Opposite effects of spinal cord stimulation in different phases of carrageenan-induced hyperalgesia

A possibly beneficial effect of spinal cord stimulation (SCS) on acute and chronic nociceptive pain is still a controversial issue. In the present experimental study, SCS was applied, with parameters similar to those used clinically, to awake but restrained rats submitted to intraplantar injection of carrageenan (CAR). The paw circumference and the threshold of paw withdrawal and/or vocalization to pressure, applied by a pair of strain gauge calibrated flat forceps to the paw, were determined before and after CAR injection. As controls, one group of rats was treated by CAR injection only. a second group was subjected to SCS after CAR injection; a third group was pretreated with SCS for 3 days before the injection; a fourth group subjected to SCS only. In the acute phase, 3 h after the injection, SCS enhanced the CAR-induced hyperalgesia and oedema. Conversely, in the subacute phase at day 3-5 after the injection, SCS suppressed the hyperalgesia which was, however less marked than in the acute phase. However, in that phase, the oedema was still increased by SCS. Pretreatment by SCS influenced neither the natural time course of hyperalgesia nor that of oedema. Circumstantial evidence suggests that the oedema augmentation by SCS in the acute phase may be the result of enhanced vasodilatation with fluid extravasation, probably associated with peripheral release of several substances, such as CGRP. The concomitant increase of the hyperalgesia may reflect an enhanced release of substance P and 5-HT, both peripherally and in the dorsal horn. The attenuation of hyperalgesia in the subacute phase may be due to an inhibitory effect of SCS on A-fibre-mediated wide-dynamic-range neuronal hyperactivity, presumably involving GABAergic mechanisms. However, a direct suppressive effect on sensitized nociceptive-specific second-order neurons cannot be excluded. (C) 1999 European Federation of Chapters of the International Association for the Study of Pain.