Molecular cloning of human neuronal nicotinic receptor alpha 3-subunit

Engineering of α-conotoxin MII-derived peptides with increased selectivity for native α6β2* nicotinic acetylcholine receptors

α6β2* Nicotinic acetylcholine receptors are expressed in selected central nervous system areas, where they are involved in striatal dopamine (DA) release and its behavioral consequences, and other still uncharacterized brain activities. α6β2* receptors are selectively blocked by the α-conotoxins MII and PIA, which bear a characteristic N-terminal amino acid tail [arginine (R), aspartic acid (D), and proline (P)]. We synthesized a group of PIA-related peptides in which R1 was mutated or the RDP motif gradually removed. Binding and striatal DA release assays of native rat α6β2* receptors showed that the RDP sequence, and particularly residue R1, is essential for the activity of PIA. On the basis of molecular modeling analyses, we synthesized a hybrid peptide (RDP-MII) that had increased potency (7-fold) and affinity (13-fold) for α6β2* receptors but not for the very similar α3β2* subtype. As docking studies also suggested that E11 of MII might be a key residue engendering α6β2* vs. α3β2* selectivity, we prepared MII[E11R] and RDP-MII[E11R] peptides. Their affinity and potency for native α6β2* receptors were similar to those of their parent analogues, whereas, for the oocyte expressed rat α3β2* subtype, they showed a 31- and 14-fold lower affinity and 21- and 3.5-fold lower potency. Thus, MII[E11R] and RDP-MII[E11R] are potent antagonists showing a degree of α6β2* vs. α3β2* selectivity in vivo.

Nicotinic receptor abnormalities of Alzheimer's disease: therapeutic implications

The neuronal nicotinic acetylcholine receptors (nAChRs) in the brain are important for functional processes, including cognitive and memory functions. The nAChRs acting as neuromodulators in communicative processes regulated by different neurotransmitters show a relatively high abundance in the human cortex, with a laminar distribution of the nAChRs of superhigh, high, and low affinity in the human cortex. The regional pattern of messenger RNA (mRNA) for various nAChR subtypes does not strictly follow the regional distribution of nAChR ligand-binding sites in the human brain. Consistent losses of nAChRs have been measured in vitro in autopsy brain tissue of Alzheimer's disease patients (AD), as well as in vivo by positron emission tomography (PET). Measurement of the protein content of nAChRs showed reduced levels of the alpha4, alpha3, and alpha7 nAChR subtypes. The finding that the alpha4 and alpha3 mRNA levels were not changed in AD brains suggests that the losses in high-affinity nicotinic-binding sites cannot be attributed to alterations at the transcriptional level of the alpha4 and alpha3 genes and that the causes have to be searched for at the translational and/or posttranslational level. The increased mRNA level of the alpha7 nAChR subtyep in the hippocampus indicates that subunit-specific changes in gene expression of the alpha7 nAChR might be associated with AD. The PET studies reveal deficits in nAChRs as an early phenomena in AD, stressing the importance of nAChRs as a potential target for drug intervention. PET ligands measuring the alpha4 nAChRs are under development. Studies of the influence of beta-amyloid on nAChRs in brain autopsy tissue from patients with the amyloid precursor protein 670/671 mutation have shown that there is no direct relationship between nAChR deficits and pathology. Treatment with cholinergic drugs in AD patients indicate improvement of the nAChRs in the brain, as visualized by PET. Further studies on neuroprotective mechanisms mediated via nAChR subtypes are exciting new avenues.

