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J Michael McIntosh

Publications and source records attributed to J Michael McIntosh.

At least 55 records · Page 3Linked to original sources
PubMed2003

The beta3 nicotinic receptor subunit: a component of alpha-conotoxin MII-binding nicotinic acetylcholine receptors that modulate dopamine release and related behaviors.

Nigrostriatal dopaminergic neurons express many nicotinic acetylcholine receptor (nAChR) subunits capable of forming multiple nAChR subtypes. These subtypes are expressed differentially along the neuron and presumably mediate diverse responses. beta3 subunit mRNA has restricted expression but is abundant in the substantia nigra and ventral tegmental areas. To investigate the potential role(s) of nicotinic receptors containing the beta3 subunit in dopaminergic tracts, we generated mice with a null mutation in the beta3 gene. We were thereby able to identify a population of beta3-dependent alpha-conotoxin MII-binding nAChRs that modulate striatal dopamine release. Changes were also observed in locomotor activity and prepulse inhibition of acoustic startle, behaviors that are controlled, in part, by nigrostriatal and mesolimbic dopaminergic activity, respectively, suggesting that beta3-containing nAChRs modulate these behaviors.

Animals↗
PubMed2003

Alpha-conotoxin PIA is selective for alpha6 subunit-containing nicotinic acetylcholine receptors.

Until now, there have been no antagonists to discriminate between heteromeric nicotinic acetylcholine receptors (nAChRs) containing the very closely related alpha6 and alpha3 subunits. nAChRs containing alpha3, alpha4, or alpha6 subunits in combination with beta2, occasionally beta4, and sometimes beta3 or alpha5 subunits, are thought to play important roles in cognitive function, pain perception, and the reinforcing properties of nicotine. We cloned a novel gene from the predatory marine snail Conus purpurascens. The predicted peptide, alpha-conotoxin PIA, potently blocks the chimeric alpha6/alpha3beta2beta3 subunit combination as expressed in oocytes but neither the muscle nor the major neuronal nAChR alpha4beta2. Additionally, this toxin is the first described ligand to discriminate between nAChRs containing alpha6 and alpha3 subunits. Exploiting the unusual intron conservation of conotoxin genes may represent a more general approach for defining conotoxin ligand scaffolds to discriminate among closely related receptor populations.

Amino Acid Sequence↗
PubMed2003

Subunit composition of functional nicotinic receptors in dopaminergic neurons investigated with knock-out mice.

Nicotinic acetylcholine receptors (nAChRs) expressed by dopaminergic (DA) neurons have long been considered as potential therapeutic targets for the treatment of several neuropsychiatric diseases, including nicotine and cocaine addiction or Parkinson's disease. However, DA neurons express mRNAs coding for most, if not all, neuronal nAChR subunits, and the subunit composition of functional nAChRs has been difficult to establish. Immunoprecipitation experiments performed on mouse striatal extracts allowed us to identify three main types of heteromeric nAChRs (alpha4beta2*, alpha6beta2*, and alpha4alpha6beta2*) in DA terminal fields. The functional relevance of these subtypes was then examined by studying nicotine-induced DA release in striatal synaptosomes and recording ACh-elicited currents in DA neurons fromalpha4, alpha6, alpha4alpha6, and beta2 knock-out mice. Our results establish that alpha6beta2* nAChRs are functional and sensitive to alpha-conotoxin MII inhibition. These receptors are mainly located on DA terminals and consistently do not contribute to DA release induced by systemic nicotine administration, as evidenced by in vivo microdialysis. In contrast, (nonalpha6)alpha4beta2* nAChRs represent the majority of functional heteromeric nAChRs on DA neuronal soma. Thus, whereas a combination of alpha6beta2* and alpha4beta2* nAChRs may mediate the endogenous cholinergic modulation of DA release at the terminal level, somato-dendritic (nonalpha6)alpha4beta2* nAChRs most likely contribute to nicotine reinforcement.

