Glutamatergic compounds for treating drug addiction: Preclinical models

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。我们先前通过研究与成瘾相关的动物模型中的mGluR 5谷氨酸受体拮抗作用和NAALADase抑制作用，研究了多巴胺能参与成瘾。我们发现，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。通过使用在体脑微透析方法，我们进一步发现，MPEP有没有影响细胞外水平的神经递质多巴胺的边缘前脑的核多巴胺在药物幼稚或可卡因熄灭的大鼠，这表明多巴胺的独立机制MPEP的行动。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。我们还研究了NAALADase抑制与成瘾相关的动物模型。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）。最近，我们研究了AMN 082，一种选择性mGluR 7激动剂，对细胞外多巴胺，γ-氨基丁酸（GABA），谷氨酸在脑髓核的影响，在体内脑微透析测量。我们发现全身或肌内给予AMN 082剂量依赖性地降低细胞外GABA，增加细胞外谷氨酸，并且对细胞外核多巴胺水平没有影响。我们发现，这些影响被MSOP，一种III组选择性mGluR拮抗剂阻断。河豚毒素（TTX）的内灌注阻断AMN 082诱导的谷氨酸的增加，但未能阻断AMN 082诱导的GABA的减少，这表明这些作用的囊泡和非囊泡GABA的起源，分别。最后，选择性GABA-B受体拮抗剂2-羟基苯氯芬的内-阿替卡因灌注不仅消除了AMN 082产生的增强的细胞外谷氨酸，但实际上减少了细胞外谷氨酸在TTX抗性的方式。我们解释这些发现表明，谷氨酸的增加是次要的GABA，这克服了mGluR 7激活诱导的抑制非囊泡谷氨酸释放的减少。我们进一步解释这些发现表明，在其对GABA和谷氨酸的调节作用，mGluR 7受体不出现调制或调节多巴胺释放的丘脑核。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。

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.