Unilateral intramuscular injections of acidic saline produce a bilateral, long-lasting hyperalgesia

This study characterizes an animal model of persistent mechanical hyperalge sia induced by repeated intramuscular injections of low pH saline. Saline a t pH 4, 5, 6, or 7.2 was injected twice, 2 to 10 days apart, into the gastr ocnemius muscle of rats. To quantify hyperalgesia, paw withdrawal latency t o radiant heat (heat hyperalgesia) and withdrawal threshold to mechanical s timuli (mechanical hyperalgesia) were measured. Two unilateral injections o f low pH saline, 5 days apart, caused a pH-dependent bilateral mechanical, but not heat, hyperalgesia that lasted 30 days. Injections given 2 and 5 da ys apart produced a significantly greater mechanical hyperalgesia than inje ctions given 10 days apart. Lidocaine injection into the gastrocnemius musc le or unilateral dorsal rhizotomy, 24 h after the second injection (pH 4), had no effect on the contralateral mechanical hyperalgesia. Minimal histopa thology was observed in the injected muscle, and changes were similar betwe en groups injected with pH 4 and pH 7.2. Thus, this new model of widespread , chronic muscle-induced pain is unrelated to tissue damage and is not main tained by continued primary afferent input from the site of injury. (C) 200 1 John Wiley & Sons, Inc.