The
circadian clock in the
suprachiasmatic nucleus (SCN) is synchronized by the 24 h, light :
dark cycle, and is reset by photic and non-photic cues. The acute effects of light in the SCN include the increase of
mRNA levels of the
circadian clock gene Per1 and a dramatic reduction of
pineal melatonin.
Neuropeptide Y (
NPY), which appears to mediate the phase-resetting effects of non-photic stimuli, prevents the ability of light, and stimuli that
mimic light, to phase shift the
circadian clock when injected into the SCN. The purpose of the present study was to determine if
NPY inhibits the ability of light to suppress
pineal melatonin. Surprisingly,
NPY injected into the SCN of hamsters mimicked the effects of light by suppressing
pineal melatonin levels. To confirm that
NPY inhibited the effects of light on the induction of Per1
mRNA levels, Per1
mRNA levels in the SCN were measured in these same animals.
NPY significantly reduced Per1
mRNA levels induced by the light
pulse. The suppression of
melatonin by
NPY appears to be mediated by the same subtype of
NPY receptors in the SCN that mediate the modulation of phase shifts. Injection of Y5
receptor agonists mimicked the effects of
NPY on
pineal melatonin, while injection of a Y2
agonist did not. Thus, these data are the first to demonstrate the
paradoxical effects of
NPY within the SCN.
NPY mimics the effects of light on
pineal melatonin and inhibits the effects of light on the induction of Per1
mRNA.
