The role of amygdala glutamate receptors in fear learning, fear-potentiated startle, and extinction.

Using a paradigm known as fear-potentiated startle, we have examined the neurobiological substrates of Pavlovian fear conditioning. In these experiments, rats are trained to fear an initially neutral stimulus by pairing that stimulus with shock. The amount of fear elicited by the stimulus [i.e., now a conditioned stimulus (CS)] is later assessed by presenting startle-eliciting noise bursts both in the presence and also the absence of the CS. After training, startle responses are typically greater in the presence of the CS. Findings reviewed here suggest that amygdala N-methyl-D-aspartate (NMDA) receptors play a key role in triggering the neural changes that support fear learning and also the loss of fear that accompanies extinction training. Amygdala (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors also participate in fear learning. However, unlike NMDA receptor antagonists, AMPA receptor antagonists also block fear-potentiated startle when infused prior to testing. Very recent data indicate that glutamate metabotropic Group II receptor agonists also block fear learning when infused into the amygdala prior to training, and block fear-potentiated startle when infused prior to testing. A fuller understanding of the role of amygdala glutamate systems in fear and fear learning may suggest novel pharmacological approaches to the treatment of clinical anxiety disorders.