Injections of the NMDA receptor antagonist aminophosphonopentanoic acid into the lateral nucleus of the amygdala block the expression of fear-potentiated startle and freezing

NMDA receptors within the amygdala play an important role in the acquisitio n and expression of conditioned fear. Because amygdaloid injections of NMDA receptor antagonists did not block the expression of every behavioral sign of fear, a discussion arose as to whether amygdaloid NMDA receptors play d ifferent roles in different kinds of fear-conditioning tasks. To clarify th e exact role of amygdaloid NMDA receptors, the present study measured the e ffects of amygdaloid NMDA receptor blockade on the two major animal models of conditioned fear. An experimental design was used that allowed simultane ous measurement of fear-potentiated startle and freezing during the same te st session after animals had undergone identical training procedures. The p resent study clearly demonstrates that injections of the NMDA receptor anta gonist AP-5 into the lateral nucleus of the amygdala significantly attenuat ed both behavioral fear responses (i.e., the amygdaloid NMDA receptors are necessary for the expression of fear-potentiated startle and freezing). The present results together with others from the literature indicate that NMD A receptors within the lateral amygdala are critically involved in normal s ynaptic transmission. It appears then that NMDA receptor antagonists may bl ock the acquisition of fear conditioning by directly interfering with norma l synaptic transmissions in the amygdala. Possible reasons for some discrep ant results in earlier studies are also discussed.