BIDIRECTIONAL MODIFICATION OF CA1 SYNAPSES IN THE ADULT HIPPOCAMPUS IN-VIVO

MEMORIES are believed to be stored by synaptic modifications. One type of activity-dependent synaptic modification, long-term potentiation ( LTP), has received considerable attention as a possible memory mechani sm, particularly in hippocampus(1). However, use-dependent decreases i n synaptic strength can store information as well. A form of homosynap tic long-term depression (LTD) has been described and widely studied i n the CA1 region of the developing hippocampus in vitro(2-4). However, the relevance of this model of LTD to memory has been questioned beca use of failures to replicate it in the adult brain in vitro(5) and, mo re recently, in vivo(6). Here we re-examine this important issue and f ind that homosynaptic LTD can in fact be elicited in the adult hippoca mpus in vivo, that it has all the properties described in immature CA1 in vitro, and that LTD and LTP are reversible modifications of the sa me Schaffer collateral synapses. Thus homosynaptic LTD is not peculiar to brain slices, nor is it only of developmental significance. Rather , our data suggest that the mechanisms of LTP and LTD may be equal par tners in the mnemonic operations of hippocampal neural networks.