Protein synthesis in neurons and glial cells of the developing rat brain: an in vivo study.

A technique for the isolation of pure neuronal perikarya and intact glial cells from cerebral cortex has been developed for routine use. The yield of neuronal perikarya and glial cells was greater from highly immature (5-10 days) rat cerebral cortex than from the cortex of older rats (18-43 days). The perikarya/glia yield ratio decreased with age indicating that, as the glial population matured, the procedure succeeded in isolating a gradually smaller proportion of the existing neurons. The perikarya/glia ratio was highest for the 5-day- old cortex in which no mature glial cells could be identified. After a 10-min pulse in vivo of intrathecally injected (14C)phenylalanine, the specific radioactivity of the neuronal proteins was higher than that of the glial proteins in the 5-, 10- and 18-day-old rat but was lower in the 43-day-old rat. The values for absolute specific radioactivity of the ''C-labelled proteins in both cell types were greater, the younger the brain. The 14C-labelling of neuronal and glial proteins in the 18-day-old rat was assessed in vivo as a function of time by determining the incorporation of (14C)phenylalanine into such proteins at 5, 10, 20 and 45 min after administration of the amino acid. The rate of incorporation of ('4C)phenylalanine into the glial cells was faster than into the neurons since higher specific radioactivities of the glial proteins could be achieved at earlier times. Also, a biphasic pattern of W-labelling of the glial proteins was noted, suggesting, perhaps, a sequen- tial involvement of the oligodendrocytes and astrocytes. Homogenates of prelabelled neuronal perikarya were fractionated into the nuclear, mitochondrial, microsomal and soluble cell sap fractions. In the 18-day-old cerebral cortex, the proteins of the microsomal fraction exhibited the highest specific radioactivity at the end of 10 min, whereas by 20 min proteins of the mitochondrial fraction were most highly labelled. The specific radioactivity of the nuclear proteins increased over the entire 45-min experimental period. On the contrary, the proteins of the soluble cell sap, in which the specific radioactivity was at all times by far the lowest, were maximally labelled by 5 min. Examination of the labelling of the neuronal subcellular fractions as a function of age revealed that at 10 min after administration of (14C)phenyIalanine, the specSc radioactivities of all l4C-labe1led proteins were highest in the youngest (5-day-old) neurons. The proteins of the microsomal fraction were most rapidly labelled at all ages. During this interval the proteins of the soluble cell sap were only moderately labelled in the 5-day-old neurons and were totally unlabelled in the 43-day-old neurons, indicating age-dependent differences in the rate of utilization of the amino acid precursor by the neurons. THE BRAIN is characterized by a high rate of protein synthesis (for reviews see LAJTHA, 1964 a, b). The rate of synthesis of cerebral proteins and of RNA is higher in the im- mature animal than in the adult (MILLER, 1969; ROBERTS, ZOMZELY and BONDY, 1970). BARONDES (1964) and YAMAGAMI, FRITZ and RAPPOPORT (1966) have shown that RNA polymerase and RNA are more active in brains of immature animals than in those of adults. SCHAIN, CARVER, COPENHAVER and UNDERDAHL (1967) examined the rate in vivo of incorporation of amino acids into pig brain and observed that the maximum This work was supported in part by training grant MH-07417 from the National Institute of