MEMORY ON LONG BUT NOT ON SHORT DAYS IS STORED IN THE RAT SUPRACHIASMATIC NUCLEUS

In order to follow adjustment of the rat circadian pacemaking system t o a change of the photoperiod, the rhythm in the light-induced c-fos e xpression was used as a marker of the intrinsic rhythmicity of the sup rachiasmatic nucleus (SCN). After a change from a long photoperiod wit h 16 h of light per hay to a short one with 8 h of light, the interval enabling high c-fos photoinduction decompressed gradually and the ful l extension by 5-6 h was achieved only within 2 weeks. After a change from a short to a long photoperiod, tile interval was compressed by 5- 6 h within 3 days. The data indicate a rapid adjustment of the photope riod dependent-state of the SCN pacemaker to long days but only a slow one to short days.