In vivo induction of striatal long-term potentiation by low-frequency stimulation of the cerebral cortex

Both long-term depression and long-term potentiation have been described at corticostriatal synapses. These long-lasting changes in synaptic strength were classically induced by high-frequency (100 Hz) electrical stimulations of cortical afferents. The purpose of the present study was to test the ab ility of corticostriatal connections to express use-dependent modifications after cortical stimulation applied at the frequency of synchronization of corticostriatal inputs observed in our in vivo preparation, i.e. the barbit urate-anesthetized rat. For this study we used an identified monosynaptic c orticostriatal pathway, between the orofacial motor cortex and its target r egion in the striatum. Intracellular recording of striatal output neurons s howed spontaneous large-amplitude oscillation-like depolarizations exhibiti ng a strong periodicity with a narrow frequency band at 5 Hz. Using the foc al electroencephalogram of the cortical region projecting to the recorded c ells, we found that membrane potential oscillations in striatal neurons wer e in phase with episodes of spontaneous cortical spindle waves. To determin e directly the pattern of activity of corticostriatal neurons, we performed intracellular recordings of electrophysiologically identified corticostria tal neurons simultaneously with the corresponding surface electroencephalog ram. We found that corticostriatal cells (n = 7) exhibited periods of spont aneous 5-Hz discharges in phase with the cortical spindle waves. Therefore, we have tested the effect of repetitive cortical stimulations at this low frequency (5 Hz, 500-1000 pulses) on the corticostriatal synaptic efficacy. In 62% of cases (eight of 13 neurons tested), this conditioning was able t o produce long-term potentiation in the corticostriatal synaptic efficacy. The mean increase of excitatory postsynaptic potential amplitude ranged fro m 13.3% to 172% (mean = 67.3%, n = 8). These results provide additional support for physiological long-term potent iation at corticostriatal connections. Furthermore, this study demonstrates that corticostriatal long-term potentiation can be induced by synchronizat ion at low frequency of cortical afferents. Our data support the concept th at the striatal output neuron may operate as a coincidence detector of conv erging cortical information. (C) 1999 IBRO. Published by Elsevier Science L td.