TRPV1 drugs alter core body temperature via central projections of primary afferent sensory neurons

Abstract

TRPV1, a capsaicin- and heat-sensitive ion channel, is expressed by peripheral nociceptors and has been implicated in various inflammatory and neuropathic pain conditions. Although pharmacological modulation of TRPV1 has attracted therapeutic interest, their utility is limited because TRPV1 agonists and antagonists thus far examined show thermo-modulatory side effects in animal models and human clinical trials. These on-target effects may result from the perturbation of TRPV1 receptors on nociceptors, which transduce signals to central thermoregulatory circuits and also release pro-inflammatory factors from their peripheral terminals, such as the vasodilative neuropeptide calcitonin gene-related peptide (CGRP). Alternatively, they may originate from the modulation of TRPV1 on vascular smooth muscle cells (vSMCs), where channel activation promotes arteriole constriction. Here, we ask which of these pathways is most responsible for the body temperature perturbations elicited by TRPV1 drugs in vivo. We address this question by selectively eliminating TRPV1 expression in sensory neurons or vSMCs and show that only the former abrogates agonist-induced hypothermia and antagonist-induced hyperthermia. Furthermore, lesioning the central projections of TRPV1-positive sensory nerve fibers also abrogates drug-mediated thermo-modulation, whereas eliminating CGRP has no effect. Thus, TRPV1 drugs alter core body temperature by modulating sensory input to the central nervous system, rather than through peripheral actions on the vasculature. These findings suggest how mechanically distinct TRPV1 antagonists may diminish inflammatory pain without affecting core body temperature.