[(3)H]Nicotine binding in peripheral blood cells of smokers is correlated with the number of cigarettes smoked per day

The principal sites for biological action of tobacco products are thought to be the nicotinic acetylcholine receptors (nAChR). Nicotinic receptor subunit genes, therefore, represent an important gene family for study in nicotine addiction. They are localized in both brain and in the periphery. In brain these receptors appear to function as modulators of synaptic transmission; the function of peripheral receptors is not known. Nicotinic receptor levels in human brain are regulated by smoking in a dose-dependent manner. In peripheral blood, nicotinic receptors are present on both lymphocytes and polymorphonuclear cells (PMN). We have compared [(3)H]nicotine binding in PMN isolated from smokers and non-smokers. [(3)H]nicotine binding was increased in smokers and was correlated, as in brain, with tobacco use. Expression of both mRNA and protein in lymphocytes and PMN, for a subset of nicotinic receptor subunits, suggests that these cell types contain both alpha4beta2 and alpha3beta4 receptors.

Expression of the alpha3 nicotinic receptor subunit mRNA in aging and Alzheimer's disease

Changes in the number of high-affinity nicotine binding sites have been widely reported in specific regions of the human brain during aging and in degenerative neurological diseases associated with aging, such as Alzheimer's disease. Nicotinic receptors are highly diverse and a description of the molecular subtypes affected in such conditions has not been achieved to date. To investigate the status of the alpha3 subunit-containing subtypes in such conditions, we assessed by in situ hybridisation the alpha3 mRNA density in the hippocampus, entorhinal cortex and thalamus of Alzheimer's patients and age-matched controls. No significant difference in the expression of the alpha3 mRNA, either qualitative or quantitative, was found between Alzheimer's individuals and controls in any of the analysed areas. This result suggests that the nicotine binding changes occurring in these areas in Alzheimer's patients are not correlated to a variation of the alpha3 mRNA in the same regions. Nevertheless, a negative correlation between the alpha3 mRNA density and the age was observed in the entorhinal cortex of both the Alzheimer's and the normal subjects, suggesting a potentially extensive decay of the alpha3-expressing neurons or loss of alpha3-containing receptors in intact neurons of the entorhinal cortex in the late elderly.

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.

Structural and functional characterization of the human alpha3 nicotinic subunit gene promoter

We describe the structural and functional features of the human alpha3 nicotinic receptor subunit promoter. A 0.35-kb region immediately upstream of the start codon was identified that when transfected in human neuroblastoma cells was able to drive the expression of the luciferase reporter gene with a strength comparable to that of the well-characterized simian virus 40 promoter/enhancer. This region displayed the features of a multistart-site, GC-rich, TATA-less, and CAAT-less promoter, containing many overlapping Sp1 and AP-2 putative binding sites. Further dissections of the 0.35-kb fragment revealed that its 3' region, specifying the 5' UT of the mRNA, plays a relevant positive effect in determining the strength of the promoter. This region contains putative cis-acting elements for AP-2, nuclear factor-kappaB, and the recently described multiple-start site element downstream-1. By mutation analysis, we showed that these sites are functional and when combined increase the promoter activity by 4-fold. The 0.35-kb promoter was found to be under the negative control of upstream sequences that include a modern Alu repeat. The alpha3 Alu repeat works as a composite region, containing both positive and negative elements that control the activity of the downstream promoter. Finally, we investigated the tissue-specific activity of the human alpha3 gene 5' regulatory sequences, showing that they are able to drive the expression of the reporter gene preferentially in neuronal cells.

Linkage of strain-specific nicotinic receptor alpha 7 subunit restriction fragment length polymorphisms with levels of alpha-bungarotoxin binding in brain

