Phase response curve of mouse locomotor activity rhythm under constant light after 8-OH-DPAT and dark pulses

Abstract

Both photic and nonphotic stimuli modulate suprachiasmatic nuclei (SCN). In the mammalian circadian system, serotonin is involved in photic as well as nonphotic regulation of circadian rhythms, but its role in this is still controversial. In our previous experiments we characterized the phase response curve for (R)-(+)-8-OH-DPAT under constant light (LL) in mice, and found its shape to resemble the PRC for dark pulses in mice under LL. This suggested to us that the mechanism responsible for the phase shifts of the locomotor activity rhythm induced by 5-HT1A agonist and by dark pulses may be the same. The aim of this work was to describe the PRC of (R)-(+)-8-OH-DPAT after the application of 5-HT1A agonist and 4-hour dark pulses in mice under LL in order to elucidate the mechanism of the action of the serotonergic pathway on SCN. The results indicated that the influence of 5-HT1A agonist and dark pulses on SCN increased during mid subjective day, whereas at 2:00 CT and 10:00 CT the effect decreased. This pattern is connected with the diurnal rhythm of 5-HT in suprachiasmatic nuclei (SCN), dorsal raphe nuclei (DRN) and midbrain raphe nuclei (MRN), as well as with the complex mechanism of dark-pulse-induced phase shifts of the locomotor activity rhythm under LL.