During the present reporting period, very significant progress was made on this research project. The existence of cannabinoid CB2 receptors in the brain has been heretofore controversial. Most evidence has heretofore suggested that only CB1 cannabinoid receptors are found in brain and central nervous system while cannabinoid CB2 receptors are restricted to the body's periphery - primarily in the immune system. However, this view has been challenged by recent claims that CB2 receptors are present in the central nervous system and by recent claims that CB2 receptors modulate synaptic activity. Therefore, we used highly selective CB2 agonists and antagonists, combined with the use of CB1 and CB2 receptor gene-deleted mice, to study CB2 involvement in cocaine's behavioral and neurochemical effects. We found that the CB2 receptor-selective agonist JWH133 attenuates intravenous cocaine self-administration in wild-type and CB1 gene-deleted mice, but not in CB2 gene-deleted mice. This effect was abolished by the CB2 receptor-selective antagonist AM630. To confirm our findings, we also used the CB2-selective agonist GW405833 and found a similar inhibition of intravenous cocaine self-administration in wild-type mice. Under progressive-ratio reinforcement conditions, we found that JWH133 inhibits incentive motivation to self-administer cocaine, as evidenced by strong reductions in the progressive-ratio break-point. Similar effects were found when JWH133 was administered intra-nasally (for direct passage into the brain via the cribiform plate) or administered by direct intracerebral microinjections of JWH133 into the nucleus accumbens. Again, the effect was seen in wild-type but not in CB2 receptor gene-deleted mice. JWH133 by itself was found to have no reinforcing or aversive effects, as assessed by intravenous self-administration and by conditioned place preference/aversion experiments. Further, JWH133 inhibited cocaine-enhanced locomotion in wild-type and CB1 gene-deleted mice, but not in CB2 gene-deleted mice. JWH133 by itself had an inhibitory effect on locmotion, both with systemic administration and with intracerebral microinjection into the nucleus accumbens in wild-type and CB1 gene-deleted mice, but not in CB2 gene-deleted mice. The CB2 selective antagonist AM630 had a stimulatory effect on locomotion, both with systemic administration and with intracerebral microinjection into the nucleus accumbens in wild-type and CB1 gene-deleted mice, but not in CB2 gene-deleted mice. JWH133 by itself inhibited extracellular nucleus accumbens dopamine as measured by real-time in vivo brain microdialysis. JWH133 also inbited basal and cocaine-enhanced extracellular nucleus accumbens dopamine as measured by real-time in vivo brain microdialysis. This effect was blocked by the CB2-selective antagonist AM630. By itself, AM630 - microinjected intracerebrally into the nucleus accumbens - aumented basal extracellular nucleus accumbens dopamine. We conclude that CB2 cannabinoid receptors exist in the brain, that CB2 receptors functionally modulate the meso-accumbens dopamine system, and that CB2 receptors functionally modulate dopamine-mediated behaviors. In addition, we used the electrical brain-stimulation reward preclinical animal model to study the rewarding and/or aversive effects of several cannabinoids and the receptor mechanisms underlying these actions in laboratory rats. We found that the mixed CB1/CB2 cannabinoid agonists delta-9-tetrahydrocannabinol (THC) and WIN55212-2 produce biphasic effects on brain reward - low doses enhancing brain reward mechanism and high doses inhibiting them. On the other hand, the selective CB1 cannabinoid receptor agonist ACEA produces only brain-reward enhancement, while the selective CB2 receptor agonist produces only brain-reward inhibition. Further, the selective cannabinoid CB1 receptor antagonist AM251 selectively blocks the enhanced brain reward produced by low dose THC or WIN55212-2, while the selective cannabinoid CB2 receptor antagonist AM630 selectively blocks the brain reward inhibition produced by high dose THC or WIN55212-2. The TRPV1 antagonist capsazepine (posited by some researchers to act via a non-CB1, non-CB2 cannabinoid receptor) fails to alter THC- or WIN55212-2-induced changes in brain reward. In addition, the CB1 receptor selective antagonist AM251, but not the CB2 receptor selective antagonist AM630, blocks ACEA-enhanced brain reward. The CB2 receptor selective antagonist AM630, but not the CB1 receptor selective antagonist AM251, blocks JWH133-induced inhibition of brain reward. Intranasal JWH133 inhibits brain reward, an effect that is blocked by intranasal co-administration of the CB2 receptor selective antagonist AM630. We conclude that cannabinoid agonists produce biphasic effects on brain reward, with low doses enhancing and high doses inhibiting brain reward mechanisms. We further conclude that cannabinoid-induced reward enhancement is mediated by activation of brain CB1 receptors, while cannabinoid-induced inhibition of brain reward mechanisms is mediated by activation of CB2 receptors in the brain. These research findings suggest that brain CB1 and CB2 receptor-linked neural systems may functionally antagonize each other in a reciprocal mutually antatagonistic manner. Such mechanistic knowledge can aid in the search for new and effective pharmacotherapeutic compounds for the treatment of drug addiction and dependence. We have also extended this work to laboratory rats, to see if brain CB2 receptors act differently in a different mammalian species. We have also started electrophysiological studies of the effects of JWH133 on dopaminergic neuronal cell firing - in single dissociated ventral tegmental area dopamine neurons, in ventral tegmental area dopamine neurons in midbrain slice preparations, and in ventral tegmental area dopamine neurons in live anesthetized animals.
|Li, Xia; Hoffman, Alexander F; Peng, Xiao-Qing et al. (2009) Attenuation of basal and cocaine-enhanced locomotion and nucleus accumbens dopamine in cannabinoid CB1-receptor-knockout mice. Psychopharmacology (Berl) 204:1-11|
|Xi, Zheng-Xiong; Spiller, Krista; Pak, Arlene C et al. (2008) Cannabinoid CB1 receptor antagonists attenuate cocaine's rewarding effects: experiments with self-administration and brain-stimulation reward in rats. Neuropsychopharmacology 33:1735-45|