Vol. 295, Issue 1, 328-336, October 2000

Cannabinoid Agonist Signal Transduction in Rat Brain: Comparison of Cannabinoid Agonists in Receptor Binding, G-Protein Activation, and Adenylyl Cyclase Inhibition¹

Christopher S. Breivogel and Steven R. Childers

Department of Physiology and Pharmacology, Center for the Neurobiological Investigation of Drug Abuse, and Center for Investigative Neuroscience, Wake Forest University School of Medicine, Winston-Salem, North Carolina

To investigate differences in agonist affinity, potency, and efficacy across rat brain regions, five representative cannabinoid compounds were investigated in membranes from three different rat brain regions for their ability to maximally stimulate [³⁵S]guanosine-5'-O-(3-thio)triphosphate (GTPS) binding and bind to cannabinoid receptors (measured by inhibition of [³H]antagonist binding) under identical assay conditions. In all three brain regions, the rank order of potency for the stimulation of [³⁵S]GTPS binding and the inhibition of [³H]SR141716A binding for these compounds were identical, with CP55940  levonantradol > WIN55212-2  ⁹-tetrahydrocannabinol (⁹-THC) > methanandamide. The rank order of efficacy was not related to potency, and relative maximal agonist effects varied across regions. Receptor binding fit to a three-site model for most agonists, stimulation of [³⁵S]GTPS binding fit to a two-site model for all agonists, and high-affinity receptor binding did not appear to produce any stimulation of [³⁵S]GTPS binding. WIN55212-2, methanandamide, and ⁹-THC also were assayed for the inhibition of adenylyl cyclase in cerebellar membranes. The rank orders of potency and efficacy were similar to those for [³⁵S]GTPS binding, but the efficacies and potencies of methanandamide and ⁹-THC compared with WIN55212-2 were higher for adenylyl cyclase inhibition, implying receptor/G-protein reserve.