Functional neuroanatomy and neural oscillations during social eavesdropping in male golden hamsters.

Social eavesdropping is a low-cost learning mechanism by which individuals extract relevant social information from social interactions between conspecifics, thereby gaining subsequent advantages in information gathering and usage. The aim of this study was to take advantage of a new hamster model of social eavesdropping to investigate behavioral consequences and neural activity in male hamsters during social eavesdropping. Bystander hamsters with a defeat experience were exposed to either a fighting interaction, a neutral encounter, or control conditions for 3 days of social eavesdropping. In Experiment 1, bystanders in the fight and neutral groups displayed more information gathering behaviors and less nonsocial behavior than control hamsters. The fight group displayed significant increases in c-Fos-positive neurons in the anterior mid-cingulate cortex (aMCC) and the piriform cortex. A slight but not significant group difference was found in their serum cortisol levels. In vivo local field potential oscillation recordings in Experiment 2 revealed that bystanders in the fight group had more delta oscillations in the aMCC during information gathering across 3-day social eavesdropping than those in the other 2 groups. Experiment 3 confirmed that 20 min of social eavesdropping on Day 1 was sufficient to evoke differential behavioral outcomes, and the behavioral responses became more prominent after 3 days of social eavesdropping. Collectively, our study confirmed that male golden hamsters are capable of social eavesdropping and indicated the involvement of aMCC delta oscillations in social eavesdropping.

Pubertal Testosterone Programs Adult Behavioral Adaptations to Sexual Experience through Infralimbic Cortex ΔFosB.

Acquisition of social proficiency entails behavioral adaptations to social experience, including both behavioral flexibility and inhibition of behaviors inappropriate in specific social contexts. Here, we investigated the contributions of testosterone and ΔFosB, a transcription factor linked to experience-dependent neural plasticity, to the adolescent maturation of social proficiency in male-female social interactions. To determine whether pubertal testosterone organizes circuits underlying social proficiency, we first compared behavioral adaptations to sexual experience in male Syrian hamsters that were deprived of testosterone during puberty (prepubertal castration; NoT@P) to those of males deprived of testosterone for an equivalent period of time in adulthood (postpubertal castration; T@P). All males were given testosterone replacement in adulthood for two weeks before sexual behavior testing, where males were allowed to interact with a receptive female once per week for five consecutive weeks. T@P males showed the expected decrease in ectopic (mis-directed) mounts with sexual experience, whereas NoT@P males did not. In addition, sexual experience induced FosB gene products expression in the infralimbic cortex (IL) in T@P, but not NoT@P, males. Overexpression of ΔFosB via an adeno-associated viral (AAV) vector in the IL of NoT@P males prior to sexual behavior testing was sufficient to produce a behavioral phenotype similar to that of experienced T@P males. Finally, overexpression of ΔFosB in IL increased the density of immature spines on IL dendrites. Our findings provide evidence that social proficiency acquired through sexual experience is organized by pubertal testosterone through the regulation of ΔFosB in the IL, possibly through increasing synaptic lability.

Brain inflammatory cytokines and microglia morphology changes throughout hibernation phases in Syrian hamster.

Hibernators tolerate low metabolism, reduced cerebral blood flow and hypothermia during torpor without noticeable neuronal or synaptic dysfunction upon arousal. Previous studies found extensive changes in brain during torpor, including synaptic rearrangements, documented both morphologically and molecularly. As such adaptations may represent organ damage, we anticipated an inflammatory response in brain during specific hibernation phases. In this study, signs of inflammation in the brain were investigated in the Syrian hamster hippocampus (Mesocricetus Auratus) both during hibernation (torpor and arousal phases) and in summer and winter euthermic animals. mRNA expression of the pro-inflammatory cytokines TNF-α, IL-6 and IL-1ß was quantified by RT-qPCR. Morphological changes of microglia were studied by immunohistochemistry staining for IBA-1. Activation of microglia based on retraction and thickening of the dendritic branches and an increase in cell body size was quantified by calculation of cell body size to total cell size ratio. Expression of pro-inflammatory cytokines was upregulated early in arousal (90 min), and normalized after 8 h of arousal. Substantial loss of microglia ramification was found throughout torpor and early arousal together with a 2-fold increase in the cell body size to total cell size ratio. Notably, microglia changes were fully reversed in late arousal (8 h) to euthermic levels. These results demonstrate an upregulation of inflammatory cytokines and signs of microglia activation during hibernation, which completely resolves by late arousal. Activation of this response may serve to prevent or offset brain damage resulting from the substantial physiological changes accompanying torpor and their rapid change during early arousal.

