Phase response relationships between light pulses and the melatonin rhythm in rats

Acute inhibition of casein kinase 1δ/ε rapidly delays peripheral clock gene rhythms

Circadian rhythms are generated through a transcription-translation feedback loop involving clock genes and the casein kinases CSNK1D and CSNK1E. In this study, we investigated the effects of the casein kinase inhibitor PF-670462 (50 mg/kg) on rhythmic expression of clock genes in the liver, pancreas and suprachiasmatic nucleus (SCN) as well as plasma corticosterone, melatonin and running behaviour in rats and compared them to the responses to a 4 h extension of the light phase. PF-670462 acutely phase delayed the rhythmic transcription of Bmal1, Per1, Per2 and Nr1d1 in both liver and pancreas by 4.5 ± 1.3 and 4.5 ± 1.2 h, respectively, 1 day after administration. In the SCN, the rhythm of Nr1d1 and Dbp mRNA expression was delayed by 4.2 and 4 h, respectively. Despite these changes, the time of peak plasma melatonin secretion was not delayed, although the plasma corticosterone rhythm and onset of wheel-running activity were delayed by 2.1 and 1.1 h, respectively. These changes are in contrast to the effects of the 4 h light extension, which resulted in delays in peak expression of the clock genes of less than 1 h and no change in the melatonin or corticosterone rhythms. The ability of the casein kinase inhibitor to bring about large phase shifts in the rhythms of major metabolic target tissues may lead to new drugs being developed to rapidly phase adjust circadian rhythms to alleviate the metabolic impact of shift work.

Enhanced circadian photoresponsiveness after prolonged dark adaptation in seven species of diurnal and nocturnal rodents

Previous studies in mice and Syrian hamsters have described an enhancement of circadian photoresponsiveness after exposure to darkness for several weeks. The present study investigated the generality of the phenomenon in 3 diurnal and 4 nocturnal rodent species. In four of the species tested, phase delays of the running-wheel activity rhythm evoked by 1-h light pulses were several-fold larger after 3 to 4 weeks of exposure to darkness than after a single day. This drastic change in photoresponsiveness has important implications for the understanding of the process of photic entrainment. Differences between species that showed a significant effect of dark adaptation and species that showed no effect were not accounted for by temporal niche (diurnal versus nocturnal) or photic sensitivity (albino versus pigmented). Further research is needed to elucidate the mechanisms responsible for inter-species differences in the occurrence of enhanced photoresponsiveness after dark adaptation and to identify the neural substrates of this phenomenon in species that exhibit it.

Activation of 5-HT2C receptors acutely induces Per gene expression in the rat suprachiasmatic nucleus at night

The suprachiasmatic nucleus (SCN) of the hypothalamus receives dense serotonergic projections from the raphe nuclei and this input has been implicated in the modulation of circadian rhythms. In the present study, we investigated the effect of 5-HT2C receptor activation on various clock genes within the suprachiasmatic nucleus, including Per1 and Per2, which have previously been demonstrated as necessary for phase shifts. Rats were exposed to light (400 lx, 15 min), administered 5-HT2C receptor agonists (+/-)-1-(4-iodo-2,5-dimethoxy-phenyl)-2-aminopropane (DOI) (2 mg/kg) or RO 60-0175 (10 mg/kg) or vehicle 4 or 10 h after dark onset (ZT16 and ZT22). The expression of Per1, Per2, Cry1, Clock, Bmal1, Dec1, Dec2 and c-fos was determined 30 and 120 min after treatment in suprachiasmatic nucleus punches by real time reverse transcription-polymerase chain reaction (RT-PCR). Light exposure induced a 7-fold increase in c-fos expression within 30 min of treatment at both ZT16 and ZT22. Per1 expression was increased 2-fold following light exposure at ZT22, whereas treatment at ZT16 had no significant effect. Per2 expression was significantly induced following light at ZT16, but was not affected at ZT22. RO 60-0175 or DOI administration induced a 5-fold change in c-fos expression at ZT16 and a 3-fold change at ZT22 within 30 min of treatment. The drug increased both Per1 and Per2 expression at ZT16, but had no effect at ZT22. These results provide evidence for 5-HT2C receptors being involved in the modulation of circadian rhythms during early night.

Serotonin, excitatory amino acids and the photic control of melatonin rhythms and SCN c-FOS in the rat

There is a growing acceptance that serotonergic pathways to the suprachiasmatic nucleus play an important role in the mediation and modulation of light entrainment of rhythms. In this study administration of the 5-HT(2A/2C) agonist (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI, 0.5 mg/kg) at mid dark caused a phase shift in the onset of the urinary excretion of 6-sulphatoxymelatonin in rats that was sustained for at least 8 days and was blocked by the specific 5-HT(2C) antagonist SB-242084. Administration of DOI (2 mg/kg) across the night resulted in the appearance of c-FOS in the nucleus of cells in the suprachiasmatic nucleus during subjective darkness, but did not cause induction at the time of expected lights on (CT0). By contrast light exposure induced c-fos throughout the night including CT0. Administration of the NMDA receptor antagonist MK-801 (3 mg/kg) prior to light pulses had no effect on c-fos in the first part of the night, but towards the expected time of lights on, became progressively more potent, such that by CT0, light induction of c-fos was almost completely inhibited. These results provide further evidence that serotonin plays a role in the mediation of light effects on rhythms in the rat.