TRESK in nociceptive neurons: functional insights and modulation of peripheral sensitivity

Abstract

[eng] Sensing stimuli from our internal and external environments is critical to react appropriately and to guarantee our survival. Primary sensory neurons from dorsal root ganglia (DRG) and trigeminal ganglia are the first connection between our tissues and the central nervous system. Different populations of primary sensory neurons are involved in the detection of specific stimuli, and the tuning of their excitability is crucial to modulate our sensitivity. TRESK background potassium channel plays a critical role in modulating the action potential firing and excitability of primary sensory neurons. It is selectively expressed in populations of neurons involved in the perception of touch (low-threshold mechanoreceptors) and pain (nociceptors) and its depletion results in enhanced pain sensitivity. Here, using RNA in situ hybridisation, calcium imaging and electrophysiological assays in DRG labelled neurons from wild type (WT) and TRESK knock-out (KO) mice, we explore the role of the channel in a population of nociceptors expressing the MrgprD receptor. We validate that TRESK is expressed in 70 % of MrgprD+ neurons and that it modulates their excitability, reducing the number of neurons activated by the MrgprD specific agonist β-alanine in male and female mice and by cold in female mice. Using calcium imaging, we also study the role of TRESK in the modulation of the responsiveness of DRG neurons to cold temperatures and specific agonists. In concordance with previous studies showing that TRESK modulates the excitability of DRG neurons, we reveal that knocking out the channel enhances the number of neurons being activated by the noxious molecule capsaicin or cold temperatures, an effect present in females but not in sensory neurons from male mice. In contrast, deleting TRESK enhances the activation of neurons by menthol and the chemical irritant AITC but only from male mice. Considering the effects of TRESK in the modulation of DRG neurons’ responsiveness to cold and chemical compounds activating nociceptors and cold-sensitive neurons, we evaluated the cold, heat and mechanical sensitivity of WT and TRESK KO mice in physiological and pathological conditions. Knocking out TRESK does not change mice's cold and plantar mechanical sensitivity but increases their heat sensitivity in physiological but not under inflammatory conditions. Surprisingly, WT mice are more sensitive to joint mechanical friction than TRESK KO animals in both physiological and osteoarthritic conditions. Finally, treatment

with the calcineurin-inhibitor Tacrolimus increases wild-type mice’s plantar sensitivity to heat and mechanical, but not to cold stimuli. The increased heat sensitivity after TRESK indirect inhibition by Tacrolimus in WT male mice resembles the heat allodynia observed in TRESK KO animals. In summary, the presented data indicates that TRESK modulates the excitability of MrgprD+ nociceptors and prevents the responsiveness to chemical compounds and cold temperatures of DRG neurons in a sex-dependent manner. Genetically depleting the channel results in increased plantar heat sensitivity and decreased joint mechanical sensitivity, but, surprisingly, does not affect cold and plantar mechanical sensitivity of mice.

[cat] El canal de potassi TRESK participa en la modulació de l'excitabilitat de les neurones sensorials primàries. S'expressa en poblacions de neurones implicades en la percepció del dolor (nociceptors) i del tacte, i la seva eliminació provoca un augment de la sensibilitat al dolor. En aquest estudi multidisciplinar, demostrem que TRESK s’expressa en nociceptors que presenten el receptor MrgprD i en modula l’excitabilitat, reduint l'activació d'aquestes neurones per un agonista selectiu de MrgprD, o pel fred en ratolins femella. També mostrem que l’expressió del canal redueix el nombre de neurones que s’activen en resposta al proalgèsic capsaïcina, i a temperatures fredes, cosa que només s'observa en ratolins femella, però no de ratolins mascle. En canvi, TRESK també redueix el nombre de neurones de ratolins mascle, però no de ratolins femella, que s’activen en resposta a mentol o al compost irritant AITC. D'altra banda, en condicions fisiològiques, eliminar TRESK no canvia les respostes dels ratolins a estímuls mecànics i de fred, però augmenta la seva sensibilitat a la calor. Tot i això, l’expressió de TRESK no protegeix als ratolins de la hipersensibilitat mecànica o a l’escalfor en un model inflamatori. Sorprenentment, tant en condicions fisiològiques com en un model d’osteoartritis, els ratolins que expressen TRESK són més sensibles a la fricció en les articulacions que els que no l’expressen. El tractament amb l'inhibidor de calcineurina Tacrolimus augmenta la sensibilitat plantar dels ratolins a la calor i als estímuls mecànics, però no al fred. La sensibilitat a la calor induïda pel tractament en els ratolins mascle que expressen TRESK és similar a la que presenten els ratolins sense el canal en condicions basals. En resum, els nostres resultats indiquen que TRESK modula l'excitabilitat dels nociceptors MrgprD+ i redueix la resposta de les neurones sensorials primàries a estímuls químics i de fred. L’eliminació o inactivació indirecta del canal provoca una major sensibilitat a la calor, però no afecta la sensibilitat mecànica plantar i al fred dels ratolins. D’altra banda, els ratolins que no expressen el canal presenten una sensibilitat mecànica reduïda a les articulacions.