The cold receptor TRPM8 activation in cerebral circulation leads to acute reversible impairment of endothelium‐dependent vasodilator function during selective head cooling

Abstract

Selective head cooling is frequently used for treatment of hypoxic‐ischemic encephalopathy in newborns. However, the effects of head cooling on cerebral microcirculation are poorly understood. Newborn pigs provide a relevant large animal model of neonatal cerebrovascular regulation. Intravital microscopy using the closed cranial window technique in anesthetized, ventilated newborn pigs of both sexes was implemented to investigate acute effects of selective head cooling on cerebral vascular function. Cerebral vascular function was tested by responses of pial arterioles, the major cerebral resistance vessels, to endothelium‐dependent vasodilator stimuli (systemic hypercapnia and topical bradykinin, glutamate, and hemin), and to sodium nitroprusside (SNP), a nitric oxide donor that produces dilation by targeting arteriolar smooth muscle via a cGMP‐dependent mechanism. Selective head cooling by ice packs lowered the brain and the core temperatures to 25.6 ± 0.3 °C and 33.5 ± 0.1 °C, respectively. Cerebral vascular function was detected consecutively during normothermia, followed by head cooling and rewarming (duration of each period ~ 2h, N=6 pigs). During the hypothermic state, the responses of pial arterioles (mean diameter, 71±5 μm) to endothelium‐dependent vasodilators were greatly reduced, whereas the arteriolar smooth muscle responses to SNP remained intact. After rewarming, all cerebral vascular responses returned to the control normothermia level. We tested the hypothesis that the cold sensing transient receptor potential melastatin channel 8 (TRPM8) which is activated by moderate cold (22–27°C) contributes to impairment of endothelium‐dependent vascular function caused by head cooling. TRPM8 expression in the cerebral endothelium was detected by Western immunoblotting and immunofluorescence techniques. The TRPM8 antagonist AMTB (50 μM) topically applied under the cranial window during the head cooling period completely prevented reduction of endothelium‐dependent cerebral vascular responses (N=5 pigs). The inhibitory effects of head cooling on cerebral vascular function were mimicked by the TRPM8 super‐agonist icilin. Icilin (2 μM) topically applied to cerebral surface during normothermia (core temperature, 37.3 ± 0.2°C) greatly inhibited endothelium‐dependent cerebral vasodilation to hypercapnia, bradykinin, glutamate, and hemin, whereas the dilator responses to SNP were not altered. The ability of icilin to block the endothelium‐dependent dilator responses of pial arterioles during the normothermia period was completely abolished in the presence of the TRPM8 antagonist AMTB (50 μM). Overall, our data demonstrate that hypothermia caused by head cooling acutely and reversibly reduces endothelium‐dependent cerebral vasodilator functions, whereas arteriolar smooth muscle functions remain intact. We provide the first evidence that TRPM8 expressed in the cerebral vascular endothelium is activated by hypothermia or by the cooling mimetic agonists thus leading to acute impairment of endothelium‐dependent regulation of cerebral blood flow during selective head cooling.Support or Funding InformationGrants R01NS101717, R01NS105655 (HP)