Acetylcholine↗
PubMed2003

Solution conformation of alphaA-conotoxin EIVA, a potent neuromuscular nicotinic acetylcholine receptor antagonist from Conus ermineus.

We report the solution three-dimensional structure of an alphaA-conotoxin EIVA determined by nuclear magnetic resonance spectroscopy and restrained molecular dynamics. The alphaA-conotoxin EIVA consists of 30 amino acids representing the largest peptide among the alpha/alphaA-family conotoxins discovered so far and targets the neuromuscular nicotinic acetylcholine receptor with high affinity. alphaA-Conotoxin EIVA consists of three distinct structural domains. The first domain is mainly composed of the Cys3-Cys11-disulfide loop and is structurally ill-defined with a large backbone root mean square deviation of 1.91 A. The second domain formed by residues His12-Hyp21 is extremely well defined with a backbone root mean square deviation of 0.52 A, thus forming a sturdy stem for the entire molecule. The third C-terminal domain formed by residues Hyp22-Gly29 shows an intermediate structural order having a backbone root mean square deviation of 1.04 A. A structurally ill-defined N-terminal first loop domain connected to a rigid central molecular stem seems to be the general structural feature of the alphaA-conotoxin subfamily. A detailed structural comparison between alphaA-conotoxin EIVA and alphaA-conotoxin PIVA suggests that the higher receptor affinity of alphaA-conotoxin EIVA than alphaA-conotoxin PIVA might originate from different steric disposition and charge distribution in the second loop "handle" motif.

Amino Acid Sequence↗
PubMed2003

Characterization of nicotinic acetylcholine receptors in cultured arterial chemoreceptor cells of the cat.

Neurotransmitters appear to be involved in chemotransmission of the carotid body, a major arterial chemoreceptor. Substantial data indicate that acetylcholine (ACh) is an excitatory neurotransmitter in the carotid body, regulating the excitability of afferent nerve endings and glomus cells (putative chemoreceptor cells). In this study we characterized properties of nicotinic ACh receptors (nAChRs) in cultured cat glomus cells using immunocytochemistry and whole cell patch clamp techniques. Cultured glomus cells expressed immunoreactivity for alpha3, alpha4, and beta2 subunits of nAChRs. An application of ACh elicited inward current. Nicotinic AChRs of glomus cells showed high affinity for ACh. The current-voltage relationship showed strong inward rectification at positive membrane potential. alpha-Conotoxin MII (20 nM), dihydro-beta-erythroidine (DHbetaE; 1 nM), and hexamethonium (300 microM) significantly inhibited ACh-induced current. These results indicate that cultured cat glomus cells possess functional nAChRs, and that their characteristics are consistent with those of alpha3, alpha4 and beta2 containing nAChRs.

Acetylcholine↗
PubMed2003

Identification of residues that confer alpha-conotoxin-PnIA sensitivity on the alpha 3 subunit of neuronal nicotinic acetylcholine receptors.

Neuronal nicotinic receptors composed of the alpha3 and beta2 subunits are at least 1000-fold more sensitive to blockade by alpha-conotoxin-PnIA than are alpha2beta2 receptors. A series of chimeric subunits, formed from portions of alpha2 and alpha3, were coexpressed with beta2 in Xenopus oocytes and tested for toxin sensitivity. We found determinants of toxin sensitivity to be widely distributed in the extracellular domain of alpha3. Analysis of receptors formed by a series of mutant alpha3 subunits, in which residues that differ between alpha3 and alpha2 were changed from what occurs in alpha3 to what occurs in alpha2, allowed identification of three determinants of alpha-conotoxin-PnIA sensitivity: proline 182, isoleucine 188, and glutamine 198. Comparison with determinants of alpha-conotoxin-MII and kappa-bungarotoxin sensitivity on the alpha3 subunit revealed overlapping, but distinct, arrays of determinants for each of these three toxins. When tested against an EC50 concentration of acetylcholine, the IC50 for alpha-conotoxin-PnIA blockade was 25 +/- 4 nM for alpha3beta2, 84 +/- 7 nM for alpha3P182Tbeta2, 700 +/- 92 nM for alpha3I188Kbeta2, and 870 +/- 61 nM for alpha3Q198Pbeta2. To examine the location of these residues within the receptor structure, we generated a homology model of the alpha3beta2 receptor extracellular domain using the structure of the acetylcholine binding protein as a template. All three residues are located on the C-loop of the alpha3 subunit, with isoleucine 188 nearest the acetylcholine-binding pocket.

