Long-Term Potentiation in Dentate Gyrus: Induction by Asynchronous Volleys in Separate Afferents

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Abstract
Long-term potentiation (LTP), a long-lasting enhancement of synaptic efficacy, is considered a model for learning and memory. In anesthetized rats, activation of dentate granule cells by stimulating either the medial or lateral perforant pathway at frequencies of 100 to 400 Hz produced LTP of the stimulated pathway preferentially at 400 Hz. However, hippocampal pathways do not normally fire at this high rate. Stimuli at 200 Hz were then applied to either the medial or lateral pathway separately, to both pathways simultaneously, or to the two pathways asynchronously so that the composite stimulus applied to the granule cell dendrite was 400 Hz. LTP was produced preferentially in the asynchronous condition. Thus, lower frequency, physiological input volleys arriving asynchronously at medial and lateral synapses can induce LTP by activating a 400-Hz sensitive mechanism capable of integrating spatially separated granule cell inputs. This may reflect how LTP is normally produced in the dentate gyrus.