Stress-induced hypoalgesia and defensive freezing are attenuated by application of diazepam to the amygdala.

Recent studies have shown that lesions of the amygdala, as well as systemic administration of benzodiazepine receptor agonists, block the hypoalgesia and defensive behavior normally observed when rats are exposed to stimuli associated with shock. The present study was conducted to determine if the direct application of a small quantity of the benzodiazepine diazepam (DZP) to the amygdala would affect defensive freezing and hypoalgesia as measured by the formalin test. Independent groups of rats were prepared with chronic cannulae aimed at the basolateral or central regions of the amygdala. Bilateral injection of DZP (30 micrograms) into the basolateral amygdala attenuated both the defensive freezing behavior and the hypoalgesia seen during an 8-min period following a series of three brief foot-shocks. The same dose of DZP applied to the central amygdala attenuated the freezing response, although this effect may have been due to limited diffusion of the drug into the basolateral region. Baseline levels of formalin-induced behavior were not affected by DZP in either group. These results support the idea that hypoalgesia is one component of an integrated defensive response shown by rats in anxiety- or fear-related situations and that the amygdala represents an important forebrain component of a neural circuit that subserves the expression of this response.