Evidence for PKD2L1‐positive neurons distant from the central canal in the ventromedial spinal cord and medulla of the adult mouse

Neurons in contact with the cerebrospinal fluid (CSF) are found around the medullo-spinal central canal (CC) in adult mice. These neurons (CSF-cNs), located within or below the ependymal cell layer, known as the stem cell niche, present a characteristic morphology with a dendrite projecting to the CC and ending with a protrusion. They are GABAergic, present an intermediate neuronal maturity and selectively express PKD2L1, a member of the transient receptor potential channel superfamily with sensory properties. Using immunohistological and electrophysiological recording techniques in mice, we characterize the properties of a new population of PKD2L1 positive cells that is distant from the CC in a zone enriched with astrocytes and ependymal fibers of the ventro-medial spinal cord and medulla. They appear around embryonic day 16 and their number increases up to early postnatal days. With development and the reorganization of the CC region, they progressively become more distant from the CC, suggesting some migratory capabilities. These neurons share functional and phenotypical properties with CSF-cNs but appear subdivided in two groups. One group, present along the midline, has a bipolar morphology and extends a long dendrite along ependymal fibers and towards the CC. The second group, localized in more ventro-lateral regions, has a multipolar morphology and no apparent projection to the CC. Altogether, we describe a novel population of PKD2L1+ neurons distant from the CC but with properties similar to CSF-cNs that might serve to sense modification in the composition of either CSF or interstitial liquid, a function that will need to be confirmed.