Muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

Increase in dopaminergic, but not serotoninergic, receptors in T-cells as a marker for schizophrenia severity

Schizophrenia is characterized by a slow deteriorating mental illness. Although the pathophysiology mechanisms are not fully understood, different studies have suggested a role for the immune system in the pathogenesis of schizophrenia. To date, an altered expression or signaling of neurotransmitters receptors is observed in immune cells during psychiatric disorders. In the present study, we investigated the expression of different serotonin and dopamine receptors in T-cells of schizophrenic and control patients. We used flow cytometry to determine the pattern of expression of dopamine (D2 and D4) and serotonine receptors (SR1A, SR1C, SR2A, SR2B), as well as serotonin transporter (ST), in T-cell subsets (CD4 and CD8). Expression of serotonin receptors and ST in T-cells of schizophrenic patients were not different from controls. However, the percentages of CD4+D4+ and CD8+D4+ were increased in schizophrenic patients as compared to controls. In addition, increased percentages of CD8+D2+ cells were also observed in schizophrenic patients, albeit this population revealed lower CD4+D2+ cells in comparison to controls. Interestingly, a relationship between clinical symptoms and immunological parameters was also observed. We showed that the Brief Psychiatric Rating Scale (BPRS), the Positive and Negative Syndrome Scale (PANSS) and the Abnormal Involuntary Movement Scale (AIMS) were positively related to CD8+D2+ cells, though AIMS was inversely related to CD4+D4+ cells. In conclusion, the alteration in the pattern of cell population and molecules expressed by them might serve as a promising biomarker for diagnosis of schizophrenia.

Acetylcholine-induced neuronal differentiation: muscarinic receptor activation regulates EGR-1 and REST expression in neuroblastoma cells

Neurotransmitters are considered part of the signaling system active in nervous system development and we have previously reported that acetylcholine (ACh) is capable of enhancing neuronal differentiation in cultures of sensory neurons and N18TG2 neuroblastoma cells. To study the mechanism of ACh action, in this study, we demonstrate the ability of choline acetyltransferase-transfected N18TG2 clones (e.g. 2/4 clone) to release ACh. Analysis of muscarinic receptors showed the presence of M1-M4 subtypes and the activation of both IP(3) and cAMP signal transduction pathways. Muscarinic receptor activation increases early growth response factor-1 (EGR-1) levels and treatments with agonists, antagonists, and signal transduction enzyme inhibitors suggest a role for M3 subtype in EGR-1 induction. The role of EGR-1 in the enhancement of differentiation was investigated transfecting in N18TG2 cells a construct for EGR-1. EGR-1 clones show increased neurite extension and a decrease in Repressor Element-1 silencing transcription factor (REST) expression: both these features have also been observed for the 2/4 clone. Transfection of this latter with EGR zinc-finger domain, a dominant negative inhibitor of EGR-1 action, increases REST expression, and decreases fiber outgrowth. The data reported suggest that progression of the clone 2/4 in the developmental program is dependent on ACh release and the ensuing activation of muscarinic receptors, which in turn modulate the level of EGR-1 and REST transcription factors.

Noradrenergic and cholinergic neural pathways mediate stress-induced reactivation of colitis in the rat

