Heterogeneity and regulation of nicotinic acetylcholine receptors

Nicotinic Receptors of Different Species Exhibit Differential Sensitivities to Nitromethylene and Organophosphate Insecticides

The effects of nitromethylene heterocycle (NMH) and organophosphate (OP) insecticides were studied on nicotinic acetylcholine receptors (nAChR) in cultured cells of different species origin, in order to examine the selectivity of these compounds at the level of the target sites. In mouse muscle BC3H1, mouse NIE-115 and human SH-SY5Y neuroblastoma, and locust thoracic ganglion cells, the neurotransmitter, acetylcholine (ACh), induces a similar transient inward current. Dependent on the cell type, the six NMHs acted as agonists and/or antagonists on nAChR. Distinct agonistic effects of NMHs on nAChR are observed on insect neurons only. Further, NMHs potently block nicotinic responses in insects, while mammalian cells are only moderately affected. In all cases, the neuronal type nAChR was more sensitive to blocking than the endplate type nAChR in mammalian cells.

Parathion and paraoxon at micromolar concentrations inhibit ACh-induced nicotinic inward currents. The insecticide, parathion, is a 50-fold more potent inhibitor than its acetyl-cholinesterase-inhibiting metabolite, paraoxon. Moderate differences in sensitivity to the blocking action of the OPs appear to exist among cells of different species. The results demonstrate that the experimental approach of fundamental electrophysiology and the use of cell lines are relevant tools for investigating the species specificity of neurotoxic compounds.

Positive and negative effects of alcohol and nicotine and their interactions: a mechanistic review

Nicotine and alcohol are two of the most commonly abused legal substances. Heavy use of one drug can often lead to, or is predictive of, heavy use of the other drug in adolescents and adults. Heavy drinking and smoking alone are of significant health hazard. The combination of the two, however, can result in synergistic adverse effects particularly in incidences of various cancers (e.g., esophagus). Although detrimental consequences of smoking are well established, nicotine by itself might possess positive and even therapeutic potential. Similarly, alcohol at low or moderated doses may confer beneficial health effects. These opposing findings have generated considerable interest in how these drugs act. Here we will briefly review the negative impact of drinking-smoking co-morbidity followed by factors that appear to contribute to the high rate of co-use of alcohol and nicotine. Our main focus will be on what research is telling us about the central actions and interactions of these drugs, and what has been elucidated about the mechanisms of their positive and negative effects. We will conclude by making suggestions for future research in this area.

Naturally-expressed nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.

Is modulation of nicotinic acetylcholine receptors by melatonin relevant for therapy with cholinergic drugs?

Melatonin, the darkness hormone, synchronizes several physiological functions to light/dark cycle. Besides the awake/sleep cycle that is intuitively linked to day/night, daily variations in memory acquisition and innate or acquired immune responses are some of the major activities linked to melatonin rhythm. The daily variation of these complex processes is due to changes in specific mechanisms. In the last years we focused on the influence of melatonin on the expression and function of nicotinic acetylcholine receptors (nAChRs). Melatonin, either "in vivo" or "in vitro", increases, in a selective manner, the efficiency of alpha-bungarotoxin (alpha-BTX)-sensitive nAChRs. Melatonin's effect on receptors located in rat sympathetic nerve terminals, cerebellum, skeletal muscle and chick retina, was tested. We observed that melatonin is essential for the development of alpha-BTX-sensitive nAChRs, and important for receptor maintenance in aging models. Taking into account that both melatonin and alpha-7 nAChRs (one of the subtypes sensitive to alpha-BTX) are involved in the development of Alzheimer's disease, here we discuss the possibility of a therapeutic strategy focused on both melatonin replacement and its potential association with cholinergic drugs.

Nicotine and nicotinic system in hypoglutamatergic models of schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with devastating consequences. It is characterized by thought fragmentation, hallucination and delusion, collectively referred to as positive symptoms. In addition, mood changes or affective disorders, referred to as negative symptoms, as well as cognitive impairments can be manifested in these patients. Arguably, modeling such a disorder in its entirety in animals might not be feasible. Despite this limitation, various models with significant construct, predictive and some face validity have been developed. One such model, based on hypoglutamatergic hypothesis of schizophrenia, makes use of administering NMDA receptor antagonists and evaluating behavioral paradigms such as sensorimotor gating. Because of very high incidence of smoking among schizophrenic patients, it has been postulated that some of these patients may actually be self medicating with tobacco's nicotine. Research on nicotinic-glutamatergic interactions using various animal models has yielded conflicting results. In this review, some of these models and possible confounding factors are discussed. Overall, a therapeutic potential for nicotinic agonists in schizophrenia can be suggested. Moreover, it is evident that various experimental paradigms or models of schizophrenia symptoms need to be combined to provide a wider spectrum of the behavioral phenotype, as each model has its inherent limitations.

Neurotransmitter receptor expression and activity during neuronal differentiation of embryonal carcinoma and stem cells: from basic research towards clinical applications

Embryonal carcinoma and embryonic stem cells have served as models to understand basic aspects of neuronal differentiation and are promising candidates for regenerative medicine. Besides being well characterized regarding the capability of embryonal carcinoma and embryonic stem cells to be precursors of different tissues, the molecular mechanisms controlling neuronal differentiation are hardly understood. Neuropeptide and neurotransmitter receptors are expressed at early stages of differentiation prior to synaptogenesis, triggering transient changes in calcium concentration and inducing neurone-specific gene expression. In vitro neuronal differentiation of embryonal carcinoma and embryonic stem cells closely resembles early neuronal development in vivo. Murine P19 EC cells are a well-characterized model for in vitro differentiation, which upon treatment with retinoic acid differentiate into neurones. Expression and activity of various receptor proteins is regulated during their differentiation. Stimulation of kinin-B2, endothelin-B, muscarinic acetylcholine, and N-methyl-D-aspartate receptors results in transient increases of intracellular free calcium concentration [Ca(2+)](i) in P19 cells undergoing neuronal differentiation, whereas embryonal cells do not respond or show a smaller change in [Ca(2+)](i) than differentiating cells. Receptor inhibition, as studied with the example of the kinin-B2 receptor, aborts neuronal maturation of P19 cells, demonstrating the crucial importance of B2 receptors during the differentiation process. Future success in obtaining desired neuronal phenotypes from pluripotent cells in vitro may offer new therapeutic perspectives for curing genetic and acquired dysfunctions of the developing and adult nervous system.

Nicotine: the link between cigarette smoking and the progression of renal injury?

Cigarette smoke (CS) is the most important source of preventable morbidity and mortality in the United States. Recent clinical studies have suggested that, in addition to being a major cardiovascular risk factor, CS promotes the progression of kidney disease. The mechanisms by which CS promotes the progression of chronic kidney disease have not been elucidated. Here we demonstrate for the first time that human mesangial cells (MCs) are endowed with the nicotinic ACh receptors (nAChRs) alpha4, alpha5, alpha7, beta2, beta3, and beta4. Studies performed in other cell types have shown that these nAChRs are ionotropic receptors that function as agonist-regulated Ca(2+) channels. Nicotine induced MC proliferation in a dose-dependent manner. At 10 (-7) M, a concentration found in the plasma of active smokers, nicotine induced MC proliferation [control, 1,328 +/- 50 vs. nicotine, 2,761 +/- 90 counts/minute (cpm); P < 0.05] and increased the synthesis of fibronectin (50%), a critical matrix component involved in the progression of chronic kidney disease. We and others have shown that, in response to PKC activation, MC synthesize reactive oxygen species (ROS) via NADPH oxidase. In the current studies we demonstrate that PKC inhibition as well as diphenyleneiodonium and apocynin, two inhibitors of NADPH oxidase, prevented the effects of nicotine on MC proliferation and fibronectin production, hence establishing ROS as second messengers of the actions of nicotine. Furthermore, nicotine increased the production of ROS as assessed by 2',7'-dichlorofluorescein diacetate fluorescence [control, 184.4 +/- 26 vs. nicotine, 281.5 +/- 26 arbitrary fluorescence units (AFU); n = 5 experiments, P < 0.05]. These studies unveil previously unrecognized mechanisms that indict nicotine, a component of CS, as an agent that may accelerate and promote the progression of kidney disease.

