^*Corresponding author. Tel: +86-25-83272525. E-mail: xudeyi1946@126.com.

5-HT₃ receptors of the central amygdala mediate the modulation of thymus function in rats

XU Min¹, CHEN Wei-Qiang¹, WANG Jiping², FOSTER David JR² and XU De-Yi¹ *

¹Department of Pharmacology, School of Basic Medicine, Southeast University, Nanjing 210009, P.R.China; ²Sansom Institute, School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, SA5000, Australia

Abstract: The aim of the present study was to investigate the modulatory role of activated 5-HT₃ receptors in the central amygdala (CeA) on mitogen concanavalin A (ConA) stimulated proliferative response of thymocytes in rats and the underlying neuroendocrine regulation circuits. 1-phenylbiguanide (PBG), a putative selective 5-HT₃ receptor agonist, was administered by intra-peritoneal (ip), bilateral intra-cerebroventricle (icv), and bilateral intra-central amygdala (ica) injection. In addition, thymocytes isolated from untreated rats were incubated with PBG (at a range of concentrations of 10^-8 ~ 10^-5 mol/L) in vitro, in the presence and absence of ConA in order to investigate any direct effect of PBG on the proliferation in vitro. The MTT method was applied to demonstrate the effect of PBG on the proliferation of thymocytes. An immunohistochemistry SABC assay was used to describe the expression profiles of c-Fos positive cells in different brain regions including the CeA, hippocampus, cortex, hypothalamus and periaqueductal gray (PAG) at 1, 2, 4 and 8 h after bilateral single-administration of PBG by ica (1.0 μg/side). Results showed that PBG (10^-8 ~ 10^-5 mol/L) had no significant influence on the proliferative responses of the isolated thymocytes in vitro, whether ConA was present or not. The ConA-stimulated proliferation of thymocytes was not significantly changed when PBG was administered by ip (0.5 mg/kg/d, for consecutive 5 d), whereas it was remarkably enhanced after bilateral icv injections of PBG (10 μg/side/d, for consecutive 5 d). Similarly, when PBG was injected bilaterally by ica (1.0 μg/side/d, for 7 d), a significantly enhanced proliferation occurred at the 1^st day and continued until reaching its peak at the 5^th day before decreasing at day 7. All of the promoting effects of PBG on the ConA-stimulated proliferation of thymocytes were reversed by pretreatment with the 5-HT₃ receptor antagonist tropisetron (TRP) 5 min prior to the administration of PBG. Interestingly, compared to the treatment with NS or PBG + TRP, after a bilateral single-administration by ica of PBG (1.0 μg/side), the number of c-Fos positive cells expressing in different brain regions significantly increased at 1 h in the CeA, 1-2 h in the hippocampus, 1-2 h in the cortex, 4 h in the hypothalamus and 8 h in the PAG, respectively, with each maximum responses at 1 h in the CeA, 2 h in the hippocampus and cortex, and 4 h in the hypothalamus. Subsequently, the number of cells expressing c-Fos gradually reduced to each minima at 4 h in the CeA, while this was 8 h in the hippocampus, cortex and hypothalamus. In conclusion, the 5-HT₃ receptors in the CeA of rats mediate the modulation of thymus function, at least partly through the neuroendocrine circuit of the limbic system-cortex-hypothalamus-PAG.

Key words: 5-HT₃ receptor; 1-phenylbiguanide; central amygdala; thymocyte; c-Fos; neuroimmunomodulation; rat

大鼠中央杏仁核5-HT₃受体参与胸腺功能调制

徐民¹ 陈伟强¹ 王基平² David JR Foster² 许德义¹*

¹东南大学基础医学院药理学教研室（南京 210009）

²国立南澳大学医药学院三申研究所 (澳大利亚, SA5000)

摘 要：本研究旨在探讨大鼠中央杏仁核内5-HT₃受体激动后对丝裂原concanavalin A（ConA）刺激的胸腺细胞增殖反应的影响及其潜在的神经内分泌调节环路。分别经大鼠腹腔、双侧侧脑室和双侧中央杏仁核（ip，icv和ica）注射选择性5-HT₃受体激动剂1-phenylbiguanide（PBG），同时制备大鼠胸腺细胞悬液与不同浓度PBG（10^-8 ~ 10^-5 mol/L）体外共同孵育。经由MTT法测定，无论有无ConA刺激，离体胸腺细胞在体外培养时其增殖反应均不受PBG（10^-8 ~ 10^-5 mol/L）所影响。而ConA刺激的胸腺细胞的增殖反应虽不受PBG ip（0.5 mg/kg/d，连续5 d）所影响，但其在双侧侧脑室注射PBG（10 μg/侧/d，icv，连续5 d）后显著增强。这一显著增强的增殖反应在双侧中央杏仁核内注射PBG（1.0 μg/侧/d，ica，7 d）时，于给药后第1日开始出现并日益明显，至第5日达到高峰，而于第7日表现为下降。在相同给药部位先于PBG 5 min给予5-HT₃受体拮抗剂tropisetron（TRP）预处理可逆转上述PBG的促胸腺细胞增殖作用。SABC免疫组织化学法显示，双侧中央杏仁核内单次注射PBG（1.0 μg/侧，ica）后，c-Fos阳性细胞在各脑区中相继出现表达（中央杏仁核：1 h；海马：1~2 h；皮层：1~2 h；下丘脑：4 h；中脑导水管周围灰质：8 h）并迅速达到各自高峰（中央杏仁核：1 h；海马：2 h；皮层：2 h；下丘脑：4 h），与相应的生理盐水对照组及TRP预处理组相比均有显著性意义。随后，这一表达在各脑区逐步减弱并消失（中央杏仁核：4 h；海马：8 h；皮层：8 h；下丘脑：8 h）。由此推论，大鼠中央杏仁核内5-HT₃受体至少部分通过边缘系统-皮层-下丘脑-中脑导水管周围灰质这一神经内分泌环路调制胸腺细胞功能。

关键词：5-HT₃受体；1-phenylbiguanide；中央杏仁核；胸腺细胞；c-Fos；神经免疫调制；大鼠