Animals↗
PubMed2003

Relating neuronal nicotinic acetylcholine receptor subtypes defined by subunit composition and channel function.

Neuronal nicotinic acetylcholine receptors (nAChRs) are widespread, diverse ion channels involved in synaptic signaling, addiction, and disease. Despite their importance, the relationship between native nAChR subunit composition and function remains poorly defined. Chick ciliary ganglion neurons express two major nAChR types: those recognized by alpha-bungarotoxin (alphaBgt), nearly all of which contain only alpha7 subunits (alpha7-nAChRs) and those insensitive to alphaBgt, which contain alpha3, alpha5, beta4, and, in some cases, beta2 subunits (alpha3*-nAChRs). We explored the relationship between nAChR composition and channel function using toxins recognizing alpha7 subunits (alphaBgt), and alpha3/beta4 (alpha-conotoxin-AuIB), or alpha3/beta2 (alpha-conotoxin-MII) subunit interfaces to perturb responses induced by nicotine, alpha7-, or alpha3-selective agonists (GTS-21 or epibatidine, respectively). Using these reagents, fast-decaying whole-cell current components were attributed solely to alpha7-nAChRs, and slow-decaying components mostly to alpha3*-nAChRs. In outside-out patches, nicotine activated brief 60- and 80-pS single nAChR channel events, and mixed-duration 25- and 40-pS nAChR events. Subsequently, 60- and 80-pS nAChR events and most brief 25- and 40-pS events were attributed to alpha7-nAChRs, and long 25- and 40-pS events to alpha3*-nAChRs. alpha3*-nAChRs lacking beta2 subunits seemed responsible for long 25 pS nAChR events, whereas those containing beta2 subunits mediated the long 40 pS nAChR events that dominate single-channel records. These results reveal greater functional heterogeneity for alpha7-nAChRs than previously expected and indicate that beta2 subunits contribute importantly to alpha3*-nAChR function. By linking structural to functional nAChR subtypes, the findings also illustrate a useful pharmacological strategy for selectively targeting nAChRs.

Animals↗
PubMed2003

Differential declines in striatal nicotinic receptor subtype function after nigrostriatal damage in mice.

Nigrostriatal damage leads to a reduction in striatal nicotinic acetylcholine receptors (nAChRs) in rodents, monkeys, and patients with Parkinson's disease. The present studies were undertaken to investigate whether these nAChR declines are associated with alterations in striatal nAChR function and, if so, to identify the receptor subtypes involved. To induce nigrostriatal damage, mice were injected with the selective dopaminergic toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We measured [(125)I]3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid isopropyl ester (RTI-121, dopamine transporter), (125)I-alpha-conotoxin MII (putative alpha 6-containing sites in the central nervous system), (125)I-epibatidine (multiple sites), 5-[(125)I]iodo-3-[2(S)-azetidinylmethoxy]pyridine-2HCl ([(125)I]A85380; beta2-containing sites), and (125)I-alpha-bungarotoxin (alpha 7-containing sites) binding in brains from control and MPTP-treated mice, as well as nAChR function by [(3)H]dopamine release, [(3)H]GABA release, and [(86)Rb(+)] efflux. After MPTP treatment, declines were observed in striatal dopamine transporter levels, both binding and functional measures of striatal alpha-conotoxin MII-sensitive nAChRs, and selected measures of striatal alpha-conotoxin MII-resistant nAChRs. In contrast, (125)I-alpha-bungarotoxin binding sites were not altered after nigrostriatal damage. The changes in striatal nAChRs were selective, with no declines in cortex, thalamus, or septum. Those striatal binding and functional measures of nAChRs that decreased with MPTP treatment correlated with dopamine transporter declines, an observation suggesting that the binding and functional changes in nAChRs are limited to dopaminergic terminals. The present results are the first to demonstrate differential alterations in nAChR subtype function after nigrostriatal damage, with a close correspondence between changes in receptor binding sites and function. These data suggest that the declines in nAChR sites observed in Parkinson's disease brains may be of functional significance.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed2003

L-DOPA treatment modulates nicotinic receptors in monkey striatum.

