Let there be "more" light: enhancement of light actions on the circadian system through non-photic pathways.

Circadian rhythms are internally generated circa 24 h rhythms. The phase of the circadian pacemaker in mammals can be adjusted by external stimuli such as the daily cycle of light, as well as by internal stimuli such as information related to the physiological and behavioral status of the organism, collectively called "non-photic stimuli". We review a large number of studies regarding photic-non-photic interactions on the circadian system, with special focus on two widely described neurotransmitters associated with non-photic input pathways: neuropeptide Y (NPY) and serotonin 5-HT. Both neurotransmitters are capable of phase advancing the master pacemaker oscillation when applied during the subjective day, as do several behavioral manipulations. Also, both are capable of inhibiting light-induced phase shifts during the subjective night, suggesting a dynamic interaction between photic and non-photic stimuli in the fine-tuning of the pacemaker function. Suppression of the NPYergic and/or serotonergic non-photic input pathways can in turn potentiate the phase-shifting effects of light. These findings pose new questions about the possibility of a physiological role for the dynamic interaction between photic and non-photic inputs. This might be particularly important in the case of circadian system adjustments under certain conditions, such as depression, shift work or jet lag.

Blockade of the NPY Y5 receptor potentiates circadian responses to light: complementary in vivo and in vitro studies.

Neuropeptide Y (NPY) is delivered to the suprachiasmatic nuclei (SCN) circadian pacemaker via an input from the thalamic intergeniculate leaflet. NPY can inhibit light-induced responses of the circadian system of Syrian hamsters. Here we studied whether an antagonist to NPY receptors can be used to potentiate photic phase shifts late in the subjective night. First we determined by in situ hybridization that both NPY Y1 and Y5 receptor mRNA are expressed in the SCN of Syrian hamsters. Second, similar to our previous findings at Zeitgeber time 14 (ZT 14, where ZT 12 was the time of lights off), we found that NPY applied at ZT 18.5 onto the SCN region of brain slices maintained in vitro could block NMDA-induced phase advances of the spontaneous firing rate rhythm, and this blocking effect was probably mediated by the Y5 receptor, since co-application of Y5 receptor antagonists completely reversed the effect of NPY, while application of a Y1 receptor antagonist had no effect under the same conditions. Third, we found that co-treatment with a Y5 receptor antagonist in vivo (s.c., 10 mg/kg) not only reversed the effect of NPY applied to the SCN in vivo through a cannula but also significantly potentiated the light-induced phase advance in the absence of NPY. This is the first report of a NPY receptor antagonist having such an effect, and indicates that NPY Y5 receptor antagonists could be clinically useful for potentiating circadian system responses to light.

Is novel wheel inhibition of per1 and per2 expression linked to phase shift occurrence?

We studied whether access to a novel running wheel in vivo could reset the suprachiasmatic nuclei (SCN) in vitro. Golden hamsters were transferred to dim red light at Zeitgeber time (ZT) 4, given their first exposure to a running wheel for 3 h, and killed at either ZT7 or ZT9. Using a brain slice preparation, the SCN firing rate rhythm in vitro was advanced relative to controls only in the slices prepared at ZT9 (phase shift: 2.36+/-0.06 h, n=4) but not ZT7 (-0.26+/-0.16 h, n=4). Transitions to dim red light or brain slice preparation at ZT7 or ZT9 alone do not shift the rhythm. Hamsters with wheels had significantly lower levels of SCN per1 mRNA compared with controls at ZT7, and lower per2 mRNA when examined at ZT9. We conclude that 3 h of novel wheel access appears to require some extended time in vivo in order for the SCN to be reset, even beyond the time when per1 mRNA levels have been altered.

The neuropeptide Y Y5 receptor mediates the blockade of "photic-like" NMDA-induced phase shifts in the golden hamster.

Circadian or daily rhythms generated from the mammalian suprachiasmatic nuclei (SCN) of the hypothalamus can be synchronized by light and nonphotic stimuli. Whereas glutamate mediates photic information, nonphotic information can in some cases be mediated by neuropeptide Y (NPY) or serotonin. NPY or serotonin can reduce the phase-resetting effect of light or glutamate; however, the mechanisms and level of interaction of these two kinds of stimuli are unknown. Here we investigate the effect of NPY on the NMDA-induced phase shift of the hamster SCN circadian neural activity rhythm by means of single-unit recording techniques. NMDA (10-100 microm) applied in the early subjective night induced phase delays in the time of peak firing, whereas doses in the millimolar range disrupted firing patterns. The NMDA-induced phase delay was blocked by coapplication of NPY (0.02-200 microm). NPY Y1/Y5 and Y5 receptor agonists, but not the Y2 receptor agonist, blocked the NMDA-induced phase delay in a similar manner as NPY. The coapplication of a Y5 but not Y1 receptor antagonist eliminated NPY blockade of NMDA-induced phase delays, suggesting that the Y5 receptor is capable of mediating the inhibitory effect of NPY on photic responses. These results indicate that nonphotic and photic stimuli may interact at a level at or beyond NMDA receptor response and indicate that the Y5 receptor is involved in this interaction. Alteration of Y5 receptor function may therefore be expected to alter synchronization of circadian rhythms to light.

