Convergent but temporally separated inputs to lateral amygdala neurons from the auditory thalamus and auditory cortex use different postsynaptic receptors: in vivo intracellular and extracellular recordings in fear conditioning pathways.

The lateral nucleus of the amygdala (LA), a key component of the fear conditioning circuitry, receives a rapid but relatively impoverished auditory input from the auditory thalamus and a slower but richer input from the auditory cortex. We examined in urethane anesthetized rats whether individual cells in the LA receive convergent inputs from these two areas, and whether different postsynaptic receptors contribute to the temporally separated excitations over the two pathways. With both extracellular and intracellular recordings, individual cells could be activated by stimulation of each pathway. In extracellular recordings iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist APV and the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist CNQX demonstrated that synaptic transmission in both pathways depends on AMPA receptors, whereas transmission in the thalamic pathway also depends on the involvement of NMDA receptors. The involvement of NMDA receptors in synaptic activation of the LA from the thalamus but not the cortex was confirmed in intracellular recordings using systemic injections of the NMDA antagonist MK-801. The slow time course of NMDA currents could provide LA cells with a mechanism to integrate the inputs arriving rapidly from the thalamus and somewhat later from the cortex, thus allowing the LA to integrate signals in the two pathways during the acquisition and expression of conditioned fear reactions.