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变化。我们以前通过研究mGluR5谷氨酸受体拮抗和NAALADase抑制来研究谷氨酸能参与成瘾。我们发现，选择性的、有效的和系统活性的mGluR5受体拮抗剂MPEP(2-甲基-6-(苯乙炔)-吡啶)阻断mGluR5谷氨酸脑受体可抑制固定比例强化条件下的可卡因自我给药，并在递进比率强化条件下抑制实验大鼠的可卡因自我给药(即显著减少实验大鼠愿意接受静脉注射可卡因的工作量)。我们进一步发现，MPEP对mGluR5谷氨酸受体的阻断显著抑制了可卡因引发的寻药行为的复发，但不能抑制由应激或先前与吸毒行为配对的环境线索引发的寻药行为的复发。通过在体脑微透析的方法，我们进一步发现MPEP对药物幼稚和可卡因戒断大鼠边缘前脑伏隔核内神经递质多巴胺的胞外水平没有影响，提示MPEP的作用机制不依赖于多巴胺。相比之下，MPEP(全身或局部注射到伏隔核内)可升高细胞外谷氨酸。此外，在药物未戒断和可卡因戒断的大鼠中，MPEP呈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，伏隔核谷氨酸的变化可能是MPEP对可卡因诱导的奖赏和可卡因引发的药物寻求行为复发的作用的基础。我们还在与成瘾有关的动物模型中研究了NAALADase抑制。NAALADase(N-乙酰化-α-连锁-酸性二肽酶；谷氨酸羧基肽酶II)是一种脑酶，能将内源性脑神经肽NAAG(N-乙酰-天冬氨酸)水解为谷氨酸(Glu)和NAA(N-乙酰-天冬氨酸)。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的释放。因此，在与成瘾相关的临床前动物模型中研究NAALADase抑制剂，对于寻找临床上有用的药物治疗药物来治疗成瘾、渴求和复发具有重要意义。因此，我们研究了3种NAALADase抑制剂-2-PMPA、GPI-16476和GPI-16477-在与成瘾有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的自身给药都没有影响，但显著地抑制了实验大鼠在药物戒毒和行为上戒除了先前静脉注射可卡因的习惯后，由可卡因引发的复发到寻找可卡因的行为。我们进一步发现，NAALADase抑制剂2-PMPA显著抑制递进比率强化条件下的可卡因自我给药(即显著减少实验室大鼠愿意为接受静脉注射可卡因而花费的工作量)。最近，我们研究了选择性mGluR7激动剂AMN082对脑伏隔核细胞外多巴胺、γ-氨基丁酸(GABA)和谷氨酸的影响。我们发现，全身或伏核内给予AMN082呈剂量依赖性地降低细胞外GABA，升高细胞外谷氨酸，而对伏核外核多巴胺水平无影响。我们发现，这些作用可被Ⅲ组选择性mGluR拮抗剂MSOP阻断。伏核内灌流河豚毒素(TTX)可阻断AMN082引起的谷氨酸升高，但不能阻断AMN082引起的GABA降低，提示这些效应分别来源于囊泡性和非囊泡性GABA。最后，伏隔内灌流选择性GABA-B受体拮抗剂2-羟基糖氯芬不仅能抑制AMN082产生的增强的细胞外谷氨酸，而且还能以TTX抵抗的方式降低细胞外谷氨酸。我们对这些发现的解读是，谷氨酸的增加是次要的，而GABA的减少克服了mGluR7激活对非囊泡性谷氨酸释放的抑制。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。总而言之，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、抗渴求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.