Light and melatonin inhibit in vivo serotonergic phase advances without altering serotonergic-induced decrease of per...

Area of Science:

• Chronobiology
• Neuroendocrinology
• Molecular Biology

Background:

• Serotonergic (5-HTergic) stimulation during daytime influences the mammalian circadian timing system.
• This stimulation can cause behavioral phase advances and alter Period (Per1 and Per2) mRNA levels in the suprachiasmatic nuclei (SCN).

Purpose of the Study:

• To investigate the interactions between light or melatonin and serotonergic stimulation.
• To examine effects on phase resetting and clock gene expression in Syrian hamsters under constant darkness.

Main Methods:

• Syrian hamsters were housed in constant darkness.
• Investigated the effects of light exposure or melatonin administration prior to 8-OH-DPAT (a 5-HT(1A/7) receptor agonist) injection.
• Measured behavioral phase advances and Per1/Per2 mRNA levels in the SCN.

Main Results:

• Both light and melatonin blocked the behavioral phase advances induced by 8-OH-DPAT.
• Neither light nor melatonin prevented the down-regulation of Per1 and/or Per2 mRNA in the SCN caused by serotonergic stimulation.
• Afferent cues differentially affect phase adjustment and clock gene expression.

Conclusions:

• Light and melatonin can dissociate behavioral responses from molecular clock gene expression changes in the SCN during daytime.
• The circadian timing system exhibits differential modulation of phase adjustment and gene expression by external cues.