COMPARISON OF THE EFFECTS OF NUCLEUS-TRACTUS-SOLITARIUS AND VENTRAL MEDIAL MEDULLA LESIONS ON ILLNESS-INDUCED AND SUBCUTANEOUS FORMALIN-INDUCED HYPERALGESIAS

We have previously demonstrated that illness-inducing agents (lipopoly saccharide (LPS)) and inflammatory agents (subcutaneous (s.c.) formali n) induce hyperalgesia by similar pathways. The present series of expe riments compared the effects of medullary lesions on these phenomena. These experiments demonstrate that s.c. formalin-induced hyperalgesia, like illness-induced hyperalgesia, is dependent on the nucleus raphe magnus (NRM) but independent of the nucleus reticularis paragigantocel lularis (NRPgc). However, these two forms of hyperalgesia differ with regards to their dependence on the nucleus tractus solitarius (NTS). I llness-induced hyperalgesia is abolished by unilateral (left) NTS lesi ons, whereas formalin-induced hyperalgesia remains unaffected by this procedure. These data provide further evidence that hyperalgesias indu ced by illness agents and by inflammatory agents are mediated by simil ar but not identical pathways. They also illustrate that neural struct ures have the capacity for opposed actions, in that both the NTS and N RM are documented to underlie hyperalgesia as well as analgesia. This capacity for opposed action may prove to be characteristic of structur es involved in pain modulation.