Sympathetic modulation of sensory nerve activity with age: human and rodent skin models

1. Sensory nerves serve an afferent role and mediate neurogenic components of inflammation and tissue repair via an axon reflex release of sensory peptides at sites of injury. Dysfunction of these nerves with age could contribute to delayed tissue healing. 2. Complementary animal and human skin models were used in the present studies to investigate changes in the modulation of sensory nerve function by sympathetic efferents during ageing. Laser Doppler flowmetry was used to monitor neurogenic skin vascular responses. 3. The animal model used skin of the hind footpad of anaesthetized rats combined with electrical stimulation of the sciatic nerve, while the human model comprised capsaicin electrophoresis to the volar surface of the forearm. Sympathetic modulation was effected by systemic phentolamine pretreatment in animals and local application in the human model. 4. The results obtained from the human model confirmed the reported decline in sensory nerve function and showed no change in sympathetic modulation with age. The results from the animal model confirm and expand results obtained from the human model. 5. The use of low (5 Hz) and high (15 Hz) frequency electrical stimulation (20 V, 2 ms for 1 min) revealed a preferential response of aged sensory nerves to low-frequency electrical stimulation parameters with differential sympathetic modulation that is dependent on the frequency of stimulation.