Deficit in hippocampal long-term potentiation in monosodium glutamate-treated rats

Rats subjected to monosodium glutamate (MSG) administration during the neonatal period present chronic neuroendocrine dysfunction associated with marked cognitive deficits. Long-term potentiation (LTP) in the hippocampus provides a model suited for the study of mammalian brain plasticity and memory formation. In the present work, we used the LTP protocol to investigate the synaptic plasticity in the hippocampal CA1 area of adult rats subjected to MSG treatment during the first 10 days of life. Synaptic transmission in CA1 area was analyzed using extracellular field recordings in response to Schaffer's collateral fiber stimulation in hippocampal slices. Animals injected with MSG exhibited a dramatic decrement of LTP field excitatory postsynaptic potentials (fEPSPs) compared to control group. Analysis of percent enhancement of fEPSP slope at 2 min after high frequency stimulation (HFS) increased by 189.3 +/- 33.2% in slices from control rats and 129.45 +/- 18.5% (p < 0.01) in slices from MSG-treated rats. Additionally, MSG-treated animals failed to maintain or consolidate LTP as revealed by a significant reduction in fEPSP slope enhancement over time after HFS. The mean fEPSP slope, 60 min after HFS, was 154.28 +/- 21% of the average baseline slope in control slices versus only 124.4 +/- 15% in MSG-treated rats (p < 0.01). At 90 min after HFS, slices from controls reached a potentiation of 44.5 +/- 2.9%, whereas the MSG group displayed an overall response enhancement of 17.65 +/- 2.7% of basal levels (p < 0.01). These findings indicate that MSG-treated rats display a chronic impairment of CA1 synaptic plasticity.