Stress upregulates TPH1 but not TPH2 mRNA in the rat dorsal raphe nucleus: identification of two TPH2 mRNA splice variants.

Serotonin is implicated in stress-related psychopathologies. Two isoforms of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase, TPH1 and TPH2, are known. We show here that in the rat dorsal raphe nucleus (DRN), the nucleus that contains the highest number of 5-HT neurons in the brain, TPH1 mRNA reveals a low level of expression but is detectable both by quantitative real-time PCR and in situ hybridization whereas in the pineal gland (PiG), TPH1 mRNA is strongly expressed. To examine effects of stress on TPH expression we exposed male Wistar rats to daily restraint stress for 1 week. As shown by quantitative real-time PCR, TPH1 mRNA is 2.5-fold upregulated by the stress in DRN but not in PiG. Using 3'-RACE, we identified two TPH2 mRNA splice variants in the rat DRN which differ in the length of their 3'-untranslated regions (UTRs). TPH2b (with a short 3'-UTR) is the predominant variant in the DRN, whereas TPH2a (with a longer 3'-UTR) shows a low abundance in this nucleus. In the PiG, only TPH2b is detectable revealing a low level of expression. Expression of both TPH2 splice variants is not affected by stress, neither in DRN nor in the PiG. These data indicate that TPH1 in the serotonergic neurons of the DRN might be relevant for stress-induced psychopathologies.