PH sensitivity of cerebrospinal fluid-contacting neurons involves the modulation of phasic and tonic currents mediated by PKD2L1 channels located in the apical process.

Abstract

Cerebrospinal fluid contacting neurons (CSFcNs) are GABAergic cells that surround the central canal (CC) of the spinal cord. They have a soma that with a sub-ependymal location and a dendritic-like process that ends as a bulb (the so-called “apical process”; ApPr) inside the CC. This particular anatomical organization, with the soma and the ApPr located in different environments, defines the main function of CSFcNs, which is to act a as multimodal sensor of cerebrospinal fluid (CSF) composition. One of the main physiological features of CSFcNs is a prominent spontaneous activity mediated by PKD2L1 (or TRPP2) channels, a non-specific cation channel of the TRP family. PKD2L1 channels have a very high single-channel conductance (around 200 pS) and can be modulated by protons and mechanical forces. These characteristics (spontaneous activity, high single-channel conductance and modulation by different stimuli) make PKD2L1 channels potential regulators of CSFcNs excitability. In this work we combine electrophysiological (patch-clamp recordings) and optical (laser photolysis and calcium imaging) methods to analyze the sensitivity of PKD211 channels to the CC pH and the resulting effects on CSFcNs excitability. We show that PKD2L1 spontaneous activity induces not only phasic, inward currents, but also a tonic current that can be modulated bidirectionally by pH and has a very important role in setting the resting membrane potential of CSFcNs. We further address the issue of the spatial segregation of the channels and show that, as expected from the special anatomy of the cells, functional PKD2L1 channels are specifically located in the ApPr. Altogether, our experiments bring new insights into the physiology of CSFcNs.