Transient receptor potential melastatin 8 channel inhibition potentiates the hypothermic response to transient receptor potential vanilloid 1 activation in the conscious mouse

Objectives: Mild decrease in core temperature (therapeutic hypothermia) provides lasting neuroprotection following cardiac arrest or cerebral ischemia. However, current methods for producing therapeutic hypothermia trigger a cold-defense response that must be countered by sedatives, muscle paralytics, and mechanical ventilation. We aimed to determine methods for producing hypothermia in the conscious mouse by targeting two transient receptor potential channels involved in thermoregulation, two transient receptor potential (TRP) channels involved in thermoregulation, TRP vanilloid 1 (TRPV1) and TRP melastatin 8 (TRPM8).

Design: Controlled prospective animal study.

Setting: Research laboratory at academic medical center.

Subjects: Conscious unrestrained young and aged male mice.

Interventions: Mice were treated with the TRPV1 agonist dihydrocapsaicin, a TRPM8 inhibitor ("compound 5"), or their combination and the effects on core temperature (Tcore) were measured by implanted thermocouples and wireless transponders.

Measurements and main results: TRPV1 agonist dihydrocapsaicin produced a dose-dependent (2-4 mg/kg s.c.) drop in Tcore. A loading dose followed by continuous infusion of dihydrocapsaicin produced a rapid and prolonged (> 6 hr) drop of Tcore within the therapeutic range (32-34°C). The hypothermic effect of dihydrocapsaicin was augmented in aged mice and was not desensitized with repeated administration. TRPM8 inhibitor "compound 5" (20 mg/kg s.c.) augmented the drop in core temperature during cold exposure (8°C). When "compound 5" (30 mg/kg) was combined with dihydrocapsaicin (1.25-2.5 mg/kg), the drop in Tcore was amplified and prolonged.

Conclusions: Activating warm receptors (TRPV1) produced rapid and lasting hypothermia in young and old mice. Furthermore, hypothermia induced by TRPV1 agonists was potentiated and prolonged by simultaneous inhibition of TRPM8.