Adrenergic and opioidergic modulation of a spinal reflex in the decerebrated rabbit

1. In the decerebrated and spinalized rabbit, electrical stimulation of the sural nerve evokes a short-latency reflex in the ipsilateral ankle extensor gastrocnemius medialis (GM) which is tonically suppressed by endogenous opioids. In the present study we have investigated the inhibitory influences affecting this reflex in non-spinalized, decerebrated rabbits. 2. In non-spinalized rabbits, the thresholds and latencies of the sural-GM reflex were significantly higher than in spinalized preparations. The opioid antagonist naloxone and the alpha-adrenoceptor antagonist idazoxan potentiated the reflex in both preparations. Naloxone was significantly more effective in spinalized rabbits whereas idazoxan had a much larger effect in non-spinalized animals. 3. When the spinal cord was sectioned in the presence of naloxone alone, the GM reflex always increased in size. An ipsilateral hemisection of the cord was as effective as total section in this respect. When the section was performed in the presence of idazoxan and naloxone, the response usually decreased in size. 4. The alpha 2-adrenoceptor agonist clonidine depressed the reflex in spinalized rabbits, an action that was reversed by idazoxan but not by naloxone. 5. These data show that in the decerebrated, non-spinalized rabbit, the sural-GM reflex is tonically suppressed by endogenous opioids, presumably acting at the segmental level, and by an ipsilateral descending pathway which involves an alpha-adrenoceptor-mediated synapse. Activity in this descending pathway masks the facilitatory effects of opioid antagonists on spinal reflexes in this preparation.