Preferable Monosodium Glutamate and Sodium Chloride Solutions do not Affect Diurnal Norepinephrine Release in the Rat Lateral Hypothalamus

Abstract

Rats prefer hypo-osmolar solutions of monosodium glutamate (MSG) to water. The unique taste of MSG, called umami, has been suggested to be involved in protein homeostasis. We have investigated diurnal intake patterns of hypo-osmolar MSG solution (0.06 M) in rats fed a protein sufficient diet. Compared to water intake, animals showed a significant preference for MSG solution during the first part of dark phase (dark phase, 19:00–7:00; first part, 19:00–2:00). In contrast, no preference was found during the final part of dark phase or during light phase. No changes in food intake between the two tested groups were found. Although rats were offered MSG solution for five consecutive days, no day-to-day variations in intake were evident, suggesting that MSG preference is unlearned. Activation of the lateral hypothalamus (LH) was previously detected during of MSG solution intake in rats. Using a microdialysis technique, we evaluated LH norepinephrine (NE) release in rats drinking water, salt solution (0.06 M) and MSG solution (0.06 M). Diurnal variations of LH NE release were found in all animals without significant differences between the three groups. These data do not support involvement of LH NE in the brain recognition circuit for preferable umami or NaCl solutions.