Evidence to date suggests that stress-induced exacerbation or relapse of intestinal inflammation in inflammatory bowel disease requires both activation of the autonomic nervous system and the activation of the immune system by the presence of previously encountered luminal antigens. The aim of the present study was to further explore these associations and to determine the role of the autonomic nervous in modulating the intestinal inflammatory response to stress. Rats healed from an initial dinitrobenzene sulfonic acid-induced colitis were given a non-colitic dose of dinitrobenzene sulfonic acid (dissolved in saline) or 0.9% saline intra-rectally and then subjected to restraint stress. Cardiac sympathovagal balance was assessed by power spectral analysis of heart rate variability data collected from telemetric electrocardiogram recordings before, during and post stress. Only rats that were stressed and received dinitrobenzene sulfonic acid showed an inflammatory relapse characterized by significant macroscopic damage and elevated myeloperoxidase activity associated with a significant infiltration of mucosal and submucosal T lymphocytes. No difference in inflammatory markers was observed in animals that received intra-rectal saline and restraint stress. Rats subjected to stress and intra-rectal dinitrobenzene sulfonic acid demonstrated an increase in sympathetic activity with a nearly four fold increase in LF:HF ratio during stress and a significant increase in heart rate. Shortly after cessation of stress, the LF:HF ratio decreased significantly, returning to baseline levels, however the heart rate remained significantly elevated over baseline levels following stress, but decreased to a level that was significantly lower than during stress. The stress/dinitrobenzene sulfonic acid-induced relapses were preventable by pre-treating rats with hexamethonium (a nicotinic cholinergic ganglion blocking agent) or the co-administration of atropine (a muscarinic cholinoceptor antagonist) and bretylium (a noradrenergic ganglion blocking agent), but was not prevented when either atropine or bretylium were administered alone. This study utilizes an established model of chemically induced colitis that when integrated with stress results in relapsing inflammatory bowel disease. Moreover, this study demonstrates that noradrenergic and cholinergic neural pathways mediate the stress response critical for the relapse of colitis.

D3 dopamine receptor mRNA is elevated in T cells of schizophrenic patients whereas D4 dopamine receptor mRNA is reduced in CD4+ -T cells

The expression of dopamine receptors was examined in purified human neutrophils, monocytes, B cells, natural killer cells and CD4+ - and CD8+ -T lymphocytes by RT-PCR. In healthy subjects, D1 and D2 receptors were not expressed in leukocytes. Real Time PCR for dopamine receptors D3 and D4 disclosed that D3 receptors are expressed in T cells and natural killer cells and D4 receptors in CD4+ -T cells. The comparison of schizophrenic patients with sex- and age-matched controls revealed a significantly higher expression of D3 receptor mRNA in T cells of schizophrenic patients, whereas D4 receptor mRNA in CD4+ -T cells was downregulated.

An in vitro investigation of the effects of the nerve agent pretreatment pyridostigmine bromide on human peripheral blood T-cell function

The current pretreatment against nerve agent poisoning deployed by the UK and US armed forces is the acetylcholinesterase (EC 3.1.1.7) inhibitor pyridostigmine bromide (PB). At higher doses, PB is also used to treat the autoimmune disease myasthenia gravis. In both cases, the therapeutic effect is mediated by inhibition of acetylcholinesterase (AChE) at cholinergic synapses. However, the location of AChE is not restricted to these sites. AChE, acetylcholine (ACh) receptors and choline acetyltransferase have been reported to be expressed by T cells, suggesting that cholinergic signalling may exert some modulatory influence on T-cell function and consequently on the immune system. The aim of this study was to investigate the role of the T-cell cholinergic system in the immunological activation process and to examine whether inhibitors of AChE such as PB affect immune function. To investigate this, human peripheral blood mononuclear cells (PBMC) were stimulated using either mitogen, cross-linking of the T-cell receptor and co-receptors with antibodies (anti-CD3/CD28) or by antigen presentation in the presence of various AChE inhibitors and ACh receptor agonists or antagonist. Several indices were used to assess T-cell activation, including the secretion of IL-2, cell proliferation and expression of CD69. Treatment with PB had no significant effect on the immunological assays selected. Physostigmine (PHY), a carbamate compound similar to PB, consistently showed inhibition of T-cell activation, but only at concentrations in excess of those required to inhibit AChE. No evidence was found to support previously published findings showing muscarinic enhancement of cell proliferation or IL-2 secretion.

Potential clozapine target sites on peripheral hematopoietic cells and stromal cells of the bone marrow

The antipsychotic drug clozapine, acts via interaction with selective neurotransmitter receptor systems. Its use however, is associated with life-threatening agranulocytosis. The mechanism by which this occurs and its possible relationship with the drug's atypicality remain unclear. As a first step in identifying mechanistic pathways involved, profiling of neurotransmitter receptors on human neutrophils, mononuclear and bone marrow stromal cells as putative targets for clozapine-mediated toxicity was undertaken. Expression of mRNA encoding dopaminergic d2, d3, d4; serotonergic 5ht2a, 5ht2c, 5ht3, 5ht6, 5ht7; adrenergic alpha1a, alpha2; histaminergic h1 and muscarinic m1, m2, m3, m4, m5 receptors was analyzed by reverse transcription-polymerase chain reaction methods. While 5ht2c, 5ht6, m1 and m2 mRNA were undetected, the presence of the other receptors indicates sites at which clozapine could bind and induce toxicity of neutrophils and stromal components which regulate granulopoiesis. The functional significance of differential receptor expression while unknown, may argue for neural regulation of hematopoiesis.

