunocytochemistry in the suprachiasmatic nucleus (SCN), the intergeniculate leaflet (IGL) of the thalamus, and the medial pretectal area of hamsters that ran vigorously in the novel wheel and would have phase-shifted. c-Fos was increased, compared to levels in a control group left in their home cages, in the IGL, and the pretectum (PT), but decreased in the SCN. No significant changes in FosB were detected in any region examined. An additional experiment argued against the possibility that the changes in c-Fos could be attributed to a rapid advance of the pacemaker to a different phase in the circadian cycle. Counts of c-Fos-positive cells in the IGL were similar in animals given pulses of running starting at CT 4.5 and starting at CT 12.5-16 (i.e., in the subjective night when they would have been active anyway). Altogether the results support the view that activation of the IGL is important in nonphotic clock resetting, and raise the possibility that the PT may also be involved in nonphotic resetting. However, the results also indicate that novelty-induced running does not alter c-Fos induction in a phase-specific manner in the IGL. The inhibition of c-Fos in the SCN by nonphotic phase-shifting events contrasts with the well-known inducing effects of light pulses. These different effects might underlie some of the interactions between nonphotic and photic zeitgebers when both act together on the circadian system.

Fos-like immunoreactivity in the circadian timing system of calorie-restricted rats fed at dawn: daily rhythms and light pulse-induced changes

Daily rhythms of pineal melatonin, body temperature, and locomotor activity are synchronized to the light-dark cycle (LD) via a circadian clock located in the suprachiasmatic nuclei (SCN). A timed caloric restriction in rats fed at dawn induces phase-advances and further phase-stabilization of these rhythms, suggesting that the circadian clock can integrate conflicting daily photic and non-photic cues. The present study investigated the daily expression of Fos-like immunoreactivity (Fos-ir) and light pulse-induced Fos-ir in the SCN, the intergeniculate leaflet (IGL) and the paraventricular thalamic nucleus (PVT) in calorie-restricted rats fed 2 h after the onset of light and in controls fed ad libitum. A daily rhythm of Fos-ir in the SCN was confirmed in control rats, with a peak approximately 2 h after lights on. At this time point (i.e. just prior to the feeding time), the level of SCN Fos-ir was lowered in calorie-restricted rats. Concomitantly, IGL Fos-ir was higher in calorie-restricted vs. control rats. In response to a light pulse during darkness, Fos-ir induction was found to be specifically (i.e. phase-dependently) lowered in the SCN and IGL of calorie-restricted rats. Observed changes of Fos-ir in the PVT were possibly related to the wake state of the animals. This study shows that repetitive non-photic cues presented in addition to a LD cycle affect the Fos expression in the circadian timing system.

Cyclophosphamide cystitis as a model of visceral pain in rats: minor effects at mesodiencephalic levels as revealed by the expression of c-fos, with a note on Krox-24

