Neuroscience
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Tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme in 5-HT synthesis in the brain, is a candidate for participation in a mechanism mediating the antidepressant effect of selective 5-HT reuptake inhibitors such as fluoxetine. Using real-time reverse transcription-polymerase chain reaction (RT-PCR) and semi-quantitative RT-PCR techniques, we have examined the effects of fluoxetine administration with drinking water (7.5 mg/kg/day) for 2, 4 and 8 weeks on TPH2 mRNA expression in the midbrain part of the dorsal raphe nucleus (DRN) and in the brainstem containing the rest of the raphe complex. Fluoxetine treatment for 4 and 8 weeks significantly increased basal TPH2 mRNA levels in the midbrain, an effect that was correlated with the appearance of antidepressant-like effects in the forced swim test. ⋯ In these animals, the swim test also produced a marked decrease in 5-HT metabolite (5-hydroxyindoleacetic acid (5-HIAA)) content in the amygdala. Fluoxetine treatment for 4 and 8, but not for 2 weeks, abolished these swim-induced changes in TPH2 and 5-HTT mRNAs levels in the midbrain and 5-HIAA content in the amygdala. The results of the present study suggest that TPH2 gene expression in the midbrain part of the DRN is implicated in depression and stress response, as well as in the antidepressant fluoxetine action.
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The intergeniculate leaflet (IGL) is a flat thalamic nucleus that responds to retinal illumination, but also to non-photic input from many brain areas. Its only known function is to modulate the circadian rhythm generated by the suprachiasmatic nucleus. Previously, the firing behavior of cells in IGL has been investigated with extra-cellular recordings, but intracellular recordings from morphologically identified mammalian IGL neurons are lacking. ⋯ This suggests that spontaneous activity was controlled by several, yet undetermined factors in addition to membrane potential. Within the IGL we found a broad range of morphologies without apparent categories and no significant correlation with activity. However, the spontaneous, usually regular, spiking and the rebound depolarization of IGL cells is typical a feature that distinguish them from neurons in ventral and from interneurons in the dorsal lateral geniculate nuclei.