The effect of lesions of the dorsolateral funiculus on formalin pain and morphine analgesia: A dose-response analysis

There has been conflicting evidence concerning the role of descending bulbospinal influences on pain and opioid analgesia in the formalin test. We examined the effect of lesions of the dorsolateral funiculus (DLF) on dose-effect relations for formalin and morphine in the formalin test. Experiment 1 showed that DLF lesions reduced the effect of 5 mg/kg morphine on pain in the tail-flick test, and eliminated morphine's effect on pain produced by 2.5% formalin. When lower concentrations of formalin were used, DLF lesions produced hyperalgesia, indicated by a left shift (1.7-fold) of the formalin concentration-response curve. In experiment 2, DLF lesions increased the pain produced by 1.25% formalin and shifted the dose-response relation for the effect of morphine on the second phase of the pain response produced by 1.25% formalin to 2.5-fold higher doses. The data show that DLF lesions increase sensitivity to the pain-inducing effect of formalin, and this accounts for a substantial component of the effect of DLF lesions on morphine analgesia.