BIDIRECTIONAL SYNAPTIC PLASTICITY IN THE RAT BASOLATERAL AMYGDALA - CHARACTERIZATION OF AN ACTIVITY-DEPENDENT SWITCH SENSITIVE TO THE PRESYNAPTIC METABOTROPIC GLUTAMATE-RECEPTOR ANTAGONIST 2S-ALPHA-ETHYLGLUTAMIC ACID

This study examines forms of activity-dependent synaptic plasticity in the basolateral amygdala in vitro and demonstrates that a brief high frequency stimulus (HFS) train can induce a switch in the direction of the enduring change in synaptic strength induced by subsequent low-fr equency stimulation (LFS). LFS (1 Hz, 15 min) of the external capsule (EC) induced a persistent 1.7-fold enhancement in the amplitude of syn aptic potentials recorded intracellularly in basolateral amygdala neur ons. The enhancement occurred gradually during the stimulation and was maintained for >30 min after termination of the stimulus train, LFS-i nduced enduring synaptic facilitation was not affected by the NMDA rec eptor antagonist D(-)-2-amino-5-phosphonopentanoate (APV; 100 mu M). B rief high-frequency EC stimulation (HFS; 100 Hz, 1 sec) induced APV-se nsitive shortterm potentiation (2.5-fold) that generally decayed withi n 10 min, When LFS was applied after recovery from the short-term pote ntiating effect of HFS (HFS/LFS), there was an initial transient (<10 min) enhancement of the synaptic response followed by persistent synap tic depression (synaptic potential amplitude reduced by 22% at 30 min) . This represents the first demonstration of stimulus-dependent long-l asting synaptic depression in the amygdala. Application of the presyna ptic (group II) metabotropic glutamate receptor antagonist 2S-alpha-et hylglutamic acid (EGLU; 50 mu M) prevented the HFS-dependent switch fr om synaptic facilitation to depression, Thus, LFS in the in vitro amyg dala slice can induce either enduring synaptic potentiation or depress ion, depending on whether a priming HFS train has been applied, This e xperience-dependent switch, a novel form of metaplasticity, is not dep endent on NMDA receptors but may require group II metabotropic glutama te receptors, In the amygdala, experiential modification of activity-d ependent long-term synaptic plasticity adds flexibility to the ways in which synaptic strength can be modified and could play a role in dive rse amygdala-dependent processes, including the formation, storage, an d extinction of emotional memory and the regulation of epileptogenesis .