EXPRESSION OF MONOCARBOXYLATE TRANSPORTER MESSENGER-RNAS IN MOUSE-BRAIN - SUPPORT FOR A DISTINCT ROLE OF LACTATE AS AN ENERGY SUBSTRATE FORTHE NEONATAL VS. ADULT BRAIN

Under particular circumstances like lactation and fasting the blood-bo rne monocarboxylates acetoacetate, beta-hydroxybutyrate, and lactate r epresent significant energy substrates for the brain, Their utilizatio n Is dependent on a transport system present on both endothelial cells forming the blood-brain barrier and on intraparenchymal brain cells, Recently, two monocarboxylate transporters, MCT1 and MCT2, have been c loned. We report here the characterization by Northern blot analysis a nd by in situ hybridization of the expression of MCT1 and MCT2 mRNAs I n the mouse brain, In adults, both transporter mRNAs are highly expres sed in the cortex, the hippocampus and the cerebellum, During developm ent, a peak in the expression of both transporters occurs around postn atal day 15, declining rapidly by 30 days at levels observed in adults , Double-labeling experiments reveal that the expression of MCT1 mRNA in endothelial cells is highest at postnatal day 15 and is not detecta ble at adult stages, These results support tile notion that monocarbox ylates are important energy substrates for the brain at early postnata l stages and are consistent with the sharp decrease in blood-borne mon ocarboxylate utilization after weaning, In addition, the observation o f a sustained intraparenchymal expression of monocarboxylate transport er mRNAs in adults, in face of the seemingly complete disappearance of their expression on endothelial cells, reinforces the view that an in tercellular exchange of lactate occurs within the adult brain.