Activation of primary afferent neurons by thermal stimulation

Abstract

The effect of thermal stimulation on primary afferent neurons and its modulation by Ruthenium Red (RR) has been investigated in the isolated perfused rabbit ear with intact neuronal connection to the animal. Capsaicin, K(+)-depolarization as well as increasing the temperature of the perfusate to 50 degrees C, increased the amount of substance P-like immunoreactivity (SP-IR) in the outflow in a calcium-dependent manner. High performance liquid chromatography (HPLC) revealed that SP-IR which was released by thermal stimulation consisted of two components, one of which co-eluted with synthetic substance P. The same two components of SP-IR were also present in extracts of the auricular nerve and were released by capsaicin. RR attenuated the effect of capsaicin and thermal stimulation but did not reduce potassium-evoked release of SP-IR. To evaluate an inhibitory action of RR on the excitation of primary afferents, the isolated perfused ear with intact neuronal connection to the anaesthetized rabbit was used. Intraarterial injection of capsaicin or bradykinin as well as superfusion of a skin area of approximately 2 cm2 with water at 53 degrees C for 1 min, produced a depressor reflex. RR attenuated the response to thermal stimulation and to capsaicin, but did not block the bradykinin-induced depressor reflex. These results demonstrate that, in the rabbit ear, thermal stimuli excite primary afferent neurons and evoke the calcium-dependent release of neuropeptides from their peripheral terminals by a mechanism which is sensitive to RR.