Activity of Hb9 Interneurons during Fictive Locomotion in Mouse Spinal Cord

• Published in: The Journal of neuroscience Vol. 29; no. 37; pp. 11601 - 11613
• Main Authors: Kwan, Alex C, Dietz, Shelby B, Webb, Watt W, Harris-Warrick, Ronald M
• Format: Journal Article
• Language: English
• Published: United States Soc Neuroscience 16.09.2009; Society for Neuroscience
• Subjects: Analysis of Variance; Animals; Behavior, Animal; Calcium - metabolism; Dopamine - pharmacology; Electric Stimulation; Functional Laterality; Green Fluorescent Proteins - genetics; Homeodomain Proteins - genetics; Homeodomain Proteins - metabolism; In Vitro Techniques; Interneurons - drug effects; Interneurons - physiology; Locomotion - genetics; Locomotion - physiology; Membrane Potentials - drug effects; Membrane Potentials - genetics; Membrane Potentials - physiology; Mice; Mice, Transgenic; Motor Neurons - physiology; N-Methylaspartate - pharmacology; Neurotransmitter Agents - pharmacology; Patch-Clamp Techniques - methods; Serotonin - pharmacology; Spinal Cord - cytology; Spinal Nerve Roots - cytology; Spinal Nerve Roots - physiology; Time Factors; Transcription Factors - genetics; Transcription Factors - metabolism
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/JNEUROSCI.1612-09.2009

Hb9 interneurons (Hb9 INs) are putative components of the mouse spinal locomotor central pattern generator (CPG) and candidates for the rhythm-generating kernel. Studies in slices and hemisected spinal cords showed that Hb9 INs display TTX-resistant membrane potential oscillations, suggesting a role in rhythm generation. To further investigate the roles of Hb9 INs in the locomotor CPG, we used two-photon calcium imaging in the in vitro isolated whole neonatal mouse spinal cord preparation to record the activity of Hb9 INs, which were subsequently stained for unambiguous genetic identification. We elicited fictive locomotion by transmitter application or by electrically stimulating the caudal tip of the spinal cord. Although most Hb9 INs were rhythmically active during fictive locomotion, their activity was sparse and they failed to fire with each cycle of the episode. If Hb9 INs are the principal pacemakers of the CPG in the hemisegment in which they are located, they should direct the firing of motor neurons, with their activity preceding that of their ipsilateral segmental ventral roots. Instead, during each locomotor cycle, onset of Hb9 IN activity lagged behind the onset of the ipsilateral ventral root burst by a mean phase of 0.21 during electrical stimulation and 0.28 during transmitter application. Whole-cell recordings in intact and hemisected spinal cords confirmed the imaging results. Our data suggest that Hb9 INs participate in fictive locomotion, but the delayed onset of activity relative to ipsilateral motoneurons suggests that Hb9 INs are unlikely to be the sole intrasegmental rhythm-generating kernel of the CPG.