Major differences between long-term potentiation and ACPD-induced slow onset potentiation in hippocampus.

We examined the effects of a long-lasting application of the selective metabotropic glutamate receptor (mGluR) agonist 1S-3R, 1-amino cyclopentane-1,3-dicarboxylic acid (ACPD) on synaptic potentials recorded from the CA1 and CA3 subfields in hippocampal slices maintained in a superfusion slice chamber. In 25% of the slices, ACPD generated an slow onset potentiation (SOP) of population EPSPs (pEPSPs) in CA1. In contrast to long-term potentiation (LTP) induced by a tetanic train, SOP was accompanied by an increase in the magnitude of the presynaptic fiber volley. Potentiation of the isolated afferent volley suggests that the expression of SOP is due to a recruitment of additional presynaptic fibers by the test stimulus caused by the persistent block of K+ channels by ACPD.