Polysynaptic slow depolarization and spiking activity elicited after induction of long-term potentiation in rat auditory cortex.

Polysynaptic activity was recorded in supragranular pyramidal neurons before and after the induction of long-term potentiation (LTP) in slices obtained from rat auditory cortex. LTP was induced by tetanic stimulation of layer IV. In the pyramidal neurons exhibiting LTP, repetitive stimulation at 50 Hz with 15 pulses triggered a slow 15-35 mV depolarization lasting 0.5-2 s with two to five spike discharges. There was no such response before the induction of LTP or in the neurons that did not exhibit LTP. Slow depolarization with spike discharges was blocked by an NMDA receptor antagonist but not by a metabotropic glutamate receptor antagonist. The reversal potential of the slow depolarization was approximately -7 mV and the membrane resistance decreased during slow depolarization, suggesting that the slow depolarization was produced by polysynaptic excitatory post-synaptic potentials. LTP was also induced by low frequency stimulation paired with a depolarizing current injection. In the pyramidal neurons exhibiting LTP after the paired stimulation, the slow depolarization amplitude was small and repetitive stimulation did not trigger spike discharges. Tetanic stimulation is expected to induce LTP in the polysynaptic neural circuits connecting many pyramidal neurons. The present findings suggest that polysynaptic activity can be generated in the potentiated neural circuits. Such activity might serve to read out the memory stored in polysynaptic neural circuits in the cerebral cortex.