Chloride homeostasis and DRG neuron excitability

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Sensory responses in primary afferent neurons have mostly been studied in the context of excitation and the influx of cations, with the induction of membrane depolarization and subsequent action potential generation/propagation. Much less attention, however, has been given to the role of chloride fluxes, which can inhibit or modulate those excitatory responses. In contrast to neurons in the central nervous system (CNS), where the importance of chloride as an inhibitory signal is already well established, in primary afferent neurons chloride homeostasis and inhibitory signals are not well understood. In the current research program, we will concentrate on chloride homeostasis in dorsal root ganglion (DRG) neurons, its modulation by oxidative stress, and its contribution to sensory function. Preliminary findings from our team using swine and mouse DRG neurons suggest that the control of chloride homeostasis in these neurons is more complex than previously thought. Using pharmacological tools and transgenic animals, we observed that the Na-K-Cl cotransporter (NKCC1) and the cystic fibrosis transmembrane conductance regulator (CFTR) act in combination to set the intracellular chloride levels. Lack of CFTR lead to increase intracellular chloride levels in DRG neurons, suggesting that this channel extrudes chloride from the cell, while NKCC1 is involved it the uptake. DRG neurons lacking CFTR function were less excitable and had impaired excitatory responses, suggesting that the combined action of these transporter is required for normal sensory function. Therefore, as short-term objectives we plan on further study the involvement of NKCC1 and CFTR in DRG neuron physiology, as well as to evaluate the possible involvement of the K-Cl cotransporters KCC2 and KCC3. We will concentrate on understanding how signals that modulate excitability (such as oxidative stress), may also affect chloride homeostasis. We are particularly interested in understanding the relationship between chloride levels and ion channel function, as well as the possible interaction between chloride transporters and signalling pathways involved in neuronal excitability. As longer-term objectives, we plan to extend our study into sensory perception. Taking our findings at the cellular level, we will test their relevance in sensory function in live animals. Thus, our research has the potential to contribute to the knowledge on the physiology and pathology of pain.