Polysynaptic long-term potentiation: a physiological role of the perforant path--CA3/CA1 pyramidal cell synapse.

Chronic stimulating and recording electrodes were implanted in the perforant path (PP), the dentate gyrus, and the CA3 and CA1 regions in adult rats. Input-output curves were constructed from PP-dentate, dentate-CA3, and CA3-CA1 responses before and after 9 high-frequency trains (400 Hz, 20 ms) delivered to PP. The stimulations produced enhancement of the population spikes at each pathway tested, lasting from 1 to 24 h. It is hypothesized that the potentiating effect in the test pathways (dentate-CA3, CA3-CA1) occurred as a result of coupling the tetanizing trains via the direct PP-CA3/CA1 synapses with the asynchronous polysynaptic volleys occurring in the intrahippocampal circuitry. The findings suggest that the physiological role of the direct PP-pyramidal cell synapse may be to enhance the effectiveness of other inputs to these neurons.