Significance of Cannabinoid CB1 Receptors in Improgan Antinociception

Improgan is a congener of the H2 antagonist cimetidine, which produces potent antinociception. Because a) the mechanism of action of improgan remains unknown and b) this drug may indirectly activate cannabinoid CB1 receptors, the effects of the CB1 antagonist/inverse agonist rimonabant (SR141716A) and 3 congeners with varying CB1 potencies were studied on improgan antinociception after intracerebroventricular (icv) dosing in rats. Consistent with blockade of brain CB1 receptors, rimonabant (Kd = 0.23 nM), and O-1691 (Kd = 0.22 nM) inhibited improgan antinociception by 48% and 70% after icv doses of 43 nmol and 25 nmol, respectively. However, 2 other derivatives with much lower CB1 affinity (O-1876, Kd = 139 nM and O-848, Kd = 352 nM) unexpectedly blocked improgan antinociception by 65% and 50% after icv doses of 300 nmol and 30 nmol, respectively. These derivatives have 600-fold to 1500-fold lower CB1 potencies than that of rimonabant, yet they retained improgan antagonist activity in vivo. In vitro dose-response curves with 35S-GTPγS on CB1 receptor-containing membranes confirmed the approximate relative potency of the derivatives at the CB1 receptor. Although antagonism of improgan antinociception by rimonabant has previously implicated a mechanistic role for the CB1 receptor, current findings with rimonabant congeners suggest that receptors other than, or in addition to CB1 may participate in the pain-relieving mechanisms activated by this drug. The use of congeners such as O-848, which lack relevant CB1-blocking properties, will help to identify these cannabinoid-like, non-CB1 mechanisms.PerspectiveThis article describes new pharmacological characteristics of improgan, a pain-relieving drug that acts by an unknown mechanism. Improgan may use a marijuana-like (cannabinoid) pain-relieving mechanism, but it is shown presently that the principal cannabinoid receptor in the brain (CB1) is not solely responsible for improgan analgesia.