A functional α-bungarotoxin receptor is present in chick cerebellum: Purification and characterization

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

Disturbed neuronal ER-Golgi sorting of unassembled glycine receptors suggests altered subcellular processing is a cause of human hyperekplexia

Recent studies on the pathogenic mechanisms of recessive hyperekplexia indicate disturbances in glycine receptor (GlyR) α1 biogenesis. Here, we examine the properties of a range of novel glycine receptor mutants identified in human hyperekplexia patients using expression in transfected cell lines and primary neurons. All of the novel mutants localized in the large extracellular domain of the GlyR α1 have reduced cell surface expression with a high proportion of receptors being retained in the ER, although there is forward trafficking of glycosylated subpopulations into the ER-Golgi intermediate compartment and cis-Golgi compartment. CD spectroscopy revealed that the mutant receptors have proportions of secondary structural elements similar to wild-type receptors. Two mutants in loop B (G160R, T162M) were functional, but none of those in loop D/β2-3 were. One nonfunctional truncated mutant (R316X) could be rescued by coexpression with the lacking C-terminal domain. We conclude that a proportion of GlyR α1 mutants can be transported to the plasma membrane but do not necessarily form functional ion channels. We suggest that loop D/β2-3 is an important determinant for GlyR trafficking and functionality, whereas alterations to loop B alter agonist potencies, indicating that residues here are critical elements in ligand binding.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

Assembly and trafficking of nicotinic acetylcholine receptors (Review)

Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.

Identification of an alternatively processed nicotinic receptor alpha7 subunit RNA in mouse brain

The data in this report describe the discovery and characterization of a previously unidentified alternatively processed RNA for the neuronal nicotinic receptor alpha7 subunit. The unique transcript contains an extra exon that arises from alternative splicing of intron nine of the alpha7 subunit RNA. The alpha7 subunit protein resulting from this alternatively processed RNA is truncated shortly after transmembrane domain three. The variant protein also has a predicted amino acid substitution in the large N-terminal domain as a consequence of a non-templated nucleotide substitution present in the variant alpha7 subunit RNA. The mechanism responsible for the nucleotide substitution is not known. Initial characterization of the variant alpha7 subunit suggests that it is expressed in mouse brain in a pattern similar to the standard alpha7 subunit although at reduced levels. The variant alpha7 subunit was also found to act as a dominant-negative effecter of normal alpha7 subunit function.

Choline blocks AMPA-induced dark cell degeneration of Purkinje neurons: potential role of the α7 nicotinic receptor

The objective of the present study was to assess the contribution of sodium influx to development of dark cell degeneration (DCD) in Purkinje neurons (PNs) following AMPA (DL-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid) receptor activation. During the course of these experiments, we observed inconsistent protection against DCD by Na(+) ion substitutes leading us to consider other potential mechanisms. A 30 min application of AMPA (30 microM, induction or trigger phase) followed by a 90-120 min AMPA-free expression period produced DCD in the majority of PNs. Substitution of NaCl with choline chloride (120 mM) produced a marked suppression of AMPA-induced toxicity. Suppression of DCD by choline was concentration dependent. Concentrations of choline as low as 10 mM effectively attenuated DCD when substituted on an equimolar basis for NaCl in the artificial cerebrospinal fluid (ACSF). Unlike choline, substitution of NMDG for NaCl failed to suppress AMPA-induced DCD. Lidocaine and TTX (tetrodotoxin), two agents that inhibit Na(+) influx failed to significantly alter DCD. Because choline is a prototypical alpha7 nicotinic receptor selective agonist, methyllycaconitine (MLA), an alpha7 receptor antagonist was tested and significantly attenuated the protective effects of choline in a concentration-dependent manner. Nicotine (100 microM) added to normal ACSF was effective in attenuating AMPA-induced toxicity. These findings suggest that DCD is not heavily dependent on Na(+)-mediated phenomena and that nicotinic alpha7 receptor activation may be neuroprotective against some types of excitotoxicity that are mediated by active cellular programs.

