Abstract

The neocortex of awake animals requires spaced and repeated tetanization to successfully induce long-term synaptic potentiation (LTP) and long-term depression (LTD). Changes in synaptic strength can be promoted by synchronous neuronal activation, and the theta-rhythm (10 Hz) may be effective at inducing these changes. To investigate if theta-patterned stimulation is effective at inducing LTD in the sensorimotor cortex standard 1 Hz stimulation, pairs of pulses (with a 100 ms inter-pulse interval) delivered every 2 seconds, or 20-pulse 10 Hz trains were delivered daily for 10 days. Results were compared with control animals that received no stimulation. Prolonged 10 Hz trains were most effective at inducing LTD. To assess if 10 Hz activity in the form of large amplitude spindle waves are affected by plasticity of neocortical synapses, evoked spindle waves were recorded before and after LTP induction. The ability of single pulses to evoke spindle waves was increased following induction of LTP. The increased activation of spindle waves may have occurred due to strengthening of Layer V horizontal projections. To directly assess changes in the strength of these projections, LTP was induced and responses were recorded from a rostro-caudal array of five electrodes following corpus callosum stimulation as well as following direct stimulation of Layer V. Long-term potentiation of monosynaptic inputs to the cortex was found, but no significant increases in field potentials evoked by direct stimulation of the horizontal projections were observed. In addition, paired-pulse facilitation was observed at all electrode sites indicating the ability of the recording array to reflect enhanced responses. This research emphasizes the importance of theta-patterned activity in bi-directional synaptic plasticity in the sensorimotor cortex and suggests that LTP may occur primarily at monosynaptic inputs to the cortex rather than at synapses within the horizontal collaterals of Layer V.