Airway-related vagal preganglionic neurons express multiple nicotinic acetylcholine receptor subunits

Nicotine acting centrally increases bronchomotor tone and airway secretion, suggesting that airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) express nicotinic acetylcholine receptors (nAChRs). In the present study, we examined the three main functionally characterized subtypes of nAChRs in the CNS, the alpha7 homomeric and alpha4beta2 heteromeric receptors. First, we characterized the expression of these subunits at the message (mRNA) and protein levels in brain tissues taken from the rNA region, the site where AVPNs are located. In addition, double labeling fluorescent immunohistochemistry and confocal laser microscopy were used to define the presence of alpha7, alpha4, and beta2 nAChRs on AVPNs that were retrogradely labeled with cholera toxin beta subunit (CTb), injected into the upper lung lobe (n=4) or extrathoracic trachea (n=4). Our results revealed expression of all three studied subunits at mRNA and protein levels within the rNA region. Furthermore, virtually all identified AVPNs innervating intrapulmonary airways express alpha7 and alpha4 nAChR subunits. Similarly, a majority of labeled AVPNs projecting to extrathoracic trachea contain alpha7 and beta2 subunits, but less than half of them show detectable alpha4 nAChR traits. These results suggest that AVPNs express three major nAChR subunits (alpha7, alpha4, and beta2) that could assemble into functional homologous or heterologous pentameric receptors, mediating fast and sustained nicotinic effects on cholinergic outflow to the airways.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Electrophysiological, pharmacological, and molecular evidence for alpha7-nicotinic acetylcholine receptors in rat midbrain dopamine neurons

Dopamine (DA) neurons located in the mammalian midbrain have been generally implicated in reward and drug reinforcement and more specifically in nicotine dependence. However, roles played by nicotinic acetylcholine receptors, including those composed of alpha7-subunits [alpha7-nicotinic acetylcholine receptors (nAChRs)], in modulation of DA signaling and in nicotine dependence are not clearly understood. Although midbrain slice recording has been used previously to identify functional alpha7-nAChRs, these preparations are not optimally designed for extremely rapid and reproducible drug application, and rapidly desensitized, alpha7-nAChR-mediated currents may have been underestimated or not detected. Here, we use patch-clamp, whole-cell current recordings from single neurons acutely dissociated from midbrain nuclei and having features of DA neurons to characterize acetylcholine-induced, inward currents that rapidly activate and desensitize, are mimicked by the alpha7-nAChR-selective agonist, choline, blocked by the alpha7-nAChR-selective antagonists, methyllycaconitine and alpha-bungarotoxin, and are similar to those of heterologously expressed, human alpha7-nAChRs. We also use reverse transcriptase-polymerase chain reaction, in situ hybridization, and immunocytochemical staining to demonstrate nAChR alpha7 subunit gene expression as message and protein in the rat substantia nigra pars compacta and ventral tegmental area. Expression of alpha7 subunit message and of alpha7-nAChR-mediated responses is developmentally regulated, with both being absent in samples taken from rats at postnatal day 7, but later becoming present and increasing over the next 2 weeks. Collectively, this electrophysiological, pharmacological, and molecular evidence indicates that nAChR alpha7 subunits and functional alpha7-nAChRs are expressed somatodendritically by midbrain DA neurons, where they may play important physiological roles and contribute to nicotine reinforcement and dependence.

The nicotinic acetylcholine receptor subtypes and their function in the hippocampus and cerebral cortex

Nicotinic acetylcholine receptors (nAChRs) are widely distributed in the central nervous system and have been implicated in multiple behavioral paradigms and pathological conditions. Nicotinic therapeutic interventions require an extensive characterization of native nAChRs including mapping of their distribution and function in different brain regions. Here, we describe the roles played by different nAChRs in affecting neuronal activity in the hippocampus and cerebral cortex. At least three distinct functional nAChR subtypes (alpha 7, alpha 4 beta 2, alpha 3 beta 4) can be detected in the hippocampal region, and in many instances a single neuron type is found to be influenced by all three nAChRs. Further, it became clear that GABAergic and glutamatergic inputs to the hippocampal interneurons are modulated via different subtypes of nAChRs. In the cerebral cortex, GABAergic inhibition to the layer V pyramidal neurons is enhanced predominantly via activation of alpha 4 beta 2 nAChR and to a minor extent via activation of alpha 7 nAChR. Such diversity offers pathways by which nicotinic drugs affect brain function.

Functional properties of homomeric, human alpha 7-nicotinic acetylcholine receptors heterologously expressed in the SH-EP1 human epithelial cell line

alpha 7-Nicotinic acetylcholine receptors (alpha 7-nAChRs) are broadly distributed in the central nervous system, where they play important roles in chemical and electrical signaling, and perhaps in neurite outgrowth, synaptic plasticity, and neuronal death/survival. To help elucidate their normal and pathophysiological roles, we have heterologously expressed human alpha 7-nAChR in transfected SH-EP1 human epithelial cells. Reverse transcription-polymerase chain reaction and mRNA fluorescence in situ hybridization analyses demonstrate expression of human alpha 7 subunits as messenger RNA. Patch-clamp recordings exploiting a novel strategy to prevent functional rundown of whole-cell peak current responses to repeated acute challenges with nicotinic agonists show successful expression of functional alpha 7-nAChR that mediate inward currents characterized by rapid phases of activation and inactivation. Concentration-response curves show that nicotine, acetylcholine, and choline are efficacious agonists at human alpha 7-nAChRs. Current-voltage relationships show inward rectification for agonist-induced currents. Human alpha 7-nAChRs exhibit some sensitivity to alpha 7-nAChR antagonists alpha-bungarotoxin (Bgt) or methyllycaconitine (MLA) when applied coincidentally with agonist, but much higher affinity block occurs when cells and alpha 7-nAChRs are pre-exposed to antagonists for 2 min before challenge with agonist. Both Bgt and MLA are competitive inhibitors of alpha 7-nAChR function. Whole-cell current peak amplitudes and half-times for inactivation of alpha 7-nAChR functional responses to nicotine are dramatically reduced in the absence of extracellular Ca2+, suggestive of high Ca2+ permeability of the alpha 7-nAChR channel. Thus, heterologously expressed human alpha 7-nAChR in mammalian cells have properties of native alpha 7-nAChR or of alpha 7-nAChR heterologously expressed in other systems and serve as excellent models for studies of molecular bases of alpha 7-nAChR function.