Nicotinic acetylcholine receptor (nAChR) activation is well known to stimulate dopamine release in the striatum. This phenomenon may be physiologically significant in the control of motor function, as well as in pathological conditions such as Parkinson's disease. An understanding of the mechanisms that influence nAChR expression and function is therefore important. Because the dopamine precursor l-DOPA is the most commonly used therapeutic agent for Parkinson's disease, we investigated the effects of l-DOPA treatment on striatal nAChR expression in unlesioned and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys. In unlesioned animals, l-DOPA (15 mg/kg) administered twice daily for 2 weeks decreased both 125I-epibatidine and [125I]iodo-3-[2(S)-azetidinylmethoxy]pyridine (A-85380) binding sites in the caudate and putamen, but did not affect 125I-alpha-CtxMII sites. alpha-CtxMII inhibition of striatal 125I-epibatidine and [125I]A-85380 binding with alpha-CtxMII suggest that there are both high- (Ki < 0.2 nM) and low-affinity (Ki > 100 nM) alpha-CtxMII-sensitive sites, as well as alpha-CtxMII-resistant sites, and that l-DOPA treatment influences only the low-affinity alpha-CtxMII-sensitive subtype. The l-DOPA effect was selective for striatal nAChRs with no change in cortical sites. Monkeys with severe nigrostriatal damage did not exhibit l-DOPA-induced declines in striatal nAChRs, suggesting that l-DOPA primarily affects nAChRs associated with dopaminergic terminals. In summary, these data show that l-DOPA treatment decreases nAChR expression, in contrast with the well established up-regulation of these sites by chronic nicotine exposure. Furthermore, they demonstrate preferential l-DOPA regulation of a novel low-affinity alpha-CtxMII-sensitive site. These declines in nAChRs with l-DOPA may be relevant to both the therapeutic and side effect profiles of l-DOPA therapy in Parkinson's disease.

Animals↗
PubMed2002

Alpha-conotoxins ImI and ImII. Similar alpha 7 nicotinic receptor antagonists act at different sites.

A novel conotoxin, alpha-conotoxin ImII (alpha-CTx ImII), identified from Conus imperialis venom ducts, was chemically synthesized. A previously characterized C. imperialis conotoxin, alpha-conotoxin ImI (alpha-CTx ImI), is closely related; 9 of 12 amino acids are identical. Both alpha-CTx ImII and alpha-CTx ImI functionally inhibit heterologously expressed rat alpha7 nAChRs with similar IC(50) values. Furthermore, the biological activities of intracranially applied alpha-CTx ImI and alpha-CTx ImII are similar over the same dosage range, and are consistent with alpha7 nAChR inhibition. However, unlike alpha-CTx ImI, alpha-CTx ImII was not able to block the binding of alpha-bungarotoxin to alpha7 nAChRs. alpha-Conotoxin ImI and alpha-bungarotoxin-binding sites have been well characterized as overlapping and located at the cleft between adjacent nAChR subunits. Because alpha-CTx ImI and alpha-CTx ImII share extensive sequence homology, the inability of alpha-CTx ImII to compete with alpha-BgTx is surprising. Furthermore, functional studies in oocytes indicate that there is no overlap between functional binding sites of alpha-CTx ImI and alpha-CTx ImII. Like alpha-CTx ImI, the block by alpha-CTx ImII is voltage-independent. Thus, alpha-CTx ImII represents a probe for a novel antagonist binding site, or microsite, on the alpha7 nAChR.

