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 been associated with the processing of nociceptive information in acute and chronic pain animal models. The aim of this study was to investigate whether intraplantar (i.pl.) injections of endotoxin (ET, 1.25 microg) 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 injections. Labeled neurons were mainly detected in the lumbar segments ipsilateral to the ET-injected leg, with a major peak (71.01 +/- 4.79 positive 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 injections. Within the laminae, the majority of positive neurons was observed at 2-3 h in laminae I and II and in deep laminae (V and VI mainly) starting at 4 h after ET injections. Rostrocaudally, labeled neurons were observed initially in L4-L5 segments (2-3 h post-ET) after which they extended to L2-L6 segments at 4 h after ET. Morphine injections either i.p. (1 or 2 mg/kg) or i.pl. (50 microg) significantly reduced ET-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 described hyperalgesia, and that both FLI and hyperalgesia are reduced by morphine in a dose-dependent manner with a maximal effect shown by the local i.pl. morphine injections.