Protective effects of nicotine on ethanol-induced toxicity in cultured cerebellar granule cells

Alcoholism is associated with a higher incidence of smoking. In addition to the stimulatory effects of both ethanol and nicotine on the mesolimbic reward pathway, nicotine's ability to counteract some of the adverse effects of ethanol (e.g. ataxia) may be a powerful incentive for alcohol consumers to increase their tobacco (nicotine) intake. The cerebellum is believed to play an important role in ethanol-induced ataxia. In this study, we sought to test the hypothesis that nicotine would protect against toxic effects of ethanol in primary cultures of cerebellar granule cells. Moreover, it was postulated that the effects of nicotine would be mediated through nicotinic receptors. Primary cultures of cerebellar granule cells were prepared from 20-day embryos obtained from timed-pregnant Sprague Dawley rats. Cells were cultured for 10 days and were then exposed for 3 days to various concentrations of ethanol with and without pretreatment with nicotine and nicotinic antagonists. Cellular toxicity was evaluated by measuring the lactate dehydrogenase level. Administration of ethanol (10-100 mM) resulted in a dose-dependent toxicity. Pretreatment with nicotine 1-20 micro M resulted in a dose-dependent protection against ethanol-induced toxicity. The effects of nicotine were blocked by pretreatment with nicotinic antagonists such as mecamylamine 1-20 micro M, dihydro-beta-erythroidine 1.0 nM-1.0 micro M and methyllycaconitine 5 nM-5 micro M in a dose-dependent manner. Thus, ethanol-induced cytotoxicity in primary cultures of cerebellar granule cells is blocked by pretreatment with nicotine. The effects of nicotine, in turn, may be blocked by nicotinic antagonists, implicating both high and low affinity nicotinic receptors in mediating the actions of nicotine. The exact mechanism of ethanol-induced toxicity and/or neuroprotection through activation of nicotinic receptors in this paradigm remains to be elucidated. The neuroprotective effect of nicotine against ethanol-induced toxicity in cerebellar neurons may be a contributing factor to the high incidence of smoking among alcoholics.

Nicotinic acetylcholine receptors on NT2 precursor cells and hNT (NT2-N) neurons

This is the first report, to our knowledge, of prominent, natural expression of nAChR alpha4, alpha6 and alpha9 subunits in a human, neuronally-committed cell line. We performed studies with specific reference to the expression of nicotinic acetylcholine receptors (nAChR) to further characterize a human, postmitotic, transplantable, with a neuronal phenotype, cell line called hNT (also called NT2-N). hNT cells acquire a distinctive neuronal phenotype upon differentiation from their NT2 precursors. Immunocytochemical studies showed that NT2 cells were strongly immunopositive for alpha4 or alpha7 subunits, moderately immunopositive for alpha3/alpha5 subunits, and weakly immunopositive for beta2 or beta4 subunits, whereas hNT neurons showed positive, strong-to-moderate immunostaining for all of these nAChR subunits. Reverse transcription-polymerase chain reaction (RT-PCR) mRNA analyses indicated that levels of alpha7 subunit messages were similar in both NT2 and hNT cells, whereas alpha2, alpha10, and beta3 subunit transcripts were not detected. Levels of alpha3, alpha5, and beta4 subunit messages were lower in hNT neurons than in NT2 precursors. However, alpha4 and beta2 subunit messages were present in NT2 precursors but were greatly induced in hNT neurons. Levels of alpha6 and alpha9 subunit messages, not detectable in NT2 precursors, rose to high levels in hNT neurons. hNT cell nAChR subunit message levels were comparable to (alpha4, alpha5, beta4) or higher than (alpha6, alpha9, beta2) levels in adult human brain. NT2 and hNT cells may provide an excellent model for studies of neurogenesis, roles played by nAChR in differentiation and neurodegeneration, and effects of neuronal differentiation on nAChR expression.

Nicotine's oxidative and antioxidant properties in CNS

Nicotine has been reported to be therapeutic in some patients with certain neurodegenerative diseases and to have neuroprotective effects in the central nervous system. However, nicotine administration may result in oxidative stress by inducing the generation of reactive oxygen species in the periphery and central nervous system. There is also evidence suggesting that nicotine may have antioxidant properties in the central nervous system. The antioxidant properties of nicotine may be intracellular through the activation of the nicotinic receptors or extracellular by acting as a radical scavenger in that it binds to iron. The possibility that nicotine might be used to treat some symptoms of certain neurodegenerative diseases underlies the necessity to determine whether nicotine has pro-oxidant, antioxidant or properties of both. This review discusses the studies that have addressed this issue, the behavioral effects of nicotine, and the possible mechanisms of action that result from nicotine administration or nicotinic receptor activation.

Inhibition of neuronal nicotinic acetylcholine receptors by La(3+)

A study was made of the effects of La(3+) on neuronal alpha 2 beta 4 nicotinic acetylcholine receptors expressed in Xenopus oocytes. La(3+) by itself (up to 10 microM) did not elicit significant membrane currents. However, La(3+) reversibly inhibited the ionic currents induced by acetylcholine (IC(50)=13.5+/-4.3 microM). When La(3+) and acetylcholine were simultaneously applied onto an oocyte, the level of inhibition of the acetylcholine response was the same as when the oocyte was first preincubated with La(3+) and then exposed to acetylcholine plus La(3+). In the presence of La(3+), the EC(50) decreased from 43.8+/-6.4 to 26.5+/-5.1 microM, suggesting a small increase in the affinity of acetylcholine for the receptors through a noncompetitive mechanism. The inhibition of acetylcholine response was independent of the membrane potential. From these results we conclude that La(3+) regulates nicotinic receptors, reversibly and noncompetitively, presumably by inhibiting allosterically the receptor through interactions at an external domain of the receptor complex.

Mass spectrometry of nicotinic acetylcholine receptors and associated proteins as models for complex transmembrane proteins

Studies were conducted to optimize matrix-assisted laser desorption/ionization, time-of-flight mass spectrometry (MALDI TOF MS) in analyzing the composition of nicotinic acetylcholine receptors (nAChR) from Torpedo californica electric tissue in their membrane-bound, detergent-solubilized, and affinity-purified states. Mass spectra obtained from nAChR-rich membrane fractions gave reasonably good representations of protein compositions indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of those same samples. Efficiency of extraction of nAChR from membranes was not markedly different for most detergents, but quality and signal size of mass spectra were clearly influenced by detergent composition and concentration, protein concentration, and MALDI matrix composition. The best spectra, allowing detection and accurate size determinations for samples containing as little as 10 fmol of pure nAChR, were obtained for samples solubilized in Triton X-100 and assayed by use of a sinapinic acid matrix. Although informative spectra could be obtained for nAChR affinity purified on alpha-cobratoxin (Naja naja siamensis) columns and extracted using sinapinic acid, superior spectra with much higher signal:noise were obtained if extraction media contained Triton X-100 or sodium dodecyl sulfate. nAChR subunit masses determined were similar regardless of the membrane-associated, detergent-solubilized, or affinity-purified state of the preparation. These studies illustrate how masses can be determined for nAChR subunits and for other protein components in Torpedo membrane preparations, such as RAPsyn and Na(+)-K(+)-ATPase alpha and beta subunits. They also provide an underpinning for streamlined analysis of the composition of complex transmembrane proteins using MALDI TOF MS.