Syrian hamster neuroplasticity mechanisms fail as temperature declines to 15 °C, but histaminergic neuromodulation persists.

Previous research suggests that hippocampal neurons in mammalian hibernators shift their major function from memory formation at euthermic brain temperatures (T b = ~37 °C) to modulation of hibernation bout duration as T b decreases. This role of hippocampal neurons during torpor is based in part on in vivo studies showing that histamine (HA) infused into ground squirrel hippocampi lengthened torpor bouts by ~50%. However, it was unclear if HA acted directly on hippocampal neurons or on downstream brain regions via HA spillover into lateral ventricles. To clarify this, we used hippocampal slices to determine if HA would modulate pyramidal neurons at low levels of synaptic activity (as occurs in torpor). We tested the hypotheses that although LTP (a neuroplasticity mechanism) could not be generated at low temperatures, HA (via H2 receptors) would increase population spike amplitudes (PSAs) of Syrian hamster CA1 pyramidal neurons at low stimulation voltages and low temperatures. PSAs were recorded following Schaffer collateral stimulation from subthreshold levels to a maximum response plateau. We found that tetanus evoked LTP at 35 °C but not 15 °C; and at temperatures from 30 to 15 °C, HA significantly enhanced PSA at near threshold levels in slices from non-hibernating hamsters housed in "summer-like" or "winter-like" conditions and from hibernating hamsters. Cimetidine (H2 antagonist) blocked HA-mediated PSA increases in 8 of 8 slices; pyrilamine (H1 antagonist) had no effect in 7 of 8 slices. These results support our hypotheses and show that HA can directly enhance pyramidal neuron excitability via H2 receptors and thus may prolong torpor bouts.

Roles of RFRP-3 in the Daily and Seasonal Regulation of Reproductive Activity in Female Syrian Hamsters.

In females, reproductive activity relies on proper integration of daily and environmental changes as well as cyclic sex-steroid feedback. This study sought to investigate the role of the hypothalamic Arg-Phe amide-related peptide (RFRP)-3 in the daily and seasonal control of reproductive activity in female Syrian hamsters by analyzing the RFRP system and investigating the effects of central administration of RFRP-3 at different reproductive stages. In long day-adapted sexually active female hamsters, the number of c-Fos-activated RFRP immunoreactive neurons was reduced in the afternoon of diestrus and proestrus; the latter was correlated with increased kisspeptin activity and the luteinizing hormone (LH) surge. Moreover, acute RFRP-3 administration decreased LH secretion when given midafternoon, before the LH surge, and had no effect at other time points of proestrus or diestrus. These data indicate that RFRP-3 exerts a tonic inhibition on LH secretion, which is lifted at the time of the preovulatory surge on the afternoon of proestrus. In short day-adapted sexually inactive female hamsters, Rfrp expression is strongly inhibited in a sex steroid-independent manner, and prolonged central infusion of RFRP-3 completely reactivated the reproductive axis through increased kisspeptin expression, gonadotropin and estradiol secretion, and gonadal weight. These findings reveal a critical role of RFRP-3 in the control of reproductive activity in female rodents and suggest that RFRP neurons, acting alongside kisspeptin neurons, are essential for proper synchronization of reproductive activity with the time of the day, the stage of the estrous cycle, and the seasonal changes in photoperiod.

Cellular changes in the hamster testicular interstitium with ageing and after exposure to short photoperiod.

The aim of this study was to evaluate the cellular changes that occur in the hamster testicular interstitium in two very different physiological situations involving testicular involution: ageing and exposure to a short photoperiod. The animals were divided into an 'age group' with three subgroups - young, adult and old animals - and a 'regressed group' with animals subjected to a short photoperiod. The testicular interstitium was characterised by light and electron microscopy. Interstitial cells were studied histochemically with regard to their proliferation, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP in situ nick end labelling (TUNEL+) and testosterone synthetic activity. We identified two types of Leydig cell: Type A cells showed a normal morphology, while Type B cells appeared necrotic. With ageing, pericyte proliferation decreased but there was no variation in the index of TUNEL-positive Leydig cells. In the regressed group, pericyte proliferation was greater and TUNEL-positive cells were not observed in the interstitium. The testicular interstitium suffered few ultrastructural changes during ageing and necrotic Leydig cells were observed. In contrast, an ultrastructural involution of Leydig cells with no necrosis was observed in the regressed group. In conclusion, the testicular interstitium of Mesocricetus auratus showed different cellular changes in the two groups (age and regressed), probably due to the irreversible nature of ageing and the reversible character of changes induced by short photoperiod.