Neuropeptide Y in the mammalian circadian system: effects on light-induced circadian responses.

The present review examine the role of neuropeptide (NPY) in the circadian system, focusing on the interactions between light and NPY, especially during the subjective night. NPY has two different effects on the circadian system of mammals. On one hand, NPY, similar to behavioral stimulation, can change the phase of the clock by itself during the subjective day. On the other hand, NPY, again similar to behavioral stimulation, can inhibit the phase-shifting effect of light during the night. These effects of NPY may occur through different receptor subtypes, the Y2 receptor mediating day-time effects and the Y5 receptor mediating night-time effects of NPY. Our results also indicate that there are differences between in vivo and in vitro studies: NPY inhibition of in vivo light-induced phase shifts was observed only late in the subjective night; however, NPY applied in vitro could block light-induced phase shifts early in the subjective night as well. Contrasting these in vivo and in vitro results led us to suggest that the time of day of maximal effect of NPY in the intact animal may be a time when exogenous administration of NPY has little effect, due to saturation of the system. This situation could be an example of how the measurable output of the clock can be affected by the behavioral state in a different way at different time points, depending not only on the clock itself but also on behavior. If verified in human beings, the ability of NPY to modulate the circadian-clock responses to light may be of clinical importance.

Neuropeptide Y applied in vitro can block the phase shifts induced by light in vivo.

The mammalian suprachiasmatic nuclei (SCN) can be synchronized by light, with direct glutamatergic input from the retina. Input to the SCN from the intergeniculate leaflet contains neuropeptide Y (NPY) and can modulate photic responses. NPY can reduce the phase-resetting effect of light or glutamate. We investigated the effect of NPY applied in vitro on light-induced phase shifts of the SCN neural activity rhythm. Light pulses delivered in vivo induced phase shifts in brain slice preparations similar to those as measured by behavioral activity rhythms. NPY applied after the light pulse blocked the phase shifts during both the early and late subjective night. NPY applied 30 min after the light pulse could block the phase delay induced by light. Our results show that NPY can inhibit photic resetting of the clock during the subjective night. The time course of this inhibitory effect suggests a mechanism downstream of the glutamate receptor.

Effects of neonatal clomipramine treatment on locomotor activity, anxiety-related behavior and serotonin turnover in Syrian hamsters.

Day-night differences in locomotor and anxiety-related behavior and brain serotonin metabolism were examined in adult Syrian hamsters that received clomipramine (15 mg/kg) or vehicle from day 8 to day 21 of life. Locomotor activity was significantly greater at the beginning of scotophase (20.00 h) than at noon (12.00 h) and it was highest in hamsters treated with clomipramine at both examined times. Significant day-night differences in anxiety-related behavior, as measured in a plus-maze paradigm, were found in saline-treated hamsters only, with higher values at night. Clomipramine-treated hamsters exhibited augmented 5-hydroxyindoleacetic acid/serotonin ratio in hypothalamus and midbrain raphe, while serotonin content decreased in frontal cortex and hypothalamic areas. The results indicate that neonatal clomipramine treatment produces a long-lasting change in locomotion and anxiety-related behavior, as well as reduces brain serotonin content in hamsters.

Neonatal clomipramine treatment of Syrian hamsters: effect on the circadian system.

The circadian behavior of male Syrian hamsters injected with the serotonin/norepinephrine reuptake inhibitor clomipramine (15 mg/kg from postnatal days 8 to 21) was examined. Clomipramine treatment significantly augmented mean activity values of wheel running rhythm, as well as delayed its acrophase. After a 6-h phase advance of the light-dark cycle, reentrainment of clomipramine-treated hamsters took significantly longer than controls. Clomipramine-treated hamsters exhibited a shorter circadian period than controls in constant light conditions, but no differences were found in constant darkness. Light pulses applied at late subjective night to clomipramine-treated hamsters caused significantly reduced phase advances as compared to controls, while no differences were found in phase delay magnitudes when light pulses were applied during early subjective night. Administration of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) at circadian time 8 significantly advanced the onset of activity to a greater extent in clomipramine-treated hamsters than in controls. The results indicate that neonatal clomipramine treatment of hamsters causes long-lasting changes in the circadian system, by increasing activity levels and by partially inhibiting light-evoked responses. An enhancement of a non-photic, serotonergic-induced response was also unveiled.

