Hippocampal Long-Lasting Potentiation Produced Single Volleys and Brief Conditioning Tetani Evoked in Separate Afferents

Cooperative effects between afferents on the induction of longlasting potentiation (LLP) of synaptic transmission were examined in the CA1 region of the hippocampal slice preparation. Synaptic activity was recorded extracellularly in the dendritic layer (stratum radiatum) as a field EPSP, and the amount of LLP produced was measured from the change in slope of the rising phase of this potential. Experiments were performed with the GABA antagonist picrotoxin in the bath solution in order to facilitate the induction of LLP. It is shown that under these conditions a test input evoked by single volleys (at 0.2 Hz) is potentiated when paired with brief tetani (2-15 impulses at 50 Hz) to a separate population of fibers in the stratum radiatum. For this potentiation to appear, the test input had to occur during the train or precede it by less than 40 msec. Maximal effects were observed with the test volley positioned in the early part of the tetanus, and were largely independent of train duration. The potentiation obtained in this manner reached a peak level after some 20 conjunction events, and its magnitude measured 10 min after conjunction was about half that which could be induced by homosynaptic tetanization. Prior homosynaptic potentiation occluded the potentiation induced by conjunction, suggesting an identity of their underlying mechanisms. A test input to the apical dendritic layer was potentiated also by inputs to the basal dendritic layer, although greater effects were observed with both inputs in the same dendritic layer. It is suggested that the conditioning effect is related to the postsynaptic depolarization created by the tetanus. The results support the recent proposal that LLP is induced by ionic flux through voltage-dependent synaptic channels.