Reversible Disruption of Pre-Pulse Inhibition in Hypomorphic-Inducible and Reversible CB1^-/- Mice

Although several genes are implicated in the pathogenesis of schizophrenia, in animal models for such a severe mental illness only some aspects of the pathology can be represented (endophenotypes). Genetically modified mice are currently being used to obtain or characterize such endophenotypes. Since its cloning and characterization CB1 receptor has increasingly become of significant physiological, pharmacological and clinical interest. Recently, its involvement in schizophrenia has been reported. Among the different approaches employed, gene targeting permits to study the multiple roles of the endocannabinoid system using knockout (-/-) mice represent a powerful model but with some limitations due to compensation. To overcome such a limitation, we have generated an inducible and reversible tet-off dependent tissue-specific CB1-/- mice where the CB1R is re-expressed exclusively in the forebrain at a hypomorphic level due to a mutation (IRh-CB1-/-) only in absence of doxycycline (Dox). In such mice, under Dox+ or vehicle, as well as in wild-type (WT) and CB1-/-, two endophenotypes motor activity (increased in animal models of schizophrenia) and pre-pulse inhibition (PPI) of startle reflex (disrupted in schizophrenia) were analyzed. Both CB1-/- and IRh-CB1-/- showed increased motor activity when compared to WT animals. The PPI response, unaltered in WT and CB1-/- animals, was on the contrary highly and significantly disrupted only in Dox+ IRh-CB1-/- mice. Such a response was easily reverted after either withdrawal from Dox or haloperidol treatment. This is the first Inducible and Reversible CB1-/- mice model to be described in the literature. It is noteworthy that the PPI disruption is not present either in classical full CB1-/- mice or following acute administration of rimonabant. Such a hypomorphic model may provide a new tool for additional in vivo and in vitro studies of the physiological and pathological roles of cannabinoid system in schizophrenia and in other psychiatric disorders.尽管已有多个基因被证实与精神分裂症（schizophrenia）的发病机制相关，但针对这类严重精神疾病的动物模型，仅能复现其病理过程的部分特征，即内表型（endophenotypes）。当前科研工作者多采用基因修饰小鼠（genetically modified mice）来获取并表征这类内表型。CB1受体（CB1 receptor）自被克隆并完成表征以来，其在生理、药理及临床领域的研究价值日益凸显。近期已有研究报道其参与了精神分裂症的发病进程。在现有各类研究手段中，基因靶向技术（gene targeting）可用于解析内源性大麻素系统的多重功能，而基因敲除（knockout (-/-)）小鼠虽为极具价值的研究模型，但存在因代偿效应带来的局限性。为克服该局限，本研究构建了一种诱导型且可逆的tet-off依赖组织特异性CB1基因敲除小鼠（IRh-CB1-/-）：在不给予多西环素（doxycycline, Dox）时，该模型小鼠的CB1受体（CB1R）仅在前脑因突变呈现低表达水平。随后，本研究分别在给予Dox、给予赋形剂的该模型小鼠，以及野生型（wild-type, WT）和经典CB1基因敲除（CB1-/-）小鼠中，分析了两种与精神分裂症相关的内表型：运动活性（在精神分裂症动物模型中表现为增强）以及惊反射前脉冲抑制（pre-pulse inhibition, PPI，在精神分裂症患者中存在缺陷）。结果显示，相较于野生型小鼠，CB1-/-与IRh-CB1-/-小鼠均表现出运动活性增强。野生型与CB1-/-小鼠的PPI反应未出现异常，而仅在给予Dox的IRh-CB1-/-小鼠中，PPI反应出现了显著且严重的缺陷。该PPI缺陷可在停止给予Dox或给予氟哌啶醇（haloperidol）治疗后得到有效逆转。本研究是学术文献中首次报道诱导型且可逆的CB1基因敲除小鼠模型。值得注意的是，经典全基因CB1敲除小鼠或急性给予利莫那班（rimonabant）的小鼠，均未出现PPI缺陷。该低表达模型可为研究大麻素系统在精神分裂症及其他精神疾病中的生理与病理功能，提供全新的体内（in vivo）及体外（in vitro）研究工具。