The evoked expression of the immediate-early gene-encoded proteins c-Fos and Krox-24 was used to study activation of mesodiencephalic structures as a function of the development of cyclophosphamide (CP) cystitis in behaving rats. This article is the third of a series and completes previously published data obtained at both spinal and hindbrain levels. CP-injected animals received a single dose of 100 mg/kg i.p. under transient volatile anesthesia and survived for 1-4 h in order to cover the entire postinjection period during which the disease develops. Survival times longer than 4 h were not used owing to ethical considerations. Results from CP-injected groups are compared with those from either noninjected controls or saline-injected animals having survived for the same times as CP-injected ones. Quantitative results come from c-fos expression. At mesodiencephalic levels a high and widespread basal c-fos expression was observed in control animals; maximum staining was observed at the midthalamic level. Four groups of nuclei were identified with regard to the density of staining. The first group included nuclei showing clustered, intensely labeled cells; these areas were restricted in extent and related to the maintenance of circadian rythms (intergeniculate leaf, suprachiasmatic nucleus, dorsal parts of either paraventricular thalamic nuclei or central gray), sleep-arousal cycle (supramamillary nucleus), or changes in arterial pressure (laterodorsal tegmental nucleus). The second group included nuclei showing scattered, moderately labeled cells; these areas were widespread at all rostrocaudal levels and related to either autonomic/neuroendocrine regulations (central gray, lateral habenula, hypothalamus) or motor behavior, orienting reflex and oculomotor coordination (unspecific subdivisions of both colliculi and their adjoining mesencephalic regions, zona incerta dorsal). The third group included nuclei with evenly distributed, faintly labeled cells; these areas, which, with few exceptions, covered almost the entire diencephalon, mainly concerned nuclei of multisensory convergence having functions in either discriminative tasks (laterodorsal and lateroposterior thalamic nuclei) or emotional responses (intralaminar and midline thalamic nuclei). The fourth group included nuclei free of labeling; these were areas that received the bulk of unimodal sensory/motor inputs (central inferior colliculus, pretectal optic nuclei, ventral medial geniculate nucleus, ventral anterior pretectal nucleus, dorsal lateral geniculate nucleus, ventrobasal complex; zona incerta ventral, parafascicular thalamic nucleus) and are thus the most discriminative regarding specific modalities. Variations in staining were of the same magnitude in both saline- and CP-injected animals. A sequential study spanning every postinjection hour revealed maximum staining at 1 h postinjection, which was followed by a progressive, time-related decrease. Increases in the number of labeled cells 1 h postinjection were significant in only a restricted number of nuclei showing low basal expression (Edinger-Westphal nucleus and paraventricular, supraoptic, and lateral hypothalamic nuclei); time-related reductions in staining that were correlated to sleep or quiescence behaviors finally resulted in staining equal to or below that seen in control animals. No structures showed significantly increased staining in relation to the full development of cystitis, i.e., with the increase of visceronociceptive inputs. Comparing the present results with those previously obtained at more caudal levels, it appears that subtelencephalic levels primarily driven by visceronociceptive inputs, i.e., those that increase and/or maintain their activity in parallel with the degree of nociception, are confined to brainstem-spinal cord junction levels and only comprise certain subdivisions of the nucleus of the solitary tract (nucleus medialis, nucleus commissuralis, and ventralmost part of area po

c-Fos expression in the rat intergeniculate leaflet: photic regulation, co-localization with Fos-B, and cellular identification

Ambient light alters the level of the transcriptional regulatory protein c-Fos in the suprachiasmatic nucleus, the site of an endogenous circadian clock in mammals, and in one other retino-recipient area, the intergeniculate leaflet of the lateral geniculate complex. Complementing previous work by ourselves and others on the photic and temporal regulation of c-Fos expression in the suprachiasmatic nucleus, the present studies investigated c-Fos regulation in the rat intergeniculate leaflet, revealing some important differences between the two brain regions. In the intergeniculate leaflet, the levels of c-fos mRNA (by in situ hybridization) and immunoreactive c-Fos protein (by immunohistochemistry) were elevated by light pulses administered either during the subjective day or subjective night. The regulation of immunoreactive Fos-B protein was similar to c-Fos, and 98% of Fos-B-expressing cells were also c-Fos-positive (by double-label immunofluorescence). By combining c-Fos immunofluorescence with stereotaxic injections of the retrograde tract tracer FluoroGold, we found photically-induced c-Fos in 15% of intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus and in 34% of those projecting to the contralateral intergeniculate leaflet. Intergeniculate leaflet cells that express c-Fos after photic stimulation appear to represent a functionally-defined population that does not correspond to anatomically-defined categories based on connectivity or peptidergic phenotype.

