Synaptic plasticity in the hippocampal area CA1-subiculum projection: Implications for theories of memory

This paper reviews investigations of synaptic plasticity in the major, and underexplored, pathway from hippocampal area CA1 to the subiculum. This bra in area is the major synaptic relay for the majority of hippocampal area CA 1 neurons, making the subiculum the last relay of the hippocampal formation prior to the cortex. The subiculum thus has a very major role in mediating hippocampal-cortical interactions. We demonstrate that the projection from hippocampal area CA1 to the subiculum sustains plasticity on a number of l evels. We show that this pathway is capable of undergoing both long-term po tentiation (LTP) and paired-pulse facilitation (PPF, a short-term plastic e ffect). Although we failed to induce long-term depression (LTD) of this pat hway with low-frequency stimulation (LFS) and two-pulse stimulation (TPS), both protocols can induce a "late-developing" potentiation of synaptic tran smission. We further demonstrate that baseline synaptic transmission can be dissociated from paired-pulse stimulation of the same pathway; we also sho w that it is possible, using appropriate protocols, to change PPF to paired -pulse depression, thus revealing subtle and previously undescribed mechani sms which regulate short-term synaptic plasticity. Finally, we successfully recorded from individual subicular units in the freely-moving animal, and provide a description of the characteristics of such neurons in a pellet-ch asing task. We discuss the implications of these findings in relation to th eories of the biological consolidation of memory. (C) 2000 Wiley-Liss, Inc.