PHASE RESPONSE CURVES TO NEUROPEPTIDE-Y IN WILDTYPE AND TAU MUTANT HAMSTERS

Neuropeptide Y (NPY)-containing fibers project from the intergeniculat e leaflet to the suprachiasmatic nucleus. NPY has been shown to phase shift the circadian locomotor activity rhythm of wildtype hamsters, pr oducing large phase advances in the subjective day and small delays in the subjective night. Previous studies have implicated this pathway i n the mediation of activity-induced resetting of the circadian clock. Homozygous tau mutant and wildtype hamsters respond very differently t o pulses of activity. Not only is the amplitude of the phase response curve exaggerated in the mutants with shifts of up to 7 h, but the sti muli are effective at different times during the cycle. Homozygous tau mutant hamsters and wildtype controls were implanted with guide cannu las aimed at the suprachiasmatic nucleus and injected with NPY at vari ous times during the circadian cycle. The responses of homozygous tau mutant hamsters to NPY resembled their responses to nonphotic stimuli in both timing and direction of phase shift. This finding provides cor relational evidence that NPY is involved in the effects of nonphotic b ehavioral events on the circadian system.