Central effects of monosodium glutamate on feeding behavior in adult long-evans rats

Abstract

Monosodium glutamate (MSG) is known as a neurotoxic molecule when injected neonatally in rats, where it produces a marked decrease in food intake and an increase in adipose tissue mass. But, in adult rats subcutaneous injections of MSG produce a small, dose-dependent increase in food intake. It is not known if this action is centrally or systemically mediated. Therefore, the feeding pattern of adult rats injected intracerebroventricularly with MSG was measured. Seven days after installation of a cannula in the right lateral ventricle, rats were injected either with artificial cerebrospinal fluid or twice with 3 mg/brain MSG within a 3-day interval. The feeding pattern was recorded via a complete computerized system during 24 h. Feeding behavior was significantly modified by MSG treatments. These effects were observed immediately after drug injections, that is, upon the first meal, as well as during the 24 h that followed. For the first meal, modifications in meal size (+285%; p = 0.0001), meal duration (×10; p = 0.0005), postmeal interval (×4; p = 0.0005), and the satiety ratio (− 50%; p = 0.01) were observed. During the 24-h postinjection period, modifications in meal number (−3; p = 0.0007), total amount of food eaten (+21%; p = 0.007), time spent eating (+40%; p = 0.007), meal duration (+53%; p = 0.005), and meal size (+44%; p = 0.01) were noted. When the two MSG injections were compared, differences were also noted. For the first meal, postmeal interval (−50%; p < 0.005) and satiety ratio (−50%; p < 0.005) were decreased after the second injection. During the 24-h postinjection period, meal duration (−50%; p < 0.05) and time spent to eat (−25%; p < 0.05) were also decreased. This study showed that MSG must be considered a potent stimulant of food intake when administered through a central route. This effect was more powerful than that observed with systemic MSG injection, even at several thousand-fold smaller doses. This stimulation of food intake might be mediated by the stimulatory effect of MSG on release of neuromodulators present in the circumventricular nuclei such as the arcuate nucleus. A small neurotoxic action of MSG is possible when the injections are repeated.