Glutamatergic compounds for treating drug addiction

在2004年10月1日至2005年9月30日期间，本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对大脑中的谷氨酸神经传递产生长期的改变。NAALADase （n -乙酰化- α -连接-酸性二肽酶；谷氨酸羧肽酶II）是一种将内源性脑神经肽NAAG （n -乙酰-天冬氨酸-谷氨酸）水解为谷氨酸和NAA （n -乙酰-天冬氨酸）的脑酶。NAAG是一种内源性mGluR3谷氨酸受体激动剂，可抑制突触前谷氨酸释放。因此，在与成瘾相关的临床前动物模型中研究NAALADase抑制剂，对于寻找治疗成瘾、渴望和复发的临床有用药物治疗药物具有重要意义。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，在固定比例强化条件下，所有3种NAALADase抑制剂对静脉注射可卡因自我给药没有影响，但在药理学上解毒和行为上消失的实验室大鼠中，它们明显抑制可卡因引发的可卡因寻求行为的复发。我们进一步发现，在递进比例强化条件下，NAALADase抑制剂2-PMPA显著抑制可卡因自我给药（即显著减少实验室大鼠为接受静脉注射可卡因而愿意付出的工作量）。我们进一步发现，选择性、强效和系统活性的mGluR5受体拮抗剂MPEP（2-甲基-6-（苯乙基）-吡啶）阻断mGluR5谷氨酸脑受体，可抑制实验室大鼠在固定比例强化条件下的可卡因自我给药，并抑制在递进比例强化条件下的可卡因自我给药。大大减少了实验鼠接受静脉注射可卡因的工作量)。我们进一步发现，MPEP阻断mGluR5谷氨酸受体可显著抑制可卡因引发的寻求药物行为的复发，但对先前与吸毒行为相关联的压力或环境线索引发的寻求药物行为的复发不起作用。我们进一步发现，使用体内脑微透析方法，MPEP对边缘前脑伏隔核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。相反，MPEP（全身或局部注入伏隔核）可提高细胞外谷氨酸。此外，MPEP剂量依赖性地抑制可卡因诱导的伏隔核细胞外谷氨酸的增加。这些数据表明，伏隔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，这些发现表明，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合适靶点。

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.