Holohean A M, Hackman J C, Davidoff R A
Neurophysiology and Spinal Cord Pharmacology Laboratories, University of Miami School of Medicine, FL, USA.
Brain Res. 1995 Dec 18;704(2):184-90. doi: 10.1016/0006-8993(95)01112-9.
Motoneuron membrane potentials were recorded from the ventral roots of isolated, hemisected frog spinal cords using sucrose gap techniques. The effects of the selective 5-HT3 agonist 2-methyl-serotonin (2-Me-5HT) on the changes in motoneuron membrane potential produced by dorsal root stimulation and by superfusion of excitatory amino acid agonists were evaluated. Application of 2-Me-5HT (100 microM) did not alter motoneuron membrane potential, but did substantially reduce (approximately 20%) the polysynaptic ventral root potentials evoked by dorsal root stimulation. 2-Me-5HT did not change motoneuron depolarizations generated by addition to the Ringer's solution of the excitatory amino acid agonists AMPA (10-30 microM), kainate (30 microM), or t-ACPD (100 microM), but NMDA-induced motoneuron depolarizations (100 microM) were significantly and reversibly reduced (approximately 20%) by exposure to 2-Me-5HT (100 microM). 2-Me-5HT-evoked decreases of NMDA depolarizations were blocked by the 5-HT3 antagonists ICS 205 930 (50-100 microM) and D-tubocurarine (3-10 microM), but not by MDL 72222 (20-100 microM), the 5-HT2 receptor antagonist ketanserin (10 microM), or the 5-HT1A/5-HT2A antagonist spiperone (10 microM). Two lines of evidence support the hypothesis that the effects of 2-Me-5HT are generated by an indirect mechanism involving interneurons: (1) TTX (0.781 microM) eliminated the effect of 2-Me-5HT on NMDA-induced motoneuron depolarizations, and (2) 2-Me-5HT reduced spontaneous ventral root potentials that result from interneuronal discharges. We attempted to establish the identity of a putative transmitter released by interneurons responsible for the effects on NMDA-depolarizations produced by 2-Me-5HT, but the AMPA receptor antagonist, CNQX (10 microM), the GABAA receptor antagonist, bicuculline (50 microM), the GABAB receptor antagonist, saclofen (100 microM), the opioid antagonist, naloxone (100 microM), and the adenosine antagonists, CPT (20-100 microM) and CSC (10-100 microM) did not alter 2-Me-5HT-induced reductions of NMDA-depolarizations. In sum, the site of interaction between 2-Me-5HT and NMDA appears to be at interneuronal locus, but the mechanism remains unclear.
采用蔗糖间隙技术，从分离并半横切的青蛙脊髓腹根记录运动神经元膜电位。评估了选择性5-羟色胺3（5-HT3）激动剂2-甲基-5-羟色胺（2-Me-5HT）对背根刺激和兴奋性氨基酸激动剂灌流所产生的运动神经元膜电位变化的影响。应用2-Me-5HT（100微摩尔）并未改变运动神经元膜电位，但显著降低了（约20%）背根刺激诱发的多突触腹根电位。2-Me-5HT并未改变因向林格氏液中添加兴奋性氨基酸激动剂α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA，10 - 30微摩尔）、海人藻酸（30微摩尔）或反式-ACPD（100微摩尔）而产生的运动神经元去极化，但暴露于2-Me-5HT（100微摩尔）时，N-甲基-D-天冬氨酸（NMDA）诱导的运动神经元去极化（100微摩尔）显著且可逆地降低了（约20%）。2-Me-5HT诱发的NMDA去极化降低被5-HT3拮抗剂ICS 205 930（50 - 100微摩尔）和筒箭毒碱（3 - 10微摩尔）阻断，但未被5-HT2受体拮抗剂酮色林（10微摩尔）、5-HT1A/5-HT2A拮抗剂螺哌隆（10微摩尔）或MDL 72222（20 - 100微摩尔）阻断。有两条证据支持2-Me-5HT的作用是通过涉及中间神经元的间接机制产生的这一假说：（1）河豚毒素（TTX，0.781微摩尔）消除了2-Me-5HT对NMDA诱导的运动神经元去极化的作用，（2）2-Me-5HT降低了由中间神经元放电引起的自发腹根电位。我们试图确定负责2-Me-5HT对NMDA去极化产生影响的中间神经元释放的假定递质的身份，但AMPA受体拮抗剂CNQX（10微摩尔）、γ-氨基丁酸A型（GABAA）受体拮抗剂荷包牡丹碱（50微摩尔）、GABAB受体拮抗剂赛氯芬（100微摩尔）、阿片受体拮抗剂纳洛酮（100微摩尔）以及腺苷拮抗剂CPT（20 - 100微摩尔）和CSC（10 - 100微摩尔）均未改变2-Me-5HT诱导的NMDA去极化降低。总之，2-Me-5HT与NMDA之间的相互作用位点似乎在中间神经元部位，但其机制仍不清楚。
