The Role of PLCδ4 in the Activity of TRPM8 Expressing Sensory Neurons

Abstract

Changes in environmental temperature are sensed by the peripheral endings of dorsal root ganglion (DRG) neurons. A subset of neurons expressing Transient Receptor Potential Melastatin 8 (TRPM8) channels function as major sensors of moderately cold temperatures. Previously it was demonstrated that Ca2+ influx through recombinant TRPM8 activates a Ca2+ sensitive phospholipase C (PLC) isoform, leading to depletion of phosphatidylinositol 4,5-bisphosphate with subsequent decrease of the TRPM8 currents (desensitization). We have found earlier that the most abundant highly Ca2+ sensitive PLC isoform in DRG neurons is PLCδ4. We performed patch-clamp experiments on TRPM8 expressing DRG neurons from PLCδ4-KO and WT animals, and identified 2 neuronal subpopulations based of their size. Small neurons had significantly larger TRPM8 current densities compared to the medium-large ones. PLCδ4-KO cells had larger currents upon menthol application and diminished desensitization. Current-clamp experiments further confirmed the differences in these subgroups. Both KO and WT small neurons had a more depolarized resting membrane potential than medium-large cells, and required smaller current injection to generate action potentials (AP) indicating their higher excitability. Positive current injection initially induced a train of AP during current pulses; increasing the current magnitude lead to a profound decline of the AP frequency in WT but KO neurons were able to generate AP at maximal frequency. AP shape and characteristics were also different for small and medium-large neurons. Negative current injection demonstrated characteristic voltage- and time- dependent rectification and rebound spike for both WT and KO small neurons. Menthol application induced transient generation of AP with frequency significantly higher in KO and larger depolarization compare to the WT neurons. Our present data support the role of PLCδ4 in the process of TRPM8 desensitization.