The effects of capsaicin injected intraperitoneally (200 μg/kg) or applied locally to the cortical surface (10-5 M) were studied on cortical potentials evoked by peripheral electrical or mechanical stimulation. Capsaicin treatment (i.p.) differentially influenced the cortical evoked potentials depending on the type of stimulation. Just after both types of capsaicin application, the responses to both kinds of stimuli decreased in amplitude. Additionally, during this time a short fall in blood pressure was observed. Half an hour later, however, only in the case of interperitoneal application the potentials evoked by electrical stimulation were facilitated, while the potentials evoked by vibrissa deflection had recovered and stayed around the control levels thereafter. In addition, the responsive cortex area activated by electrical stimulation became enlarged after the i.p. injection of capsaicin, while that of the cortex region activated by mechanical stimulation did not change significantly. Capsaicin applied locally to the cortex resulted neither in the facilitation of evoked potentials nor in the enlargement of the responsive cortical area. The present findings are the first to demonstrate that the i.p. (but not local) administration of capsaicin, in low dosage, differentially influences the cortical responses evoked by electrical and mechanical stimulation of somatosensory afferents.
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