Antagonism of nicotinic acetylcholine receptors by inhibitors of monoamine uptake

A study was made of the effects of several monoamine-uptake inhibitors on membrane currents elicited by acetylcholine (ACh-currents) generated by rat neuronal alpha2beta4 and mouse muscle nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes. For the two types of receptors the monoamine-uptake inhibitors reduced the ACh-currents albeit to different degrees. The order of inhibitory potency was norfluoxetine > clomipramine > indatraline > fluoxetine > imipramine > zimelidine > 6-nitro-quipazine > trazodone for neuronal alpha2beta4 AChRs, and norfluoxetine > fluoxetine > imipramine > clomipramine > indatraline > zimelidine > trazodone > 6-nitro-quipazine for muscle AChRs. Thus, the most potent inhibitor was norfluoxetine, whilst the weakest ones were trazodone, 6-nitro-quipazine and zimelidine. Effects of the tricyclic antidepressant imipramine were studied in more detail. Imipramine inhibited reversibly and non-competitively the ACh-current with a similar inhibiting potency for both neuronal alpha2beta4 and muscle AChRs. The half-inhibitory concentrations of imipramine were 3.65 +/- 0.30 microM for neuronal alpha2beta4 and 5.57 +/- 0.19 microM for muscle receptors. The corresponding Hill coefficients were 0.73 and 1.2 respectively. The inhibition of imipramine was slightly voltage-dependent, with electric distances of approximately 0.10 and approximately 0.12 for neuronal alpha2beta4 and muscle AChRs respectively. Moreover, imipramine accelerated the rate of decay of ACh- currents of both muscle and neuronal AChRs. The ACh-current inhibition was stronger when oocytes, expressing neuronal alpha2beta4 or muscle receptors, were preincubated with imipramine alone than when it was applied after the ACh-current had been generated, suggesting that imipramine acts also on non-activated or closed AChRs. We conclude that monoamine-uptake inhibitors reduce ACh-currents and that imipramine regulates reversibly and non- competitively neuronal alpha2beta4 and muscle AChRs through similar mechanisms, perhaps by interacting externally on a non-conducting state of the AChR and by blocking the open receptor-channel complex close to the vestibule of the channel. These studies may be important for understanding the regulation of AChRs as well as for understanding antidepressant- and side-effects of monoamine-uptake inhibitors.

Depressive characteristics of FSL rats: involvement of central nicotinic receptors

Antidepressant effects of acute or chronic nicotine treatments in swim test immobility of Flinders sensitive line (FSL) rats, an animal model of depression, were recently demonstrated (Tizabi et al. Psychopharmacology 142:193, 1999). In the present study we sought to determine whether the antidepressant effects of nicotine could be blocked by the nicotinic antagonist, mecamylamine (MEC). Moreover, the effects of chronic nicotine treatment on [3H]cytisine binding in discrete brain regions of FSL and their control Flinders resistant line (FRL) rats were also evaluated. Adult male FSL rats were treated with MEC (0.5 mg/kg) 20 min prior to an acute or chronic nicotine administration. MEC by itself did not affect the immobility in swim test. However, it completely blocked the acute or chronic nicotine effects. Daily nicotine injection (0.4 mg/kg/day for 14 days) resulted in an increase in [3H]cytisine binding primarily in the FRL rats. An increase in nicotinic receptor binding following chronic nicotine administration is believed to reflect desensitization of these receptors. These findings, coupled with previous observation of higher basal nicotinic receptors in FSL rats, further support the involvement of central nicotinic receptors in depressive characteristics of these rats. Moreover, the data suggest therapeutic potential for selective nicotinic receptor agonists in depressive disorders.

Modulatory role of presynaptic nicotinic receptors in synaptic and non-synaptic chemical communication in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated channels closely related to but distinct from the muscle nAChRs. Recent progress in neurochemical and pharmacological methods supports the hypothesis of presynaptically located nAChRs on axon terminals and indicates that the major effect of nAChR is the modulation rather than processing of fast synaptic transmission. Strong neurochemical evidence indicate that the most important function of presynaptic nAChRs in either synaptic or non-synaptic localization is to increase transmitter release initiated by axonal firing, or directly induce Na(+) and Ca(2+) influx followed by a depolarization sufficient to activate local voltage-sensitive Ca(2+) channels, as a result transmitter of vesicular origin will be released. Therefore, it is somewhat expected that nicotine-induced transmitter release of different monoamines including norepinephrine (NE), dopamine (DA), serotonin (5-HT) can be tetrodotoxin (TTX)- and [Ca(2+)](o)-sensitive. However, some of the nAChR agonists at higher concentrations (1, 1-dimethyl-4-phenylpiperazinium (DMPP) and lobeline), besides their effects on presynaptic nAChRs, are able to inhibit the uptake of NE and 5-HT into nerve terminals, thereby their transmitter releasing effects are extended in time and space. The effect on the uptake process is different from classical nicotinic actions, not being sensitive to nAChR antagonism, but can be prevented by selective uptake blockers or reduced temperature. Considering neurochemical, pharmacological and electrophysiological evidence it seems likely that presynaptic nAChRs on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit. This is supported by the observation that nicotinic agonists have no presynaptic effect on transmitter release in knockout mice lacking the beta2 nAChR subunit gene. The essential brain function lies not only in impulse transmission within a hard-wired neuronal circuitry but also within synaptic and non-synaptic communication subjected to presynaptic modulation. Since the varicose noradrenergic, dopaminergic, serotonergic, glutamatergic and cholinergic axon terminals mainly do not make synaptic contact, but their varicosities are equipped with nAChRs and these non-synaptically localized receptors are of high affinity, it is suggested that nicotine inhaled during smoking might exert its behavioral, psychological, neurological and neuroendocrinological effects via these receptors.

Modulation of nicotinic acetylcholine receptors by strychnine

Strychnine, a potent and selective antagonist at glycine receptors, was found to inhibit muscle (alpha1beta1gammadelta, alpha1beta1gamma, and alpha1beta1delta) and neuronal (alpha2beta2 and alpha2beta4) nicotinic acetylcholine receptors (AcChoRs) expressed in Xenopus oocytes. Strychnine alone (up to 500 microM) did not elicit membrane currents in oocytes expressing AcChoRs, but, when applied before, concomitantly, or during superfusion of acetylcholine (AcCho), it rapidly and reversibly inhibited the current elicited by AcCho (AcCho-current). Although in the three cases the AcCho-current was reduced to the same level, its recovery was slower when the oocytes were preincubated with strychnine. The amount of AcCho-current inhibition depended on the receptor subtype, and the order of blocking potency by strychnine was alpha1beta1gammadelta > alpha2beta4 > alpha2beta2. With the three forms of drug application, the Hill coefficient was close to one, suggesting a single site for the receptor interaction with strychnine, and this interaction appears to be noncompetitive. The inhibitory effects on muscle AcChoRs were voltage-independent, and the apparent dissociation constant for AcCho was not appreciably changed by strychnine. In contrast, the inhibitory effects on neuronal AcChoRs were voltage-dependent, with an electrical distance of approximately 0.35. We conclude that strychnine regulates reversibly and noncompetitively the embryonic type of muscle AcChoR and some forms of neuronal AcChoRs. In the former case, strychnine presumably inhibits allosterically the receptor by binding at an external domain whereas, in the latter case, it blocks the open receptor-channel complex.

Antidepressants noncompetitively inhibit nicotinic acetylcholine receptor function

Nicotinic acetylcholine receptors (nAChRs) are diverse members of the neurotransmitter-gated ion channel superfamily and play critical roles in chemical signaling throughout the nervous system. The present study establishes for the first time the acute functional effects of sertraline (Zoloft), paroxetine (Paxil), nefazodone (Serzone), and venlafaxine (Effexor) on two human and one chick nAChR subtype. This study also confirms previous findings of nAChR functional block by fluoxetine (Prozac). Function of human muscle-type nAChR (alpha1/beta gammadelta) in TE671/RD cells, human autonomic nAChR (alpha3/beta4alpha5 +/- beta2) in SH-SY5Y neuroblastoma cells, or chick V274T mutant alpha7-nAChR heterologously expressed in native nAChR-null SH-EP1 epithelial cells was measured using 86Rb+ efflux assays. Functional blockade of human muscle-type and autonomic nAChRs is produced by each of the drugs in the low to intermediate micromolar range, and functional blockade of chick V274T-alpha7-nAChR is produced in the intermediate to high micromolar range. Functional blockade is insurmountable by increasing agonist concentrations at each nAChR subtype tested for each of these drugs, suggesting noncompetitive inhibition of nAChR function. These studies open the possibilities that nAChR subtypes in the brain could be targets for therapeutic antidepressants and could play roles in clinical depression.