Unexpected c-fos gene expression in the suprachiasmatic nucleus of mice entrained to a skeleton photoperiod

Several authors have suggested that the transcriptional regulatory protein c-Fos might be part of the mechanism for photic entrainment of the circadian pacemaker in the suprachiasmatic nucleus (SCN) to environmental light:dark cycles. This hypothesis has been based on evidence gathered using single light pulses administered acutely to animals free-running in constant darkness. In order to begin to analyze SCN c-fos gene expression in animals during steady-state entrainment to photic cycles, we exposed male BALB/c mice to a skeleton photoperiod consisting of two 1-h light pulses separating a long (14 h) and a short (8 h) dark interval. The cycle was designed so that stable entrainment could be achieved in either one of two patterns (with rhythmic locomotor activity occurring during either the long or the short dark interval); SCN c-fos mRNA levels could then be measured during entrainment to light pulses at different phases of the circadian cycle, while controlling for the duration of preceding darkness. We found that c-fos was induced equally well by a light pulse that represented ZT 12 or ZT 3. The ZT 12 pulse functioned as an entraining pulse, because animals free-ran after it was removed from the lighting regimen, whereas removing the ZT 3 pulse caused little or no phase shift of activity onset. The data confirm that the expression of SCN c-Fos is not itself sufficient to reset rhythm phase, and they indicate that the role of this gene in the mechanism of photic entrainment is not yet fully understood.

Resetting of the circadian clock by a conditioned stimulus

Environmental light is the dominant temporal cue for the entrainment of circadian rhythms. In mammals, light entrains circadian rhythms by daily resetting a pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Although it is widely held that phase resetting by light involves cellular elements within the SCN that are uniquely responsive to photic cues, we now report that non-photic cues that reliably precede the onset of light can, through associative learning, come to activate these elements. In rats, a neutral non-photic stimulus paired with light in pavlovian conditioning trials was capable of eliciting cellular and behavioural effects characteristic of phase-dependent resetting of the pacemaker by light, the expression of the transcription factor Fos in SCN cells, and phase shifts in free-running activity and temperature rhythms. Thus an associative learning process, pavlovian conditioning, provides a means whereby environmental cues that predict light onset can come to mimic the effect of light on the SCN pacemaker and thereby bring about entrainment of circadian rhythms.

FosB in the suprachiasmatic nucleus of the Syrian and Siberian hamster

The suprachiasmatic nucleus (SCN) generates circadian rhythms of behavior and hormone secretion in mammals, and integrates responses to light and nonphotic stimuli to synchronize such rhythms with the external environment. Previous studies have demonstrated a close association between the induction of the immediate early gene (IEG) c-fos in the SCN by light and phase shifts of circadian rhythms induced by light, but nonphotic stimuli (e.g., arousal), which also cause phase shifts, do not increase c-fos expression in the SCN. Because c-fos is now known to be a member of a large family of IEGs which can regulate transcription and thus cellular function, the aim of the current study was to determine whether induction of another member of this immediate early gene family, fosB, is associated with photic and nonphotic phase shifts. An antiserum that recognizes a unique peptide sequence derived from FosB was produced so that the expression of fosB could be investigated in cells within the SCN by immunocytochemical detection of its protein product. The regional distribution of FosB-immunoreactive (ir) cells in the SCN of Syrian and Siberian hamsters was broadly similar to that for c-Fos-ir cells. However, whereas c-fos expression in the SCN was constitutively low, but could be massively induced by light at particular circadian phases, FosB-ir cells were present at all circadian phases studied, irrespective of photic stimulation, and light only produced marginal increases in the number of FosB-ir cells compared with nonstimulated controls. Moreover, blockade of glutamatergic neurotransmission by pretreatment of hamsters with the NMDA receptor antagonist MK801 significantly reduced photic induction of c-Fos-ir cells, but did not influence the number of FosB-ir cells in the SCN. Finally, an arousing nonphotic stimulus known to cause phase advances in wheel-running behavior in Syrian hamsters did not alter significantly the number of FosB-ir cells in the SCN. These observations indicate that light and nonphotic stimuli are not potent regulators of fosB expression in the SCN. However, because fosB and c-fos can be present in the SCN at the same time after a light pulse, these studies indicate the potential for interactions with each other and with members of the Jun family in the regulation of the circadian timing system.

