Regulation of feeding behaviour in monosodium glutamate (MSG) treated mice

Neonatal monosodium glutamate alters circadian organization of feeding, food anticipatory activity and photic masking in the rat

In rodents, parenteral administration of monosodium glutamate (MSG) induces marked degeneration of the retina and arcuate nucleus (AN) and disrupts daily rhythms of food intake. We quantified the effects of neonatal MSG (2 mg/g SC, postnatal days 1, 3, 5, 7, 9) on the expression of feeding and activity rhythms in adult rats under schedules of light-dark (LD), constant dark (DD), restricted daily feeding and total food deprivation. AN lesions were confirmed by neuropeptide Y (NPY) immunocytochemistry and Nissl stain. Compared to age-matched control rats, the amplitude (quantified as LD ratios) of daily food intake and food-bin activity rhythms was significantly attenuated in MSG rats in LD 12:12 and on the first day of DD. Control rats, but not MSG rats, showed lower amplitude rhythms in DD compared to LD. The phase angle of feeding and activity rhythms did not differ between groups in either condition. In a short LD cycle (2:2), control rats, but not MSG rats, showed significant inhibition (masking) of activity during the 2 h light periods. When food access was restricted to a 4 h daily meal, MSG rats showed enhanced expression and persistence of food-entrained anticipatory activity rhythms by comparison with control rats. These results indicate that attenuation of daily feeding rhythms in MSG rats is due in part to loss of direct inhibitory effects of light on behavior, and that the AN likely modulates, but does not mediate entrainment of feeding-related rhythms to daily cycles of LD or food access.

Adrenalectomy prevents obesity in glutamate-treated mice

Mice treated with monosodium glutamate (MSG) in the neonatal period grow into obese, stunted adults without overeating. We have previously demonstrated normal control of brown adipose tissue (BAT) thermogenic function in the MSG-treated mouse and have concluded that thermoregulation at a lower than normal body temperature for most of the time is a major cause of its obesity. The objective of the present experiments was to find out whether adrenalectomy would prevent obesity in the MSG-treated mouse, as it does in hyperphagic obese rodents, and whether the thermoregulatory anomaly would be prevented by this procedure. MSG-treated mice that were adrenalectomized at 5 wk of age and studied at 10 wk of age did not become obese. Adrenalectomy increased body temperature of MSG-treated mice to normal (male mice) or almost normal (female mice). Adrenalectomy increased BAT mitochondrial guanosine 5'-diphosphate binding in MSG-treated mice, indicative of an increased thermogenic state, but had the same effect in control mice. We conclude that obesity in the MSG-treated mouse is secondary to the high level of corticosterone in its blood, which raises its metabolic efficiency, an effect of corticosterone also seen in normal lean mice, and causes it to thermoregulate at a low energy-conserving level. This latter effect is peculiar to the MSG-treated mouse and is not seen in corticosterone-treated normal mice.