Inbred mouse strains have been shown to differ in their levels of brain alpha-bungarotoxin binding. These differences in alpha-bungarotoxin receptors have been shown to correlate with an animal's sensitivity to nicotine-induced seizures. Recent studies have shown that the alpha 7 nicotinic acetylcholine receptor subunit is the major alpha-bungarotoxin binding site in rodent brain. In this report, we examined whether mouse strains that differ in levels of alpha-bungarotoxin binding and sensitivity to nicotine-induced convulsions also differ for the alpha 7 subunit. A full-length murine alpha 7 cDNA was cloned and sequenced and found to be identical to that of a mouse alpha 7 cDNA recently reported. Subsequently, a comparison of alpha 7 cDNA sequences and RNA species was performed between two strains (C3H/2 and DBA/2) that differ in levels of brain alpha-bungarotoxin binding and sensitivity to nicotine-induced seizures. The only difference observed was a single nucleotide difference in the open reading frame of alpha 7 that does not affect the primary amino acid sequence. Inbred strains were also surveyed for restriction fragment length polymorphisms at the alpha 7 locus. Strain-specific polymorphisms were identified, and F2 and backcross animals from a classic genetic cross between C3H/2 and DBA/2 mice were compared for the inheritance of alpha 7 genotype and alpha-bungarotoxin receptor levels. A significant association between genotype and receptor levels was observed in both, the F2 and backcross generations. These results indicate that alpha 7 genotype is an important determinant of alpha-bungarotoxin receptor levels.

Comparative structure of human neuronal alpha 2-alpha 7 and beta 2-beta 4 nicotinic acetylcholine receptor subunits and functional expression of the alpha 2, alpha 3, alpha 4, alpha 7, beta 2, and beta 4 subunits

cDNA clones encoding human neuronal nicotinic acetylcholine receptor alpha 2, alpha 3, alpha 4, alpha 5, alpha 6, alpha 7, beta 2, beta 3, and beta 4 subunits were isolated from brainstem, hippocampus, prefrontal cortex, substantia nigra, thalamus, and IMR32 libraries. Human alpha 2 and alpha 6 and full-length beta 3 and beta 4 clones have not been previously reported. Deduced amino acid sequences of the alpha 2, alpha 6, beta 3, and beta 4 predicted mature peptides are 503 residues (56.9 kDa), 464 residues (53.7 kDa), 440 residues (50.8 kDa), and 477 residues (54.1 kDa), respectively. These sequences show 84 (alpha 2), 87 (alpha 6), 89 (beta 3), and 84% (beta 4) identity to the corresponding rat sequences. The amino termini of the human alpha 2 and beta 3 mature peptides contain 23 and six additional residues, respectively, compared to those of rat alpha 2 and beta 3. Recombinant receptors were expressed in Xenopus laevis oocytes injected with in vitro transcripts encoding either alpha 7 alone or alpha 2, alpha 3, or alpha 4 in pairwise combination with beta 2 or beta 4. Inward currents were elicited by the application of acetylcholine (1-100 microM) and other agonists; these responses were blocked 65-97% by application of 10 microM d-tubocurare, confirming functional expression of human nicotinic receptors.

Expression of alpha subunit genes of nicotinic acetylcholine receptors in human lymphocytes

Using immunohistochemistry and in situ hybridisation, we have studied whether alpha-subunits of nicotinic acetylcholine receptors (nAChRs) are expressed in human lymphocytes. Cells were isolated by differential low speed gradient centrifugation from heparinised venous blood of 10 healthy volunteers. Receptor sites were visualised using the monoclonal antibody WF6 which specifically recognises alpha-isoforms from several species including man. For visualisation of transcripts, digoxigenin-labelled cRNA probes for alpha 4- and alpha 3-subunits were used. Immunostaining revealed specific binding of WF6 to isolated human lymphoid cells. The antibody was bound to most cells and concentrated preferentially in the perinuclear/surface region. The immunoreactivity resembled that observed after application of an antibody recognising CD4 surface proteins which was conducted for comparison. In situ-hybridisation revealed that the alpha 4-subunit genes of nAChRs was expressed in lymphocytes of all probands. The alpha 3-subunit was found, with lower intensity than alpha 4-transcripts, in eight of the 10 individuals. Control incubations with corresponding sense cRNAs were negative. It is concluded that human lymphocytes are able to express alpha-subunit genes of nAChRs.

