Differential innervation of superficial versus deep laminae of the dorsal horn by bulbo-spinal serotonergic pathways in the rat.

Convergent data showed that bulbo-spinal serotonergic projections exert complex modulatory influences on nociceptive signaling within the dorsal horn. These neurons are located in the B3 area which comprises the median raphe magnus (RMg) and the lateral paragigantocellular reticular (LPGi) nuclei. Because LPGi 5-HT neurons differ from RMg 5-HT neurons regarding both their respective electrophysiological properties and responses to noxious stimuli, we used anatomical approaches for further characterization of the respective spinal projections of LPGi versus RMg 5-HT neuron subgroups. Adult Sprague-Dawley rats were stereotaxically injected into the RMg or the LPGi with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). The precise location of injection sites and RMg vs LPGi spinal projections into the different dorsal horn laminae were visualized by PHA-L immunolabeling. Double immunofluorescent labeling of PHA-L and the serotonin transporter (5-HTT) allowed detection of serotonergic fibers among bulbo-spinal projections. Anterograde tracing showed that RMg neurons project preferentially into the deep laminae V-VI whereas LPGi neuron projections are confined to the superficial laminae I-II of the ipsilateral dorsal horn. All along the spinal cord, double-labeled PHA-L/5-HTT immunoreactive fibers, which represent only 5-15% of all PHA-L-immunoreactive projections, exhibit the same differential locations depending on their origin in the RMg versus the LPGi. The clear-cut distinction between dorsal horn laminae receiving bulbo-spinal serotonergic projections from the RMg versus the LPGi provides further anatomical support to the idea that the descending serotonergic pathways issued from these two bulbar nuclei might exert different modulatory influences on the spinal relay of pain signaling neuronal pathways.

Ethanol-induced epigenetic regulations at the Bdnf gene in C57BL/6J mice.

High ethanol intake is well known to induce both anxiolytic and anxiogenic effects, in correlation with chromatin remodeling in the amygdaloid brain region and deficits in cell proliferation and survival in the hippocampus of rodents. Whether only moderate but chronic ethanol intake in C57BL/6J mice could also have an impact on chromatin remodeling and neuroplasticity was addressed here. Chronic ethanol consumption in a free choice paradigm was found to induce marked changes in the expression of genes implicated in neural development and histone post-translational modifications in the mouse hippocampus. Transcripts encoding neural bHLH activators and those from Bdnf exons II, III and VI were upregulated, whereas those from Bdnf exon VIII and Hdacs were downregulated by ethanol compared with water consumption. These ethanol-induced changes were associated with enrichment in both acetylated H3 at Bdnf promoter PVI and trimethylated H3 at PII and PIII. Conversely, acetylated H3 at PIII and PVIII and trimethylated H3 at PVIII were decreased in ethanol-exposed mice. In parallel, hippocampal brain-derived neurotrophic factor (BDNF) levels and TrkB-mediated neurogenesis in the dentate gyrus were significantly enhanced by ethanol consumption. These results suggest that, in C57BL/6J mice, chronic and moderate ethanol intake produces marked epigenetic changes underlying BDNF overexpression and downstream hippocampal neurogenesis.