Contralateral paw sensitization following injection of endothelin-1: effects of local anesthetics differentiate peripheral and central processes

Subcutaneous injection of the peptide endothelin-1 (ET-1) into the rat's footpad is known to cause rapid, transient ipsilateral mechanical and thermal sensitization and nocifensive hind paw flinching. Here we report that local injection of ET-1 (2 nmoles) into one hind paw slowly sensitizes the contralateral paw to chemical and mechanical stimulation. There was a 1.5–2-fold increase in the hind paw flinching response, over that from the first injection, to a second injection of the same dose of ET-1 delivered 24 h later into the contralateral paw. A similar increase in the number of flinches during the second phase of the response to formalin also occurred in the contralateral paw 24 h after ET-1. The contralateral paw withdrawal threshold to von Frey hairs was lowered by ∼55% at 24 h after ipsilateral ET-1 injection. ET-1 injected s.c. at a segmentally unrelated location, the nuchal midline, caused no sensitization of the paws, obviating a systemic route of action. Local anesthetic block of the ipsilateral sciatic nerve during the period of initial response to ipsilateral ET-1 prevented contralateral sensitization, indicating the importance of local afferent transmission, although ipsilateral desensitization was not changed. These findings suggest that peripheral ET-1 actions lead to central sensitization that alters responses to selected stimuli.