Incorporation in Vivo of Intracisternally Injected ³³p₁, Into Phospholipids of Rat Brain¹

The incorporation in vivo of intracisternally injected 33 P i into individual phospholipids of rat brain was studied at intervals ranging froml min to 8 days after injection. At 1 min after injection there was significant incorporation of “Pi into PA and the phosphoino‐sitides, with the pattern of incorporation TPI > DPI > PA > PI. There was a 10‐min lag period before significant labelling of the remaining phospholipids. In general, the extent of incorporation measured during the first 8 days after injection paralleled the quantity of each phospholipid in the brain, except for the values of PA and the phosphoinositides which were all higher than would be predicted by this criterion and reached maximal incorporation sooner than the other phospholipids. The possible correlation of the high metabolic activities of PA and the phosphoinositides with neuronal function, as well as the importance of short‐term experiments in clarifying such relationships are discussed.