Glutamatergic compounds for treating drug addiction: Preclinical models

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。我们之前通过研究与成瘾相关的动物模型中的mGluR5谷氨酸受体拮抗和NAALADase抑制，研究了谷氨酸能在成瘾中的作用。我们发现，mGluR5受体拮抗剂MPEP（2-甲基-6-（苯乙基）吡啶）阻断mGluR5谷氨酸脑受体抑制固定比例强化条件下的可卡因自我给药，并抑制实验室大鼠(即：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。通过体内脑微透析方法，我们进一步发现MPEP对未用药或可卡因灭活大鼠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。我们还研究了与成瘾有关的动物模型中NAALADase的抑制作用。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）。最近，我们研究了选择性mGluR7激动剂AMN082对大脑伏隔核细胞外多巴胺、γ -氨基丁酸（GABA）和谷氨酸的影响，这是通过体内脑微透析测量的。我们发现全身或伏隔核内给予AMN082剂量依赖性地降低细胞外GABA，增加细胞外谷氨酸，而对细胞外伏隔核多巴胺水平没有影响。我们发现这些作用被MSOP阻断，MSOP是一种iii组选择性mGluR拮抗剂。伏隔区内灌注河豚毒素（TTX）阻断了amn082诱导的谷氨酸增加，但未能阻断amn082诱导的GABA减少，提示这些作用分别来自水疱和非水疱GABA。最后，选择性GABA-B受体拮抗剂2-羟氯芬在伏隔核内灌注不仅消除了AMN082产生的增强的细胞外谷氨酸，而且实际上以ttx耐药的方式降低了细胞外谷氨酸。我们解释这些发现表明谷氨酸的增加是继发于GABA的减少，这克服了mGluR7激活诱导的非囊状谷氨酸释放的抑制。我们进一步解释这些发现表明，与其对GABA和谷氨酸的调节作用相反，mGluR7受体似乎不调节或调节伏隔核多巴胺的释放。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。

Studies have shown that chronic or repeated cocaine administration produces long-term alterations in glutamate neurotransmission in the brain. We had previously studied glutamatergic involvement in addiction by studying mGluR5 glutamate receptor antagonism and NAALADase inhibition in animal models relating to addiction. We 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. By using in vivo brain microdialysis methods, we further found that 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. We also studied NAALADase inhibition in animal models relating to addiction. 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). More recently, we studied the effects of AMN082, a selective mGluR7 agonist, on extracellular dopamine, gamma-aminobutyric acid (GABA), and glutamate in the nucleus accumbens of the brain as measured by in vivo brain microdialysis. We found that systemic or intra-accumbens administration of AMN082 dose-dependently lowered extracellular GABA, increased extracellular glutamate, and had no effect on extracellular nucleus accumbens dopamine levels. We found that these effects were blocked by MSOP, a group III-selective mGluR antagonist. Intra-accumbens perfusion of tetrodotoxin (TTX) blocked the AMN082-induced increases in glutamate, but failed to block the AMN082-induced reduction in GABA, suggesting vesicular and non-vesicular GABA origins for these effects, respectively. Finally, intra-accumbens perfusion of the selective GABA-B receptor antagonist 2-hydroxysaclofen not only abolished the enhanced extracellular glutamate produced by AMN082, but actually decreased extracellular glutamate in a TTX-resistant manner. We interpret these findings to suggest that the increase in glutamate is secondary to the decrease in GABA, which overcomes mGluR7 activation-induced inhibition of non-vesicular glutmate release. We further interpret these findings to suggest that, in contrast to its modulatory effect on GABA and glutamate, the mGluR7 receptor does not appear to modulate or regulate nucleus accumbens dopamine release. 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.