FOS-LIKE IMMUNOREACTIVITY INDUCED BY INTRAPLANTAR INJECTION OF ENDOTOXIN AND ITS REDUCTION BY MORPHINE

C-Fos-like immunoreactivity (FLI) in the central nervous system, has b een associated with the processing of nociceptive information in acute and chronic pain animal models. The aim of this study was to investig ate whether intraplantar (i.pl.) injections of endotoxin (ET, 1.25 mu g) can induce FLI in the lumbar spinal cord of rats and to assess the effects of morphine injection on c-fos expression. FLI was studied in various groups of rats at 2, 3, 4, 6, 9 and 24 h following ET injectio ns, Labeled neurons were mainly detected in the lumbar segments ipsila teral to the ET-injected leg, with a major peak (71.01 +/- 4.79 positi ve neurons) at 4 h and a second peak (29.87 +/- 5.97 positive neurons) at 9 h followed by a recovery to the baseline at 24 h after ET inject ions. Within the laminae, the majority of positive neurons was observe d at 2-3 h in laminae I and II and in deep laminae (V and VI mainly) s tarting at 4 h after ET injections. Rostrocandally, labeled neurons we re observed initially in L-4-L-5 segments (2-3 h post-ET) after which they extended to L-2-L-6 segments at 4 h after ET. Morphine injections either i.p. (1 or 2 mg/kg) or i.pl. (50 mu g) significantly reduced E T-induced hyperalgesia and simultaneously the FLI. The maximum effect was observed on labeled neurons in the deep laminae (V and VI mainly). We conclude that local injections of ET can induce FLI in the lumbar spinal cord with a temporal and spatial patterns comparable to the des cribed hyperalgesia, and that both FLI and hyperalgesia are reduced by morphine in a dose-dependent manner with a maximal effect shown by th e local i.pl. morphine injections. (C) 1997 Elsevier Science B.V.