Native nicotinic acetylcholine receptors in human Imr32 neuroblastoma cells: functional, immunological and pharmacological properties

IMR32 cells express two classes of surface nicotinic receptors: those labelled with high affinity by [125I]neuronal toxin, and those labelled by [125I]alpha-bungarotoxin. Whole-cell patch-clamp recordings indicate that both classes of receptor are able to elicit inward currents that are totally blocked by d-tubocurarine but only partially blocked by alpha-bungarotoxin. In IMR32 cells, nicotine induces an increase in the intracellular level of free Ca2+. This increase, which is also completely blocked by d-tubocurarine and only partially blocked by alpha-bungarotoxin and Cd2+, is due to extracellular calcium influx through both the nicotinic receptors and the voltage-activated Ca2+ channels. By using subunit-specific polyclonal antibodies, we have demonstrated that the alpha-bungarotoxin receptors contain the alpha 7 subunit, but none of the other subunits whose transcripts are present in IMR32 cells. The pharmacological profile of these human alpha 7-containing alpha-bungarotoxin receptors is similar to that observed in the native chick alpha 7 receptor, but there are also some species-specific differences.

Distribution of nicotinic receptors in the human hippocampus and thalamus

Neuronal nicotinic acetylcholine receptors consist of different subunits, alpha and beta, with different subtype arrangement corresponding to distinct pharmacological and functional properties. The expression of alpha 3, alpha 7 and beta 2 mRNA in the human brain was studied by in situ hybridization and compared to [3H]nicotine, [3H]cytisine and [125I]alpha-bungarotoxin binding in contiguous sections. The beta 2 probe showed a strong hybridization signal in the granular layer of the dentate gyrus and in the CA2/CA3 region of the hippocampus and in the insular cortex, and a signal of lower intensity in the subicular complex and entorhinal cortex. The alpha 3 probe showed strong hybridization in the dorsomedial, lateral posterior, ventroposteromedial and reticular nuclei of the thalamus, and a weak signal in the hippocampal region and in the entorhinal, insular and cingular cortex. The amount of alpha 7 mRNA was high at the level of the dentate granular layer and the CA2/CA3 region of the hippocampus, in the caudate nucleus and in the pulvinar and ventroposterolateral nuclei of the thalamus. [3H]Nicotine and [3H]cytisine binding appeared to be identical in anatomical distribution and relative intensity. It was high in the thalamic nuclei, the putamen and in the hippocampal formation in the subicular complex and the stratum lacunosum moleculare. The level of [125I]alpha-bungarotoxin binding was particularly high in the hippocampus and in the pyramidal cells of the CA1 region, but was relatively low in the subicular complex. Our data indicate that in the human brain nicotinic receptor subtypes have discrete distributions, which are in part different from those of other species.

Cellular distribution of nicotinic acetylcholine receptor subunit mRNAs in the human cerebral cortex as revealed by non-isotopic in situ hybridization

The pharmacology of telencephalic nicotinic acetylcholine receptors (nAChRs) has become an important issue in recent years. While in the human brain a direct pharmacological assessment is difficult to achieve the visualization of nAChRs has been enabled by histochemical techniques providing an ever increasing and improving resolution. Receptor autoradiography was used to visualize binding sites on the level of cortical layers whereas immunohistochemistry has allowed for the cell type-specific and ultrastructural localization of receptor protein. Further investigations have to elucidate the cellular sites of NAChR biosynthesis by visualizing subunit-specific transcripts. Using autopsy samples of the human precentral cortex (Area 4) as a paradigm we have applied digoxigenin-labeled cRNA probes to localize transcripts for the alpha 3- and alpha 4-1-subunits of the nAChR. In accordance with findings in the monkey cortex, the alpha 3-subunit seems to be expressed mainly in pyramidal neurons of layers III-VI of the human cerebral cortex. Transcripts for the alpha 4-1-subunit, by contrast, appear to be present in a large number of neurons throughout all layers of the cerebral cortex, consonant with its ubiquitous distribution in the rodent brain. The present findings show that also in human autopsy brains the cell type-specific detection of nAChR transcripts is possible. For the future, this technique will enable to investigate the expression of receptor transcripts in diseased human brains as compared to controls.

