Differences in opioidergic inhibition of spinal reflexes and Fos expression evoked by mechanical and chemical noxious stimuli in the decerebrated rabbit

Noxious mechanical and chemical stimuli were applied to the toes of the lef t hind limb of decerebrated, spinalized rabbits and their effects on a hind limb spinal withdrawal reflex and expression of Foe-like immunoreactivity in the spinal cord were measured. The animals were prepared so as to minimi ze nociceptive inputs arising from surgery. A single crush stimulus applied with a pair of haemostatic forceps caused long-lasting (c. 20 min) inhibit ion of reflexes evoked in medial gastrocnemius motoneurons by electrical st imulation of the skin at the heel. Naloxone (0.25 mg/kg i.v.) increased ref lexes to more than 1000% of pre-drug controls and reversed crush-evoked inh ibition. Mustard oil applied to the toes had no consistent effects on the h eel-gastrocnemius reflex before or after naloxone. Both crush and mustard o il stimuli gave rise to unilateral increases in the number of Fos-immunopos itive profiles in the superficial dorsal horn of spinal segments L7 and S1. There were significantly more Fos-immunoreactive elements in the central a nd lateral parts of lamina I of both segments in animals receiving the crus h stimulus than there were in animals receiving the mustard oil stimulus. I mmunochemical localization of enkephalins in rabbit spinal cord showed a de nse network of fibres and terminals in laminae I and II, accompanied by inf requent but distinctly stained neuronal cell bodies. The same pattern, with increased numbers of visible cell bodies, was seen after treatment with co lchicine. The present data show that tonic and stimulus-evoked opioidergic inhibition of the heel-gastrocnemius reflex of the rabbit are not epiphenomena of sur gical preparation of the hindlimb. Opioid-mediated inhibition of the heel-g astrocnemius withdrawal reflex of the rabbit was evoked by noxious mechanic al but not by chemical stimulation of the toes. Of these stimuli, the forme r gave rise to greater activation of neurons in central and lateral lamina I of segments L7 and S1, the region of termination of afferent fibres from the heel and the location of some enkephalin-positive neuronal cell bodies. Thus, noxious mechanical stimulation of the toes elicits inhibition of the heel-gastrocnemius withdrawal reflex, probably via activation of enkephali nergic neurons in the lateral half of lamina I in the L7 and S1 segments. ( C) 1999 IBRO. Published by Elsevier Science Ltd.