Vol. 303, Issue 1, 314-322, October 2002

Inhibition of Improgan Antinociception by the Cannabinoid (CB)₁ Antagonist N-(Piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A): Lack of Obligatory Role for Endocannabinoids Acting at CB₁ Receptors

L. B. Hough, J. W. Nalwalk, R. Stadel, H. Timmerman, R. Leurs, B. C. Paria, X. Wang and S. K. Dey

Center for Neuropharmacology and Neuroscience, Albany Medical College, Albany, New York (L.B.H., J.W.N., R.S.); Department of Pharmacochemistry, Leiden/Amsterdam Center for Drug Research, Vrije University, Amsterdam, The Netherlands (R.L., H.T.); and Departments of Pediatrics and Molecular and Integrative Physiology, Ralph L. Smith Research Center, Kansas University Medical Center, Kansas City, Kansas (B.C.P., X.W., S.K.D.)

Improgan, a nonopioid antinociceptive agent, activates descending, pain-relieving mechanisms in the brain stem, but the receptor for this compound has not been identified. Because cannabinoids also activate nonopioid analgesia by a brain stem action, experiments were performed to assess the significance of cannabinoid mechanisms in improgan antinociception. The cannabinoid CB₁ antagonist N-(piperidin-1-yl)-5-(4-chloro phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) induced dose-dependent inhibition of improgan antinociception on the tail-flick test after i.c.v. administration in rats. The same treatments yielded comparable inhibition of cannabinoid {R-(+)-(2,3-dihydro-5-methyl-3-[(4-mor pholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl)methanone monomethanesulfonate, WIN 55,212-2} analgesia. Inhibition of improgan and WIN 55,212-2 antinociception by SR141716A was also observed in Swiss-Webster mice. Radioligand binding studies showed no appreciable affinity of improgan on rat brain, mouse brain, and human recombinant CB₁ receptors, ruling out a direct action at these sites. To test the hypothesis that CB₁ receptors indirectly participate in improgan signaling, the effects of improgan were assessed in mice with a null mutation of the CB₁ gene with and without SR141716A pretreatment. Surprisingly, improgan induced complete antinociception in both CB₁ (/) and wild-type control [CB₁ (+/+)] mice. Furthermore, SR141716A inhibited improgan antinociception in CB₁ (+/+) mice, but not in CB₁ (/) mice. Taken together, the results show that SR141716A reduces improgan antinociception, but neither cannabinoids nor CB₁ receptors seem to play an obligatory role in improgan signaling. Present and previous studies suggest that ⁹-tetrahydrocannabinol may act at both CB₁ and other receptors to relieve pain, but no evidence was found indicating that improgan uses either of these mechanisms. SR141716A will facilitate the study of improgan-like analgesics.