Two distinct classes of functional 7-containing nicotinic receptor on rat superior cervical ganglion neurons

Nicotinic acetylcholine receptors (nAChRs) that bind alpha-bungarotoxin (alpha Bgt) were studied on isolated rat superior cervical ganglion (SCG) neurons using whole-cell patch clamp recording techniques. Rapid application of ACh onto the soma of voltage clamped neurons evoked a slowly desensitizing current that was reversibly blocked by alpha Bgt (50 nM). The toxin-sensitive current constituted on average about half of the peak whole-cell response evoked by ACh. Nanomolar concentrations of methyllycaconitine blocked the alpha Bgt-sensitive component of the ACh-evoked current as did intracellular dialysis with an anti-alpha 7 monoclonal antibody. The results indicate that the slowly reversible toxin-sensitive response elicited by ACh arises from activation of an unusual class of alpha 7-containing receptor (alpha 7-nAChR) similar to that reported previously for rat intracardiac ganglion neurons. A second class of functional alpha 7-nAChR was identified on some SCG neurons by using rapid application of choline to elicit responses. In these cases a biphasic response was obtained, which included a rapidly desensitizing component that was blocked by alpha Bgt in a pseudo-irreversible manner. The pharmacology and kinetics of the responses resembled those previously attributed to alpha 7-nAChRs in a number of other neuronal cell types. Experiments measuring the dissociation rate of 125I-labelled alpha Bgt from SCG neurons revealed two classes of toxin-binding site. The times for toxin dissociation were consistent with those required to reverse blockade of the two kinds of alpha Bgt-sensitive response. These results indicate that rat SCG neurons express two types of functional alpha 7-nAChR, differing in pharmacology, desensitization and reversibility of alpha Bgt blockade.

Neuronal alpha-bungarotoxin receptors are alpha7 subunit homomers

Nicotinic acetylcholine receptors in the nervous system are heterogeneous with distinct pharmacological and functional properties resulting from differences in post-translational processing and subunit composition. Because of nicotinic receptor diversity, receptor purification and biochemical characterization have been difficult, and the precise subunit composition of each receptor subtype is poorly characterized. Evidence is presented that alpha-bungarotoxin (Bgt)-binding nicotinic receptors found in pheochromocytoma 12 (PC12) cells are pentamers composed solely of alpha7 subunits. Metabolically labeled, affinity-purified Bgt receptors (BgtRs) consisted of a single 55 kDa band on SDS gels, which was recognized by anti-alpha7 antibodies on immunoblots. Isoelectric focusing separated the 55 kDa band into multiple spots, all recognized by anti-alpha7 antibodies and, therefore, each a differentially processed alpha7 subunit. Cell-surface BgtR subunits, cross-linked to each other and (125)I-Bgt, migrated on gels as a ladder of five bands with each band a multiple of an alpha7 subunit monomer. Similar characteristics of BgtRs from rat brain suggest that they, like PC12 BgtRs, are alpha7 pentamers containing differentially processed alpha7 subunits.

Characterisation of the binding of [3H]methyllycaconitine: a new radioligand for labelling alpha 7-type neuronal nicotinic acetylcholine receptors

Methyllycaconitine (MLA), a norditerpenoid alkaloid isolated from Delphinium seeds, is one of the most potent non-proteinacious ligands that is selective for alpha bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors (nAChR). [3H]MLA bound to rat brain membranes with high affinity (Kd = 1.86 +/- 0.31 nM) with a good ratio of specific to non-specific binding. The binding of [3H]MLA was characterised by rapid association (t 1/2 = 2.3 min) and dissociation (t 1/2 = 12.6 min) kinetics. The radioligand binding displayed nicotinic pharmacology, consistent with an interaction with alpha bungarotoxin-sensitive nAChR. The snake alpha-toxins, alpha bungarotoxin and alpha cobratoxin, displaced [3H]MLA with high affinity (Ki = 1.8 +/- 0.5 and 5.5 +/- 0.9 nM, respectively), whereas nicotine was less potent (Ki = 6.1 +/- 1.1 microM). The distribution of [3H]MLA binding sites in crudely dissected rat brain regions was identical to that of [125I] alpha bungarotoxin binding sites, with a high binding site density in hippocampus and hypothalamus, but low density in striatum and cerebellum. [3H]MLA also labelled a sub-population of binding sites which are not sensitive to the snake alpha toxins, but which did not differ significantly from the major population with respect to their other pharmacological properties or regional distribution. [3H]MLA, therefore, is a novel radiolabel for characterising alpha 7-type nAChR. A good signal to noise ratio and rapid binding kinetics provide advantages over the use of radiolabelled alpha bungarotoxin for rapid and accurate equilibrium binding assays.