Desensitization of catecholamine release. The novel catecholamine release-inhibitory peptide catestatin (chromogranin a344-364) acts at the receptor to prevent nicotinic cholinergic tolerance

Nicotinic cholinergic receptors undergo desensitization upon repeated or prolonged exposure to agonist. We investigated the effects of a novel chromogranin A catecholamine release-inhibitory fragment, catestatin (chromogranin A344-364), on agonist-induced desensitization of catecholamine release from pheochromocytoma cells. In a dose-dependent fashion, the nicotinic antagonist catestatin blocked agonist desensitization of both catecholamine release (IC50 approximately 0.24 microM) and 22Na+ uptake (IC50 approximately 0.31 microM), the initial step in nicotinic cationic signal transduction; both secretion inhibition and blockade of desensitization were noncompetitive with agonist. Desensitizing effects of the nicotinic agonists nicotine and epibatidine were blocked. This antagonist action was specific to desensitization by nicotinic agonists, since catestatin did not block desensitization of catecholamine release induced by agents which bypass the nicotinic receptor. Hill plots with slopes near unity suggested noncooperativity for catestatin effects on both nicotinic responses (secretory antagonism and blockade of desensitization). Human, bovine, and rat catestatins (as well as substance P) had similar potencies. IC50 values for secretion inhibition and blockade of desensitization paralleled each other (r = 0.76, n = 10 antagonists, p = 0.01) for several noncompetitive nicotinic antagonists. Peptide nicotinic antagonists (catestatins, substance P) were far more potent inhibitors of both secretion (p = 0.019) and desensitization (p = 0.005) than nonpeptide antagonists (trimethaphan, hexamethonium, procaine, phencyclidine, cocaine, or clonidine), and the peptides displayed enhanced selectivity to block desensitization versus secretion (p = 0.003). We conclude that catestatin is a highly potent, dose-dependent, noncompetitive, noncooperative, specific inhibitor of nicotinic desensitization, an effect which may have implications for control of catecholamine release.

Genomic organization and partial duplication of the human alpha7 neuronal nicotinic acetylcholine receptor gene (CHRNA7)

The human alpha7 neuronal nicotinic acetylcholine receptor gene (HGMW-approved symbol CHRNA7) has been characterized from genomic clones. The gene is similar in structure to the chick alpha7 gene with 10 exons and conserved splice junction positions. The size of the human gene is estimated to be larger than 75 kb. A putative promoter 5' of the translation start in exon 1 has been cloned and sequenced. The promoter region lacks a TATA box and has a high GC content (77%). Consensus Sp1, AP-2, Egr-1, and CREB transcription factor binding sites appear to be conserved between bovine and human genes. The alpha7 nAChR gene was found to be partially duplicated, with both loci mapping to the chromosome 15q13 region. A yeast artificial chromosome contig was constructed over a genetic distance of 5 cM that includes both alpha7 loci and the region between them. Four novel exons are described, located in genomic clones containing the partially duplicated gene. The duplicated sequences, including the novel exons, are expressed in human brain.

Interaction of the nicotinic agonist (R,S)-3-pyridyl-1-methyl-2-(3-pyridyl)-azetidine (MPA) with nicotinic acetylcholine receptor subtypes expressed in cell lines and rat cortex

The interaction of the nicotinic agonist (R,S)-3-pyridyl-1-methyl-2-(3-pyridyl)-azetidine (MPA) with different nicotinic acetylcholine receptor (nAChR) subtypes was studied in cell lines and rat cortex. MPA showed an affinity (Ki = 1.21 nM) which was higher than anatoxin-a > (-)-nicotine > (+)-[R]nornicotine > (-)-[S]nornicotine > and (+)-nicotine, but lower than cytisine (Ki = 0.46 nM) in competing for (-)-[3H]nicotine binding in M10 cells, which stably express the recombinant alpha4beta2 nAChR subtype. A one-binding site model was observed in all competing experiments between (-)-[3H]nicotine binding and each of the agonists studied in M10 cells. MPA showed a 13-fold higher affinity for (-)-[3H]nicotine binding sites compared to the [3H]epibatidine binding sites in rat cortical membranes. In human neuroblastoma SH-SY5Y cells, which predominantly express the alpha3 nAChR subunit mRNA, MPA displaced [3H]epibatidine binding from a single population of the binding sites with an affinity in the same nM range as that observed MPA in displacing [3H]epibatidine binding in rat cortical membranes. Chronic treatment of M10 cells with MPA significantly up-regulated the number of (-)-[3H]nicotine binding sites in a concentration dependent manner. Thus MPA appears to have higher affinity to alpha4-subunit containing receptor subtype than alpha3-subunit containing receptor subtype of nAChRs. Furthermore MPA binds to alpha4beta2 receptor subtype with higher affinity than (-)-nicotine and behaves, opposite to cytisine, as a fult agonist in up-regulating the number of nAChRs.

Peptidergic activation of transcription and secretion in chromaffin cells. Cis and trans signaling determinants of pituitary adenylyl cyclase-activating polypeptide (PACAP)

Pituitary adenylyl cyclase-activating polypeptide (PACAP) is a potent endogenous secretagogue for chromaffin cells. Chromogranin A is the major soluble core component in secretory vesicles. Since chromogranin A is secreted along with catecholamines, we asked whether PACAP regulates expression of the chromogranin A gene in PC12 rat chromaffin cells, so as to resynthesize the just-secreted protein, and whether such biosynthetic regulation is coupled mechanistically to catecholamine secretion. PACAP activated the endogenous chromogranin A gene by four- to fivefold. Proportional results (seven- to eightfold activation) were obtained with a transfected 1,200-bp mouse chromogranin A promoter/luciferase reporter construct. A series of chromogranin A promoter 5' deletion mutant/luciferase reporter constructs narrowed down the PACAP response element to a proximal region containing the cAMP response element (CRE box), at (-71 bp)5'-TGACGTAA-3'(-64 bp). Site-directed point mutations of the CRE site suppressed PACAP-induced trans-activation of the promoter. Thus, the proximal CRE box is entirely necessary for the chromogranin A promoter response to PACAP. Transfer of the CRE box to a neutral, heterologous promoter also conferred activation by PACAP, suggesting that the CRE domain is also sufficient to mediate the transcriptional response to PACAP. Expression of a dominant-negative mutant (KCREB) of the CRE-binding factor CREB markedly diminished trans-activation of the chromogranin A promoter by PACAP. Cotransfection of expression plasmids encoding the protein kinase A inhibitor, or an inactive protein kinase A (PKA) catalytic beta subunit, inhibited both forskolin and PACAP activation of chromogranin A transcription, revealing that PACAP-induced trans-activation is highly dependent on PKA. By contrast, inhibition of protein kinase C (by chronic exposure to phorbol ester) had no effect on transcriptional activation by PACAP. The potent PACAP/vasoactive intestinal peptide (VIP) type I receptor antagonist PACAP6-38 impaired both chromogranin A transcription or catecholamine secretion triggered by PACAP38, while the PACAP/VIP type II receptor antagonist (p-Chloro-D-Phe6, Leu17)-VIP had little or no ability to antagonize the PACAP38 effect. The agonist VIP was approximately 100- to 1,000-fold less potent than PACAP in stimulating either secretion or transcription. Thus, PACAP-evoked chromogranin A transcription and catecholamine secretion are likely mediated by the PACAP/VIP type I receptor isoform. Although the calcium channel antagonists Zn2+ (100 microM), nifedipine (10 microM), or ruthenium red (10 microM), or the cytosolic calcium chelator BAPTA-AM (50 microM) each strongly impaired PACAP-induced secretion, transcriptional activation of chromogranin A remained unaltered. Therefore, we propose that PACAP signals to chromogranin A transcription through the CRE in cis, and through PKA and CREB in trans. By contrast, a pathway involving cytosolic calcium entry through L-type voltage-dependent channels is required for PACAP to evoke catecholamine secretion.

