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 be en monitored in rabbits undergoing surgical preparation for electrophysiolo gical experimentation under Saffan anaesthesia. Reflexes were evoked by per cutaneous electrodes inserted at the heel and recorded as electromyograph s ignals from the ipsilateral medial gastrocnemius muscle. Two levels of surg ery were carried out. The "full surgical" preparation was performed under d eep Saffan anaesthesia. The trachea, carotid artery, jugular Vein and intra thecal space (via a small laminectomy at L1) were cannulated, the animals w ere decerebrated by suction, and the left hindlimb was immobilized by screw clamps applied to the tibia and the femur. The sciatic nerve and its branc hes were exposed by bisection of the posterior biceps muscle and the anaest hetic was withdrawn. In the "reduced surgery" preparation, procedures were carried out with a lighter lever of Saffan anaesthesia and operated tissues were infiltrated with local anaesthetic. Only the cannulations were perfor med in these animals. The excitability of the heel-gastrocnemius reflex dec lined throughout the full surgical preparation, with the median threshold i ncreasing 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 Most of this effect was reversed after surgery was compl ete and anaesthesia withdrawn subsequent to decerebration. There were no si gnificant changes in reflex excitability during the reduced surgery prepara tion (n = 15). Animals prepared by each of these protocols were given incre asing intrathecal doses of either the selective alpha(2)-adrenoceptor antag onist RX 821002 (0.3 to 300 mu g) or the serotonin/5-hydroxytryptamine (5-H T)(1A)-receptor antagonist WAY-100635 (0.01 to 30 mu g). Both drugs caused significant, dose-dependent increases in reflex responses, to four to six t imes 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 bot h preparations investigated and is not therefore an epiphenomenon of the wa y in which the animals were prepared. (C) 1999 IBRO. Published by Elsevier Science Ltd.