Nicotinic antagonist administration into the ventral hippocampus and spatial working memory in rats

Nicotinic acetylcholine receptors are important for maintaining optimal memory performance. In order to more fully characterize the involvement of nicotinic systems in memory, the contributions of nicotinic acetylcholine receptor subtypes were investigated. This study targeted the alpha 7 and alpha 4 beta 2 nicotinic receptors in the ventral hippocampus, an area known to be important for spatial working memory. Antagonists of alpha 7 and alpha 4 beta 2 receptors were locally infused into the ventral hippocampus of rats and the effects on memory were examined with the radial-arm maze. The subtype-specific competitive antagonists infused into separate groups of rats were methyllycaconitine citrate (an alpha 7 antagonist) and dihydro-beta-erythroidine hydrobromide (an alpha 4 beta 2 antagonist). Their effects on radial-arm maze performance were contrasted with the non-specific competitive antagonist, D-tubocurarine chloride. Significant deficits in radial-arm maze choice accuracy performance were found at 78.7 micrograms/side for methyllycaconitine and at 106.9 micrograms/side for dihydro-beta-erythroidine. Increased response latency was also seen at these doses. Tubocurarine induced seizures at doses previously reported to have no effect. Wet dog shakes were seen in most rats at 0.1 microgram/side with tubocurarine, 26.3 micrograms/side with methyllycaconitine and 106.9 micrograms/side with dihydro-beta-erythroidine. This study suggests that both alpha 7 and alpha 4 beta 2 nicotinic acetylcholine receptor subtypes are involved in working memory formation and that the hippocampus is a critical site for nicotinic cholinergic involvement in memory function, though the high doses of antagonists needed to produce the memory impairment may have had less than completely specific effects.

Neuronal alpha-bungarotoxin receptors differ structurally from other nicotinic acetylcholine receptors

We have characterized the alpha-bungarotoxin receptors (BgtRs) found on the cell surface of undifferentiated pheochromocytoma (PC12) cells. The PC12 cells express a homogeneous population of alpha7-containing receptors that bind alpha-Bgt with high affinity (Kd = 94 pM). The BgtRs mediate most of the response elicited by nicotine, because the BgtR-specific antagonists methyllycaconitine and alpha-Bgt block approximately 90% of the whole-cell current. The binding of nicotinic agonists to cell-surface BgtRs was highly cooperative with four different agonists showing Hill coefficients in the range of 2.3-2.4. A similar agonist binding cooperativity was observed for BgtR homomers formed from chimeric alpha7/5HT3 subunits expressed in tsA 201 cells. Two classes of agonist binding sites, in the ratio of 4:1 for PC12 cell BgtRs and 3:1 for alpha7/5HT3 BgtRs, were revealed by bromoacetylcholine alkylation of the reduced sites on both PC12 BgtRs and alpha7/5HT3 BgtRs. We conclude from this data that PC12 BgtRs and alpha7/5HT3 homomers contain at least three distinguishable agonist binding sites and thus are different from other nicotinic receptors.

