Basal and carrageenan-induced pain behavior in Sprague-Dawley, Lewis and Fischer rats.

Individual differences in pain sensitivity are believed to reflect the interplay of many factors, including genetics. Inbred rat strains can be used to study the impact of genetic factors on pain sensitivity. Inbred Lewis (LEW) and Fischer 344 (FIS) rat strains display profound and contrasting alterations in neuroendocrine, immunological and behavioral responses to stressors. Because of the established interactions between stressors, the neuroendocrine system, the immune system and pain processing pathways, we hypothesized that LEW and FIS rats would differ in their pain sensitivity. Pain sensitivity was assessed using several behavioral pain assays in untreated and carrageenan-inflamed LEW and FIS rats, and in the outbred Sprague-Dawley (SD) rat. The results showed that at baseline, FIS rats were the most sensitive to mechanical stimulation (the von Frey monofilament test) and the least sensitive to noxious heat pain (the Hargreaves radiant heat test). After intraplantar administration of carrageenan, LEW rats showed the least, and FIS rats showed the greatest, thermal hyperalgesia and mechanical allodynia/hyperalgesia. Hindlimb muscle grip force and tail-flick latencies did not differ across the three strains, either before or after carrageenan. These results demonstrate differences in basal and carrageenan-induced pain sensitivity in LEW, FIS and SD rats, which extend earlier findings that genetic factors modulate both basal and inflammatory pain. The results further demonstrate that basal pain sensitivity can be predictive of inflammatory pain sensitivity, with the direction of the effect dependent upon the pain measure.