NEUROPEPTIDE-Y MICROINJECTED INTO THE SUPRACHIASMATIC REGION PHASE-SHIFTS CIRCADIAN-RHYTHMS IN CONSTANT DARKNESS

The geniculohypothalamic tract (GHT) is a projection from the intergen iculate leaflet to the suprachiasmatic nucleus (SCN). The GHT exhibits neuropeptide Y (NPY) immunoreactivity and appears to communicate phot ic information to the SCN. Microinjection of NPY into the SCN has been found to phase shift circadian rhythms of hamsters housed in constant light in a manner similar to the phase shifts produced by pulses of d arkness or triazolam injections. In the present study, NPY was injecte d into the SCN of Syrian hamsters housed in constant darkness and was found to produce phase shifts similar to those seen in hamsters housed in constant light. Microinjections were not followed by wheel running during the subjective day (the time when NPY microinjections are foll owed by significant phase advances). These data suggest that NPY produ ces phase shifts by some mechanism other than by inducing wheel runnin g or by inhibiting the response of SCN neurons to light and supports a role for NPY in nonphotic shifting of the circadian clock.