Photoperiodic dependent changes in the number of neurons containing mRNA encoding neuropeptide Y in the intergeniculate leaflet of the Syrian hamster

The integeniculate leaflet (IGL) is a distinct division of the lateral geni culate complex that participates in the regulation of the circadian rhythm through its projections to the circadian pacemaker located in the suprachia smatic nuclei of the hypothalamus. A high number of neuropeptide Y (NPY) ce ll bodies has been described in the IGL by immunohistochemistry and in situ hybridization. The present study investigated whether NPY in the IGL is in fluenced by the length of the daily photoperiod. By using in situ hybridiza tion we show a significant increase of the number of NPY mRNA containing ne urons in the mid-part of the IGL of Syrian hamsters maintained in a short p hotoperiod compared to those kept in a long photoperiod. On the other hand, NPY mRNA expression per cell in the IGL is similar in both photoperiods te sted. (C) 1998 Elsevier Science B.V. All rights reserved.