Misu Y, Goshima Y, Miyamae T, Furukawa N, Okumura Y, Sugiyama Y, Shimizu M
Department of Pharmacology, Yokohama City University School of Medicine, Japan.
Nihon Yakurigaku Zasshi. 1997 Oct;110 Suppl 1:159P-164P. doi: 10.1254/fpj.110.supplement_159.
DOPA has been proposed to be a neurotransmitter of the primary baroreceptor afferents terminating in the NTS and a neuromodulator in striata of rats. Pre- and post-synaptic recognition sites for DOPA itself may exist. DOPA methyl ester (DOPA ester), a potent competitive antagonist for DOPA in continuously superfused slices or within a several min when microinjected in the NTS has some limitation in in vivo use because of its unstableness as a prodrug for DOPA. We have explored to find stable and potent antagonists for DOPA. At first, we tried to clarify whether or not newly synthesized DOPA esters with bulky structure such as DOPA cyclohexyl ester (CHE), DOPA cyclopentyl ester (CPE) and DOPA cyclopentyldimethyl ester (CPDME) antagonize depressor responses to DOPA microinjected into the NTS, and to what degree these analogues 1 microM perfused via probes are converted to DOPA during striatal microdialysis in urethane-anesthetized rats. Then, we analyzed mode of antagonism of some candidate in the NTS system. During striatal microdialysis, DOPA ester 1 microM increased extracellular levels of DOPA to an 100 fold higher level at a peak of the 1st 20 min sample after the perfusion. Conversion ratio of CHE, CPE and CPDME, compared to DOPA ester, was less than 1/5 at the peak of the 3rd sample, 1/3.3 at the peak of the 2nd, and 1/2.7 at the peak of the 3rd, respectively. In the NTS, all analogues 1 microgram microinjected abolished depressor responses to DOPA 60 ng. Time required to recover a base was 20 min for CHE and 10 min for DOPA ester, CPE and CPDME. At 100 ng, CHE inhibited peak depressor responses to DOPA 60 ng by 53%, CPE by 40%, and CPDME by 24%, compared to DOPA ester by 22%. Then, DOPA 10-300 ng microinjected produced dose-dependent hypotension. DOPA ester 300 ng, microinjected 1 min previously, shifted a dose-response curve for DOPA 30-300 ng to the right without reduction of the maximum response, showing a competitive mode of antagonism. CHE 30 ng competitively antagonized depressor responses to DOPA 18-300 ng without reduction of the maximum response. CHE 100 ng further shifted the dose-response curve for DOPA 18-100 ng to the right with 25% reduction of the maximum response. Antagonistic activity of CHE 100 ng was equipotent with DOPA ester 300 ng. CHE is a competitive antagonist including partially some noncompetitive components. A possible component might be related to NMDA receptors, since high concentrations of CHE displaced selective binding of [3H]-MK-801, an antagonist. In conclusion, CHE is suitable for the purpose.
多巴胺（DOPA）被认为是终止于孤束核（NTS）的初级压力感受器传入神经的神经递质，也是大鼠纹状体中的一种神经调质。DOPA自身可能存在突触前和突触后识别位点。DOPA甲酯（DOPA酯），在连续灌流切片中或微量注射到NTS内几分钟时，是DOPA的一种强效竞争性拮抗剂，但由于其作为DOPA前体药物的不稳定性，在体内使用存在一定局限性。我们一直在探索寻找稳定且强效的DOPA拮抗剂。首先，我们试图弄清新合成的具有庞大结构的DOPA酯，如DOPA环己酯（CHE）、DOPA环戊酯（CPE）和DOPA环戊基二甲酯（CPDME），是否能拮抗微量注射到NTS中的DOPA引起的降压反应，以及在乌拉坦麻醉的大鼠纹状体微透析过程中，通过探针灌流的1 microM这些类似物在多大程度上会转化为DOPA。然后，我们分析了NTS系统中一些候选物的拮抗模式。在纹状体微透析过程中，1 microM的DOPA酯在灌流后的第1个20分钟样本峰值时，使细胞外DOPA水平升高到100倍以上。与DOPA酯相比，CHE、CPE和CPDME的转化率在第3个样本峰值时分别小于1/5、第2个样本峰值时为1/3.3、第3个样本峰值时为1/2.7。在NTS中，微量注射1微克的所有类似物都消除了对60纳克DOPA的降压反应。恢复到基线所需时间，CHE为20分钟，DOPA酯、CPE和CPDME为10分钟。在100纳克时，与22%的DOPA酯相比，CHE抑制对60纳克DOPA的峰值降压反应53%，CPE抑制40%，CPDME抑制24%。然后，微量注射10 - 300纳克的DOPA产生剂量依赖性低血压。提前1分钟微量注射300纳克的DOPA酯，使30 - 300纳克DOPA的剂量 - 反应曲线向右移动，而最大反应未降低，显示出竞争性拮抗模式。30纳克的CHE竞争性拮抗对18 - 300纳克DOPA的降压反应，最大反应未降低。100纳克的CHE进一步使18 - 100纳克DOPA的剂量 - 反应曲线向右移动，最大反应降低25%。100纳克CHE的拮抗活性与300纳克DOPA酯相当。CHE是一种竞争性拮抗剂，部分包括一些非竞争性成分。一个可能的成分可能与N - 甲基 - D - 天冬氨酸（NMDA）受体有关，因为高浓度的CHE取代了拮抗剂[3H] - MK - 801的选择性结合。总之，CHE适合该目的。
