Acoustic stimulation in vivo and corticotropin-releasing factor in vitro increase tryptophan hydroxylase activity in the rat caudal dorsal raphe nucleus.

Exposure of rats to unpredictable loud sound pulses increases activity of the rate-limiting enzyme for serotonin synthesis, tryptophan hydroxylase (TPH), in the median raphe nucleus (MnR) and a mesolimbocortical serotonergic system. Corticotropin-releasing factor (CRF)-induced activation of a subset of serotonergic neurons in the caudal dorsal raphe nucleus (DR) may underlie stress-related increases in TPH activity in the MnR and a mesolimbocortical serotonergic system. An in vivo acoustic stimulation paradigm and an in vitro brain slice preparation were designed to test the hypothesis that stress-related stimuli and CRF receptor activation have convergent actions on TPH activity in the caudal DR (DRC). We measured 5-hydroxytryptophan (5-HTP) accumulation as an index of TPH activity following inhibition of aromatic amino acid decarboxylase (using NSD-1015). To examine effects of acoustic stimulation on TPH activity, male Wistar rats, pretreated with NSD-1015, were exposed to a 30 min sham, predictable or unpredictable acoustic stimulation paradigm; brains were frozen and microdissected for analyses of tissue 5-HTP concentrations in subregions of the DR. To examine the effect of CRF receptor activation on TPH activity, freshly prepared brain slices were exposed to CRF (0-2000 nM) for 10 min in the presence of NSD-1015, then frozen and microdissected for analysis of tissue 5-HTP concentrations. Increases in TPH activity in the DRC, but not other subregions, were observed in both paradigms. These findings are consistent with the hypothesis that stress-related increases in TPH activity are mediated via effects of CRF or CRF-related neuropeptides on a mesolimbocortical serotonergic system originating in the DRC.