Elimination of glutamatergic transmission from Hb9 interneurons does not impact treadmill locomotion

Lina M Koronfel,Angelita Alcos,Robert M Brownstone,Christopher E Henderson,Kevin C Kanning

doi:10.1038/s41598-021-95143-y

Abstract

The spinal cord contains neural circuits that can produce the rhythm and pattern of locomotor activity. It has previously been postulated that a population of glutamatergic neurons, termed Hb9 interneurons, contributes to locomotor rhythmogenesis. These neurons were identified by their expression of the homeobox gene, Hb9, which is also expressed in motor neurons. We developed a mouse line in which Cre recombinase activity is inducible in neurons expressing Hb9. We then used this line to eliminate vesicular glutamate transporter 2 from Hb9 interneurons, and found that there were no deficits in treadmill locomotion. We conclude that glutamatergic neurotransmission by Hb9 interneurons is not required for locomotor behaviour. The role of these neurons in neural circuits remains elusive.

Highlights

The spinal cord contains neural circuits that can produce the rhythm and pattern of locomotor activity
We demonstrated that glutamatergic transmission by Hb9 interneurons (Hb9 INs) does not contribute to treadmill locomotion of varying speeds
At early embryonic stages (E9; Fig. 1D), TAM led to widespread expression of tdTom in the spinal cord, presumably due to early expression of Hb9 outside of the motor neuron (MN) lineage in the caudal neural plate at this time point (Fig. 1D)

Summary

Introduction

The spinal cord contains neural circuits that can produce the rhythm and pattern of locomotor activity. It was found that distinct populations of spinal neurons express Hb9: MNs (somatic and SPNs) and Hb9 INs. As Hb9 INs were shown to be glutamatergic, to be positioned in the ventromedial upper lumbar spinal cord where locomotor rhythm generation occurs[8], and to have membrane properties that could support pacemakertype activity, it was proposed that they could have a role in locomotor rhythm generation [ 6,7 reviewed in9]. As Hb9 INs were shown to be glutamatergic, to be positioned in the ventromedial upper lumbar spinal cord where locomotor rhythm generation occurs[8], and to have membrane properties that could support pacemakertype activity, it was proposed that they could have a role in locomotor rhythm generation [ 6,7 reviewed in9] To test this hypothesis, it should be possible to study locomotor activity following genetic functional removal of Hb9 INs from spinal circuits, for example using a binary strategy to eliminate vGluT2 in neurons that express Hb9. Excision of vGluT2 using H b9cre mice would not be limited to Hb9-expressing neuronal populations, making interpretation of results using that strategy problematic

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