ASSOCIATIVE LONG-TERM POTENTIATION IN PIRIFORM CORTEX SLICES REQUIRESGABA(A) BLOCKADE

Previous studies have demonstrated that NMDA-dependent, long-term pote ntiation (LTP) can be induced in both afferent and intrinsic associati on fiber systems in the piriform (primary olfactory) cortex. In this r eport we demonstrate that an associative form of LTP can be induced by coactivation of these two systems, which terminate on adjacent apical dendritic segments of pyramidal cells. Potentiating stimulus trains w ere delivered to either afferent or association fibers, and weak shock s, which were nonpotentiating when delivered alone, were delivered to the other pathway. Under control recording conditions where homosynapt ic (single pathway) LTP is consistently evoked, coincident application of these stimuli failed to induce LTP of the weak shock response. How ever, after local blockade of the fast, GABA(A)-mediated IPSP, associa tive LTP was consistently produced in both directions. Induction was b locked by D-2-amino-5-phosphonovaleric acid, indicating that it is dep endent on activation of NMDA receptors. It is speculated that afferent and association fibers are segregated on different dendritic segments of pyramidal cells in piriform cortex to allow regulation of associat ive LTP by way of centrifugal inputs that modulate the activity of GAB Aergic interneurons.