The alpha-bungarotoxin-binding nicotinic acetylcholine receptor from rat brain contains only the alpha7 subunit

When expressed in Xenopus oocytes, the rat alpha7 subunit forms homo-oligomeric nicotinic acetylcholine receptors, which are blocked by alpha-bungarotoxin. Since the pharmacological and physiological properties of the alpha7 receptor expressed in oocytes are similar to those of the alpha-bungarotoxin-sensitive nicotinic currents recorded from neuronal preparations and the distribution patterns of alpha7 mRNA and alpha-bungarotoxin-binding sites in the rat brain are very similar, alpha7 is thought to be the main component of the alpha-bungarotoxin-binding nicotinic receptor in the mammalian brain. However, while alpha7 is found in purified alpha-bungarotoxin-binding complexes from rat brain or PC12 cells, other proteins copurify with it. Therefore, the question whether alpha7 forms a homo-oligomeric alpha-bungarotoxin-binding nicotinic receptor in the mammalian brain remains. We have developed and characterized affinity-purified polyclonal antibodies and used these antibodies in Western blot analyses of alpha-bungarotoxin-binding proteins purified from rat brains. We report here that our experimental data support the current working hypothesis that the alpha-bungarotoxin-binding nicotinic receptor is a homo-oligomer of alpha7 subunits in the rat brain.

Expression of alpha 7 neuronal nicotinic receptors during postnatal development of the rate cerebellum

Several lines of evidence suggest that alpha-bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors may play a developmental role by modulating plasticity in neuronal circuits. The alpha 7 subunit, a main component of these receptors, is expressed in most regions of the brain, including the cerebellum, where it is present almost exclusively in Purkinje cells and deep cerebellar nuclei. Purkinje cells constitute the only efferent pathway of the cerebellum and their development involves complex interactions, which have been extensively studied. They therefore provide a potentially useful model for analysis of development plasticity which could be influenced by alpha 7 neuronal nicotinic receptors. In the present study a previously characterized monoclonal antibody (mAb 307) has been used to determine the temporal pattern of expression of the alpha 7 subunit in the developing rat cerebellum. No detectable alpha 7 immunoreactivity is found between P0 and P2. Between P3 and P5, however, the Purkinje cell layer shows moderate immunolabeling. alpha 7 expression in this layer increases rapidly between P8 and P15. This increase in alpha 7 staining, which overlaps in time with important developmental and synaptogenic events, is not uniform throughout the cerebellar cortex. Thus, between P3 and P5 all Purkinje cells are weakly labeled, while at later stages (P8-P15) immunolabeling becomes more intense, but at the same time, disappears from Purkinje cells in rostral lobules. In addition, a very well defined pattern for discontinuous or columnar labeling is detected in regions of the Purkinje cell layer where alpha 7 subunits were being expressed. Finally, at P20, alpha 7 subunit labeling is found again in all Purkinje cells, although with lower intensity. These results suggest that alpha 7 receptor expression is developmentally regulated, with a time course that parallels the final differentiation of Purkinje cells. In addition, the heterogeneous spatial distribution of alpha 7-containing nicotinic receptors indicates that, during cerebellar maturation, these cells may receive different signals that modulate receptor gene expression in a very specific way.

alpha-Bungarotoxin-sensitive nicotinic receptors on bovine chromaffin cells: molecular cloning, functional expression and alternative splicing of the alpha 7 subunit

Chromaffin cells from the bovine adrenal medulla express alpha-bungarotoxin-sensitive acetylcholine receptors whose subunit composition is unknown. Northern blot analysis showed that the alpha 7 subunit, a main component of these alpha-bungarotoxin-sensitive acetylcholine receptors in avian and rat brain, is expressed in chromaffin cells. The cDNA of this bovine alpha 7 subunit was cloned by polymerase chain reaction amplification of adrenal medulla RNA for detailed characterization of structure and function. The protein-coding region revealed 92% amino acid sequence identity to rat alpha 7 and 89% to chicken alpha 7 subunits. The alpha-bungarotoxin affinity of alpha 7 homomers expressed in Xenopus oocytes was similar to that observed previously with native chromaffin alpha-bungarotoxin-sensitive acetylcholine receptors. Cross-linking and sucrose gradient experiments suggested that, like the muscular and neuronal acetylcholine receptors; the alpha 7 receptor has a pentameric structure. Upon activation with nicotinic agonists the alpha 7 receptor exhibited rapidly desensitizing cation currents that were blocked by nicotinic antagonists and showed inward rectification. The amplification of adrenal medulla RNA by reverse transcription-polymerase chain reaction methods revealed an alternatively spliced isoform of the bovine alpha 7 subunit, where the exon that codes for the M2 transmembrane segment was skipped during mRNA processing. Oocyte expression of this isoform does not yield functional channels. However, this alternative mRNA exhibits dose-dependent inhibition of alpha 7 homomer expression when coinjected with the undeleted isoform.

