MIRROR PAIN IN THE FORMALIN TEST - BEHAVIORAL AND 2-DEOXYGLUCOSE STUDIES

Subcutaneous injection of a dilute formaldehyde solution (5 or 10%) in to a hind paw induced, in the majority of rats, the appearance of 'mir ror pain': licking the contralateral untreated hind paw 10-60 min afte r injection. Contralateral licking activity was much less frequent tha n the ipsilaterally directed one, but the overall intensities of the t wo responses were positively correlated. Qualitatively, the two behavi ours were similar. Functional activity levels of the lumbar spinal cor d, as revealed by the 2-deoxyglucose (2-DG) technique, were increased bilaterally over the first hour after unilateral hind limb formalin in jection in unanesthetized, freely moving rats. The enhancement of the [C-14]2-DG uptake could be detected both in dorsal and ventral horns, as well as in the gray matter surrounding the central canal, and the a nterolateral and dorsolateral funiculi. These metabolic changes may re flect an enhancement of the functional activity of both interneuronal pools and units projecting to supraspinal centers, giving rise to a re ferred contralateral pain.