Aging attenuates diurnal variation in hamster locomotion, anxiolysis and GABA turnover.

A decrease in amplitude of wheel running circadian rhythms was found in old (18 month old) Syrian hamsters, as compared with young (3 month old.) animals. In a plus-maze paradigm, amplitude of variation of anxiety-related variables (2400 vs. 1600 h) was significantly impaired in aged hamsters. Cerebral cortex, hypothalamic and pineal gamma-aminobutyric acid (GABA) turnover was higher at night, amplitude of variation being significantly smaller in aged hamsters. The results further support the existence of impaired amplitude of circadian rhythms in aged Syrian hamsters.

Daily rhythms in spontaneous and diazepam-induced anxiolysis in Syrian hamsters.

The diurnal variations in spontaneous and diazepam-induced anxiolysis and exploratory behavior were examined in Syrian hamsters in a plus-maze paradigm. The administration of diazepam or flunitrazepam augmented the percentage of time spent in the open arms, the percentage of entries to the open arms, and the number of crosses to both arms, whereas ethyl-beta-carboline injection decreased them. These three behavioral parameters showed significant daily variations, with the maxima being found at night (2400-0400 h). Flumazenil (5 mg/kg) injected at 0400 h decreased significantly the percentage of time spent in open arms and of entries to the open arms, without affecting significantly the total number of entries to both arms. Day-night differences in anxiety-related behavior persisted in hamsters kept under constant darkness for 3 days. Diazepam (0.5 mg/kg) increased the time spent in the open arms at 1600 and 2000 h only, and augmented the percent of entries to the open arms at 2000 h only. The total number of entries to both arms was augmented significantly by diazepam at all time intervals tested, except for 0400 h. The results indicate that Syrian hamsters exhibited significant diurnal changes in anxiolysis-related behavior in the plus-maze paradigm.

Electrodermal response in nonglabrous skin of freely moving rats: mediation by the sympathetic nervous system and evaluation in an animal model of depression.

Electrodermal responses in the facial region of freely moving rats were recorded bilaterally. After a nociceptive stimulus (ammonia vapor exposure), the response (a transient negative potential followed by a longer-lasting positive potential) attained a similar amplitude on both sides. Surgical sympathetic denervation of facial skin by ipsilateral superior cervical ganglionectomy (SCGx) significantly decreased the electrodermal response. When an inferior cervical ganglionectomy was performed in addition to SCGx, a further decrease in electrodermal response was observed. Pretreatment of unilaterally SCGx rats with atropine blunted the electrical response in the control side to levels similar to those found in the SCGx side. Treatment with phenoxybenzamine or propranolol was ineffective. Skin potential responses were measured in adult rats administered with clomipramine from the 8th to the 21st day of life and exhibiting a long-lasting syndrome resembling human depression. Clomipramine-injected rats developed larger skin potential responses to sound stimulation than controls while responses to ammonia vapor were similar in both groups, as well as the habituation rate after repetitive exposure to ammonia vapor. The results indicate that some of the altered electrodermal responses found in depressed patients are detectable in the clomipramine animal model of endogenous depression.

Daily variations in GABA receptor function in Syrian hamster cerebral cortex.

The existence of diurnal changes in postsynaptic expression of gamma-aminobutyric acid (GABA) type A receptors was assessed in cerebral cortex of Syrian hamsters by measuring [3H]GABA binding and the influx of 36Cl- in synaptoneurosomes. A diurnal variation in dissociation constant of [3H]GABA binding to cerebral cortex membranes, and the absence of diurnal differences in maximal number of sites, were found. When the nycthemeral changes in muscimol-stimulated 36Cl- uptake by cortical synaptoneurosomes were assessed, a maximum occurred late at night (i.e. 0400 h). At 1600 h, micromolar concentrations of flunitrazepam potentiated significantly the influx of chloride induced by muscimol, while at 0400 h flunitrazepam did not exert any significant effect on 36Cl- uptake. The results indicate that postsynaptic type A GABAergic activity peaked at nocturnal hours in the cerebral cortex of Syrian hamsters.