Tryptophan hydroxylase 2 and Bcl-xL in the rat raphe nucleus after acute and chronic forced swim stress

Abstract

Stressful events induce changes in the expression of numerous genes and their protein products in the brain. However, the mechanisms mediating these changes, as well as their significance for the development of stress-induced depression or for coping with stress, remain obscure. Evidence for the sensitivity of tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme of the serotonin (5-HT) pathway, to stress is concisely reviewed, as well as the neuroprotective function of the antiapoptotic protein Bcl-xL in the brain. The goal of our experiments was to investigate the gene and protein expression of TPH2 and Bcl-xL in the dorsal (DRN) and median (MRN) raphe nuclei during repeated stress events. Gene (RT PCR) and protein (immunohistochemistry) expression was assessed 24 hours after the second and fourteenth forced swim sessions. The increase in TPH2 protein expression observed after the second swim stress exposure might reflect the protective action of Bcl-xL. During the subsequent stressful events, the stress-induced increase in Bcl-xL expression decreased. This effect was associated with the weakening of serotonergic neuron function evidenced by the compensatory activation of the TPH2 gene expression without TPH2 protein increase. Thus, shortand long-term forced swimming resulted in qualitatively different alterations in brain expression of TPH2 and Bcl-xL, suggesting their specific roles during acute and chronic stages in the development of stress-induced psychopathology. These changes may constitute a component of the mechanisms underlying elevated tph2 gene expression in depressed patients.