Complex regional influence of photoperiod on the nycthemeral functioning of the dorsal and median raphe serotoninergic system in the Syrian hamster

The Syrian hamster (Mesocricetus auratus) is a widely used species for the study of biological clock synchronization and photoperiodism. The serotoninergic system arising from the median (MnR) and the dorsal raphe (DR) is a major actor in circadian clock synchronization. This serotoninergic system is also associated with functions and behaviours influenced by seasonal changes. The aim of the present study was to assess the influence of photoperiod on the daily functioning of the MnR and DR serotoninergic system. The morphology of both raphe nuclei was analysed in hamsters kept under long and short photoperiod by immunocytochemical detection of two markers of the serotoninergic system, serotonin and tryptophan hydroxylase (TPH, the rate-limiting enzyme of serotonin synthesis). The morphological analysis revealed a fairly complex morphological organization of the DR and MnR along their caudo-rostral extent. This morphological organisation was similar in the two photoperiods. However, quantification of several markers of serotonin (5-HT) synthesis in the DR and MnR revealed a time-dependent functioning of serotoninergic cells that was locally influenced by photoperiod. In particular, the significant daily variations of tph2-mRNA and TPH levels in the rostral MnR, and of the 5-HT/5-HIAA (5-hydroxyindoleacetic acid) ratio within the suprachiasmatic nuclei, were abolished under short photoperiod. The results are discussed with regard to the known physiological role of the serotoninergic system on the biological clock.