NOVEL ACTIONS OF INVERSE AGONISTS

目前有相当大的兴趣和争论，是否逆激动剂代表仅仅是药理学好奇或是否他们有生理相关的作用和治疗潜力。争论的产生是因为反激动作用主要在受体过度表达或突变（构成活性突变）的人工系统中被证明。在这些非自然系统中，逆激动剂通常会降低“基础”效应剂的活性，因为它会降低配体非依赖性（组成型）受体的活性。然而，越来越多的证据表明，逆受体激动剂具有新的作用，不仅仅是降低基础效应活性，而且这些新作用发生在非突变受体以自然密度表达的系统中。我们发现，尽管5-HT2C反向激动剂不会降低表达低水平人类5-HT2C受体的CHO细胞的基础效应活性，但它们会增强5-HT2C受体系统和内源性表达的嘌呤能受体系统的反应性。这些数据表明，逆激动剂的细胞作用可能是通过激活受体系统介导的，而受体系统不是这些药物的直接靶点。由于许多以前被认为是拮抗剂的治疗性和研究性药物最近被证明（或怀疑）是逆激动剂，这些新作用可能对其治疗效果很重要。为了探索反向激动剂在生理相关模型系统中的这些新作用，我们提出：1)验证5-HT2A和5-HT2C受体在大脑中以及在细胞培养模型中以自然密度表达时构成活性的假设。此外，我们将测试假设，这些受体系统部分脱敏由于效应系统的成分的亚敏感；2)在生理相关的模型系统中，验证与配体无关的5-HT2A和5-HT2C受体活性导致具有相同受体后信号成分的其他非靶受体系统的本构性脱敏的假设。我们将测量5-HT2A/2C反向激动剂改变受体激活反应的能力，这些受体不是5-HT2A/2C反向激动剂的直接靶点（例如嘌呤能、毒碱和肾上腺素能受体），但已知它们激活相同的细胞效应途径。这项研究将提供有关逆激动剂细胞效应的重要信息，这将有助于我们更好地了解各种治疗和研究药物的作用。

Currently there is considerable interest and debate as to whether inverse agonists represent merely a pharmacological curiosity or whether they have physiologically relevant actions and therapeutic potential. The debate arose because inverse agonism has been demonstrated primarily in artificial systems where receptors are overexpressed or mutated (constitutively active mutants). In these non-natural systems, inverse agonists typically reduce "basal" effector activity as a result of reducing ligand-independent (constitutive) receptor activity. However, evidence is beginning to emerge which suggests that inverse agonists have novel actions that extend beyond simply reducing basal effector activity and that these novel effects occur in systems where non-mutated receptors are expressed at natural densities. We have found that even though 5-HT2C inverse agonists do not reduce basal effector activity in CHO cells expressing low levels of the human 5-HT2C receptor, they enhance the responsiveness of both the 5-HT2C receptor system and the endogeously expressed purinergic receptor system. These data suggest that cellular actions of inverse agonists may be mediated through activation of receptor systems that are not direct targets for these drugs. Since many therapeutic and investigational drugs that were previously thought to be antagonists have recently been demonstrated (or are suspected) to be inverse agonists, these novel actions may be important for their therapeutic efficacy. To explore these novel actions of inverse agonists in physiologically relevant model systems, we propose to: 1) test the hypothesis that 5-HT2A and 5-HT2C receptors are constitutively active in brain and when expressed at natural densities in cell culture models. Further, we will test the hypothesis that these receptor systems are partially desensitized due to subsensitivity of components of the effector system; and 2) test the hypothesis that ligand-independent 5-HT2A and 5-HT2C receptor activity, in physiologically relevant model systems, leads to constitutive desensitization of other non- target receptor systems that share the same post-receptor signaling components. We will measure the capacity of 5-HT2A/2C inverse agonists to alter responses to activation of receptors which are not direct targets for 5-HT2A/2C inverse agonists (e.g. purinergic, muscarinic, and adrenergic receptors), but which are known to activate the same cellular effector pathways. This study will provide important information about the cellular effects of inverse agonists which should help us to better understand the actions of a variety of therapeutic and investigational drugs.