Cahusac P M B, Senok S S, Hitchcock I S, Genever P G, Baumann K I
Department of Psychology, University of Stirling, Stirling FK9 4LA, Scotland, UK.
Neuroscience. 2005;133(3):763-73. doi: 10.1016/j.neuroscience.2005.03.018.
Specific immunohistochemical staining for NMDA receptor NR2A/B subunits was found in the outer root sheath layer of rat sinus hair (whisker) follicle. Co-localization with CK 20 confirmed that Merkel cells were stained. The NR2A/B staining seen on Merkel cells was pericellular. In addition it appeared that NF70-positive staining was in close proximity to, but did not colocalise with NR2A/B immunoreactivity, indicating that NR2A/B was only expressed by Merkel cells and not their adjacent nerve terminals. Merkel cells and the nerve terminals have previously been associated with electrophysiological recordings from slowly adapting type I (St I) mechanoreceptor unit activity. Pharmacological experiments with isolated sinus hairs using a wide range of ionotropic glutamate receptor antagonists found that only certain NMDA receptor blockers depressed St I unit responses to mechanical stimuli. AMPA/kainate receptor antagonists (CNQX and NBQX, 100 microM) had no effect, nor did classical competitive NMDA receptor antagonists, D-AP5 (600 microM) and R-CPP (100 microM), nor the NMDA glycine site antagonist 5,7-dichlorokynurenic acid (100 microM). The only effective NMDA receptor blockers were those selective for the polyamine site: ifenprodil (IC50 20 microM) and Ro 25-6981 (IC50 approximately 50 microM), and the associated ion channel: MK 801, ketamine and (+/-)-1-(1,2-diphenylethyl)piperidine (IC50 < 100 microM). The two enantiomers of MK 801 were equipotent. All effects were long lasting, consistent with their non-/uncompetitive actions. The most potent drug tested, ifenprodil, at an effective dose of 30 microM, had a mean recovery time of 74 min. A three-fold increase in drug concentration was required to depress St II units (associated with non-synaptic lanceolate endings). Changes in Zn2+ did not affect St I unit responses. These data suggest that unconventional NMDA receptors are involved in St I unit responses, but question the notion of a glutamatergic synapse between the Merkel cell and nerve terminal.
在大鼠鼻窦毛发（触须）毛囊的外根鞘层中发现了NMDA受体NR2A/B亚基的特异性免疫组化染色。与细胞角蛋白20的共定位证实了默克尔细胞被染色。在默克尔细胞上看到的NR2A/B染色是围绕细胞的。此外，似乎NF70阳性染色与NR2A/B免疫反应性紧密相邻，但不共定位，这表明NR2A/B仅由默克尔细胞表达，而不由其相邻的神经末梢表达。默克尔细胞和神经末梢以前与慢适应性I型（St I）机械感受器单位活动的电生理记录有关。使用多种离子型谷氨酸受体拮抗剂对分离的鼻窦毛发进行的药理学实验发现，只有某些NMDA受体阻滞剂会抑制St I单位对机械刺激的反应。AMPA/海人酸受体拮抗剂（CNQX和NBQX，100微摩尔）没有效果，经典的竞争性NMDA受体拮抗剂D-AP5（600微摩尔）和R-CPP（100微摩尔）也没有效果，NMDA甘氨酸位点拮抗剂5,7-二氯犬尿氨酸（100微摩尔）也没有效果。唯一有效的NMDA受体阻滞剂是对多胺位点有选择性的那些：ifenprodil（IC50为20微摩尔）和Ro 25-6981（IC50约为50微摩尔），以及相关的离子通道：MK 801、氯胺酮和（±）-1-（1,2-二苯乙基）哌啶（IC50<100微摩尔）。MK 801的两种对映体效力相当。所有作用都是持久的，与其非/非竞争性作用一致。测试的最有效药物ifenprodil，在有效剂量30微摩尔时，平均恢复时间为74分钟。需要将药物浓度提高三倍才能抑制St II单位（与非突触性柳叶状末梢相关）。锌离子的变化不影响St I单位反应。这些数据表明非常规NMDA受体参与了St I单位反应，但对默克尔细胞与神经末梢之间谷氨酸能突触的概念提出了质疑。
