We have performed time-resolved degenerate four-wave mixing (DFWM) experiments in 75Nb_{2}O_{5 }-20TeO_{2}-5ZnO glasses dopedby Er_{2}O_{3} at different excitation intensities and lattice temperatures. DFWM signal exhibits three peaks at high excitation intensities, where a main peak appears at zero time delay and two rather weak side peaks locate symmetrically at the negative and positive time delay, respectively. The main peak is attributed to local-field effect and two side peaks areattributed to Coulomb interaction (CI).