Behavioral suppression induced by oral administration of monosodium L-glutamate in rats

Effect of water restriction on the development of hypothalamic lesions in weanling rodents given MSG. I. Drinking behaviour and physiological parameters in mice (Mus musculus)

Deprivation of water overnight (ON: 19.00-09.00 h) induced approx. 20% haemoconcentration in weanling ICR strain mice and resulted in avid consumption of fluid offered for 30 min subsequently, with delayed restoration of serum osmolality and sodium to normal levels in those offered 8% w/v monosodium L-glutamate monohydrate (MSG) aqueous solution. Neuronal necrosis of the arcuate nucleus of the hypothalamus was found in 12/180 dehydrated mice deprived ON and subsequently consuming 3.0 g or more MSG/kg body weight offered hypertonic MSG solution as the sole source of drinking water. The lesions were less severe than in mice given MSG s.c. or by forced intubation. Mice deprived ON and subsequently consuming MSG solution with water in a free choice experiment, and those deprived during daytime (DT: 07.00-21.00 h) and subsequently consuming MSG solution, with or without water, developed fewer and less severe lesions than did mice deprived ON and subsequently consuming MSG solution solely. The provision of other nutrients (e.g. glucose, arginine) reduced glutamate-induced hypothalamic lesions and elevation of plasma L-glutamate. The findings are in accord with earlier observations that hypothalamic lesions do not result even in this most susceptible species from the 'voluntary' consumption of MSG by physiologically normal animals. No lesions were produced even in mice deprived of food and water ON and subsequently consuming up to 7.3 g MSG/kg body weight in diet and in soup.

Effects of intraventricular glutamic acid on the acquisition, performance, and extinction of an operant response, and on general activity

This study was designed to investigate the effects of intracerebral injections of glutamic acid on acquisition and extinction of a bar-press response, performance of that response, and behavior activity. Nineteen food-deprived rats bearing chronic ventricular cannulae were tested in operant conditioning chambers or in an open field. Just prior to each testing session each animal was injected intraventricularly with 10 mul of either normal or acidified saline solution (control groups), or 10 mul of 100 mMol glutamic acid solution. Acquisition of the bar-pressing response by the glutamic acid group was significantly retarded as compared to the control groups, and the responding of the glutamic acid group was suppressed during the beginning of the first few sessions of testing on a fixed-interval schedule and during extinction. Other behavioral measures, such as operant level, performance of the response on a continuous schedule, performance on fixed-interval and extinction schedules except during the start of the first few sessions, and behavioral activity, were either unaffected or only slightly affected. It is concluded that interference with the normal neurophysiological activity of glutamic acid in the central nervous system interferes with learning and suppresses behavioral output in certain situations.

Learned taste aversions induced by high doses of monosodium L-glutamate

Learned taste aversions, as measured by increased time to complete 100 licks of a sweetened condensed milk solution, were demonstrated by laboratory rats 4 days after consumption of the milk solution paired with high oral doses of monosodium 1-glutamate (MSG). The hesitancy of the rats to consume milk on the test session cannot be simply attributed to direct action of the drug on motivation (e.g., hunger) or to drug debilitation. MSG has been reported to occasionally cause aversive effects in humans (Chinese restaurant syndrome), and the present experiments demonstrate that the effects of MSG are aversive to laboratory rats as well.