PSA-NCAM in the posterodorsal medial amygdala is necessary for the pubertal emergence of attraction to female odors in male hamsters.

During puberty, attention turns away from same-sex socialization to focus on the opposite sex. How the brain mediates this change in perception and motivation is unknown. Polysialylated neural cell adhesion molecule (PSA-NCAM) virtually disappears from most of the central nervous system after embryogenesis, but it remains elevated in discrete regions of the adult brain. One such brain area is the posterodorsal subnucleus of the medial amygdala (MePD). The MePD has been implicated in male sexual attraction, measured here as the preference to investigate female odors. We hypothesize that PSA-NCAM gates hormone-dependent plasticity necessary for the emergence of males' attraction to females. To evaluate this idea, we first measured PSA-NCAM levels across puberty in several brain regions, and identified when female odor preference normally emerges in male Syrian hamsters. We found that MePD PSA-NCAM staining peaks shortly before the surge of pubertal androgen and the emergence of preference. To test the necessity of PSA-NCAM for female odor preference, we infused endo-neuraminidase-N into the MePD to deplete it of PSAs before female odor preference normally appears. This blocked female odor preference, which suggests that PSA-NCAM facilitates behaviorally relevant, hormone-driven plasticity.

Spontaneous nonneoplastic lesions in control Syrian hamsters in three 24-month long-term carcinogenicity studies.

Information about the incidence of spontaneously occurring, nonneoplastic background findings in Syrian hamsters is essential if Syrian hamsters are to be used for toxicity studies. Male and female Syrian hamsters of the strain Han:AURA from the Fraunhofer Institute for Toxicology and Experimental Medicine (ITEM) breeding colony were maintained as control animals for carcinogenicity studies and were examined for the presence of nonneoplastic background findings either when they died or when the study was terminated. The nonneoplastic background lesions observed at an incidence of >50% (high), >25% (moderate), and >10% (low) in either male or female animals or in both sexes in one or more long-term studies are detailed. The results are compared to previous published reports of nonneoplastic, spontaneous background lesions in Syrian hamsters. Background information about the incidence of background lesions in Syrian hamsters on short- and long-term studies is useful to both toxicologists and toxicological pathologists.

Suppression of progesterone-enhanced hyperactivation in hamster spermatozoa by γ-aminobutyric acid.

It has been recently shown that mammalian spermatozoa were hyperactivated by steroids, amines and amino acids. In the present study, we investigated whether hyperactivation of hamster sperm is regulated by progesterone (P) and γ-aminobutyric acid (GABA). Although sperm hyperactivation was enhanced by P, GABA significantly suppressed P-enhanced hyperactivation in a dose-dependent manner. Suppression of P-enhanced hyperactivation by GABA was significantly inhibited by an antagonist of the GABAA receptor (bicuculline). Moreover, P bound to the sperm head, and this binding was decreased by GABA. Because the concentrations of GABA and P change in association with the estrous cycle, these results suggest that GABA and P competitively regulate the enhancement of hyperactivation through the GABAA receptor.

Hormonal changes and energy substrate availability during the hibernation cycle of Syrian hamsters.

Animals have to adapt to seasonal variations in food resources and temperature. Hibernation is one of the most efficient means used by animals to cope with harsh winter conditions, wherein survival is achieved through a significant decrease in energy expenditure. The hibernation period is constituted by a succession of torpor bouts (hypometabolism and decrease in body temperature) and periodic arousals (eumetabolism and euthermia). Some species feed during these periodic arousals, and thus show different metabolic adaptations to fat-storing species that fast throughout the hibernation period. Our study aims to define these metabolic adaptations, including hormone (insulin, glucagon, leptin, adiponectin, GLP-1, GiP) and metabolite (glucose, free fatty acids, triglycerides, urea) profiles together with body composition adjustments. Syrian hamsters were exposed to varied photoperiod and temperature conditions mimicking different phases of the hibernation cycle: a long photoperiod at 20 °C (LP20 group), a short photoperiod at 20 °C (SP20 group), and a short photoperiod at 8 °C (SP8). SP8 animals were sampled either at the beginning of a torpor bout (Torpor group) or at the beginning of a periodic arousal (Arousal group). We show that fat store mobilization in hamsters during torpor bouts is associated with decreased circulating levels of glucagon, insulin, leptin, and an increase in adiponectin. Refeeding during periodic arousals results in a decreased free fatty acid plasma concentration and an increase in glycemia and plasma incretin concentrations. Reduced incretin and increased adiponectin levels are therefore in accordance with the changes in nutrient availability and feeding behavior observed during the hibernation cycle of Syrian hamsters.