Cholinergic pathways modulate experimental dinitrobenzene sulfonic acid colitis in rats

Recent studies have suggested that neuroimmune interactions modulate intestinal mucosal immune responses. In the current study, we examined the role of cholinergic pathways in modulating the severity of acute dinitrobenzene sulfonic acid colitis, using pharmacological agents to suppress acetylcholinesterase in Sprague-Dawley rats, and evaluating the colitis in the cholinergic hyperresponsive Flinder's sensitive line rats and their control counterparts, the Flinder's resistant line. Colitis was induced by intrarectal dinitrobenzene sulfonic acid (80 mg x ml(-1) in 50% ethanol); controls received intrarectal saline. Sprague-Dawley rats received an acetylcholinesterase inhibitor, physostigmine (50 microg x kg(-1) s.c.) or neostigmine (50 microg x kg(-1) s.c.), 30 min prior to intrarectal dinitrobenzene sulfonic acid; controls received saline vehicle. On day 5, the macroscopic damage score, myeloperoxidase activity (an estimate of granulocyte infiltration) and smooth muscle thickness were evaluated in the inflamed colonic segment. Significant increases in macroscopic damage score and colonic smooth muscle thickness were observed in Sprague-Dawley and Flinder's Resistant Line rats on day 5 following intrarectal dinitrobenzene sulfonic acid compared to saline controls. Increased myeloperoxidase activity was also observed in dinitrobenzene sulfonic acid-treated Sprague-Dawley rats and Flinder's Resistant Line rats. In contrast, Flinder's Sensitive Line rats failed to demonstrate a significant rise in macroscopic damage, smooth muscle layer thickness, or myeloperoxidase activity on day 5 following intrarectal dinitrobenzene sulfonic acid when compared to saline-treated Flinder's Sensitive Line controls. Neostigmine and physostigmine treatment prior to intrarectal dinitrobenzene sulfonic acid significantly attenuated macroscopic damage score, myeloperoxidase activity and smooth muscle thickness on day 5 compared to colitic Sprague-Dawley controls. Significantly greater reductions in myeloperoxidase activity were observed with physostigmine vs. neostigmine pretreatment. These data suggest that cholinergic pathways modulate the acute colonic inflammatory response associated with the dinitrobenzene sulfonic acid model, with central pathways exerting a greater protective effect relative to peripheral pathways. Further studies are required to determine the contributions of sites in the nervous system and neuro-effector junctions.

Structure and transcription of the human m3 muscarinic receptor gene

We have isolated and characterized the human m3 muscarinic receptor gene and its promoter. Using 5' rapid amplification of cDNA ends (RACE), internal polymerase chain reaction (PCR), and homology searching to identify EST clones, we determined that the cDNA encoding the m3 receptor comprises 4,559 bp in 8 exons, which are alternatively spliced to exclude exons 2, 4, 6, and/or 7; the receptor coding sequence occurs within exon 8. Analysis of P1 artificial chromosome (PAC) and bacterial artificial chromosome (BAC) clones and of PCR- amplified genomic DNA, and homology searching of human chromosome 1 sequence provided from the Sanger Centre (Hinxton, Cambridge, UK) revealed that the m3 muscarinic receptor gene spans at least 285 kb. A promoter fragment containing bp -1240 to +101 (relative to the most 5' transcription start site) exhibited considerable transcriptional activity during transient transfection in cultured subconfluent, serum-fed canine tracheal myocytes, and 5' deletion analysis of promoter function revealed the presence of positive transcriptional regulatory elements between bp -526 and -269. Sequence analysis disclosed three potential AP-2 binding sites in this region; five more AP-2 consensus binding motifs occur between bp -269 and +101. Cotransfection with a plasmid expressing human AP-2alpha substantially increased transcription from m3 receptor promoter constructs containing 526 or 269 bp of 5' flanking DNA. Furthermore, m3 receptor promoter activity was enhanced by long-term serum deprivation of canine tracheal myocytes, a treatment that is known to increase AP-2 transcription-promoting activity in these cells. Together, these data suggest that expression of the human m3 muscarinic receptor gene is regulated in part by AP-2 in airway smooth muscle.

