Opioidergic and adrenergic modulation of formalin-evoked spinal c-fos mRNAexpression and nocifensive behavior in the rat

Fos protein expression has been used to reflect neuronal activation in pain processing pathways although analgesics may uncouple behavioral and Fos re sponses. We determine whether formalin-induced spinal c-fos mRNA expression (Northern blotting) correlates with nocifensive behavior following pretrea tment with morphine, the alpha(2)-adrenoceptor agonist dexmedetomidine. or their respective antagonists naloxone and atipamezole. Both opiate and alph a(2)-adrenoceptor agonists reduced formalin-induced c-fos gene transcriptio n and nocifensive behavior via their cognate receptors. Unexpectedly, block ade of either the opiate or alpha(2)-adrenergic receptors, alone, caused an increase in formalin-evoked c-fos mRNA: while blocking the opiate receptor had no effect on formalin-induced behavior, alpha(2)-adrenoceptor block ha d an analgesic effect, indicating discordance between c-fos message transcr iption and nocifensive behavior. We concluded that the formalin-induced spi nal c-fos signal was a poor predictor of the behavioral response to pharmac ological manipulation of pain processing pathways. (C) 1999 Elsevier Scienc e B.V. All rights reserved.