Amino Acid Sequence↗
PubMed2002

Identification of the nicotinic receptor subtypes expressed on dopaminergic terminals in the rat striatum.

Neuronal nicotinic acetylcholine receptors (nAChRs) expressed on mesostriatal dopaminergic neurons are thought to mediate several behavioral effects of nicotine, including locomotion, habit learning, and reinforcement. Using immunoprecipitation and ligand-binding techniques, we have shown that both alpha6beta2* and alpha4(nonalpha6)beta2* nAChRs are expressed in the caudate-putamen and that only alpha6* nAChRs can bind alpha-conotoxin MII and methyllycaconitine with affinities of 1.3 and 40 nm, respectively. Further studies performed on 6-hydroxydopamine-lesioned striatum led to the identification of nAChR subtypes selectively expressed on dopaminergic terminals [alpha4alpha5beta2, alpha4alpha6beta2(beta3), and alpha6beta2(beta3)], nondopaminergic neuronal structures (alpha2alpha4beta2), or both structures (alpha4beta2). The identification of the nAChRs expressed on striatal dopaminergic terminals opens up the possibility of developing selective nAChR ligands active on dopaminergic systems and associated diseases, such as Parkinson's disease.

Animals↗
PubMed2002

Alpha-conotoxin GIC from Conus geographus, a novel peptide antagonist of nicotinic acetylcholine receptors.

Many venomous organisms produce toxins that disrupt neuromuscular communication to paralyze their prey. One common class of such toxins comprises nicotinic acetylcholine receptor antagonists (nAChRs). Thus, most toxins that act on nAChRs are targeted to the neuromuscular subtype. The toxin characterized in this report, alpha-conotoxin GIC, is a most striking exception. The 16-amino acid peptide was identified from a genomic DNA clone from Conus geographus. The predicted mature toxin was synthesized, and synthetic toxin was used in all studies described. alpha-Conotoxin GIC shows no paralytic activity in fish or mice. Furthermore, even at concentrations up to 100 microm, the peptide has no detectable effect on the human muscle nicotinic receptor subtype heterologously expressed in Xenopus oocytes. In contrast, the toxin has high affinity (IC(50) approximately 1.1 nm) for the human alpha3beta2 subunit combination, making it the most neuronally selective nicotinic antagonist characterized thus far. Although alpha-conotoxin GIC shares some sequence similarity with alpha-conotoxin MII, which is also a potent alpha3beta2 nicotinic antagonist, it is much less hydrophobic, and the kinetics of channel block are substantially different. It is noteworthy that the nicotinic ligands in C. geographus venom fit an emerging pattern in venomous predators, with one nicotinic antagonist targeted to the muscle subtype (thereby causing paralysis) and a second nicotinic antagonist targeted to the alpha3beta2 nAChR subtype (possibly inhibiting the fight-or-flight response).

Amino Acid Sequence↗
PubMed2002

Involvement of the alpha3 subunit in central nicotinic binding populations.

The alpha3 subunit gene was one of the first neuronal nicotinic acetylcholine receptor (nAChR) subunits to be cloned (Boulter et al., 1986), but direct evidence of alpha3 subunit contributions to mammalian central nAChR populations has not been presented. The studies reported here used mice engineered to contain a null mutation in the alpha3 nAChR subunit gene (Xu et al., 1999) to examine the involvement of the alpha3 subunit in central nAChR populations. Heterologously expressed alpha3beta2 and alpha3beta4 nAChRs are pharmacologically similar to native [125I]alpha-conotoxin MII (alpha-CtxMII)-binding and 3-(2(S)-azetidinylmethoxy)pyridine dihydrochloride (A85380)-resistant [125I]epibatidine-binding nAChR subtypes, respectively. The hypothesis that both native sites are alpha3-subtype nAChRs was tested using quantitative autoradiography in alpha3-null mutant mice. Somewhat surprisingly, deletion of the alpha3 nAChR subunit gene did not affect expression of the great majority of [125I]alpha-CtxMII-binding sites, indicating that they do not correspond to heterologously expressed alpha3beta2 nAChRs. The only exception to this was observed in the habenulointerpeduncular tract, where alpha3-dependent [125I]alpha-CtxMII binding was observed. This finding may suggest the presence of an additional, minor nicotinic population in this pathway. In contrast, most -resistant [125I]epibatidine-binding nAChRs were dependent on alpha3 gene expression, suggesting that they do indeed correspond to an alpha3 nAChR subtype. However, widespread but lower levels of alpha3-independent -resistant [125I]epibatidine binding were also seen. Again, this may indicate the existence of an additional, minor population of non-alpha3 -resistant sites.

