5-HT(3) receptors in the central amygdala mediate the modulation of thymus function in rats

The aim of the present study was to investigate the modulatory role of activated 5-HT(3) 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-HT(3) receptor agonist, was administered by intraperitoneal (i.p.), bilateral intracerebroventriclular (i.c.v.), and bilateral intracentral amygdala (i.c.a.) injection. In addition, thymocytes isolated from untreated rats were incubated with PBG (at a range of concentrations of 1x10(-8)-1x10(-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. MTT method was applied to demonstrate the effect of PBG on the proliferative response of thymocytes. An immunohistochemical 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 i.c.a. (1.0 microg/side). Results showed that PBG (1x10(-8)-1x10(-5) mol/L) had no significant influence on the proliferative responses of the isolated thymocytes in vitro, no matter ConA was present or not. The proliferation of thymocytes stimulated by ConA was not significantly changed when PBG was administered by i.p. (0.5 mg/kg per day, for consecutive 5 d), whereas it was remarkably enhanced after bilateral i.c.v. injection of PBG (10 microg/side per day, for consecutive 5 d). Similarly, when PBG was injected bilaterally by i.c.a. (1.0 microg/side per day, for 1 d or consecutive 3, 5 and 7 d), a significantly enhanced proliferation occurred on the 1st day and continued until reaching its peak on the 5th day before decreasing on the 7th day. All of the promoting effects of PBG on the ConA-stimulated proliferation of thymocytes were reversed by pretreatment with the 5-HT(3) receptor antagonist tropisetron (TRP) 5 min prior to the administration of PBG. Interestingly, compared to the treatment with normal saline or TRP + PBG, after a bilateral single-administration of PBG (1.0 microg/side) by i.c.a., the number of c-Fos-positive cells 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 response 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 the minimum at 4 h in the CeA, and at 8 h in the hippocampus, cortex and hypothalamus. In conclusion, the 5-HT(3) 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.