Tonic adrenergic and serotonergic inhibition of a withdrawal reflex in rabbits subjected to different levels of surgical preparation.

The excitability of the heel-gastrocnemius withdrawal reflex pathway has been monitored in rabbits undergoing surgical preparation for electrophysiological experimentation under Saffan anaesthesia. Reflexes were evoked by percutaneous electrodes inserted at the heel and recorded as electromyograph signals from the ipsilateral medial gastrocnemius muscle. Two levels of surgery were carried out. The "full surgical" preparation was performed under deep Saffan anaesthesia. The trachea, carotid artery, jugular vein and intrathecal space (via a small laminectomy at L1) were cannulated, the animals were decerebrated by suction, and the left hindlimb was immobilized by screw clamps applied to the tibia and the femur. The sciatic nerve and its branches were exposed by bisection of the posterior biceps muscle and the anaesthetic was withdrawn. In the "reduced surgery" preparation, procedures were carried out with a lighter level of Saffan anaesthesia and operated tissues were infiltrated with local anaesthetic. Only the cannulations were performed in these animals. The excitability of the heel-gastrocnemius reflex declined throughout the full surgical preparation, with the median threshold increasing from 0.8 to 4.2 mA (n=19) and responses to suprathreshold stimuli reducing in size. Most of this effect was reversed after surgery was complete and anaesthesia withdrawn subsequent to decerebration. There were no significant changes in reflex excitability during the reduced surgery preparation (n = 15). Animals prepared by each of these protocols were given increasing intrathecal doses of either the selective alpha2-adrenoceptor antagonist RX 821002 (0.3 to 300 microg) or the serotonin/5-hydroxytryptamine (5-HT)1A-receptor antagonist WAY-100635 (0.01 to 30 microg). Both drugs caused significant, dose-dependent increases in reflex responses, to four to six times pre-drug control in both groups of animals. There were no differences in the effects on reflexes of either drug between the preparations. Thus, surgical preparation of decerebrated rabbits for electrophysiological recording results in depression of hindlimb withdrawal reflexes, although much of this effect did not persist beyond the completion of surgery. Tonic monoaminergic inhibition of reflexes was present to the same extent in both preparations investigated and is not therefore an epiphenomenon of the way in which the animals were prepared.