Neuropeptides phase shift the mammalian circadian pacemaker

Area of Science:

• Neuroscience
• Chronobiology
• Peptide Signaling

Background:

• The suprachiasmatic nucleus (SCN) is the master circadian pacemaker in mammals.
• Several neuropeptides, including gastrin-releasing peptide (GRP1-27), peptide histidine isoleucine (PHI), and vasoactive intestinal polypeptide (VIP), are found in SCN neurons.
• The functional role of these colocalized peptides in regulating circadian rhythms remains to be fully elucidated.

Purpose of the Study:

• To investigate the influence of microinjected GRP1-27, PHI, and VIP on the circadian locomotor activity rhythms of Syrian hamsters.
• To determine if these peptides exert phase-dependent effects on circadian timing.
• To assess potential synergistic interactions between GRP1-27, PHI, and VIP in modulating circadian rhythms.

Main Methods:

• Syrian hamsters were maintained under constant light conditions and allowed to free run.
• GRP1-27, PHI, VIP, or a combination thereof were microinjected into the SCN region at different circadian times (CT).
• Locomotor activity rhythms were monitored, and phase shifts were quantified.

Main Results:

• All tested peptides (GRP1-27, VIP, PHI) induced phase-dependent phase shifts in hamster locomotor activity rhythms, irrespective of lighting conditions.
• GRP1-27 demonstrated dose-dependent phase-delaying effects at CT12-14, which were blocked by a specific antagonist.
• A neuropeptide cocktail of GRP1-27, PHI, and VIP did not show synergistic effects; its impact was comparable to GRP1-27 alone and less than the sum of individual peptide effects.

Conclusions:

• GRP1-27, PHI, and VIP can independently modulate circadian rhythms by inducing phase shifts.
• The observed effects are phase-dependent and not influenced by constant lighting.
• There is no evidence of synergistic interactions among these colocalized neuropeptides in regulating the circadian pacemaker.