Peripheral pain enhances the effects of incentive downshifts

Abstract

Physical pain (induced by tissue damage) and psychological pain (induced by surprising incentive loss) share a set of common neural substrates, but little is known about their interactions. The present research studied such interactions using the formalin test to induce physical pain and consummatory successive negative contrast (cSNC) to induce psychological pain. In the formalin test, animals receive an intradermal injection of formalin (1%) in a hind paw. In cSNC, rats with free access to 32% sucrose show a sharp suppression of drinking behavior after a downshift to 4% sucrose, compared to rats that always receive 4% sucrose. In Experiment 1, formalin administration before the first and second 32-to-4% sucrose downshift trials enhanced cSNC. In Experiment 2, a similar treatment before the first downshift trial after a 16-to-4% sucrose downshift, which normally produces little or no evidence of cSNC, significantly increased cSNC. In Experiment 3, using a 32-to-4% sucrose downshift procedure similar to that of Experiment 1, no effects were observed following formalin administration immediately after Trial 11. Thus, no evidence was found that the effects of physical pain on cSNC were caused by changes in memory consolidation. The procedures used in these experiments offer a new approach to study the neural substrates of interactions between physical and psychological pain.