Unilateral knife cuts produce ipsilateral suppression of responsiveness to pain in the formalin test

The present experiment investigated the possibility that unilateral brain interventions would produce unilateral deficits in responsiveness to painful stimulation. One hundred and four male rats were subjected to coronal knife cuts in the right or left hemisphere or to control operations. All rats were injected subdermally in the dorsal surface of both forepaws with 0.05 ml of 5% formalin. Responsiveness to pain was scored on a 4-point scale by rating the spatial position of each injected forepaw relative to the floor. Rats subjected to cuts through (a) the medial forebrain bundle (MFB) and the medial portion of the internal capsule (IC), (b) the thalamus, and (c) through both of these regions obtained greatly reduced pain scores on the ipsilateral forepaw. Most of these rats did not orient to innocuous tactile stimuli applied to the contralateral side of the body and turned ipsilaterally in a rotation bowl. Cuts involving the IC and MFB produced accumulation of substance P-like immunoreactivity in the IC rostral to the cut and accumulation of acetylcholinesterase in the MFB caudal to the cut. A motor deficit cannot be excluded as a factor contributing to the asymmetry observed in the formalin test. However, it is possible that the lower pain scores on the ipsilateral forepaw indicate a disturbance in the neural projections involved in transmission and perception of pain induced by formalin.