Conditional hypoalgesia is attenuated by naltrexone applied to the periaqueductal gray.

The ventrolateral periaqueductal gray (vPAG) is an important component in a brainstem system involved in the endogenous modulation of nociception and defensive behavior. The present study was conducted to determine if opioid receptors within the vPAG contribute to the hypoalgesia seen in rats during presentation of a Pavlovian CS for footshock. Independent groups of animals received microinjections of either 5.0 micrograms naltrexone HCl or vehicle into the vPAG prior to being placed in an observation chamber in which shock had been delivered 24 h earlier. Nociceptive reactivity was measured with the formalin test. Naltrexone treatment attenuated conditional hypoalgesia but did not affect formalin-induced behavior in non-shocked rats. Naltrexone had no effect on the amount of defensive freezing behavior observed during the test session. These results indicate that conditional hypoalgesia as measured by the formalin test involves the activation of documented brainstem antinociceptive systems.