Effects of delta-9-tetrahydrocannabinol and (R)-methanandamide on open-field behavior in rats

This study compared the effects of (R)-methanandamide, an analog of the mammalian brain constituent anandamide, and delta-9-tetrahydrocannabinol on the open-field behavior of male Sprague-Dawley rats. Rats were individually housed with free access to food and water. Animals were treated with 0, 1, 3, and 5.6 mg/kg delta-9-tetrahydrocannabinol given i.p. 30 min pre-session; and 0, 3, 10, and 18 mg/kg (R)-methanandamide, 15 min pre-session. The behavioral categories recorded were ambulation (the number of squares crossed), rearing (the number of times the rat stood erect on its hind-legs), latency (the time in seconds to leave the starting area, the circle in the center of the field), circling (the number of times the animal turned around its vertical axis, 0.5 point given for each 180 degrees turn), grooming (the number of cleaning bouts), urination and defecation (the number of urination spots and fecal boli deposited during the 5 min observation period). Delta-9-tetrahydrocannabinol was more potent than (R)-methanandamide, but otherwise the effects of delta-9-tetrahydrocannabinol and (R)-methanandamide were similar, with one exception; whereas delta-9-tetrahydrocannabinol produced dose-related increases in circling, (R)-methanandamide did not increase circling over the doses examined. The delta-9-tetrahydrocannabinol-induced increase in circling was blocked by the central cannabinoid receptor CB1 antagonist SR 141716. The differential effects with regard to circling may indicate that there are qualitative behavioral differences in the effects of delta-9-tetrahydrocannabinol and (R)-methanandamide.