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Abstract
The geniculohypothalamic tract (GHT) is a projection from the intergeniculate leaflet
to the suprachiasmatic nucleus (SCN). The GHT exhibits neuropeptide Y (NPY) immunoreactivity
and appears to communicate photic 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 darkness or triazolam
injections. In the present study, NPY was injected 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 followed
by significant phase advances). These data suggest that NPY produces phase shifts
by some mechanism other than by inducing wheel running or by inhibiting the response
of SCN neurons to light and supports a role for NPY in nonphotic shifting of the circadian
clock.