PICROTOXIN PRODUCES A CENTRAL PAIN-LIKE SYNDROME WHEN MICROINJECTED INTO THE SOMATO-MOTOR CORTEX OF THE RAT

In this study, we report the possibility of producing marked electroco rticographic changes and ''pain-like'' reactions, when the GABAA antag onist picrotoxin is microinjected unilateraly into the rat somato-moto r SmI cortex in the region of the hind paw. After the microinjection, we observed continuous seizure isolated spikes, spikes-and-waves, burs ts, and pain-like reactions, almost exclusively confined to the hind p aw. These reactions consisted of lifting off the floor, licking of the paw palm or digits, biting, paw tremors, and a peculiar paw position that we called ''turn-in'' paw. We also noted other behaviors, such as ''limping,'' ''neglected'' paw, or rearing. The ''pain-like'' charact er of these manifestations was suggested by the fact that similar qual itative and quantitative data occurred consequent to the administratio n of 2.5% diluted formalin into the palm of the hind paw in different rats. Bringing together the electrocorticographic events and the behav ioral reactions produced by SmI picrotoxin indicated that there was no obvious correlation between the phenomena, except that the tremor was always associated with the bursts. Sensory denervation of the hind pa w, produced by sciatic and saphenous nerve transections, did not signi ficantly modify either the ictal activity or the behavior. Finally, mi croinjection of naloxone prior to picrotoxin did not change the cortic al events, but greatly diminished the ''pain-like'' reactions. All the se results favor the cortical microinjection of a GABA(A) receptor ant agonist as a good rat model for studying pain of ''central'' origin. T hey emphasize the possible role of the SmI cortex in such a phenomenon , and the deficit of cortical GABAergic processing, which can include an opioid link. Copyright (C) 1996 Elsevier Science Inc.