Distribution of neuronal nicotinic receptor subunits in human brain

Neuronal nicotinic acetylcholine receptors (nAchRs) are multimeric proteins constituted of two different subunits, alpha and beta, with different subtypes arrangement and different pharmacological and functional properties. nAchRs mediate neurotransmission in many central and peripheral synapses and appear to be affected in human degenerative disorders. We have studied the distribution of nAchR in human brain, particularly in the hippocampus and thalamus, by binding of 3H-nicotine and 3H-cytisine and by in situ hybridization with human alpha 3 and beta 2 nAchR subunits of mRNA. An alpha 3 probe shows a strong hybridization signal in the thalamus, while a beta 2 probe has a good signal at the level of the enthorinal cortex, hippocampus and in caudate and putamen. The alpha 3 and beta 2 mRNA localization is different from that described in other species. 3H-nicotine and 3H-cytisine binding were very similar in terms of anatomical distribution and comparable to the binding described in other animal species. The binding of the two ligands was distributed over the areas labeled by the alpha 3 and beta 2 probes and did not completely overlap with either of the subunits.

Human nicotinic receptors--their role in aging and dementia

Multiple nicotinic receptors seem to exist in brain as revealed by neurophysiological, neurochemical, molecular and immunological studies. The mechanisms for their involvement in higher functions including learning and memory are still relatively unknown. The nicotinic receptor subtypes in human brain undergo changes during aging. Deficits of brain nicotinic receptors have been traced in neurodegenerative disorders as Alzheimer's disease and Parkinson's disease. Brain imaging studies in patients and neurochemical studies in autopsy brain tissue from Alzheimer patients reveal significant losses of the nicotinic receptors. New therapeutic compounds tried in Alzheimer's disease, aiming to increase cholinergic activity in the brain, act via the nicotinic receptors in brain. Augmentation of nicotinic receptor function in brain might be of importance for alleviating some of the cognitive impairments in Alzheimer's disease.

Nicotine-related brain disorders: the neurobiological basis of nicotine dependence

1. This paper was written at a moment when the dependence liability of nicotine, the psychoactive component from tobacco, was the center of a dispute between the tobacco manufacturing companies and the scientific community (Nowak, 1994a-c). Without being comprehensive, it tries to summarize evidence compiled from several disciplines within neuroscience demonstrating that nicotine produces a true psychiatric disease, behaviorally expressed as dependence to the drug (American Psychiatric Association, 1994). Nicotine dependence has a biological substratum defined as "neuroadaptation to nicotine." 2. The first part of the article defines terms such as "abuse," "tolerance," "dependence," and "withdrawal." It discusses clinical and experimental facts at the whole-organism level, showing that animals and humans will seek and self-administer nicotine because of its rewarding properties. 3. The second part discusses the neurobiological basis of neuroadaptation to nicotine. It presents information on neuroanatomical circuits which may be involved in nicotine-related brain disorders, such as the mesocorticolimbic pathway and the basal forebrain-frontal cortex pathway. It also discusses work from several laboratories, including our own, that support the notion of a molecular basis for neuroadaptative changes induced by nicotine in the brain of a chronic smoker. 4. Although still under experimental scrutiny, the hallmark of neuroadaptation to nicotine is up-regulation of nicotinic receptors, possibly due to nicotine-induced desensitization of their function (Marks et al., 1983; Schwartz and Kellar, 1985). A correlation between these plastic changes and the behavioral data obtained from animal and human experiments is still needed to understand dependence to nicotine fully.