Amino acids within residues 181-200 of the nicotinic acetylcholine receptor alpha1 subunit involved in nicotine binding

Structural determinants of L-[3H]nicotine binding to the sequence flanking Cys 192 and Cys 193 of the Torpedo acetylcholine receptor alpha1 subunit were investigated using synthetic peptides (residues 181-200) and fusion proteins (residues 166-211). Nicotine binding at a single concentration (30 nM) was compared with 71 peptides and fusion proteins in which individual amino acids at positions 181-200 were substituted. Substitution of Lys 185, Tyr 190, Cys 192, Cys 193, Thr 196, and Tyr 198 resulted in the greatest reduction in nicotine binding. Equilibrium binding of [3H]nicotine to peptide 181-200 revealed a binding component with an apparent KD of 1.2 microM. Substitution of Lys 185 (with Glu), His 186, Tyr 190, Cys 192, Cys 193, and Tyr 198 resulted in a significant reduction in affinity. Affinity was not affected significantly by substitution of Arg 182, Lys 185 (with Gly or Arg), Val 188, Tyr 189, Pro 194, Asp 195, Thr 196, and Asp 200. It is concluded that Lys 185, His 186, Tyr 190, Cys 192, Cys 193, and Tyr 198 play the greatest role in nicotine binding to residues 181-200 of the alpha1 subunit. Previous studies have implicated Tyr 190, Cys 192, Cys 193, and Tyr 198 in agonist binding to the acetylcholine receptor. These results confirm a role for these residues and also demonstrate a function for Lys 185 and His 186 in nicotine binding.

Comparison of the regional expression of nicotinic acetylcholine receptor alpha7 mRNA and [125I]-alpha-bungarotoxin binding in human postmortem brain

Neuronal nicotinic acetylcholine receptors are expressed in the human central nervous system. A specific subtype of this receptor family, the alpha7 nicotinic acetylcholine receptor, is thought to be the principal alpha-bungarotoxin (alphaBTX)-binding protein in mammalian brain. Although the expression of this receptor subtype has been characterized in rat, no study has specifically compared the expression of both the alpha7 gene and the localization of BTX binding sites in human brain. Expression of alpha7 mRNA and receptor protein in human postmortem brain tissue was examined by in situ hybridization and [125I]-alpha-bungarotoxin autoradiography, respectively, with particular emphasis on regions associated with sensory processing. Regions with high levels of both alpha7 gene expression and [125I]-alphaBTX binding include the nucleus reticularis of the thalamus, the lateral and medial geniculate bodies, the basilar pontine nucleus, the horizontal limb of the diagonal band of Broca, the nucleus basalis of Meynert, and the inferior olivary nucleus. High-to-moderate levels of alpha7 probe hybridization were also seen in the hippocampus and the cerebral cortex; however, there was a reduced or variable degree of [125I]-alphaBTX binding in these regions compared with the level of probe hybridization. In most brain regions, [125I]-alphaBTX binding was localized to neuronal cell bodies similar in morphology to those that exhibited alpha7 hybridization, suggesting that the high-affinity [125I]-alphaBTX binding sites in the human brain are likely to be principally composed of alpha7 receptor subtypes.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

Assigning functions to residues in the acetylcholine receptor channel region (review)

This review is concerned with the functional domains of the nicotinic acetylcholine receptor (AChR) involved in ion permeation. These comprise the ion pore and its gate. The latter allows the channel to be almost exclusively closed in the absence of agonist and favours ion flux in its presence. Early photoaffinity labelling experiments using open-channel blockers and site-directed mutagenesis studies identified M2 of each AChR subunit as the transmembrane domain lining the walls of the ion pore. Several biochemical, electrophysiological, and mutagenesis studies as well as molecular modelling and in vitro studies of ion channel formation with synthetic peptides corroborate these findings. Point mutations combined with electrophysiological techniques have contributed to dissecting the AChR channel region assigning functions to individual amino acid residues, thus revealing structural and functional stratification of the M2 channel domain. Specific residues have been found to be structural determinants of conductance, ion selectivity, gating, and desensitization. The three-dimensional structure of the AChR protein at 9A resolution suggests a possible arrangement of the M2 alpha-helices in the open and closed states, respectively. In spite of the current wealth of knowledge on the AChR ion channel stemming from the combination of experimental approaches discussed in this review, the mechanistic structure by which the interaction with the agonist favours the opening of the cationic channel remains unknown.

Characterization of nicotinic receptors involved in the release of noradrenaline from the hippocampus

The pharmacological features of putative nicotinic acetylcholine receptor sites involved in the release of [3H]noradrenaline were assessed in rat hippocampus. The effect of nicotinic agonists to induce [3H]noradrenaline release was examined in superfused slices. The nicotinic agonists (-)-epibatidine, (+)-anatoxin-a, dimethylphenylpiperazinium, (-)-nicotine and (-)-lobeline released [3H]noradrenaline. The dose-response curves to nicotinic agonists were bell shaped, and indicated that their functional efficacies and potency vary across agonists. Maximal efficacy was seen with dimethyl-phenylpiperazinium and lobeline (Emax values two to three times higher than other agonists). The rank order of potency for the agonists to release [3H]noradrenaline was (-)-epibatidine > (+)- anatoxin-a > dimethylphenylpiperazinium > cytisine > nicotine > (-)-lobeline. The nicotinic acetylcholine receptor antagonists (n-bungarotoxin > mecamylamine > (+)-tubocurarine > hexamethonium > alpha-bungarotoxin = dihydro-beta-erythroidine) and tetrodotoxin antagonized the effect of dimethylphenylpiperazinium to release [3H]noradrenaline. The results, based on these pharmacological profiles, suggest the possible involvement of alpha 3 and beta 2 nicotinic acetylcholine receptor subunits in the control of [3H]noradrenaline release from hippocampal slices. The absence of effect of alpha-bungarotoxin and alpha-conotoxin-IMI excludes the possible involvement of nicotinic acetylcholine receptors containing the alpha 7 subunit. The release of [3H]noradrenaline by dimethylphenylpiperazinium was Ca2+ dependent. Nifedipine failed to prevent the dimethylphenylpiperazinium-induced release of [3H]noradrenaline, but Cd2+, omega-conotoxin and Ca(2+)-free conditions significantly reduced the dimethylphenylpiperazinium-induced release, suggesting that N-type voltage-sensitive Ca2+ channels are involved in the nicotinic acetylcholine receptor response. These voltage-sensitive Ca2+ channels are activated by the local depolarization produced by sodium influx through the nicotinic channels activated by dimethylphenylpiperazinium. Thus, the observed tetrodotoxin sensitivity of dimethylphenylpiperazinium-induced release of [3H]noradrenaline can be explained either by local depolarization and subsequent generation of action potentials at the preterminal area or that these nicotinic acetylcholine receptors are located on interneurons rather than directly on noradrenergic terminals.

