Long-term depression in the sensorimotor cortex induced by repeated delivery of 10 Hz trains in vivo.

Memory consolidation in the neocortex is thought to be mediated in part by bi-directional modifications of synaptic strength. The sensorimotor cortex shows marked spontaneous activity near 10 Hz during both waking and sleep in the form of electroencephalographic spindle waves, and is also sensitive to electrical activation of inputs at 10 Hz. Induction of long-term synaptic depression in corpus callosum inputs to layer V of the sensorimotor cortex of the awake, adult rat requires repeated low-frequency stimulation over many days. To determine if 10 Hz stimulation may facilitate the induction of long-term depression, we compared the amounts of long-term depression induced by conventional 1 Hz trains, repeated delivery of 450 pairs of stimulation pulses using a 100 ms interpulse interval, and 45 short, 2 s, 10 Hz trains. Each pattern was delivered daily for 10 days and was matched for total duration and number of pulses. Changes in synaptic responses were assessed by monitoring field potentials evoked by stimulation of the corpus callosum. A facilitation of synaptic responses in layer V was observed during delivery of both paired-pulse trains and 10 Hz trains. There was no significant difference in long-term depression induced by 1 Hz stimulation and repeated paired-pulse stimulation, but 10 Hz trains induced significantly greater long-term depression than 1 Hz trains in both the early monosynaptic and late polysynaptic field potential components. The effectiveness of short 10 Hz trains for the induction of long-term depression suggests that synchronous population activity at frequencies near 10 Hz such as spindle waves may contribute to endogenous synaptic depression in sensorimotor cortex.