Cellular colocalization of Fos and neuropeptide Y in the intergeniculate leaflet after nonphotic phase-shifting events

Nonphotic and photic stimuli that phase shift circadian rhythms were presented to hamsters, Mesocricetus auratus. The nonphotic stimulus was a 3-h pulse of novelty-induced wheel running starting at circadian time 4-5. The photic stimulus used was a 0.5 h light pulse starting at circadian time 18. Double immunocytochemistry was used to determine the neurochemical phenotype of cells in the intergeniculate leaflet that were activated by these stimuli. Both the nonphotic and the photic phase-shifting stimuli induced the expression of c-fos in the intergeniculate leaflet compared to unstimulated controls. However, after nonphotic stimulation, Fos-like immunoreactivity was common in neurons that also were NPY positive. Such colocalization of Fos and NPY after photic stimuli was rare. These findings suggest that the NPY pathway from the intergeniculate leaflet to the suprachiasmatic nucleus carries information about nonphotic events.

c-fos and jun-B mRNAs are transiently expressed in fetal rodent suprachiasmatic nucleus following dopaminergic stimulation

We examined the time-course of expression of c-fos and jun-B mRNAs in the fetal rat suprachiasmatic nuclei (SCN) following maternal cocaine injection on gestational day 20. Both c-fos and jun-B mRNA levels increased within 20 min, peaked at 40 min and declined to baseline by 120 min after cocaine treatment (30 mg/kg). In mice, the D1-dopamine agonist, SKF 38393, induced c-fos and jun-B mRNAs in the fetal SCN and striatum. Regulated expression of immediate early genes in the fetal SCN may play a role in entrainment of the fetal clock.

Circadian regulation of Fos B is different from c-Fos in the rat suprachiasmatic nucleus

We measured immunoreactive Fos B protein levels in the ventrolateral subdivision of the rat suprachiasmatic nucleus (SCN) as a function of light and time of day. Immunohistochemistry revealed high levels of Fos B that were uniformly expressed throughout the 12 h:12 h light-dark cycle. Levels remained high in constant darkness and were modestly increased (about 2-fold) after a 2 h light pulse administered during the subjective night, but not after a light pulse during the subjective day. Fos B and c-Fos immunoreactivities could be colocalized within individual SCN cell nuclei using a double-label immunofluorescence method. Thus, despite their structural similarities, these two members of the fos gene family exhibit different patterns of expression in the rat SCN. These and previous data suggest that Fos/Jun DNA-binding complexes in the SCN are composed of constant, as well as variable, protein components; in at least some SCN cells, light-induced changes in the composition of these constituent proteins may lead to altered transcription of target genes.

Physiological mechanisms regulating photic induction of Fos-like protein in hamster suprachiasmatic nucleus

Immediate early genes including c-fos are selectively induced in cells of the hamster suprachiasmatic nucleus (SCN) by nocturnal light stimulation, suggesting that the Fos protein may play a role in the photic entrainment of circadian rhythms. To examine the physiological regulation of the induction of c-fos in the SCN, we studied the effects of antagonists of excitatory amino acids (EAA) receptors on photic induction of Fos-like immunoreactivity (Fos-lir) in the hamster SCN. We also examined the effects of electrical stimulation of the intergeniculate leaflet (IGL) to see whether neural input from IGL to SCN is involved in the induction of Fos protein in SCN cells. The results indicate that for most SCN cells EAA receptors mediate photic input involved in Fos induction but that another mechanism affects cells in restricted area of the caudal SCN. The neurochemical mechanisms and pathways by which these cells are activated by light remain undetermined.

Specific destruction of the serotonergic afferents to the suprachiasmatic nuclei prevents triazolam-induced phase advances of hamster activity rhythms

Administration of Triazolam (Tz)--a short acting benzodiazepine (BZ)--induces permanent phase-shifts in locomotor activity of golden hamsters (Mesocricetus auratus). However, the target area(s) as well as the mechanism involved in the Tz-induced changes are not known. Previous results indicated that raphe nuclei (RN) would appear to be a likely site for Tz-induced phase shifts. Therefore, we specifically destroyed the 5-HT fibers connecting the RN with the SCN--the site of the endogenous mammalian clock--by microinjections of the selective neurotoxin 5,7 dihydroxytryptamine (5,7-DHT) at the level of SCN. Infusion of 5,7-DHT resulted in long lasting damage of the ascending serotonergic projection from RN to the hypothalamus. Subsequently, the phase-shifting effect of Tz was investigated. Only complete or almost complete depletion of the 5-HT input to the SCN was accompanied with a pronounced reduction of the phase shift together with a significant reduction of wheel-running activity during the 6 h following Tz injection. Our present results support the view that the 5-HT innervation of the SCN represents an essential link in the phase-shifting action following peripheral Tz injections.

