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

5-羟色胺（5-HT）转运体（SERT）负责通过细胞外液（ECF）对5-HT的高亲和力摄取来终止5-羟色胺能神经传递，因此在确定ECF中5-HT的浓度中至关重要。在SERT上起作用的药物，如选择性血清素再摄取抑制剂，可以改善几种神经精神疾病，包括抑郁症和某些焦虑症。SERT也是诸如“摇头丸”（MDMA）等滥用药物的主要作用部位。然而，个体对这些药物的反应存在显著差异，其原因尚不清楚。例如，一些人对抗抑郁药物治疗反应良好，而另一些人则不然。此外，某些人会过量服用MDMA，其剂量对其他人只会产生轻微影响。现在已经确定SERT的等位基因变异会影响SERT的活性，并且可能与许多精神疾病和/或个体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中5-HT清除的功能和适应性影响。使用体内高速时温法，我们发现5-HT1B自身受体可以调节ECF中5-HT的清除，并且SERT密度的改变可以改变5-HT的清除率。本提案将利用SERT或5-HT1B受体基因零突变的两种小鼠（SERT敲除（KO）和5-HT1B受体KO小鼠），在这些动物体内获得SERT调控和基因表达改变对药物敏感性的影响的新见解。据预测，在两种KO菌株的杂合子中（分别表达少50%的SERTs和5-HT1B受体），在基础条件下5-HT的清除率与野生型小鼠没有差异，但在对药物挑战的反应中会显示缺陷。我们的一般假设是，作为SERT或5-HT1B受体密度降低的适应性后果，5-HT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菌株三种基因型中5-HT清除动力学（KT和Vmax）的能力来确定。高速计时电流法将用于测量体内5-羟色胺的清除。定量放射自显影术将用于评估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疾病和成瘾的病理生物学以及SERT等位基因变异个体的药物治疗选择提供重要信息。

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.