Endocannabinoid brain mechanisms and addiction

在2006年10月1日至2007年9月30日期间，该研究项目取得了重大进展。在上述报告期间，我们研究了AM251（一种新型高选择性大麻素CB1受体拮抗剂）在恢复动物模型中对可卡因引发的药物寻求行为复发的影响，以及在恢复条件下可卡因诱导的细胞外多巴胺、谷氨酸和γ -氨基丁酸（GABA）变化的影响。我们发现，系统给药AM251选择性地抑制可卡因诱导的寻求奖励行为的恢复，而不是蔗糖加蔗糖提示诱导的恢复。AM251没有单独触发恢复。边缘前脑伏隔核或背纹状体局部微灌注AM251也能抑制可卡因诱发的恢复。AM251单独以电压依赖性钠通道依赖性的方式剂量依赖性地升高伏隔核谷氨酸。AM251不影响伏隔核多巴胺和GABA。AM251预处理对可卡因诱导的伏隔核谷氨酸增加具有剂量依赖性，但对多巴胺没有抑制作用。LY341495阻断伏隔核代谢性谷氨酸mGluR2/3受体略微促进了可卡因增强的谷氨酸释放，但阻断了AM251对可卡因触发的恢复的拮抗作用。结果表明：1)在可卡因消失条件下，CB1大麻素受体对伏隔核谷氨酸释放具有强直性抑制作用；2) AM251阻断CB1大麻素受体可抑制可卡因增强的伏隔核谷氨酸释放和可卡因触发的恢复；3)这些效应可能是由am251诱导伏隔核谷氨酸释放增加（去抑制）的突触前mGluR2/3自身受体的激活介导的。我们进一步发现，在递进比例强化条件下，AM251剂量依赖性地降低了实验室大鼠静脉注射可卡因自我给药的断点（30-70%）。使用CB1受体拮抗剂SR141716可使临界点降低35%，但无统计学意义。在固定比例（FR2）强化条件下，AM251和SR141716均未改变可卡因自我给药。AM251还显著且剂量依赖地抑制（25-90%）可卡因增强的脑刺激奖励，而SR141716仅在SR141716测试的最高剂量下减弱可卡因的奖励增强作用，并且仅产生40%的衰减。在所有测试剂量下，单独使用AM251对脑刺激奖励没有影响，而单独使用高剂量的SR141716显著抑制脑刺激奖励。这些数据表明，AM251阻断CB1受体的剂量依赖性抑制了可卡因的奖赏效应（通过静脉注射可卡因自我给药和脑电刺激奖励来评估），而SR141716在很大程度上是无效的。我们认为，高剂量SR141716对脑奖励的抑制可能是临床报告中使用SR141716治疗的人出现烦躁不安和/或抑郁的基础。我们得出结论，AM251或其他更有效和选择性的CB1大麻素受体拮抗剂作为潜在有效的抗可卡因药物值得进一步研究。

During the period 01 Oct 06 to 30 Sept 07, significant progress was made on this research project. During said reporting period, we studied the effects of AM251, a novel highly selective cannabinoid CB1 receptor antagonist, on cocaine-triggered relapse to drug-seeking behavior using the reinstatement animal model and on cocaine-induced changes in extracellular dopamine, glutamate, and gamma-aminobutyric acid (GABA) under reinstatement conditions. We found that systemic administration of AM251 selectively inhibited cocaine-induced, but not sucrose plus sucrose cue-induced, reinstatement of reward-seeking behavior. AM251 alone did not trigger reinstatement. Local micro-perfusion of AM251 into the nucleus accumbens of the limbic forebrain or the dorsal striatum also inhibited cocaine-triggered reinstatement. AM251 alone dose-dependently elevated nucleus accumbens glutamate in a voltage-dependent sodium-channel-dependent manner. AM251 did not affect nucleus accumbens dopamine or GABA. Pretreatment with AM251 dose-dependently inhibited cocaine-induced increases in nucleus accumbens glutamate but not in dopamine. Blockade of nucleus accumbens metabotropic glutamate mGluR2/3 receptors by LY341495 slightly facilitated cocaine-enhanced glutamate release but blocked the antagonism of cocaine-triggered reinstatement by AM251. We conclude that: 1) CB1 cannabinoid receptors exert tonic inhibition over nucleus accumbens glutamate release under cocaine-extinction conditions; 2) blockade of CB1 cannabinoid receptors by AM251 inhibits cocaine-enhanced nucleus accumbens glutamate release and cocaine-triggered reinstatement; and 3) these effects appear to be mediated by activation of presynaptic mGluR2/3 autoreceptors secondary to AM251-induced increase (disinhibition) of nucleus accumbens glutamate release. We further found that AM251 dose-dependently lowered (by 30-70%) the break-point for intravenous cocaine self-administration under progressive-ratio reinforcement conditions in laboratory rats. Administration of the CB1 receptor antagonist SR141716 lowered the break-point by 35%, which did not reach statistical significance. Neither AM251 nor SR141716 altered cocaine self-administration under fixed-ratio (FR2) reinforcement conditions. AM251 also significantly and dose-dependently inhibited (by 25-90%) cocaine-enhanced brain stimulation reward, while SR141716 attenuated cocaine's reward-enhancing effect only at the highest dose of SR141716 tested and only producing a 40% attenuation. AM251 alone, at all doses tested, had no effect on brain stimulation reward, while high doses of SR141716 alone significantly inhibited brain reward. These data suggest that blockade of CB1 receptors by AM251 dose-dependently inhibits cocaine's rewarding effects (as assessed by both progressive-ratio intravenous cocaine self-administration and by electrical brain stimulation reward), whereas SR141716 is largely ineffective. We believe that the inhibition of brain reward produced by high doses of SR141716 may underlie clinical reports of dysphoria and/or depression in humans treated with SR141716. We conclude that AM251 or other more potent and selective CB1 cannabinoid receptor antagonists deserve further study as potentially effective anti-cocaine medications.