Molecular cloning of a human neuronal nicotinic acetylcholine receptor beta 3-like subunit

Three cDNA clones homologous to rat neuronal nicotinic receptors were identified in a human brain stem library by screening at low stringency with a mixture of rat alpha 5, beta 2 and beta 4 subunit probes. Positive clones were further analysed by hybridisation at higher stringency with individual rat nicotinic receptor probes. One positive clone was found to encode a protein that exhibited characteristic features of a member of the ligand-gated ion channel class of protein subunits, and showed 89% homology with a rat beta 3 neuronal nicotinic receptor subunit. Northern blots demonstrated the presence of a 1.7 kb transcript in RNA extracted from adult human Pons. This clone has therefore been designated the human beta 3 neuronal nicotinic receptor subunit.

Neuronal-type nicotinic receptors in human neuroblastoma and small-cell lung carcinoma cell lines

A beta subunit of the neuronal nicotinic receptor, sharing 88% homology with the rat beta 4 subunit, has been cloned from a human neuroblastoma cell line. The gene encoding the human beta 4 subunit is expressed in association with the alpha 3 gene in neuroblastoma and small-cell lung carcinoma cell lines. Patch-clamp experiments and radioligand binding assays confirm that these neuroendocrine tumor cell lines express functional neuronal nicotinic receptors. We suggest that these receptors might play a crucial role in the control of neurotransmitter and hormone secretion from neurosecretory human tumors.

Distribution of nicotinic receptors in cynomolgus monkey brain and ganglia: localization of alpha 3 subunit mRNA, alpha-bungarotoxin and nicotine binding sites

The distribution of nicotinic receptors in the brain and ganglia of the Cynomolgus monkey was studied by in situ hybridization and receptor autoradiography. A 35S-labeled antisense riboprobe for the mRNA of the alpha 3 subunit of the human nicotinic receptor, [3H]L-nicotine and [125]alpha-bungarotoxin were used as markers. The highest levels of alpha 3-mRNA were observed in the hippocampus, the medial habenula, the lateral geniculate, the granular layer of the cerebellum, as well as in the pineal gland; moderate levels were found in other nuclei of the thalamus and in the deeper layers of the cerebral cortex. High-affinity binding sites for [3H]L-nicotine were observed mainly in the thalamus. The distribution of [125I]alpha-bungarotoxin binding sites was different from that observed for alpha 3-mRNA and [3H]L-nicotine; they were most abundant in a few specific thalamic nuclei, in the medial habenula and in lamina I of the cerebral cortex. The localization of these three markers was also investigated in the sympathetic, parasympathetic and sensory ganglia of the monkey. Intense labeling was observed for alpha 3-mRNA and for [125I]alpha-bungarotoxin in the sympathetic and parasympathetic ganglia, whereas no positive signal was seen in the ganglion of Gasser. [3H]L-nicotine binding was not detected in any of the ganglia examined. High levels of mRNA for the alpha 3 subunit of the nicotinic receptor were also detected in human sympathetic ganglia. Comparison between alpha 3-mRNA distribution and [3H]L-nicotine binding suggests that in the Cynomolgus monkey brain, the alpha 3 subunit may participate in the formation of more than one nicotinic receptor subtype: a high-affinity binding site for [3H]L-nicotine in the thalamus, and other sites with low affinity for nicotine in the medial habenula and cerebral cortex. Both the alpha 3-mRNA and the [125I]alpha-bungarotoxin are highly expressed in the sympathetic ganglia; however, since no information is presently available on the intraneuronal cellular localization, it cannot be established whether or not they are both present at synaptic sites.

Chromosomal localization of seven neuronal nicotinic acetylcholine receptor subunit genes in humans

We have determined the chromosomal location of seven human neuronal nicotinic acetylcholine receptor subunit genes by genomic Southern analysis of hamster/human somatic cell hybrid DNAs. The beta 2 subunit gene was localized to human chromosome 1, the alpha 2 and beta 3 subunit genes were localized to human chromosome 8, the alpha 3, alpha 5, and beta 4 subunit genes were localized to human chromosome 15, and the alpha 4 subunit gene was localized to human chromosome 20. Mapping of the beta 2 subunit gene to chromosome 1 establishes a syntenic group with the amylase gene locus on human chromosome 1 and mouse chromosome 3, while mapping of the alpha 3 subunit gene to chromosome 15 confirms the existence of a syntenic group with the mannose phosphate isomerase gene locus on human chromosome 15 and mouse chromosome 9.