Animals↗
PubMed2002

Distribution and pharmacology of alpha 6-containing nicotinic acetylcholine receptors analyzed with mutant mice.

The alpha6 subunit of the nicotinic acetylcholine receptor (nAChR) is expressed at very high levels in dopaminergic (DA) neurons. However, because of the lack of pharmacological tools selective for alpha6-containing nAChRs, the role of this subunit in the etiology of nicotine addiction remains unknown. To provide new tools to investigate this issue, we generated an alpha6 nAChR knock-out mouse. Homozygous null mutants (alpha6-/-) did not exhibit any gross neurological or behavioral deficits. A careful anatomic and molecular examination of alpha6-/- mouse brains demonstrated the absence of developmental alterations in these animals, especially in the visual and dopaminergic pathways, where the alpha6 subunit is normally expressed at the highest levels. On the other hand, receptor autoradiography revealed a decrease in [3H]nicotine, [3H]epibatidine, and [3H]cytisine high-affinity binding in the terminal fields of retinal ganglion cells of alpha6-/- animals, whereas high-affinity [125I]alpha-conotoxinMII (alphaCtxMII) binding completely disappeared in the brain. Moreover, inhibition of [3H]epibatidine binding on striatal membranes, using unlabeled alphaCtxMII or cytisine, revealed the absence of alphaCtxMII-sensitive and cytisine-resistant [3H]epibatidine binding sites in alpha6-/- mice, although the total amount of binding was unchanged. Because alphaCtxMII, a toxin formerly thought to be specific for alpha3beta2-containing nAChRs, is known to partially inhibit nicotine-induced dopamine release, these results support the conclusion that alpha6 rather than alpha3 is the partner of beta2 in the nicotinic modulation of DA neurons. They further show that alpha6-/- mice might be useful tools to understand the mechanisms of nicotine addiction, although some developmental compensation might occur in these mice.

Alkaloids↗
PubMed2002

5-Iodo-A-85380 binds to alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors (nAChRs) as well as alpha4beta2* subtypes.

Recent work suggests that 5-iodo-A-85380, a radioiodinated analog of the 3-pyridyl ether A-85380, represents a promising imaging agent for non-invasive, in vivo studies of alphaAbeta2* nicotinic acetylcholine receptors (nAChRs; *denotes receptors containing the indicated subunits), because of its low non-specific binding, low in vivo toxicity and high selectivity for alpha4beta2* nAChRs. As an approach to elucidate nAChR subtypes expressed in striatum, we carried out competitive autoradiography in monkey and rat brain using 5-[125I]iodo-A-85380 ([125I]A-85380) and [125I]alpha-conotoxin MII, a ligand that binds with high affinity to alpha6* and alpha3* nAChRs, but not to alpha4beta2* nAChRs. Although A-85380 is reported to be selective for alpha4beta2* nAChRs, we observed that A-85380 completely inhibited [125I]alpha-conotoxin MII binding in rat striatum and that A-85380 blocked >90% of [125I] alpha-conotoxin MII sites in monkey caudate and putamen. These results suggest that A-85380 binds to non-alpha4beta2* nAChRs, including putative alpha6* nAChRs. Experiments to determine the percentage of [125I]A-85380 sites that contain alpha-conotoxin MII-sensitive (alpha6beta2*) nAChRs indicate that they represent about 10% of [125I]A-85380 sites in rodent striatum and about 30% of sites in monkey caudate and putamen. These data are important for identifying alterations in nicotinic receptor subtypes in Parkinson's disease and other basal ganglia disorders both in in vitro and in in vivo imaging studies.

