Evidence for the involvement of docosahexaenoic acid in cholinergic stimulated signal transduction at the synapse

[4,5-H-3]Docosahexaenoic acid ([H-3]DHA) or [9,10-H-3]palmitic acid ([H-3]PAM) was infused intravenously for 5 min to awake, adult male rats before and after treatment with arecoline (15 mg/kg, i.p.), a cholinergic agonist. Animals were killed 15 min post-infusion, the brains were rapidly removed and subcellular fractions were obtained after sucrose density centrifugation. In control animals, [H-3]DHA and [H-3]PAM were incorporated into the synaptosomal fractions, representing 50%-60% of total membrane label. Most remaining membrane label (30%-40%) was in the microsomal fraction. Both fractions contained the synaptic marker synaptophysin. The remaining 10% of radioactivity was in the myelin and mitochondrial fractions. Arecoline significantly increased [H-3]DHA entry into the synaptosomal fractions by 100% and into the microsomal fraction by 50%. In these fractions 60%-65% of the [H-3]DHA was in phospholipid, the rest corresponding to free fatty acid and diacylglycerol. In contrast, arecoline did not change [3H]PAM incorporation into any brain fraction. These results demonstrate that plasma [3H]DHA incorporation is selectively increased into synaptic membrane phospholipids of the rat brain in response to cholinergic activation. The increased incorporation of DHA but not of PAM into synaptic membranes in response to cholinergic stimulation indicates a primary role for DHA in phospholipid mediated signal transduction at the synapse involving activation of phospholipase A(2) and/or C.