Impact of wheel running on chronic ethanol intake in aged Syrian hamsters.

INTRODUCTION: Alcohol dependence in aging populations is seen as a public health concern, most recently because of the significant proportion of heavy drinking among "Baby Boomers." Basic animal research on the effects of aging on physiological and behavioral regulation of ethanol (EtOH) intake is sparse, since most of this research is limited to younger models of alcoholism. Here, EtOH drinking and preference were measured in groups of aged Syrian hamsters. Further, because voluntary exercise (wheel-running) is a rewarding substitute for EtOH in young adult hamsters, the potential for such reward substitution was also assessed. METHODS: Aged (24 month-old) male hamsters were subjected to a three-stage regimen of free-choice EtOH (20% v/v) or water and unlocked or locked running wheels to investigate the modulatory effects of voluntary wheel running on EtOH intake and preference. Levels of fluid intake and activity were recorded daily across 60 days of experimentation. RESULTS: Prior to wheel running, levels of EtOH intake were significantly less than levels of water intake, resulting in a low preference for EtOH (30%). Hamsters with access to an unlocked running wheel had decreased EtOH intake and preference compared with hamsters with access to a locked running wheel. These group differences in EtOH intake and preference were sustained for up to 10 days after running wheels were re-locked. DISCUSSION: These results extend upon those of our previous work in young adult hamsters, indicating that aging dampens EtOH intake and preference. Voluntary wheel running further limited EtOH intake, suggesting that exercise could offer a practical approach for managing late-life alcoholism.

Photic sensitivity for circadian response to light varies with photoperiod.

The response of the circadian system to light varies markedly depending on photic history. Under short day lengths, hamsters exhibit larger maximal light-induced phase shifts as compared with those under longer photoperiods. However, effects of photoperiod length on sensitivity to subsaturating light remain unknown. Here, Syrian hamsters were entrained to long or short photoperiods and subsequently exposed to a 15-min light pulse across a range of irradiances (0-68.03 µW/cm(2)) to phase shift activity rhythms. Phase advances exhibited a dose response, with increasing irradiances eliciting greater phase resetting in both conditions. Photic sensitivity, as measured by the half-saturation constant, was increased 40-fold in the short photoperiod condition. In addition, irradiances that generated similar phase advances under short and long days produced equivalent phase delays, and equal photon doses produced larger delays in the short photoperiod condition. Mechanistically, equivalent light exposure induced greater pERK, PER1, and cFOS immunoreactivity in the suprachiasmatic nuclei of animals under shorter days. Patterns of immunoreactivity in all 3 proteins were related to the size of the phase shift rather than the intensity of the photic stimulus, suggesting that photoperiod modulation of light sensitivity lies upstream of these events within the signal transduction cascade. This modulation of light sensitivity by photoperiod means that considerably less light is necessary to elicit a circadian response under the relatively shorter days of winter, extending upon the known seasonal changes in sensitivity of sensory systems. Further characterizing the mechanisms by which photoperiod alters photic response may provide a potent tool for optimizing light treatment for circadian and affective disorders in humans.

The role of endogenous H2S formation in reversible remodeling of lung tissue during hibernation in the Syrian hamster.

During hibernation, small mammals alternate between periods of metabolic suppression and low body temperature ('torpor') and periods of full metabolic recovery with euthermic temperatures ('arousal'). Previously, we demonstrated marked structural remodeling of the lung during torpor, which is rapidly reversed during arousal. We also found that cooling of hamster cells increased endogenous production of H(2)S through the enzyme cystathionine-ß-synthase (CBS). H(2)S suppresses the immune response and increases deposition of collagen. Therefore, we examined inflammatory markers and matrix metalloproteinase (MMP) activity in relation to CBS expression and H(2)S levels in lungs of euthermic and hibernating Syrian hamsters. Lung remodeling during torpor was confirmed by a strong increase in both collagenous and non-collagenous hydroxyproline content. The number of leukocytes in lung was unchanged in any phase of hibernation, while adhesion molecules VCAM-1 and ICAM-1, and the inflammatory marker NF-κB (P65) were modestly upregulated in torpor. Gelatinase activity was decreased in lungs from torpid animals, indicating inhibition of the Zn(2+)-dependent MMP-2 and MMP-9. Moreover, expression of CBS and tissue levels of H(2)S were increased in torpor. All changes normalized during arousal. Inhibition of gelatinase activity in torpor is likely caused by quenching of Zn(2+) by the sulphide ion of H(2)S. In accord, inhibition of CBS normalized gelatinase activity in torpid animals. Conversely, NaHS decreased the gelatinase activity of euthermic animals, which was attenuated by excess Zn(2+). Similar results were obtained on the activity of the Zn(2+)-dependent angiotensin converting enzyme. Our data indicate that increased production of H(2)S through CBS in hamster lungs during torpor contributes to remodeling by inhibition of gelatinase activity and possibly by suppression of the inflammatory response. Although administration of H(2)S is known to induce metabolic suppression in non-hibernating mammals ('suspended animation'), this is the first report implying endogenous H(2)S production in natural hibernation.

