Second-order fear conditioning prevented by blocking NMDA receptors in amygdala

Jonathan C. Gewirtz () and Michael Davis
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Jonathan C. Gewirtz: Yale University, Ribicoff Research Facilities of the Connecticut Mental Health Center
Michael Davis: Yale University, Ribicoff Research Facilities of the Connecticut Mental Health Center

Nature, 1997, vol. 388, issue 6641, 471-474

Abstract: Abstract Antagonists of NMDA (N-methyl-D-aspartate)-type glutamate receptors disrupt several forms of learning1,2,3,4,5,6,7,8. Although this might indicate that NMDA-receptor-mediated processes are critical for synaptic plasticity, there may be other mechanisms by which NMDA-receptor antagonism could interfere with learning1,9,10,11,12. For instance, fear conditioning would be blocked by microinfusion of the NMDA-receptor antagonist AP5 (D,L-2-amino-5-phosphonovalerate) into the basolateral amygdala6,13,14 if AP5 inhibited routine synaptic transmission, thereby reducing the ability of stimuli to activate amygdala neurons15,16. In second-order fear conditioning17,18, the reinforcer is a fear-eliciting conditioned stimulus rather than an unconditioned stimulus. Expression of conditioned fear is amygdala-dependent19,20 and so provides a behavioural assessment of the ability of the reinforcer to activate amygdala neurons in the presence of AP5. We report here that intra-amygdala AP5 actually enhances expression of conditioned fear to the conditioned stimulus that provides the reinforcement signal for second-order conditioning. Nevertheless, acquisition of second-order fear conditioning is completely blocked. Our findings strongly support the view that NMDA receptors are critically involved in synaptic plasticity.

Date: 1997
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DOI: 10.1038/41325

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