Expression of alpha-bungarotoxin receptor subtypes in chick central nervous system during development

Chick central nervous system (CNS) expresses alpha-bungarotoxin (alpha Bgtx) receptors. We have recently reported the purification and characterization of two alpha Bgtx receptor subtypes, alpha 7 and alpha 7-alpha 8 from chick optic lobe (COL). In order to study whether other alpha Bgtx receptor subtypes are present in other areas of the chick CNS, as well as their developmental expression, we used anti-alpha 7 and anti-alpha 8 subunit-specific antibodies to study alpha Bgtx receptors at different developmental stages in COL, brain and retina. We found that only the alpha 7 and alpha 7-alpha 8 subtypes are present at all developmental stages in chick COL and brain, where they represent 90% of all the alpha Bgtx receptors at embryonic day 19 and 1 day post hatching (D1). In chick retina, an alpha 8 subtype representing 50% of all alpha Bgtx receptors at D1 is present in addition to the alpha 7 and alpha 7-alpha 8 subtypes, and the expression of this alpha 8 subtype increases during neurodevelopment.

alpha-Bungarotoxin blocks the nicotinic receptor mediated increase in cell number in a neuroendocrine cell line

Exposure of H69 small cell lung carcinoma cells to nicotinic agonists resulted in a significant increase (up to 100%) in cell number after 6 to 12 days. The effect of nicotine (10(-8) M to 10(-4) M) was both dose and time dependent as was that of another nicotinic agonist cytisine (10(-6) M to 10(-4) M). Interestingly, both the nicotine and cytisine induced increases in H69 cell number were blocked by alpha-bungarotoxin, as well as d-tubocurarine a nicotinic blocker which appears to interact with most nicotinic receptors. These results suggest that the nicotine induced increase in cell number is mediated through an interaction at the nicotinic alpha-bungarotoxin receptor. This idea is further supported by experiments which show (1) that H69 cells possess high affinity alpha-bungarotoxin sites (Kd = 25 nM, Bmax = 10.4 fmol/10(6) cells) with the characteristics of a nicotinic alpha-bungarotoxin receptor and (2) that the potencies of nicotinic receptor ligands in the alpha-bungarotoxin binding assay were similar to those observed in the functional studies. Northern analysis showed that mRNA for alpha 7, a putative nicotinic alpha-bungarotoxin binding subunit, and for alpha 5 were present in H69 cells. The present data provide further evidence that nicotine increases cell number in small cell lung carcinoma and are the first to show that this effect is mediated through an interaction at the nicotinic alpha-bungarotoxin receptor population. These results suggest that the alpha-bungarotoxin site may be involved in modulating proliferative responses in neuroendocrine derived SCLC cells.

