  The $^1 S_0$ pairing of symmetric nuclear matter is discussed in the frame
work of relativistic nuclear theory with Dyson-Schwinger equations (DSEs). The
in-medium nucleon and meson propagators are treated in a more self-consistent
way through meson polarizations. The screening effects on mesons due to
in-medium nucleon excitation are found to reduce the $^1S_0$ pairing gap and
shift remarkably the gap peak to low density region.