Animals↗
PubMed2002

Characterization of [(125) I]epibatidine binding and nicotinic agonist-mediated (86) Rb(+) efflux in interpeduncular nucleus and inferior colliculus of beta2 null mutant mice.

The beta2 nicotinic acetylcholine receptor subunit null mutation eliminated most high affinity [(3) H]epibatidine binding in mouse brain, but significant binding remained in accessory olfactory nucleus, medial habenula, inferior colliculus and interpeduncular nucleus. Residual [(125) I]epibatidine binding sites in the inferior colliculus and interpeduncular nucleus were subsequently characterized. Inhibition of [(125) I]epibatidine binding by 12 agonists and six antagonists was very similar in these regions. Most acetylcholine-stimulated (86) Rb(+) efflux is eliminated in thalamus and superior colliculus of beta2 null mutants, but significant activity remained in inferior colliculus and interpeduncular nucleus. This residual activity was subsequently characterized. The 12 nicotinic agonists tested elicited concentration-dependent (86) Rb(+) efflux. Epibatidine was the most potent agonist. Cytisine was also potent and efficacious. EC(50) values for quaternary agonists were relatively high. Cytisine-stimulated (86) Rb(+) efflux was inhibited by six classical nicotinic antagonists. Mecamylamine and D-tubocurarine were most potent, while decamethonium was the least potent. Agonists and antagonists exhibited similar potency in both brain regions. Alpha-bungarotoxin (100 nm) did not significantly inhibit cytisine-stimulated (86) Rb(+) efflux, while the alpha3beta4 selective antagonist, alphaConotoxinAuIB, inhibited a significant fraction of the response in both brain regions. Thus, beta2 null mutant mice express residual nicotinic activity with properties resembling those of alpha3beta4*-nAChR.

Acetylcholine↗
PubMed2002

Declines in different beta2* nicotinic receptor populations in monkey striatum after nigrostriatal damage.

In the present study we used the nicotinic ligand 5-iodo-A-85380 [5-iodo-3(2(S)-azetidinylmethoxy)pyridine], which selectively binds to beta2-containing nicotinic acetylcholine receptors, to elucidate the nicotinic receptor subtypes affected by nigrostriatal damage in the monkey. Autoradiographic studies in control monkeys showed that 5-[(125)I]A-85380 ([(125)I]A-85380) binds throughout the brain with the characteristics of a nicotinic receptor ligand. Competition experiments with cytisine and nicotine yielded K(i) values of approximately 1 and 10 nM, respectively, with complete inhibition of [(125)I]A-85380 binding at a 10(-6) M concentration of these ligands. In contrast, alpha-conotoxin MII blocked radioligand binding in the striatum by 30% at the highest concentrations, suggesting that a subset of striatal [(125)I]A-85380 sites are alpha-conotoxin MII-sensitive. Monkeys treated with the nigrostriatal neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine showed a selective decrease in striatal [(125)I]A-85380 sites, with a 42% reduction in the caudate and putamen of animals with moderate nigrostriatal lesioning and a 53% decline in the striatum of severely lesioned animals. Our previous work had demonstrated that there were two populations of nicotinic receptors eliminated after nigrostriatal damage, an alpha-conotoxin MII-sensitive and an alpha-conotoxin MII- resistant subtype. Analysis of both striatal [(125)I]A-85380 and [(125)I]epibatidine competition studies extend our earlier studies by demonstrating that the alpha-conotoxin MII-sensitive sites eliminated after moderate nigrostriatal lesioning appear to be composed of two nicotinic receptor subtypes. The data may be important for potential therapeutic approaches because they suggest that there are at least three populations of nicotinic receptors in monkey striatum, of which two are selectively vulnerable to nigrostriatal damage, while the third is more resistant.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