Day-Night Changes in Neuropeptide Y Receptor Binding in the Suprachiasmatic Nucleus of the Golden Hamster

The suprachiasmatic nucleus (SCN) of the hypothalamus is a circadian pacemaker that receives input from the retinohypothalamic tract (RHT) and the lateral geniculate nucleus via the geniculohypothalamic tract (GHT). Neuropepbde Y (NPY)-containing fibers in the GHT project to the ventrolateral SCN and modulate circadian rhythmicity. Fluctuation of neuropeptide Y within the SCN has been documented in rats, with peak NPY levels occurring dunng light-dark transitions. In hamsters, local administration ofNPY within the SCN produces phase shifts in circadian wheel running rhythms that are Similar to the phase shifts produced by pulses of darkness and transitions from light to dark