Involvement of the dorsal periaqueductal gray in the loss of fear-potentiated startle accompanying high footshock training.

The amplitude of acoustic startle is markedly enhanced by cues signaling moderately intense footshocks but, surprisingly, not by cues signaling higher intensity footshocks. Previous findings suggest that the ineffectiveness of high footshock training may involve activation of the dorsal periaqueductal gray (PAG). As a means of evaluating this possibility, rats trained with moderate (0.6 mA) footshocks were later tested after intra-PAG infusion of an excitatory nontoxic dose of kainic acid. Kainic acid significantly reduced fear-potentiated startle relative to vehicle controls. In a 2nd experiment, the effect of dorsal PAG lesions on fear-potentiated startle to cues paired with 0.6-mA and 1.6-mA footshocks was evaluated. Dorsal PAG lesions prevented the disruptive effects of high footshock training. Together, these results suggest that dorsal PAG activation mediates the loss of potentiated startle accompanying high footshock training.