SYNAPTIC PLASTICITY IN THE BASOLATERAL AMYGDALA INDUCED BY HIPPOCAMPAL-FORMATION STIMULATION IN-VIVO

Several studies suggest that axonal projections from the hippocampal f ormation (HF) to the basolateral amygdala (BLA) play a role in Pavlovi an fear conditioning to contextual conditional stimuli, We have used e lectrophysiological techniques to characterize neuronal transmission i n these projections in urethane-anesthetized rats. Single-pulse electr ical stimulation of the ventral angular bundle (VAB), which carries pr ojections from the HF to the BLA, reliably evoked a biphasic extracell ular field potential in the BLA that consisted of an early, negative a nd a late, positive component. The negative component of the field pot ential occurred at a short latency (3-8 msec), was both temporally and spatially correlated with VAB-evoked multiple-unit discharges in the BLA, and exhibited properties typical of a monosynaptic response, Infu sion of lidocaine or glutamate receptor antagonists into the BLA atten uated VAB-evoked field potentials, indicating that they are generated by local synaptic glutamatergic transmission, Both paired-pulse stimul ation and brief trains of high-frequency stimulation (HFS) induced a s hort-lasting facilitation of BLA field potentials, whereas longer and more numerous trains of HFS produced an enduring, NMDA receptor-depend ent long-term potentiation (LTP) of the potentials, The induction of L TP was accompanied by a decrease in paired-pulse facilitation (PPF), s uggesting a presynaptic modification underlying its expression, Electr olytic lesions placed in regions of the HF that project to the BLA or excitotoxic lesions placed in the BLA eliminated Pavlovian fear condit ioning to a contextual conditional stimulus. The critical role of both structures in context conditioning implicates plasticity at HF-BLA sy napses in this form of learning.