Cass W A, Zahniser N R, Flach K A, Gerhardt G A
Department of Pharmacology, University of Colorado Health Sciences Center, Denver 80262.
J Neurochem. 1993 Dec;61(6):2269-78. doi: 10.1111/j.1471-4159.1993.tb07469.x.
In vivo electrochemistry was used to investigate the mechanisms contributing to the clearance of locally applied dopamine in the dorsal striatum and nucleus accumbens of urethane-anesthetized rats. Chronoamperometric recordings were continuously made at 5 Hz using Nafion-coated carbon fiber electrodes. When a finite amount of dopamine was pressure-ejected at 5-min intervals from a micropipette adjacent to the electrode, transient and reproducible dopamine signals were detected. Substitution of L-alpha-methyldopamine, a substrate for the dopamine transporter but not for monoamine oxidase, for dopamine in the micropipette did not substantially alter the time course of the resulting signals. This indicates that metabolism of locally applied dopamine to 3,4-dihydroxy-phenylacetic acid is not responsible for the decline in the dopamine signal. Similarly, changing the applied oxidation potential from +0.45 to +0.80 V, which allows for detection of 3-methoxytyramine formed from dopamine via catechol-O-methyltransferase, had little effect on signal amplitude or time course. In contrast, lesioning the dopamine terminals with 6-hydroxydopamine, or locally applying the dopamine uptake inhibitors cocaine or nomifensine before pressure ejection of dopamine, significantly increased the amplitude and time course of the dopamine signals in both regions. The effects of cocaine and nomifensine were greater in the nucleus accumbens than in the dorsal striatum. Local application of lidocaine and procaine had no effect on the dopamine signals. Initial attempts at modeling resulted in curves that were in qualitative agreement with our experimental findings. Taken together, these data indicate that (1) uptake of dopamine by the neuronal dopamine transporter, rather than metabolism or diffusion, is the major mechanism for clearing locally applied dopamine from the extracellular milieu of the dorsal striatum and nucleus accumbens, and (2) the nucleus accumbens is more sensitive to the effects of inhibitors of dopamine uptake than is the dorsal striatum.
采用体内电化学方法研究了在乌拉坦麻醉大鼠的背侧纹状体和伏隔核中，局部应用的多巴胺清除机制。使用涂有Nafion的碳纤维电极以5 Hz的频率连续进行计时电流记录。当每隔5分钟从与电极相邻的微量移液器中压力喷射一定量的多巴胺时，检测到瞬态且可重复的多巴胺信号。用L-α-甲基多巴胺（一种多巴胺转运体的底物，但不是单胺氧化酶的底物）替代微量移液器中的多巴胺，并未显著改变所得信号的时间进程。这表明局部应用的多巴胺代谢为3,4-二羟基苯乙酸并不是多巴胺信号下降的原因。同样，将施加的氧化电位从+0.45 V改变为+0.80 V（这允许检测通过儿茶酚-O-甲基转移酶由多巴胺形成的3-甲氧基酪胺），对信号幅度或时间进程几乎没有影响。相比之下，用6-羟基多巴胺损伤多巴胺末梢，或在压力喷射多巴胺之前局部应用多巴胺摄取抑制剂可卡因或诺米芬辛，显著增加了两个区域中多巴胺信号的幅度和时间进程。可卡因和诺米芬辛在伏隔核中的作用比在背侧纹状体中更大。局部应用利多卡因和普鲁卡因对多巴胺信号没有影响。最初的建模尝试得到的曲线与我们的实验结果在定性上一致。综上所述，这些数据表明：（1）神经元多巴胺转运体对多巴胺的摄取，而非代谢或扩散，是从背侧纹状体和伏隔核的细胞外环境中清除局部应用多巴胺的主要机制；（2）伏隔核对多巴胺摄取抑制剂的作用比背侧纹状体更敏感。
