Intra-amygdala infusion of the N-methyl-D-aspartate receptor antagonist AP5 blocks acquisition but not expression of fear-potentiated startle to an auditory conditioned stimulus.

The fear-potentiated startle paradigm, in which the amplitude of the startle reflex is enhanced in the presence of a stimulus previously paired with footshock, was used to measure aversive conditioning after intra-amygdala infusion of the competitive N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5). Infusion of 2.5 micrograms/side AP5 immediately before five noise-footshock pairings on each of 2 consecutive days dose-dependently blocked acquisition or consolidation of auditory fear-potentiated startle, consistent with previous results from our laboratory obtained with a visual stimulus. Somatosensory or auditory transmission deficits do not appear to be induced by intra-amygdala AP5 because rats reacted normally to footshocks and showed reliable potentiated startle expression after pretesting AP5 infusion at a dose that blocked acquisition. Together with earlier reports, these data suggest that an NMDA-dependent process localized in or near the amygdala may be necessary for the acquisition of conditioned fear across different sensory modalities.