Effect of neonatal MSG treatment on day-night alkaline phosphatase activity in the rat duodenum

The day-night variation of food intake and alkaline phosphatase (AP) activi ty was studied in the duodenum of rats neonatally treated with monosodium g lutamate (MSG) and saline-treated (control) rats. The animals were kept und er light-dark conditions (light phase from 09:00 h to 21:00 h) with free ac cess to food. AP activity was cytophotometrically analyzed in the brush-bor der of enterocytes separated fi om the tip, middle and cryptal part of the villi every 6 h over a 24-hour period. In comparison with the controls, MSG -treated rats consumed about 40 % less food during the dark period and thei r 24-hour food intake was thus significantly lowered (P<0.001). On the othe r hand, the nocturnal feeding habit showed a similar pattern: food consumpt ion was high during the night (65 % vs. 75 %) and the lowest consumption wa s found during the light phase (35 % vs. 25 %) in MSG-treated and control r ats, respectively. In agreement with the rhythm of food intake, the highest AP activity was observed during the dark phase and was lowest during the l ight phase in both groups of animals. These significant day-night variation s showed nearly the same pattern in the enterocytes of all observed parts a long the villus axis. In comparison with the controls, a permanent increase of AP activity was observed in neonatal MSG-treated rats. This increase wa s more expressive during the dark phase of the day in the cryptal (P<0.001) and middle part of the villus (P<0.01). From the viewpoint of feeding, thi s enzyme in MSG-treated rats was enhanced in an inverse relation to the amo unt of food eaten i.e. despite sustained hypophagia the mean AP activity in the enterocytes along the villus axis was higher than in the control anima ls during all investigated periods. The present results suggest that the in creased AP activity in MSG-treated rats is probably not a consequence of ac tual day-night eating perturbations but could be a component of a more gene ral effect of MSG. This information contributes to better understanding of the function of intestinal AP and its relation to day-night feeding changes especially in connection with the MSG syndrome.