We report electrical conductivity measurements of Polymethyl-methacrylate filled by onion-like carbon particles with primary particle size of $approx 5$ nm. We shown that the conductivity $sigma$ is exceptionally high even at very low loadings, and that its low-temperature dependence follows a Coulomb gap regime at atmospheric pressure and an activated behavior at a pressure of $2$ GPa. We interpret this finding in terms of the enhancement under the applied pressure of the effective dielectric permittivity within the aggregates of onion-like carbons, which improves the screening of the Coulomb interaction and reduces the optimal hopping distance of the electrons.
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