Consumption of a high dietary dose of monosodium glutamate fails to affect extracellular glutamate levels in the hypothalamic arcuate nucleus of adult rats

We examined the effects of systemic or oral ad libitum monosodium glutamate (MSG) administration on glutamate levels in plasma, and on glutamate release from the arcuate nucleus of the hypothalamus (estimated using brain microdialysis). Systemic MSG administration (0.25, 0.5, 1 or 2 g/kg, i.p.) to adult rats caused dose-dependent increases in glutamate levels within arcuate nucleus dialysates. These levels increased during the initial 20 min after systemic MSG administration, and peaked during the second 20-min interval (maximally to 116 +/- 7%, 146 +/- 15%, 790 +/- 191% and 1230 +/- 676% of basal values, respectively). Plasma glutamate levels, measured simultaneously, were increased maximally during the initial 20 min after MSG administration. These increases were 10-, 13-, 76- and 163-fold after doses of 0.25, 0.5, 1 and 2 g/kg, i.p., respectively. In feeding experiments, consumption of 2.3 g/kg of MSG by previously-trained rats during an 1-h period increased plasma glutamate levels to 352 +/- 61% of basal values 140 min after the start of the feeding period. No changes were observed in glutamate levels of arcuate nucleus dialysates. These findings may explain why ad libitum dietary consumption of MSG apparently lacks neurotoxic potential.