The effects of naloxone administered into the periaqueductal gray on shock-elicited freezing behavior in the rat.

Freezing behavior that occurs following footshock was found to increase in rats in which naloxone was injected into the ventrolateral region of the mesencephalic periaqueductal gray (PAG) area of the brain prior to footshock administration. Since naloxone administered into the ventrolateral region of the PAG induced minimal freezing in rats which did not receive footshock, the results suggest that the effect of naloxone on shock-induced freezing is not due to a nonspecific decrease in motor activity. Naloxone had no effect on freezing when injected into the dorsolateral region of the PAG. The data are consistent with the theory that conditioned fear induces opioid mediated analgesia, and that the ventrolateral region of the PAG is an important component of a pain-inhibitory system involved in this analgesia.