DIFFERENTIAL CONTRIBUTION OF THE 2 PHASES OF THE FORMALIN TEST TO THEPATTERN OF C-FOS EXPRESSION IN THE RAT SPINAL-CORD - STUDIES WITH REMIFENTANIL AND LIDOCAINE

Injection of formalin in the rat hindpaw produces two phases of nocice ptive behavior. Although it is generally agreed that the first phase r esults from direct chemical activation of nociceptive primary afferent fibers, the factors that contribute to the second phase are not estab lished. In the present study, we monitored the expression of the c-fos protein to evaluate whether the pattern of activity of dorsal horn ne urons differs as a result of ongoing afferent activity during the two phases. To selectively block the first or second phase, we respectivel y used remifentanil, a potent and short acting opiate agonist, and QX- 314, a quaternary derivative of lidocaine, which does not cross the bl ood brain barrier. We also evaluated the effect of eliminating nocicep tive behavior in both phases using both remifentanil and lidocaine or a combination of local anesthetics, bupivicaine and quaternary lidocai ne. In all groups, formalin (5%, 50 mu l) was injected subcutaneously into the plantar surface of the hindpaw. To assess the nociceptive beh avior produced by formalin, we monitored the number of flinches. Injec tion of remifentanil during the first phase completely blocked the fir st phase formalin-evoked nociceptive behavior, and had no effect on th e second phase. Injection of lidocaine during the interphase completel y blocked second phase nociceptive behavior. As expected, when remifen tanil was administered during the first phase and lidocaine during the second phase, all formalin-evoked nociceptive behavior was blocked. T he same was true for rats that received injections of bupivicaine and lidocaine during phases 1 and 2, respectively. In laminae I-II of the L4-L5 segment, the magnitude of the decrease in Fos expression was com parable for remifentanil (26.5%) and lidocaine (27.3%); the decrease w as greater when both remifentanil and lidocaine were administered (50. 5%), and even greater when bupivicaine and lidocaine were used (74.2%) . In laminae V-VI, remifentanil, by itself, decreased c-fos expression by 39.4%; for lidocaine alone, the decrease was 58.4%. We did not obs erve further significant decreases when both remifentanil and lidocain e, or bupivacaine and lidocaine were injected (69.7% and 74.6%, respec tively). Our results not only provide strong evidence that activity du ring the second phase is necessary for maintaining the maximal express ion of c-fos in the spinal cord, but also reveal significant regional differences in the central patterns of activity generated during the t wo phases. These results also confirm our previous reports that c-Sos expression is not eliminated when the behavioral manifestation of the noxious stimulus is completely blocked.