Electrophysiology: a method to investigate the functional properties of ligand-gated channels

Ligand-gated channels (LGCs) play a fundamental role in the fast transmission of electrical activity from neuron to neuron and/or to effector cells. Studies of LGCs in isolation have become possible since the identification of genes coding for these membrane proteins together with the establishment of reconstitution techniques in host systems. Methods for electrophysiological investigations of LGCs reconstituted either in the Xenopus oocytes or stably tranfected in cell lines are discussed. Functional studies of reconstituted receptors enable fast determination of LGCs' pharmacological profiles and comparison of their physiological properties. Combination of molecular engineering with physiological measurements allows studies with unpreceeding resolution and it is now possible to examine at the amino-acid level the contribution of some residues in the formation of the ligand-binding site or the ionic channel domains.

Interactions between tachykinins and diverse, human nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChR) are diverse members of the ligand-gated ion channel superfamily of neurotransmitter receptors and play critical roles in chemical signaling throughout the nervous system. Reports of effects of substance P (SP) on nAChR function prompted us to investigate interactions between several tachykinins and human nAChR subtypes using clonal cell lines as simple experimental models. Acute exposure to SP inhibits carbamylcholine- or nicotine-stimulated function measured using 86Rb+ efflux assays of human ganglionic (alpha 3 beta 4) nAChR expressed in SH-SY5Y neuroblastoma cells (IC50 approximately 2.3 microM) or of human muscle-type (alpha 1 beta 1 gamma delta) nAChR expressed in TE671/RD clonal cells (IC50 approximately 21 microM). SP also acutely blocks function of rat ganglionic nAChR expressed in PC12 pheochromocytoma cells (IC50 approximately 2.1 microM). Neurokinin A and eledoisin inhibit function (extrapolated IC50 values between 60 and 160 microM) of human muscle-type or ganglionic nAChR, but neurokinin B does not, and neither human nAChR is as sensitive as PC12 cell alpha 3 beta 4-nAChR to eledoisin or neurokinin A inhibition. At concentrations that produce blockade of nAChR function, SP fails to affect binding of [3H]acetylcholine to human muscle-type or ganglionic nAChR. SP-mediated blockade of rat or human ganglionic nAChR function is insurmountable by increasing agonist concentrations. Collectively, these results indicate that tachykinins act noncompetitively to inhibit human nAChR function with potencies that vary across tachykinins and nAChR subtypes. They also indicate that tachykinin actions at nAChR could further contribute to complex cross-talk between nicotinic cholinergic and tachykinin signals in regulation of nervous system activity.

Burn injury alters beta-adrenergic receptor and second messenger function in rat ventricular muscle

OBJECTIVES

The molecular pharmacologic bases for the attenuated cardiovascular and metabolic responses to catecholamines, after burn injury, have not been elucidated. In the present study, myocardial tissues were used as a model of beta-adrenergic receptors to study burn injury-induced alterations in receptors and in signal transduction.

DESIGN

Prospective study, randomized to treatment and control groups.

SETTING

University-hospital research laboratory.

SUBJECTS

Male Sprague-Dawley rats (180 to 210 g).

INTERVENTIONS

A 50% body surface area burn or sham-burn was administered to the rats.

MEASUREMENTS AND MAIN RESULTS

Myocardial membranes were isolated at 24 hrs, 7 days and 14 days after 50% body surface area scald or sham injury. (-)125I-iodocyanopindolol was used to assess maximal binding capacity and affinity of the beta-adrenergic receptor. Basal and stimulated concentrations of second messenger, cyclic adenosine monophosphate (cAMP), were also assessed. Production of cAMP during isoproterenol stimulation tested the integrity of the beta-adrenergic receptor-mediated signal transduction. Forskolin, which stimulates adenylate cyclase enzyme directly (bypassing the receptor and G protein) to produce cAMP, tested the efficacy of the enzyme itself. Maximal binding capacity was unaltered between the experimental and control groups, but the affinity (mean +/- SEM) was significantly decreased in burned animals at 7 days (125.4 +/- 15.5 picomoles [pmol]; p = .01) and at 14 days (216.7 +/- 50.7 pmol; p = .001) compared with controls (75.5 +/- 8.4 pmol). In different set experimental and control groups, basal concentrations of cAMP in myocardial membranes were significantly decreased in burned animals at 7 days (control 38.6 +/- 4.2 vs. 5.8 +/- 0.9 pmol/mg of protein/min; p = .003) and at 14 days (control 47.4 +/- 3.2 vs 28.3 +/- 6.6 pmol/mg of protein/min; p = .002). The forskolin (direct)-stimulated synthesis of cAMP was decreased in burned animals at 24 hrs (control 339.0 +/- 40.5 vs. 214.4 +/- 16.6 pmol/mg of protein/min; p = .01), at 7 days (control 289.0 +/- 34.4 vs. 32 +/- 13.0 pmol/mg of protein/min; p = .01), and at 14 days (control 322.9 +/- 28.6 vs. 137.0 +/- 46.1 pmol/mg of protein/min; p = .01). The isoproterenol or receptor-mediated stimulation of cAMP production was also significantly (p < .001) impaired in burned animals compared with controls at 24 hrs (control 134.7 +/- 11.9 vs. 83.1 +/- 13.3 pmol/mg of protein/min), and at 14 days (control 128.2 +/- 7.2 vs. 92.8 +/- 17.7 pmol/mg of protein/min).

CONCLUSION

The etiology of the decreased responses in the myocardium to exogenous and endogenous beta-adrenergic receptor agonists after burn injury may be attributed to decreased affinity for ligands, and also to impaired receptor-mediated signal transduction and to decreased adenylate cyclase enzyme activity, resulting in decreased basal and stimulated second messenger (cAMP) production.

Synaptic and extrasynaptic distribution of two distinct populations of nicotinic acetylcholine receptor clusters in the frog cardiac ganglion

We examined the distribution of neuronal nicotinic acetylcholine receptor clusters in relation to synaptic sites on autonomic neurons in the frog heart using immunofluorescence techniques and laser scanning confocal microscopy. Acetylcholine receptor clusters were visualized using the rat anti-Electrophorus acetylcholine receptor monoclonal antibody no. 22 and cyanine 3.18-labelled goat anti-rat secondary antibody. Synaptic boutons were labelled with the mouse anti-synaptic vesicle protein SV2, monoclonal antibody no. 10h and cyanine 5.18-labelled goat anti-mouse secondary antibody. Acetylcholine receptor clusters on the neuronal surface exist in two populations that vary in size, staining intensity, and surface distribution. The more prominent population consists of large, brightly stained clusters numbering 30 +/- 15 per cell, while the second class is smaller and less brightly stained and numbers over 100 per cell. The large clusters tend to be organized into groups of 2-6 members. This arrangement results from the fact that 80% of the large clusters colocalize at synaptic boutons and that single boutons can have several associated clusters. The remaining 20% of large/bright acetylcholine receptor clusters are extrasynaptic, but they, too, are clustered and are found in close proximity to synaptic boutons. The small/dim acetylcholine receptor clusters are randomly distributed over the cell surface. The large/bright synaptic acetylcholine receptor clusters presumably underlie fast excitatory synaptic transmission. The small/dim clusters and the large/bright extrasynaptic clusters may represent intermediates in the metabolism of large/bright synaptic clusters.

