Peripheral CaV2.2 channels in skin regulate prolonged heat hypersensitivity during neuroinflammation.

Abstract

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuro-immune cell signaling but their roles in specific behavioral responses are not fully elucidated. Voltage-gated Ca_V2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal (id) capsaicin via IL-1α cytokine signaling. Ca_V2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the id capsaicin animal model. Here, we show that Ca_V2.2 channels are also critical for heat hypersensitivity induced by id Complete Freund's Adjuvant (CFA). Id CFA, a model of chronic neuroinflammation, involves ongoing cytokine signaling for days leading to pronounced edema and hypersensitivity to sensory stimuli. Peripheral Ca_V2.2 channel activity in skin was required for the full development and week-long time course of heat hypersensitivity induced by id CFA but, paw edema and mechanical hypersensitivity were independent of Ca_V2.2 channel activity. CFA induced increases in several cytokines in hind paw fluid including IL-6 which was also dependent on Ca_V2.2 channel activity. Using IL-6 specific neutralizing antibodies in vivo we show that IL-6 contributes to heat hypersensitivity and, neutralizing both IL-1α and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between Ca_V2.2 channel activity and the release of IL-6 in skin and show that Ca_V2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in skin.Significance statement Neuroinflammation can lead to chronic maladaptive pain. Neurotransmitters, ion channels, cytokines, and cytokine receptors are implicated in neuron-immune signaling, but their importance in mediating specific behavioral responses are not fully elucidated. We show that the activity of peripheral Ca_V2.2 calcium ion channels in skin play a unique role in the induction and maintenance of heat hypersensitivity in the CFA model of prolonged neuroinflammation, but they are not involved in the development of edema and mechanical hypersensitivity. Blocking peripheral Ca_V2.2 channel activity reduces local cytokine levels in hind paws injected with CFA including IL-6, and neutralizing IL-6 reduces CFA-induced heat hypersensitivity. Our studies define key signaling molecules that act locally in skin to trigger and maintain heat hypersensitivity during chronic neuroinflammation.