Pharmacology and biophysical properties of alpha 7 and alpha 7-alpha 8 alpha-bungarotoxin receptor subtypes immunopurified from the chick optic lobe

Two chick optic lobe alpha-bungarotoxin receptor subtypes (alpha 7 and alpha 7-alpha 8) were immunopurified using polyclonal antibodies raised against synthetic peptides of chick alpha 7 and alpha 8 alpha-bungarotoxin receptor subunits. The alpha 7 subtype contained the M(r) 57,000 alpha 7 subunit, and represented 60-70% of the alpha-bungarotoxin receptors; the alpha 7-alpha 8 subtype contained the M(r) 57,000 alpha 7 and alpha 8 subunits, and represented only 20-25% of the receptors. Both subtypes also had an additional M(r) 52,000 subunit. The affinity of these subtypes for alpha-bungarotoxin as well as antagonists was similar. However, the alpha 7-alpha 8 subtype displayed consistently higher affinities for agonists. When reconstituted in planar lipid bilayers, the alpha 7-alpha 8 subtype displayed several conductance states of 10-50 pS; the alpha 7 subtype had only one conductance state of 45 pS. The alpha 7-alpha 8 subtype was activated by lower agonist concentrations than the alpha 7 subtype. When expressed in Xenopus oocytes, the alpha 8 subunit formed functional homomeric receptors that desensitized rapidly. These channels were blocked by alpha-bungarotoxin and displayed a higher affinity for agonists than the alpha 7 homomeric receptor. Taken together, these data indicate that at least two alpha-bungarotoxin subtypes are present in the chick optic lobe. They operate as ligand-gated channels and display different agonist sensitivities and kinetics/conductance properties.

Neuronal acetylcholine receptors that bind alpha-bungarotoxin with high affinity function as ligand-gated ion channels

Neuronal membrane components that bind alpha-bungarotoxin with high affinity can increase intracellular levels of free calcium, demonstrating the components function as nicotinic receptors. Though such receptors often contain the alpha 7 gene product, which by itself can produce ionotropic receptors in Xenopus oocytes, numerous attempts have failed to demonstrate an ion channel function for the native receptors on neurons. Using rapid application of agonist, we show here that the native receptors are ligand-gated ion channels which are cation selective, prefer nicotine over acetylcholine, and rapidly desensitize. Much of the calcium increase caused in neurons by the receptors under physiological conditions appears to result from their depolarizing the membrane sufficiently to trigger calcium influx through voltage-gated channels.

A role for the nicotinic alpha-bungarotoxin receptor in neurite outgrowth in PC12 cells

The addition of nicotine decreased neuritic outgrowth in PC12 cells in culture. This effect occurs as early as one day after addition of nicotine to the culture medium in a concentration-dependent manner. The nicotine-induced decline in neurite outgrowth was prevented by d-tubocurarine (10(-4) M) indicating that the effect was mediated through a nicotinic receptor. alpha-Bungarotoxin (10(-8) M) was also able to inhibit the nicotine-induced decrease in process formation in a dose-dependent manner. The concentrations of alpha-bungarotoxin required to affect process outgrowth correlated with those required to inhibit radiolabelled alpha-bungarotoxin binding. alpha-Bungarotoxin had no effect on [3H]noradrenaline release, a functional response mediated through the alpha-bungarotoxin-insensitive neuronal nicotinic acetylcholine receptor, suggesting that alpha-bungarotoxin specifically interacts with the neuronal alpha-bungarotoxin receptor. The present results suggest a functional role for the neuronal nicotinic alpha-bungarotoxin receptor in neurite outgrowth.

Homomeric and native alpha 7 acetylcholine receptors exhibit remarkably similar but non-identical pharmacological properties, suggesting that the native receptor is a heteromeric protein complex

Sucrose gradient analysis of chick acetylcholine receptor (AChR) alpha 7 subunits expressed in oocytes indicates that they form pharmacologically active homomers of the same size as native alpha 7 AChRs, a size compatible with a complex of five alpha 7 subunits. By immunoisolating the [35S]methionine-labeled alpha 7 subunits we also demonstrate that they do not appear to assemble with endogenous Xenopus AChR subunits. Pharmacological characterization of detergent-solubilized brain alpha 7 AChRs and alpha 7 homomers reveals that they have similar but nonidentical properties. The pharmacological difference is most accentuated for cytisine (approximately 50-fold). Thus, at least in E18 chicken brain, most or all of the native alpha 7 AChRs do not appear to be homomeric.