In vivo immunomodulation by peripheral adrenergic and cholinergic agonists/antagonists in rat and mouse models

Our work is devoted to defining relationships between the immune system and the adrenergic and cholinergic systems in vivo. In the rat model, we have shown that the cells of different immune compartments express the genes of a defined set of adrenergic/cholinergic receptors, and it was shown that lymphocytes are a site of non-neuronal production of norepinephrine and acetylcholine. Furthermore, using implantable slow-release tablets containing adrenergic or cholinergic agonists/antagonists, distinct and partly opposite effects were observed on peripheral immune functions. Concerning sympathetic immunoregulation, our data--in contrast to those of other studies--suggest that an enhanced adrenergic tonus leads to immunosuppression primarily via alpha 2-receptor-mediated mechanisms. Beta-blockade strongly enhances this effect, most likely by inhibition of pineal melatonin synthesis. In recent experiments on the kinetics it was found that the continuous alpha-adrenergic treatment entails a strong suppression of cellular responsiveness during the first few hours, which is increasingly followed by a general loss of lymphocytes in blood and lymphoid organs most likely due to enhanced apoptosis. More recently, we have extended our studies to the mouse model. First data obtained with RNAse protection assays suggest a biphasic effect on the gene expression of several cytokines in spleen cells due to adrenergic in vivo treatment.

Migraine patients show an increased density of dopamine D3 and D4 receptors on lymphocytes

Recent studies have revealed peculiar functional and genetic features of dopamine receptors in migraine. As peripheral blood lymphocytes (PBL) may represent a tool for peripheral detection of neuroreceptors, we compared the expression of dopamine D3 (DRD3) and D4 (DRD4) receptors on PBL in migraine patients and in healthy controls using radioligand binding assay techniques in the presence of antidopamine D2-like receptor antibodies. The dopamine D2-like receptor agonist [3H]7-OH-DPAT was used as a radioligand. An increased density of both DRD3 (P=0.0006) and DRD4 (P=0.002) on PBL was observed in migraineurs compared with controls. This up-regulation might reflect central and/or peripheral dopamine receptor hypersensitivity due to hypofunction of the dopaminergic system. These findings support the view that dopamine D2-like receptors are involved in the determination of the so-called migraine trait, which may help to elucidate several clinical features of the disease.

Low-level exposure to methylmercury modifies muscarinic cholinergic receptor binding characteristics in rat brain and lymphocytes: physiologic implications and new opportunities in biologic monitoring

Methylmercury (MeHg) affects several parameters of cholinergic function. These alterations are thought to play a role in MeHg neurotoxicity. In vitro experiments have indicated that MeHg acts as a strong competitive inhibitor of radioligand binding to muscarinic cholinergic receptors (mAChRs) in rat brain. Furthermore, rat brain mAChRs share several pharmacologic characteristics of similar receptors present on lymphocytes. Using the muscarinic antagonist [(3)H]quinuclidinyl benzilate (QNB) to label receptors, we investigated the in vivo interactions of MeHg with rat brain mAChRs. We also investigated whether MeHg-induced central mAChR changes are reflected by similar alterations in splenic lymphocytes. Exposure to low doses of MeHg--0.5 or 2 mg/kg/day in drinking water--for 16 days significantly increased (20-44% of control) mAChRs density (B(max)) in the hippocampus and cerebellum without affecting receptor affinity (K(d)). The effect of MeHg did not occur immediately; it was not apparent until 2 weeks after the termination of treatment. No significant changes in [(3)H]QNB binding were observed in the cerebral cortex. In splenic lymphocytes, mAChR density was remarkably increased (95-198% of control) by day 14 of MeHg exposure and remained enhanced 14 days after the cessation of treatment. These results suggest up-regulation of mAChRs in selected brain regions (hippocampus and cerebellum) after prolonged low-level ingestion of MeHg in rats. These cerebral effects are delayed in onset and are preceded by a marked increase in density of mAChRs on lymphocytes. In chronic MeHg exposure, peripheral lymphocytes may represent a sensitive target for the interaction of MeHg with mAChRs and, therefore, may be predictive indicators of later adaptive response involving cerebral mAChRs. Additionally, the effect of MeHg on lymphocyte mAChRs in vivo indicates that this receptor system should be investigated further as a possible target for MeHg immunotoxicity.