Neuronal-type alpha-bungarotoxin receptors and the alpha 5-nicotinic receptor subunit gene are expressed in neuronal and nonneuronal human cell lines

alpha-Bungarotoxin (alpha Bgtx) is a toxin known to interact with muscle nicotinic receptors and with some neuronal nicotinic receptors. We show that alpha Bgtx binding sites are also expressed in nonmuscle and nonneuronal human cells, including small cell lung carcinoma and several epithelial cell lines. These receptors are immunologically related to the alpha Bgtx receptors of unknown function described in the nervous system and in the IMR32 neuroblastoma cell line and are distinct from muscle nicotinic receptors. We have also cloned from IMR32 cells the human alpha 5-nicotinic receptor subunit, which is supposed to participate in the formation of alpha Bgtx receptors. Transcripts corresponding to the alpha 5-subunit gene were found not only in neuroblastoma cells but also in all the cell lines expressing alpha Bgtx receptors, with the exception of the TE671 cell line, whose nicotinic receptor subunits are of the muscle type. We conclude that both alpha Bgtx receptors and the alpha 5-nicotinic subunit gene are not neuron-specific, as previously thought, but are expressed in a number of human cell lines of various origin.

Alpha 4-2 beta 2 and other nicotinic acetylcholine receptor subtypes as targets of psychoactive and addictive drugs

1. Xenopus oocytes were injected with various muscle and neuronal nicotinic acetylcholine receptor (ACh receptor, cholinoceptor) subunit RNA combinations and their pharmacological properties studied using two-electrode voltage clamp. The functional expression of one of these combinations, rat alpha 4-2 beta 2, has not been previously described. The alpha 4-2 mRNA is a splicing variant transcribed from the alpha 4 gene. In the experiments reported here, the alpha 4-2 beta 2 subtype was functionally indistinguishable from the alpha 4-1 beta 2 subtype. 2. For each subtype, the relative potency of nicotine compared with acetylcholine was obtained by estimating the relative concentration of nicotine which would elicit the same current response as 0.1 microM Ach. The ratios of these concentrations (nicotine: ACh) for the mouse muscle ACh receptor-(alpha 1 beta 1 gamma delta) was 96.1:1. In contrast, the ratios for the rat neuronal subtypes were: alpha 2 beta 2, 1.01:1; alpha 3 beta 2, 2.01:1; alpha 4 beta 2, 0.76:1 and alpha 4-2 beta 2, 0.76:1. The much greater relative nicotine sensitivity of the neuronal subtypes as compared with muscle receptors illustrates their potential to mediate the psychoactive and addictive effects of nicotine. However, it does not appear that the differences in relative nicotinic sensitivity among the neuronal receptors themselves can be used as a simple discriminative tool in neuronal tissue. 3. The slopes of the log dose-log response curves at low ACh concentrations were all greater than 1 but less than 2, suggesting that at least two agonist binding sites mediate the functional response of each hetero-oligomer. 4. The response of all the neuronal subtypes to ACh could be inhibited by the psychoactive drugs mecamylamine, amitriptyline, phencyclidine, trifluoperazine and promethazine. With the exception of the very potent antagonist, mecamylamine, the degree of block of the peak current to ACh produced by 10 microM concentrations of these drugs was remarkably similar (around 50%). 5. The degree of inhibition produced when the antipsychotic drug, trifluoperazine, was co-applied with ACh increased as the duration of application increased. Such an effect was not observed with promethazine, a related phenothiazine derivative which does not have antipsychotic actions.

