ANANDAMIDE, AN ENDOGENOUS CANNABINOID, INHIBITS CALCIUM CURRENTS AS APARTIAL AGONIST IN N18 NEUROBLASTOMA-CELLS

Anandamide (arachidonyl ethanolamide) has been identified as an endoge nous ligand of cannabinoid receptors on the basis of its ability to di splace H-3-labeled synthetic cannabinoid in a binding assay. One well characterized cellular action of cannabinoids is inhibition of hormona lly stimulated adenylyl cyclase. Another action of synthetic cannabino ids is potent, stereospecific, and reversible inhibition of N-type cal cium currents (I(Ca)) in the NG108-15 neuroblastoma-glioma cell line v ia a pertussis toxin (PTX)-sensitive pathway, independently of cAMP me tabolism. Here we used the N18 neuroblastoma cell line and the whole-c ell voltage-clamp technique to show that anandamide also potently inhi bits N-type I(Ca) in a PTX-sensitive fashion. As with the cannabinomim etic aminoalkylindole WIN 55,212-2, inhibition by anandamide was volta ge dependent and N-ethylmaleimide sensitive. However, anandamide was l ess efficacious than either WIN 55,212-2 or the nonclassical cannabino id CP 55,940. Indeed, anandamide appears to act as a partial agonist a t the cannabinoid receptor. Application of WIN 55,212-2 always caused further inhibition Of I(Ca) in cells exposed to a maximally effective concentration of anandamide, and application of anandamide always caus ed a partial recovery of I(Ca) in cells exposed to a maximally effecti ve concentration of WIN 55,212-2. This partial agonist property of ana ndamide suggests that, although anandamide inhibits N-type I(Ca) via a PTX-sensitive G protein, its actions as a neuromodulator in the intac t animal may be more complex than would be inferred by extrapolating t he results of in vivo studies with (-)-DELTA9-tetra-hydrocannabinol or synthetic cannabinoids.