Permanent changes of the visually evoked responses in the superior colliculus after early postnatal treatment with monosodium-L-glutamate.

10 rats of the Long-Evans strain were treated from the first to the eleventh postnatal day (pd) with daily subcutaneous injections of 4 mg/g body weight monosodium-L-glutamate (MSG) in aqua dest. 10 rats were used as controls and received isotonic NaCl-solution on the same days. When rats were 4 months old, electrodes were implanted in different depths of the superior colliculus (CS) to investigate averaged evoked potentials by flash (VEP) and by click stimuli (AEP). The VEP in the superficial layers of the CS of MSG rats were strongly reduced in their amplitudes and irregularly shaped with the most prominent result that the primary negative component N28 was missing. The deeper recordings were made, the less this polarity-reversed primary component was affected with still significant amplitude reduction in the deepest layer. Shape and peak times of the AEP from the same electrodes in different depths were not significantly changed. However, the amplitudes of the early negative-positive and the second negative-positive deflections were enhanced in all MSG treated rats, and the amplitude proportions between VEP and AEP were in favour of AEP, in contrast to the controls. Heteromodal interactions of paired click and flash with an interval of 100 ms resulted in nearly total suppression of the VEP, in contrast to controls in which VEP were rather uninfluenced by the preceding click. Also the behaviour-dependent reduction of VEP during grooming and exploratory behaviour was much stronger in MSG treated rats compared with controls. The evoked potential analyses revealed greater interindividual heterogeneity of VEP in MSG treated rats than in controls.