Systemic insulin administration evokes sympathoexcitatory actions but the mechanisms underlying these observations are unknown. We reported that insulin sensitizes the response of thin-fibre primary afferents, as well as the dorsal root ganglion (DRG) that subserve them, to mechanical stimuli. However, little is known about the effects of insulin on primary neuronal responses to chemical stimuli. TRPV1, whose agonist is capsaicin (CAP), is widely expressed on chemically-sensitive metaboreceptors and/or nociceptors. The aim of this investigation was to determine the effects of insulin on CAP-activated currents in small DRG neurons and CAP-induced action potentials in thin-fibre muscle afferents of normal healthy rodents. Additionally, we investigated whether insulin potentiates sympathetic nerve activity (SNA) responses to CAP. In whole cell patch-clamp recordings from cultured mice DRG neurons in vitro, the fold change in CAP-activated current from pre- to post-application of insulin (n = 13) was significantly (P<0.05) higher than those with a vehicle control (n = 14). Similar results were observed in single-fibre recording experiments ex vivo as insulin potentiated CAP-induced action potentials compared to vehicle controls (n = 9 per group, P<0.05). Furthermore, insulin receptor blockade with GSK1838705, significantly suppressed the insulin-induced augmentation in CAP-activated currents (n = 13) as well as the response magnitude of CAP-induced action potentials (n = 9). Likewise, the renal SNA response to CAP after intramuscular injection of insulin (n = 8) was significantly (P<0.05) greater compared to vehicle (n = 9). The findings suggest that insulin potentiates TRPV1 responsiveness to CAP at the DRG and muscle tissue levels, possibly contributing to the augmentation in sympathoexcitation during activities such as physical exercise.

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