Thymopoietin, a thymic polypeptide, potently interacts at muscle and neuronal nicotinic alpha-bungarotoxin receptors

Current studies suggest that several distinct populations of nicotinic acetylcholine (ACh) receptors exist. One of these is the muscle-type nicotinic receptors with which neuromuscular nicotinic receptor ligands and the snake toxin alpha-bungarotoxin interact. alpha-Bungarotoxin potently binds to these nicotinic receptors and blocks their function, two characteristics that have made the alpha-toxin a very useful probe for the characterization of these sites. In neuronal tissues, several populations of nicotinic receptors have been identified which, although they share a nicotinic pharmacology, have unique characteristics. The alpha-bungarotoxin-insensitive neuronal nicotinic receptors, which may be involved in mediating neuronal excitability, bind nicotinic agonists with high affinity but do not interact with alpha-bungarotoxin. Subtypes of these alpha-toxin-insensitive receptors appear to exist, as evidenced by findings that some are inhibited by neuronal bungarotoxin whereas others are not. In addition to the alpha-bungarotoxin-insensitive sites, alpha-bungarotoxin-sensitive neuronal nicotinic receptors are also present in neuronal tissues. These latter receptors bind alpha-bungarotoxin with high affinity and nicotinic agonists with an affinity in the microM range. The function of the nicotinic alpha-bungarotoxin receptors are as yet uncertain. Thymopoietin, a polypeptide linked to immune function, appears to interact specifically with nicotinic receptor populations that bind alpha-bungarotoxin. Thus, in muscle tissue where alpha-bungarotoxin both binds to the receptor and blocks activity, thymopoietin also potently binds to the receptor and inhibits nicotinic receptors-mediated function. In neuronal tissues, thymopoietin interacts only with the nicotinic alpha-bungarotoxin site and not the alpha-bungarotoxin-insensitive neuronal nicotinic receptor population. These observations that thymopoietin potently and specifically interacts with nicotinic alpha-bungarotoxin-sensitive receptors in neuronal and muscle tissue, together with findings that thymopoietin is an endogenously occurring agent, could suggest that this immune-related polypeptide represents a ligand for the alpha-bungarotoxin receptors. The function of thymopoietin at the alpha-bungarotoxin receptor is as yet uncertain; however, a potential trophic, as well as other roles are suggested.

Nicotinic and muscarinic subtypes in the human brain: changes with aging and dementia

Different effects of normal aging on muscarinic and nicotinic receptor subtypes were observed in postmortem brain tissue from different regions of the human brain. A significant decrease in M1 and M2 receptors was found in cerebral cortex, while the M1 and especially the M2 receptors increased with age in the thalamus. A similar pattern of changes was also observed when using (-)3H-nicotine as ligand for nicotinic receptors in the cortex and thalamus. No significant changes in nicotinic receptor binding were observed with age in the cortex or thalamus when using 3H-acetylcholine as ligand. Nicotinic and muscarinic receptors in the brain are not equally affected in dementia disorders. A marked loss of high affinity nicotinic receptors was observed in cortical tissue from patients with Alzheimer's disease and with multi-infarct dementia (MID). The muscarinic receptors were (both M1 and M2) increased in Alzheimer cortical tissue while they were decreased in MID.

Pharmacological and functional diversity of neuronal nicotinic acetylcholine receptors

Recent molecular cloning studies have identified several genes encoding alpha and beta subunits of the nicotinic acetylcholine receptor. These genes have distinct, although overlapping, patterns of expression in the brain and peripheral ganglia. Multiple nicotinic receptors with distinct pharmacological and functional properties can be made in oocytes by pairwise combination of different alpha-type subunits with different beta-type subunits. Both alpha and beta subunits contribute to the pharmacological and functional diversity. Evan Deneris and colleagues explain how oocyte expression studies, in concert with immunological and electrophysiological analysis in vivo, are beginning to reveal the subunit compositions of different neuronal nicotinic receptor subtypes.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.