Implantation and monitoring of a novel telemetry unit in the Syrian golden hamster model.

Radiotelemetry allows for real-time remote monitoring of biological parameters in freely moving laboratory animals. The HD-X11 transmitter is a novel telemetry device that enables simultaneous collection of body temperature, activity, blood pressure, electrocardiogram (ECG), and other biopotentials in small animal models. Previously, researchers could only collect either blood pressure or ECG parameters; prioritizing the signal of most interest or increasing the number of animals on study to capture both signals at one time. This new device eliminates the need for separate animal groups for assorted measurements and allows for a more complete cardiovascular assessment. Evaluation of the transmitter from both surgical and data collection perspectives indicates that the HD-X11 transmitter can be a useful tool to researchers in a wide range of scientific and medical fields.

Reversible remodeling of lung tissue during hibernation in the Syrian hamster.

During hibernation, small rodents such as hamsters cycle through phases of strongly suppressed metabolism with low body temperature (torpor) and full restoration of metabolism and body temperature (arousal). Remarkably, the repetitive stress of cooling-rewarming and hypoxia does not cause irreversible organ damage. To identify adaptive mechanisms protecting the lungs, we assessed histological changes as well as the expression and localization of proteins involved in tissue remodeling in lungs from Syrian hamsters at different phases of hibernation using immunohistochemical staining and western blot analysis. In torpor (early and late) phase, a reversible increased expression of smooth muscle actin, collagen, angiotensin converting enzyme and transforming growth factor-ß was found, whereas expression of the epidermal growth factor receptor and caveolin-1 was low. Importantly, all these alterations were restored during arousal. This study demonstrates substantial alterations in protein expression mainly in epithelial cells of lungs from hibernating Syrian hamsters. These structural changes of the bronchial airway structure are termed airway remodeling and often occur in obstructive lung diseases such as asthma, chronic obstructive pulmonary disease (COPD) and lung fibrosis. Unraveling the molecular mechanism leading to reversal of airway remodeling by the end of torpor may identify possible therapeutic targets to reduce progression of this process in patients suffering from asthma, chronic obstructive pulmonary disease and lung fibrosis.

Information theory and the neuropeptidergic regulation of seasonal reproduction in mammals and birds.

Seasonal breeding in the temperate zone is a dramatic example of a naturally occurring change in physiology and behaviour. Cues that predict periods of environmental amelioration favourable for breeding must be processed by the brain so that the appropriate responses in reproductive physiology can be implemented. The neural integration of several environmental cues converges on discrete hypothalamic neurons in order to regulate reproductive physiology. Gonadotrophin-releasing hormone-1 (GnRH1) and Kisspeptin (Kiss1) neurons in avian and mammalian species, respectively, show marked variation in expression that is positively associated with breeding state. We applied the constancy/contingency model of predictability to investigate how GnRH1 and Kiss1 integrate different environmental cues to regulate reproduction. We show that variation in GnRH1 from a highly seasonal avian species exhibits a predictive change that is primarily based on contingency information. Opportunistic species have low measures of predictability and exhibit a greater contribution of constancy information that is sex-dependent. In hamsters, Kiss1 exhibited a predictive change in expression that was predominantly contingency information and is anatomically localized. The model applied here provides a framework for studies geared towards determining the impact of variation in climate patterns to reproductive success in vertebrate species.

Play fighting and corticotropin-releasing hormone in the lateral septum of golden hamsters.

