Vol. 293, Issue 1, 289-295, April 2000

Synthesis and Characterization of a Fluorescent Substrate for the N-Arachidonoylethanolamine (Anandamide) Transmembrane Carrier¹

Shanmugam Muthian, Kasem Nithipatikom, William B. Campbell and Cecilia J. Hillard

Department of Pharmacology and Toxicology, Medical College of Wisconsin, Milwaukee, Wisconsin

N-Arachidonoylethanolamine (AEA) is a proposed endogenous ligand of the central cannabinoid receptor (CB1). Previous studies indicate that AEA is translocated across membranes via a process that has the characteristics of carrier-mediated facilitated diffusion. To date, studies of this mechanism have relied on [³H]AEA as a substrate for the carrier. We have synthesized an analog of AEA, SKM 4-45-1, that is nonfluorescent in the extracellular environment. When SKM 4-45-1 is exposed to intracellular esterases, it is de-esterified and becomes fluorescent. We have carried out studies to demonstrate that SKM 4-45-1 accumulation in cells occurs via the AEA carrier. SKM 4-45-1 is accumulated by both cerebellar granule cells and C6 glioma cells. Uptake of SKM 4-45-1 into C6 glioma is inhibited by AEA (IC₅₀=53.8 ± 1.8 µM), arachidonoyl-3-aminopyridine amide (IC₅₀=10.1 ± 1.4 µM), and arachidonoyl-4-hydroxyanilineamide (IC₅₀=6.1 ± 1.3 µM), all of which also inhibit [³H]AEA accumulation. Conversely, [³H]AEA accumulation by cerebellar granule cells is inhibited by SKM 4-45-1 with an IC₅₀ of 7.8 ± 1.3 µM. SKM 4-45-1 is neither a substrate nor inhibitor of fatty acid amide hydrolase, an enzyme that catabolizes AEA. SKM 4-45-1 does not bind the CB1 cannabinoid receptor at concentrations <10 µM. In summary, the cellular accumulation of SKM 4-45-1 occurs via the same pathway as AEA uptake and provides an alternative substrate for the study of this important cellular process.