Glutamatergic compounds for treating drug addiction

在2004年10月1日至2005年9月30日期间，该研究项目取得了重大进展。最近的研究表明，长期或反复服用可卡因会导致大脑中谷氨酸神经传递的长期改变。NAALADase（N-乙酰化-α-连接-酸性二肽酶;谷氨酸羧肽酶II）是一种脑酶，其将内源性脑神经肽NAAG（N-乙酰基-乙酰基-谷氨酸）水解为谷氨酸和NAA（N-乙酰基-天冬氨酸）。NAAG是一种内源性mGluR 3谷氨酸受体激动剂，可抑制突触前谷氨酸释放。因此，NAALADase抑制剂在与成瘾相关的临床前动物模型中的研究在寻找用于治疗成瘾、渴望和复发的临床有用的药物制剂中是有意义的。因此，我们研究了3种NAALADase抑制剂- 2-PMPA，GPI-16476和GPI-16477 -在与成瘾相关的动物模型中的作用。我们发现，所有3种NAALADase抑制剂在固定比例强化条件下对静脉内可卡因自我给药没有影响，但在实验室大鼠中显着抑制可卡因引发的可卡因寻求行为复发，这些大鼠已经脱毒并从先前的静脉内可卡因服用习惯中行为消失。我们进一步发现，NAALADase抑制剂2-PMPA在渐进比率强化条件下（即，显著减少了实验室大鼠愿意花费以接受静脉内可卡因输注的工作量）。我们进一步发现，在实验室大鼠中，通过选择性的、有效的和全身活性的mGluR 5受体拮抗剂MPEP（2-甲基-6-（苯乙炔基）-吡啶）阻断mGluR 5谷氨酸脑受体抑制了在固定比率强化条件下的可卡因自我给药，并且抑制了在渐进比率强化条件下的可卡因自我给药（即，显著减少了实验室大鼠愿意花费以接受静脉内可卡因输注的工作量）。我们进一步发现，MPEP对mGluR 5谷氨酸受体的阻断显著抑制了可卡因引发的药物寻求行为的复发，但对先前与吸毒行为配对的压力或环境线索引发的药物寻求行为的复发没有抑制。我们进一步发现，使用体内脑微透析方法，MPEP对未用药或可卡因熄灭大鼠的边缘前脑中脑核中神经递质多巴胺的细胞外水平没有影响，这表明MPEP的作用机制与多巴胺无关。相比之下，MPEP（全身或局部给药到丘脑核）升高细胞外谷氨酸。此外，MPEP剂量依赖性地抑制可卡因诱导的增加，在药物幼稚和可卡因熄灭大鼠的脑桥核细胞外谷氨酸。这些数据表明，核谷氨酸的改变可能是MPEP对可卡因诱导的奖励和可卡因引发的药物寻求行为复发的作用的基础。总之，这些研究结果表明，大脑中的谷氨酸神经递质系统可能是开发潜在的抗成瘾，抗渴望和抗复发药物的适当目标。

During the period 01 Oct 04 to 30 Sept 05, significant progress was made on this research project. Recent studies have shown that chronic or repeated cocaine administration produces long-term alterations in glutamate neurotransmission in the brain. NAALADase (N-acetylated-alpha-linked-acidic dipeptidase; glutamate carboxypeptidase II) is a brain enzyme which hydrolyzes the endogenous brain neuropeptide NAAG (N-acetyl-aspartyl-glutamate) to glutamate and NAA (N-acetyl-aspartate). NAAG is an endogenous mGluR3 glutamate receptor agonist, which inhibits presynaptic glutamate release. Therefore, studies of NAALADase inhibitors in preclinical animal models relating to addiction are of interest in the search for clinically useful pharmacotherapeutic agents for the treatment of addiction, craving, and relapse. Consequently, we studied the effects of 3 NAALADase inhibitors - 2-PMPA, GPI-16476, and GPI-16477 - in animal models relating to addiction. We found that all 3 NAALADase inhibitors had no effect on intravenous cocaine self-administration under fixed-ratio reinforcement conditions, but significantly inhibited cocaine-triggered relapse to cocaine-seeking behavior in laboratory rats who has been pharmacologically detoxified and behaviorally extinguished from their prior intravenous cocaine-taking habits. We further found that the NAALADase inhibitor 2-PMPA significantly inhibits cocaine self-administration under progressive-ratio reinforcement conditions (i.e., significantly reduces the amount of work that laboratory rats are willing to expend to receive intravenous cocaine infusions). We further found that blockade of the mGluR5 glutamate brain receptor by the selective, potent, and systemically-active mGluR5 receptor antagonist MPEP (2-methyl-6-(phenylethynyl)-pyridine) inhibits cocaine self-administration under fixed-ratio reinforcement conditions and inhibits cocaine self-administration under progressive-ratio reinforcement conditions in laboratory rats (i.e., significantly reduces the amount of work that laboratory rats are willing to expend to receive intravenous cocaine infusions). We further found that blockade of the mGluR5 glutamate receptor by MPEP significantly inhibits relapse to drug-seeking behavior triggered by cocaine, but not relapse to drug-seeking behavior triggered by either stress or environmental cues previously paired with drug-taking behavior. We further found that, using in vivo brain microdialysis methods, MPEP has no effect on extracellular levels of the neurotransmitter dopamine in the nucleus accumbens of the limbic forebrain in either drug-naive or cocaine-extinguished rats, suggesting a dopamine-independent mechanism underlying MPEP's actions. In contrast, MPEP (administered either systemically or locally into the nucleus accumbens) elevates extracellular glutamate. Furthermore, MPEP dose-dependently inhibited cocaine-induced increases in nucleus accumbens extracellular glutamate in both drug-naive and cocaine-extinguished rats. These data suggest that alterations in nucleus accumbens glutamate may underlie MPEP's actions on cocaine-induced reward and cocaine-triggered relapse to drug-seeking behavior. In all, these findings suggest that the glutamate neurotransmitter system in the brain may be an appropriate target-of-action for the development of potential anti-addiction, anti-craving, and anti-relapse medications.