Calcium-dependent induction of neuronal tryptophan hydroxylase (tph2) transcription and inhibition by 5-HT1A autoreceptor activation

F. REMES LENICOV; P. R. ALBERT

Washington DC

Conferencia; 35th Annual Neuroscience Meeting; 2005

Society for Neuroscience

The neuronal isoform of tryptophan hydroxylase (TPH2) is the rate-limiting enzyme in serotonin (5-HT) biosynthesis in the brain. Transcriptional control of TPH2 expression is expected to influence the 5-HT output of serotonergic neurons and low 5-HT levels could predispose to depression. However the transcriptional regulation of this key enzyme has not been characterized. To study TPH2 regulation we have used RN46A cells, which express both TPH1 and TPH2 RNA. RN46A cells are derived from embryonic rat raphe neurons and can be differentiated to express an enhanced serotonergic phenotype. We quantified TPH2 RNA levels in rat raphe RN46A cells by real time RT-PCR. Levels of TPH2 RNA were normalized to GAPDH RNA. Upon differentiation for 7 days, RN46A cells showed a 3-4 fold induction of TPH2 RNA level, hence differentiated RN46A cells were examined. Depolarization of the cells using 40 mM KCl induced a 4- to 5-fold increase in TPH2 RNA within 3 h. Similarly, calcium mobilization using the ionophore ionomycin induced a concentration dependent increase in TPH2 RNA of up to 4-fold basal within 3 h, suggesting a key role for calcium in the induction of TPH2 RNA. Previous studies established that activation of 5-HT1A and 5-HT1B autoreceptors decreased 5-HT synthesis, but the mechanisms are unclear. RN46A cells stably transfected with 5-HT1A receptor were treated for 24 h with specific 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), which inhibited TPH2 expression by up to 40% in a concentration-dependent manner. Since 5-HT1A autoreceptors inhibit calcium entry, the role of calcium in 5-HT1A action will be addressed. We conclude that TPH2 expression is induced by calcium mobilizing agents and inhibited by 5-HT1A signaling that reduces calcium, providing the first mechanistic evidence for calcium dependent regulation of TPH2. We suggest that activation of autoreceptors and decrease of TPH2 RNA levels may explain 5-HT-mediated decrease in 5-HT levels following antidepressant treatment.