N-Docosahexaenoylethanolamine (synaptamide): Carbon-14 radiolabeling and metabolic studies.

N-Docosahexaenoylethanolamine (synaptamide) is structurally similar to the endocannabinoid N-arachidonoylethanolamine (anandamide), but incorporates the omega-3 22:6 fatty acid docosahexaenoic acid (DHA) in place of the omega-6 20:4 fatty acid arachidonic acid (AA). Some brain membrane lipid effects may be mediated via synaptamide. In competition experiments with mouse brain homogenate in vitro, we found that synaptamide was an order-of-magnitude poorer inhibitor of radioactive anandamide hydrolysis than was anandamide itself. Also, enzyme-mediated hydrolysis of synaptamide was observed to occur at a slower rate than for anandamide. We have synthesized synaptamide radiolabeled with carbon-14 in both the ethanolamine ([α,β-C]synaptamide) and in the DHA ([1-C]synaptamide) moieties. The brain penetration, distribution, and metabolism of radiolabeled synaptamide were studied in mice in vivo relative to anandamide, DHA, and AA. Brain uptake of labeled synaptamide was greater than for labeled DHA, consistent with previous studies of labeled anandamide and AA in our laboratory. After administering either isotopomer of radiolabeled synaptamide, radiolabeled phospholipids were found in mouse brain. Pretreatment of mice with PF3845, a potent, specific inhibitor of fatty acid amide hydrolase (FAAH), eliminated formation of labeled phospholipids measured after 15min, suggesting that synaptamide is hydrolyzed nearly exclusively by FAAH, though it is a poorer substrate for FAAH than anandamide.