Khan Imran M, Wennerholm Michelle, Singletary Erin, Polston Kimberley, Zhang Limin, Deerinck Tom, Yaksh Tony L, Taylor Palmer
Department of Pharmacology, University of California, San Diego, CA 92093-0636, USA.
J Neurocytol. 2004 Sep;33(5):543-56. doi: 10.1007/s11068-004-0516-6.
A variety of studies indicate that spinal nicotinic acetylcholine receptors modulate the behavioral and autonomic responses elicited by afferent stimuli. To examine the location of and role played by particular subtypes of nicotinic receptors in mediating cardiovascular and nociceptive responses, we treated neonatal and adult rats with capsaicin to destroy C-fibers in primary afferent terminals. Reduction of C-fiber terminals was ascertained by the loss of isolectin B4, CGRP and vanilloid receptors as monitored by immunofluorescence. Receptor autoradiography shows a reduction in number of epibatidine binding sites following capsaicin treatment. The reduction is particularly marked in the dorsal horn and primarily affects the class of high affinity epibatidine binding sites thought to modulate nociceptive responses. Accompanying the loss of terminals and nicotinic binding sites were significant reductions in the expression of alpha 3, alpha 4, alpha 5, beta 2 and beta 4 nicotinic receptor subunits in the superficial layers of the spinal cord as determined by antibody staining and confocal microscopy. The loss of nicotinic receptors that follows capsaicin treatment results in attenuation of the nociceptive responses to both spinal cytisine and epibatidine. Capsaicin treatment also diminishes the capacity of cytisine to desensitize nicotinic receptors mediating nociception, but it shows little effect on intrathecal nicotinic agonist elicited pressor and heart rate responses. Hence, our data suggest that alpha 3, alpha 4, alpha 5, beta 2 and beta 4 subunits of nicotinic receptors are localized in the spinal cord on primary afferent terminals that mediate nociceptive input. A variety of convergent data based on functional studies and subunit expression suggest that alpha 3 and alpha 4, in combination with beta 2 and alpha 5 subunits, form the majority of functional nicotinic receptors on C-fiber primary afferent terminals. Conversely, spinal nicotinic receptors not located on C-fibers play a primary role in the spinal pathways evoking spinally coordinated autonomic cardiovascular responses.
多项研究表明，脊髓烟碱型乙酰胆碱受体可调节传入刺激引发的行为和自主反应。为了研究特定亚型烟碱受体在介导心血管和伤害性反应中的位置及作用，我们用辣椒素处理新生大鼠和成年大鼠，以破坏初级传入终末中的C纤维。通过免疫荧光监测异凝集素B4、降钙素基因相关肽（CGRP）和香草酸受体的缺失来确定C纤维终末的减少情况。受体放射自显影显示，辣椒素处理后，埃博霉素结合位点数量减少。这种减少在背角尤为明显，主要影响被认为可调节伤害性反应的高亲和力埃博霉素结合位点类别。伴随终末和烟碱结合位点的丧失，通过抗体染色和共聚焦显微镜检测发现，脊髓表层中α3、α4、α5、β2和β4烟碱受体亚基的表达显著降低。辣椒素处理后烟碱受体的丧失导致对脊髓金雀花碱和埃博霉素的伤害性反应减弱。辣椒素处理还降低了金雀花碱使介导伤害感受的烟碱受体脱敏的能力，但对鞘内烟碱激动剂引发的升压和心率反应影响较小。因此，我们的数据表明，烟碱受体的α3、α4、α5、β2和β4亚基定位于介导伤害性输入的初级传入终末的脊髓上。基于功能研究和亚基表达的各种趋同数据表明，α3和α4与β2和α5亚基结合，构成了C纤维初级传入终末上大多数功能性烟碱受体。相反，位于C纤维上的脊髓烟碱受体在引发脊髓协调自主心血管反应的脊髓通路中起主要作用。
