6533b826fe1ef96bd12846e4

RESEARCH PRODUCT

A repetitive intracortical microstimulation pattern induces long-lasting synaptic depression in brain slices of the rat primary somatosensory cortex.

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Repetitive intracortical microstimulation (ICMS) applied to the rat primary somatosensory cortex (SI) in vivo was reported to induce reorganization of receptive fields and cortical maps. The present study was designed to exam- ine the effect of such an ICMS pattern applied to layer IV of brain slices containing SI on the efficacy of synaptic in- put to layer II/III. Effects of ICMS on the synaptic strength was quantified for the first synaptic component ( s1) of cor- tical field potentials (FPs) recorded from layer II/III of SI. FPs were evoked by stimulation in layer IV. The pattern of ICMS was identical to that used in vivo. However, stimula- tion intensity had to be raised to induce an alteration of syn- aptic strength. In brain slices superfused with standard ACSF, repetitive ICMS induced a short-lasting (60 min) re- duction of the amplitude (-37%) and the slope (-61%) ofs1 evoked from the ICMS site, while the amplitude and the slope of s1 evoked from a control stimulation site in cortical layer IV underwent a slow onset increase (13% and 50%, respectively). In brain slices superfused with ACSF con- taining 1.25 µM bicuculline, ICMS induced an initial strong reduction of the amplitude (-50%) and the slope (-79%) of s1 evoked from the ICMS site. These effects decayed to a sustained level of depression by -30% (amplitude) and -60% (slope). In contrast to experiments using standard ACSF, s1 evoked from the control site was not affected by ICMS. The presynaptic volley was not affected in either of the two groups of experiments. A conventional high fre- quency stimulation (HFS) protocol induced input-specific long-term potentiation (LTP) of the amplitude and slope of s1 (25% and 76%, respectively). Low frequency stimula- tion (LFS) induced input-specific long-term depression (LTD) of the amplitude and slope of s1 (24% and 30%, re- spectively). Application of common forms of conditioning stimulation (HFS and LFS) resulted in LTP or LTD of s1, indicating normal susceptibility of the brain slices studied to the induction of common forms of synaptic plasticity. Therefore, the effects of repetitive ICMS on synaptic FP components were considered ICMS-specific forms of short- lasting (standard ACSF) or long-lasting synaptic depression (ACSF containing bicuculline), the latter resembling neo- cortical LTD. Results of this study suggest that synaptic de- pression of excitatory mechanisms are involved in the corti- cal reorganization induced by repetitive ICMS in vivo. An additional contribution of an ICMS-induced modification of inhibitory mechanisms to cortical reorganization is dis- cussed.