Dissociable effects of lidocaine injection into medial versus lateral thalamus in tail-flick and formalin pain tests

Lidocaine was injected into the centromedial (CM) or ventroposterolateral (VPL) thalamic nuclei of alert, unrestrained rats prior to investigation in the formalin and tail-flick tests. Regional anesthesia of CM resulted in a reduction of formalin-induced pain behaviors, while injection into VPL had no significant effect. In the tail-flick test, however, lidocaine injection into VPL caused a significant decrease in withdrawal latencies, indicating increased sensitivity to noxious stimulation. Lidocaine injection into CM was ineffective in the tail-flick test. These results provide evidence of the relatively greater importance of the ‘non-specific’ thalamic nuclei in the expression of tonic pain, and of the VPL in the mediation of withdrawal reflexes to phasic pain.

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