Temporal regulation of light-induced Fos and Fos-like protein expression in the ventrolateral subdivision of the rat suprachiasmatic nucleus

We measured c-fos messenger RNA levels and Fos protein immunoreactivity in the suprachiasmatic nucleus of rats as a function of light and time of day. Immunohistochemistry demonstrated a daily rhythm of immunoreactive Fos in the ventrolateral subdivision of the suprachiasmatic nucleus of animals entrained to a 12 h/12 h light-dark cycle; expression was low during the dark phase, peaked about 2 h after light onset at dawn, and remained elevated at an intermediate level for the remainder of the light phase. Immunoblots of nuclear extracts showed a 54,000 mol. wt band that increased in density from the dark phase to the early light phase and decreased again during the late light phase. In situ hybridization using a radiolabeled cDNA probe revealed a c-fos messenger RNA signal that was detected as early as 15 min after dawn, prominent at 30 min, and absent by 2 h. The expression of c-fos messenger RNA and Fos immunoreactivity in the suprachiasmatic nucleus depended on the presence of ambient light. In rats entrained to two daily 1-h light pulses corresponding to dawn and dusk ("skeleton" photoperiod) instead of the complete light-dark cycle, immunoreactive Fos was elicited by the dawn pulse alone and was less persistent than during the complete photoperiod. In rats free-running in constant darkness, c-fos messenger RNA and Fos immunoreactivity were stimulated by 2-h light pulses administered only during the subjective night and early subjective day, but not by light pulses during the middle or late subjective day or in the absence of light pulses.

Fos protein expression in the circadian clock is not associated with phase shifts induced by a nonphotic stimulus, triazolam

Recent studies have shown that light-induced phase shifts of the circadian rhythm of locomotor activity are associated with c-fos expression in the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) of the lateral geniculate nucleus of rodents. In order to determine whether c-fos expression is necessary for the phase shifting effects of a non-photic stimulus, we assessed Fos-like immunoreactivity (Fos-lir) in the SCN and IGL at various times after an injection of the short-acting benzodiazepine, triazolam, at circadian time (CT) 6; i.e. at a time when triazolam induces an acute increase in locomotor activity and maximal phase advances in the circadian rhythm of locomotor activity. Specific Fos-lir staining was not observed in the SCN or IGL regions of any animals treated with triazolam or vehicle at any time point examined. These results indicate that exposure to an activity-inducing stimulus at circadian times when this stimulus induces phase shifts does not induce Fos protein synthesis in the SCN or IGL regions.

Compositional changes of AP-1 DNA-binding proteins are regulated by light in a mammalian circadian clock

Recent reports have shown that the nuclear phosphoprotein Fos is induced by light in a mammalian circadian clock, the suprachiasmatic nucleus. To learn how light and circadian phase affect the binding of Fos to DNA, we analyzed the photic and temporal regulation of immunoreactive Jun protein expression and AP-1 DNA-binding activity in the rat suprachiasmatic nucleus. Immunohistochemistry and gel mobility shift assays suggest that AP-1 activity during the night and after a light pulse consists of constant, as well as variable, protein components; JunD could be identified as a constituent of both dark- and light-activated binding complexes, whereas binding by JunB and Fos could be implicated only after photic stimulation. Since JunD or JunB could be colocalized with Fos in individual suprachiasmatic nucleus cell nuclei, light may be acting in at least some suprachiasmatic nucleus cells by altering AP-1 protein composition rather than binding site occupancy.