5-HT Transporter Function In Vivo: Studies Using KO Mice

5-羟色胺(5-羟色胺)转运体(SERT)负责通过高亲和力摄取细胞外液(ECF)中的5-羟色胺(5-HT)来终止5-羟色胺能神经传递，因此在测定ECF中5-羟色胺(5-HT)的浓度中起关键作用。作用于SERT的药物，如选择性5-羟色胺再摄取抑制剂，可以改善几种神经精神障碍，包括抑郁症和某些焦虑障碍。SERT也是“摇头丸”(MDMA)等滥用药物的主要行动场所。然而，个人对这些药物的反应存在显著的差异，原因尚不清楚。例如，一些人对抗抑郁药物治疗反应良好，而另一些人则不是。此外，某些人会过量服用MDMA，剂量只会对其他人产生轻微影响。SERT中的等位基因变异现在已经被发现影响SERT的活性，并可能与一些精神障碍和/或个人对药物治疗的反应有关。本研究的主要目的是研究基因诱导的SERT和5-HT1B受体表达减少对体内ECF清除5-羟色胺的功能和适应性的影响。利用体内高速计时电流法，我们发现5-HT1B自身受体可以调节ECF对5-羟色胺的清除，SERT密度的改变也可以改变5-羟色胺的清除速率。这项建议将利用两个SERT或5-HT1B受体基因零突变的小鼠(SERT基因敲除(KO)和5-HT1B受体KO小鼠)，在体内获得对SERT的调节和基因表达变化对药物敏感性的影响的新见解。据预测，在两个KO品系的杂合子(分别表达50%的SERT和5-HT1B受体)中，5-羟色胺的清除率在基础条件下与野生型小鼠没有差异，但在药物挑战的反应中会出现缺陷。我们的一般假设是，作为SERT或5-HT1B受体密度降低的适应性结果，5-羟色胺清除动力学的变化将改变这些小鼠对精神药物的敏感性。敏感性的变化将通过这些药物影响每种KO菌株的三种基因型的5-羟色胺清除动力学(KT和Vmax)的能力来衡量。将使用高速计时电流法测量体内5-羟色胺的清除量。定量放射自显影将用于评估SERT基因突变对5-HT1B受体密度的影响，反之亦然。这些小鼠对5-羟色胺摄取的改变以及这些小鼠对精神药物的反应将为我们更好地理解神经精神障碍和成瘾的病理生物学以及选择具有SERT等位基因变异的药物治疗提供重要信息。

The serotonin (5-HT) transporter (SERT) is responsible for terminating serotonergic neurotransmission by high-affinity uptake of 5-HT from extracellular fluid (ECF) and is therefore critical in determining concentrations of 5-HT in ECF. Drugs that act at the SERT, such as selective serotonin reuptake inhibitors, ameliorate several neuropsychiatric disorders, including depression and certain anxiety disorders. The SERT is also a primary site of action of drugs of abuse such as "Ecstasy" (MDMA). However, there is remarkable variability in the response of individuals to these drugs and the reason for this is not clear. For example, some individuals respond well to antidepressant treatment while others do not. Also, certain people will overdose on MDMA at a dose that will only mildly affect others. Allelic variations in the SERT have now been identified that influence SERT activity and may be associated with a number of psychiatric disorders and/or to an individual's response to drug treatment. The main goal of this proposal is to study the functional and adaptive consequences of genetically- induced reductions in the expression of the SERT and 5-HT1B receptor on the clearance of 5-HT from ECF in vivo. Using in vivo high-speed chronoamperometry we have found that the 5-HT1B autoreceptor can regulate clearance of 5-HT from ECF and also, that alterations in the density of the SERT can change the clearance rate of 5-HT. This proposal will take advantage of two lines of mice with null mutations of the SERT or the 5-HT1B receptor gene (the SERT knockout (KO) and 5-HT1B receptor KO mice), to gain new insight into both the regulation of the SERT and the effect of altered gene expression on drug sensitivity in these animals in vivo. It is predicted that in heterozygotes of both KO strains (which express 50 percent fewer SERTs and 5-HT1B receptors respectively), clearance rates of 5-HT will not differ from wild-type mice under basal conditions, but defects will be revealed in response to pharmacologic challenge. Our general hypothesis is that changes in the kinetics of 5-HT clearance as an adaptive consequence of reduced SERT or 5-HT1B receptor density will alter the sensitivity of these mice to psychotropic drugs. Changes in sensitivity will be indexed by the ability of these drugs to influence the kinetics (KT and Vmax) of 5-HT clearance in the three genotypes of each KO strain. High-speed chronoamperometry will be used to measure 5-HT clearance in vivo. Quantitative autoradiography will be used to assess the effect of genetic mutation of the SERT on 5-HT1B receptor density and vice- versa. Alterations in the uptake of 5-HT by these mice and in the response of these mice to psychotropic drugs will provide information important to our better understanding of the pathobiology of neuropsychiatric disorders and addiction as well as to the selection of drug treatment in individuals with allelic variations of the SERT.