Sensory and sympathetic nerve contributions to the cutaneous vasodilator response from a noxious heat stimulus

We investigated the roles of sensory and noradrenergic sympathetic nerves on the cutaneous vasodilator response to a localized noxious heating stimulus. In two separate studies, four forearm skin sites were instrumented with microdialysis fibres, local heaters and laser‐Doppler probes. Skin sites were locally heated from 33 to 42°C or rapidly to 44°C (noxious). In the first study, we tested sensory nerve involvement using EMLA cream. Treatments were as follows: (1) control 42°C; (2) EMLA 42°C; (3) control 44°C; and (4) EMLA 44°C. At the EMLA‐treated sites, the axon reflex was reduced compared with the control sites during heating to 42°C (P < 0.05). There were no differences during the plateau phase (P > 0.05). At both the sites heated to 44°C, the initial peak and nadir became indistinguishable, and the EMLA‐treated sites were lower compared with the control sites during the plateau phase (P < 0.05). In the second study, we tested the involvement of noradrenergic sympathetic nerves in response to the noxious heating using bretylium tosylate (BT). Treatments were as follows: (1) control 42°C; (2) BT 42°C; (3) control 44°C; and (4) BT 44°C. Treatment with BT at the 42°C sites resulted in a marked reduction in both the axon reflex and the secondary plateau (P < 0.05). At the 44°C sites, there was no apparent initial peak or nadir, but the plateau phase was reduced at the BT‐treated sites (P < 0.05). These data suggest that both sympathetic nerves and sensory nerves are involved during the vasodilator response to a noxious heat stimulus.