This study was focused on determining the possible role of corticotropin-releasing hormone (CRH) on play fighting in juvenile golden hamsters. As no specific neural sites have been proposed, we looked for changes in CRH innervations at the peak of play-fighting activity on postnatal day 35 (P-35) from a week before on P-28. We noted that the increase in play-fighting activity between these two dates was associated with a 100% increase of the density of CRH fibers within the lateral septum. We, then, tested the possible role of CRH receptors on play fighting within the lateral septum through microinjections of alpha-helical CRH, a CRH receptor antagonist (either 0, 30, or 300 ng), directly into the area. The treatments inhibited play-fighting attacks and pins as well as reduced the duration of time that the resident hamsters spent in contact with the intruders, though locomotor activity remained unaffected. The possible source of CRH release in the lateral septum was addressed by quantification of CRH neurons also labeled with a marker of cellular activity, c-Fos, after consummation of play fighting. CRH neurons in the horizontal part of the diagonal band, an area reciprocally connected with the lateral septum, showed a 75% increase in double labeling with c-Fos as compared to controls. Together, these data show that CRH receptors in the lateral septum have a general role on play fighting, not just facilitating its consummation, but also likely enhancing appetitive aspects as well. In addition, this effect is associated with enhanced CRH availability in the area and enhanced neuronal activity within interconnected areas.

Growth-differentiation factor-8 (GDF-8) in the uterus: its identification and functional significance in the golden hamster.

Transforming growth factor-beta superfamily regulates many aspects of reproduction in the female. We identified a novel member of this family, growth-differentiation factor 8 (GDF-8) in the 72 h post coital uterine fluid of the golden hamster by proteomic techniques. Uterine GDF-8 mRNA decreased as pregnancy progressed while its active protein peaked at 72 h post coitus (hpc) and thereafter stayed at a lower level. At 72 hpc, the GDF-8 transcript was localized to the endometrial epithelium while its protein accumulated in the stroma. Exogenous GDF-8 slowed down proliferation of primary cultures of uterine smooth muscle cells (SMC) and endometrial epithelial cells (EEC). In addition, GDF-8 attenuated the release of LIF (leukaemia inhibiting factor) by EEC. As for the embryo in culture, GDF-8 promoted proliferation of the trophotoderm (TM) and hatching but discouraged attachment. Our study suggests that GDF-8 could regulate the behavior of preimplantation embryos and fine-tune the physiology of uterine environment during pregnancy.

Immunolocalization of NGF and its receptors trkA and p75 in the oviducts of golden hamsters during the estrous cycle.

To investigate the physiological roles of nerve growth factor (NGF) in the oviduct of golden hamsters during the estrous cycle, the localization of NGF and its receptors, trkA and p75, were determined by immunohistochemistry. Positive staining of NGF, trkA, and p75 was present in epithelial cells and muscle cells of the infundibulum, ampulla, and isthmus in the oviduct. The intensities of the immunohistochemical signals for NGF, trkA, and p75 did not markedly change in any segment of the oviduct during the estrous cycle. These results suggest that NGF may play autocrine/paracrine roles in oviductal transport, fertilization, capacitation of spermatozoa and early embryonic development in the oviduct of golden hamsters.

Daytime gating in the Syrian hamster pineal gland.

Melatonin synthesis in rodents is tightly regulated at the transcriptional level by stimulatory and inhibitory transcription factors. Among them, phosphorylated cAMP-related element binding protein (pCREB) and inducible cAMP early repressor (ICER), a strong inhibitor of cAMP-related element-driven genes, have an antagonistic action in activating/inhibiting the transcription of the Aa-nat gene, which is an important enzyme in melatonin synthesis. In the Syrian hamster, a rodent displaying a seasonal control of reproduction, melatonin synthesis is strongly gated to the second part of the night. Indeed, exogenous adrenergic stimulation is unable to stimulate Aa-nat gene transcription and melatonin synthesis during daytime. In the present study, we investigated whether ICER may be the cause of this daytime repression by comparing the dynamic of ICER and the adrenergic regulation of two genes whose expression is rapidly activated by cAMP-dependant mechanisms, c-fos and Icer. Adrenergic induction of c-fos and Icer expression was not possible during daytime, except at early day. ICER immunoreactivity was elevated throughout the daily cycle but reached the highest levels at early day, when gene expression can be induced by adrenergic agonists. Additionally, CREB phosphorylation was subjected to the same daily gating with an adrenergic induction occurring in the early but not in the late day. Taken together, our results indicate that the diurnal gating of pineal activity in the Syrian hamster is not caused by the repressor ICER and that it may occur at the level of noradrenergic receptor signalling.