International Journal of Comparative Psychology

Rats received training in the consummatory successive negative contrast (cSNC) situation in which access to 32% sucrose solution during ten daily trials is followed by a downshift to 4% sucrose. Separate groups were exposed to the hot plate test for pain sensitivity immediately after either the first or the second downshift trial (Trials 11 and 12, respectively). Rats exhibited hypoalgesia after Trial 12 downshift, but not after Trial 11. These results suggest that cSNC induces the release of endogenous opioids that cause hypoalgesia, but only after some experience with the downshifted solution. This interpretation is supported by experiments demonstrating that opioid agonists reduce cSNC, whereas opioid antagonists enhance it.