Preferential in vivo incorporation of [3H]arachidonic acid from blood in rat brain synaptosomal fractions before and after cholinergic stimulation.

Awake adult male rats were infused intravenously with [3H]arachidonic acid for 5 min, with or without prior administration of an M1 cholinergic agonist, arecoline (15 mg/kg i.p.). Methylatropine was also administered (4 mg/kg s.c.) to control and arecoline-treated animals. At 15 min postinfusion, the animals were killed, brains were removed and frozen, and subcellular fractions were obtained from homogenates of whole brain. Total radioactivity and radioactivity in various lipid classes were determined for each fraction following normalization for exposure by use of a unidirectional incorporation coefficient, k*brain. In control animals, incorporation was greatest in synaptosomal and microsomal fractions, accounting for 50 and 30% of total label incorporated into membrane lipids, respectively. Arecoline increased incorporation in these two fractions by up to 400% but did not increase incorporation into the myelin, mitochondrial, or cytosolic fractions. Of the incorporated radioactivity, 50-80% was in phospholipid in microsomal and synaptosomal fractions, indicating that phospholipid is the major lipid affected by cholinergic stimulation. These results demonstrate that plasma [3H]arachidonic acid is preferentially incorporated into phospholipids of synaptosomal and microsomal fractions of rat brain. Cholinergic stimulation increases incorporation into these fractions, likely by activation of phospholipase A2 and/or C in association with acyltransferase activity, Thus, intravenously infused radiolabeled arachidonic acid can be used to examine synapse-mediated changes in brain phospholipid metabolism in vivo.