Effects of 2-mercaptoacetate on ingestive behavior in 18- and 21-day-old rats

Abstract

Previous work has demonstrated that mercaptoacetate (MA), which interferes with oxidation of fatty acids, can produce increases in intake in both adult and preweanling rats between 12 and 15 days of age. In both pups and adults, the increased intake is related to a decrease in the latency to initiate intake, but the duration of the effects of MA and the effective doses of MA differ in young pups compared to adult rats. Thus, in the present experiments, we examined older pups to determine the effects of administration of MA on ingestive behavior and energy-related markers (blood glucose, free fatty acids, and β-hydroxybutryrate levels and weight loss) during the weaning transition. Pups aged 18 or 21 days received an i.p. injection of a 0, 22.8, 45.6 or 68.4 mg/kg MA, and after 1 h consumed a milk diet from the floor of a test container. Unlike younger pups or adult rats, MA did not stimulate intake in 18- or 21-day-old pups in a 30 or 60 min test, instead, the higher doses significantly suppressed intake. In addition, while latency to initiate intake was not reduced, primarily due to the very short latencies observed in control pups, the duration of intake was decreased by the higher doses of MA. Finally, MA did produce changes in fatty acid oxidation in pups at both ages, but baseline levels of free fatty acids and β-hydroxybutryrate differed across the ages, as did the pattern of changes produced by MA. The results suggest that in contrast to the effects observed in both younger pups and adult rats, during the early weaning period exogenous alterations of fatty acid oxidation fail to stimulate intake. In addition, similar to effects seen in young pups, high doses of MA can significantly suppress intake. These differences in responding to MA during the weaning period may reflect heightened sensitivity to similar signals produced endogenously as a result of developmental changes in diet and/or metabolism or may result from heightened sensitivity to aversive properties of administration of MA.