Associative synaptic potentiation and depression: Quantification of dissociable modifications in the hippocampal dentate gyrus favors a particular class of synaptic modification equations

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Abstract
This report further characterizes associative long-term synaptic modification of the ipsilateral and contralateral synapses formed by the bilateral entorhinal cortical (EC) projection to the dentate gyrus (DG). The experimental model is the anesthetized hooded rat. The quatntitative results qualify this system as a model for studying the rules of associative synaptic modification formulated in terms of individual synapses. Bilateral DG microelectrodes recorded both ipsilateral and contralateral EC-DG responses before and after brief, high-frequency EC conditioning stimulatin. The weak contralateral pathway receivewd high-frequency coditioning before, during, or after similar conditioning of the strong, converging ipsilateral pathway. Statistical analyses revealed two types of significant, dissociated synaptic modifications, which depend on the relationship of the ipsilateral and contralateral afferents. First, contralateral EC-DG responses potentiated, depressed, or showed no change when the collateral ipsilateral responses concurrently either potentiated or remained unchanged. Correlation and contingency table analysis indicated that changes in the contralateral synaptic responses are not well predicted by changes at either neighboring synapses of the converging ipsilateral pathway or at synapses of the collateral ipsilateral pathway. The contingencies of associated pre- and postsynaptic activation determined by the conditioning paradigm, however accurately predicted the altered synaptic responses of both ipsilateral and contralateral rately predicted the altered synaptic responses of both ipsilateral and contralateral EC-DG pathways. The results imply that associative synaptic modification in the EC-DG system is specific to individual synapses and requires both appropriate presynaptic and postsynaptic activation. Because this system provides suitable controls for nonspecific dissociable, the EC-DG system can be used to study further those rules of activity-dependent associative modification that are formulated in terms of individual synapses. The discussion briefly considers published rules modification, pointing out several rules that are not consistent with the ecperimental observations and one that agrees with the present results.