Radioligand binding assay of M1-M5 muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

Analysis of lymphocyte muscarinic cholinergic receptors using quantitative techniques such as radioligand binding assay is made difficult due to the low density of these sites and the lack of subtype-specific selectivity of most available muscarinic ligands. In this study, a combined kinetic and equilibrium labeling technique recently developed for brain tissue was used for labeling the five muscarinic cholinergic receptor subtypes in intact human peripheral blood lymphocytes. No specific muscarinic M1 receptor binding was detectable in human peripheral blood lymphocytes using [3H]-pirenzepine as a ligand. Labeling of M2-M5 muscarinic receptors using [3H]N-methyl-scopolamine (NMS) by occluding various receptor subtypes with muscarinic antagonist and mamba venom resulted in the labeling of M2-M5 receptors in brain as well as in human peripheral blood lymphocytes. The relative density of different receptor subtypes was M3 > M5 > M4 > M2. The development of a reproducible technique for assaying muscarinic cholinergic receptor subtypes expressed by human peripheral blood lymphocytes may contribute to clarify their role in lymphocyte function.

Relative amounts of mRNA encoding four subtypes of muscarinic receptors (m2-m5) in human peripheral blood mononuclear cells

It is known that lymphocytes express functional muscarinic cholinergic receptors. In this study, RT-PCR method was applied to study the presence and relative levels of mRNA encoding muscarinic receptor subtypes in human peripheral blood mononuclear cells (PBMCs). Our results, confirmed by DNA sequencing, demonstrate the presence of m2, m3, m4, and m5 receptor subtypes in human PBMCs. The relative levels of muscarinic receptor subtypes fit the following pattern: m3 > m5 > m4 > m2. Our data provide strong evidence confirming previous pharmacological studies that suggested the existence of several subtypes of muscarinic receptors on human PBMCs. We cannot exclude the possibility that expression of receptor subtype depends on the lineage and/or activation status of the cell.

Labeling of dopamine D3 and D4 receptor subtypes in human peripheral blood lymphocytes with [3H]7-OH-DPAT: a combined radioligand binding assay and immunochemical study

Molecular biology studies have demonstrated that human peripheral blood lymphocytes express dopamine D2-like receptors belonging to the D3 and D4 receptor subtypes, whereas the characterization of these receptors using radioligand binding assay techniques provided conflicting results. The preferential dopamine D3 receptor agonist [3H]7-hydroxy-N, N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT) was used recently for labeling lymphocyte dopamine D3 receptor. However, the selectivity of this compound for the D3 receptor was questioned. In this study we have investigated human peripheral blood lymphocyte dopamine receptor subtypes labeled by [3H]7-OH-DPAT using a conventional radioligand binding assay technique and antibodies against dopamine D2-like receptor subtypes. [3H]7-OH-DPAT was specifically bound to intact human peripheral blood lymphocytes with a dissociation constant (Kd) value of 0.32 + 0.03 nM and a maximum density of binding sites (Bmax) of 18.2 + 0.8 fmol/2 x 10(6) cells. [3H]7-OH-DPAT binding was unaffected by antibodies against dopamine D2 and D2S receptors. Anti-dopamine D3 and D4 receptor antibodies reduced [3H]7-OH-DPAT binding by about 53% and 32% respectively. Combination of anti D3 and D4 receptor antibodies reduced remarkably [3H]7-OH-DPAT binding. The above results suggest that the dopamine receptor agonist [3H]7-OH-DPAT labels dopamine D3 and D4 receptor subtypes in human peripheral blood lymphocytes. The use of antibodies raised against dopamine receptor subtypes in combination with radioligand binding assay may contribute to define receptor subtypes expressed by human peripheral blood lymphocytes in health and disease.