NEUROPEPTIDE-Y BLOCKS LIGHT-INDUCED PHASE ADVANCES BUT NOT DELAYS OF THE CIRCADIAN ACTIVITY RHYTHM IN HAMSTERS

In mammals, the suprachiasmatic nuclei (SCN) are the anatomical site o f localization of the light-entrainable circadian clock responsible fo r the generation of daily rhythms in physiology and behavior. In addit ion to direct retinohypothalamic innervation, the SCN receive a promin ent projection of fibers from the intergeniculate leaflet (IGL) of the thalamus, the geniculohypothalamic tract (GHT), some of which contain the neurotransmitter, neuropeptide Y (NPY). Since the GHT has been su ggested to play a role in the modulation of photic entrainment of the SCN circadian clock in rodents, we investigated the effects of local a dministration of NPY into the region of the SCN on light-induced phase shifts of the free-running activity rhythm in hamsters. Injection of 60 nmol of NPY into the SCN region 10 min prior to light exposure at c ircadian time 19 completely blocked light-induced phase advances. Simi lar treatment at circadian time 14 had no significant effect on the ma gnitude of light-induced phase delays. Injection of NPY at either time point without light exposure did not alter circadian phase. The findi ngs support a modulatory role for NPY in the photic entrainment of the SCN circadian clock. (C) 1997 Elsevier Science Ireland Ltd.