CHARACTERIZATION OF HB9 INTERNEURONS: BEHAVIOUR AND ANATOMY
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Central pattern generators (CPGs) are neuronal circuits that are responsible for the generation of rhythmic behaviours such as locomotion. It has been suggested that a population of spinal interneurons (INs), known as Hb9 INs, are possible rhythm generators of the locomotor CPG. Whether these glutamatergic INs (expressing vesicular glutamate transporter vGluT2) are indeed involved in generating the rhythm of locomotion remains unknown. To address this question, we used genetic strategies to silence the output of Hb9 INs. Hb9::CreERT2 animals were crossed for two generations with vGluT2flox/flox animals. Injection of tamoxifen generated animals that lack vGluT2 in the terminals of Hb9 expressing cells (Hb9 -VGluT2OFF). There were no significant differences observed in treadmill locomotion of Hb9-VGluT2OFF animals compared to heterozygotes and wild-type littermates. To further assess their possible role, the distribution of glutamatergic boutons of Hb9 INs was analyzed in Hb9::CreERT2; Rosa26fs-synaptophysin-tdTom mouse model. The boutons of Hb9 INs were distributed in the rostrocaudal axis of the spinal cord, and selectively in medial lamina VIII, lamina IX and ventral lamina X of upper lumbar segments. While their role appears to be limited in locomotor activity, the locations of Hb9 IN boutons may provide insight into the role of these interneurons.