Electrophysiological observations on the effects of potassium ions on the response of intradental nerves to dentinal tubular flow in the cat

Temperature increase significantly enhances nociceptive responses of C-fibers to ATP, high K+, and acidic pH in mice

It is well established that temperature affects the functioning of almost all biomolecules and, consequently, all cellular functions. Here, we show how temperature variations within a physiological range affect primary afferents' spontaneous activity in response to chemical nociceptive stimulation. An ex vivo mouse hind limb skin-saphenous nerve preparation was used to study the temperature dependence of single C-mechanoheat (C-MH) fibers' spontaneous activity. Nociceptive fibers showed a basal spike frequency of 0.097 ± 0.013 Hz in control conditions (30°C). Non-surprisingly, this activity decreased at 20°C and increased at 40°C, showing moderate temperature dependence with Q₁₀∼2.01. The fibers' conduction velocity was also temperature-dependent, with an apparent Q₁₀ of 1.38. Both Q₁₀ for spike frequency and conduction velocity were found to be in good correspondence with an apparent Q₁₀ for ion channels gating. Then we examined the temperature dependence of nociceptor responses to high K⁺, ATP, and H⁺. Receptive fields of nociceptors were superfused with solutions containing 10.8 mM K⁺, 200 μM ATP, and H⁺ (pH 6.7) at three different temperatures: 20, 30, and 40°C. We found that at 30 and 20°C, all the examined fibers were sensitive to K⁺, but not to ATP or H⁺. At 20°C, only 53% of fibers were responsible for ATP; increasing the temperature to 40°C resulted in 100% of sensitive fibers. Moreover, at 20°C, all observed fibers were silent to pH, but at 40°C, this number was gradually increased to 87.9%. We have found that the temperature increase from 20 to 30°C significantly facilitated responses to ATP (Q₁₀∼3.11) and H⁺ (Q₁₀∼3.25), leaving high K⁺ virtually untouched (Q₁₀∼1.88 vs. 2.01 in control conditions). These data suggest a possible role of P2X receptors in coding the intensity of non-noxious thermal stimuli.