Simultaneous comparison of nicotinic receptor antagonists on three nicotinic acetylcholine receptors

The relative potencies of several nicotinic cholinoceptor antagonists in producing tetanic fade and reduction of striated muscle contraction were investigated in the isolated guinea-pig oesophagus as well as the guinea-pig and rat phrenic nerve-diaphragm preparations. Contractile smooth muscle responses to vagal stimulation, which involves ganglionic activation, were also measured simultaneously with striated muscle responses in the oesophagus. The relative potency for inhibiting the response of oesophageal smooth muscle to vagal stimulation (20 Hz) was trimetaphan > mecamylamine > hexamethonium > tubocurarine > pancuronium. For oesophageal striated muscle, production of tetanic fade at 100 Hz and reduction in peak tetanic tension at 20 or 100 Hz showed a similar relative potency; pancuronium > tubocurarine > mecamylamine > trimetaphan > hexamethonium and similar results were obtained in the guinea-pig diaphragm for the antagonists investigated (pancuronium, tubocurarine and mecamylamine). In the rat phrenic nerve-diaphragm preparation, production of tetanic fade at 50 Hz and reduction in twitch or tetanic tension all showed the relative potency; tubocurarine > pancuronium > mecamylamine > trimetaphan > hexamethonium. These findings indicate differences in the nicotinic cholinoceptor subtypes involved in vagal ganglionic responses and those in tetanic fade.

Analysis of nicotinic acetylcholine receptor subunits in the cochlea of the mouse

The present study was designed to catalogue and compare nicotinic receptor subunit messages in the mammalian cochlea. Fourteen nicotinic acetylcholine receptor subunit messages were examined by polymerase chain reaction (PCR) analysis and nucleotide sequencing. Total RNA was extracted from the auditory organs of 14- to 18-day-old CBAJ mice, and mRNA was purified using oligo-dT cellulose. After reverse transcription, resulting cDNA was amplified by PCR with the use of primers specific for the nucleotide sequences representing the following nicotinic receptor subunits: muscle types alpha 1, beta 1, gamma, delta and epsilon and neuronal types alpha 2,3,4,5,6,7 and beta 2,3,4. cDNA from cochlear tissue corresponding to the muscle-type receptor subunit beta 1 and to neuronal-type receptor subunits alpha 2,4,5,6 and beta 2,3 was amplified, whereas cDNA for muscle types alpha 1, gamma, delta and epsilon and neuronal types alpha 3,7 and beta 4 was not. All PCR products were homologous in nucleotide sequence to the corresponding reference cDNAs from which the primers were designed. The current results indicate that nicotinic acetylcholine receptor (nAChR) subunits that are similar or identical to the stated muscle and neuronal types are expressed in the murine cochlea. The presence of messages corresponding to the muscle-type beta 1 and neuronal-type nAChR subunits may be correlated with the atypical cholinergic response of cochlear hair cells to agonists and antagonists.

Physostigmine, galanthamine and codeine act as 'noncompetitive nicotinic receptor agonists' on clonal rat pheochromocytoma cells

The acetylcholine esterase inhibitor (-)-physostigmine has been shown to act as agonist on nicotinic acetylcholine receptors from muscle and brain, by binding to sites on the alpha-polypeptide that are distinct from those for the natural transmitter acetylcholine (Schröder et al., 1994). In the present report we show that (-)-physostigmine, galanthamine, and the morphine derivative codeine activate single-channel currents in outside-out patches excised from clonal rat pheochromocytoma (PC12) cells. Although several lines of evidence demonstrate that the three alkaloids act on the same channels as acetylcholine, the competitive nicotinic antagonist methyllycaconitine only inhibited channel activation by acetylcholine but not by (-)-physostigmine, galanthamine or codeine. In contrast, the monoclonal antibody FK1, which competitively inhibits (-)-physostigmine binding to nicotinic acetylcholine receptors, did not affect channel activation by acetylcholine but inhibited activation by (-)-physostigmine, galanthamine and codeine. The three alkaloids therefore act via binding sites distinct from those for acetylcholine, in a 'noncompetitive' fashion. The potency of (-)-physostigmine and related compounds to act as a noncompetitive agonist is unrelated to the level of acetylcholine esterase inhibition induced by these drugs. (-)-Physostigmine, galanthamine and codeine do not evoke sizable whole-cell currents, which is due to the combined effects of low open-channel probability, slow onset and slow inactivation of response. In contrast, they sensitize PC12 cell nicotinic receptors in their submaximal response to acetylcholine. While the abundance of nicotinic acetylcholine receptor isoforms expressed in PC12 cells excludes identification of specific nicotinic acetylcholine receptor subtypes that interact with noncompetitive agonists, the identical patterns of single-channel current amplitudes observed with acetylcholine and with noncompetitive agonists suggested that all PC12 cell nicotinic acetylcholine receptor subtypes that respond to acetylcholine also respond to noncompetitive agonist. The action of noncompetitive agonists therefore seems to be highly conserved between nicotinic acetylcholine receptor subtypes, in agreement with the high level of structural conservation in the sequence region harboring major elements of this site.

Erysodine, a competitive antagonist at neuronal nicotinic acetylcholine receptors

Erysodine, an erythrina alkaloid related to dihydro-beta-erythroidine, was found to be a more potent inhibitor of [3H]cytisine binding at neuronal nicotinic acetylcholine receptors but a less potent inhibitor of [125I]alpha-bungarotoxin binding at muscle-type nicotinic acetylcholine receptors than dihydro-beta-erythroidine. Erysodine was a competitive, reversible antagonist of (-)-nicotine-induced dopamine release from striatal slices and inhibited (-)-nicotine-induced 86Rb+ efflux from IMR-32 cells. Erysodine was equipotent with dihydro-beta-erythroidine in the dopamine release assay but 10-fold more potent in the 86Rb+ efflux assay, suggesting differential subtype selectivity for these two antagonists. Erysodine, systemically administered to mice, entered the brain and significantly attentuated nicotine's hypothermic effects and its anxiolytic-like effects in the elevated plus-maze test. There was greater separation between antagonist and toxic doses for erysodine than for dihydro-beta-erythroidine, perhaps because of erysodine's greater selectivity for neuronal receptors. In rats, erysodine prevented both the early developing decrease and the late-developing increase in locomotor activity produced by (-)-nicotine. The potent and competitive nature of erysodine's antagonism together with its ability to enter the brain after systemic administration suggest that erysodine may be a useful tool in characterizing neuronal nicotinic acetylcholine receptors.

Monovalent and divalent cation permeability and block of neuronal nicotinic receptor channels in rat parasympathetic ganglia

Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh-evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5-6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.

Diversity and patterns of regulation of nicotinic receptor subtypes

In our studies we explored the functional relevance of nAChR diversity, in part from the perspective of nAChR as ideal targets for regulatory influences, including those mediated via actions of ligands at other "interacting" receptors. We explored possible mechanisms for nAChR regulation and roles played by nAChR subtype and subunit diversity in those processes. We showed that regulatory factors can influence nAChR numbers at transcriptional and posttranscriptional levels and can affect nAChR function and subcellular distribution. We also demonstrated that nAChR expression can be influenced (1) by nicotinic ligands, (2) by second messengers, (3) by growth factors, (4) by agents targeting the nucleus, and (5) by agents targeting the cytoskeleton. We found common effects of some regulatory influences on more than one nAChR subtype, and we found instances where regulatory influences differ for different cell and nAChR types. Even from the very limited number of these initial studies, it is evident that nAChR subunit and subtype diversity, which alone can provide diversity in nAChR functions, localization, and ligand sensitivity, dovetails with diversity in cellular signaling mechanisms that can affect nAChR expression to amplify the potential functional plasticity of cholinoceptive cells. As examples, we discussed potential roles for nAChR diversity and regulatory plasticity in synapse remodeling and in changes in neuronal circuit conditions. These examples illustrate how nAChR diversity could play important roles in the regulation of nervous system function.