Antagonism of nicotinic receptors of rat chromaffin cells by N,N, N-trimethyl-1-(4-trans-stilbenoxy)-2-propylammonium iodide: a patch clamp and ligand binding study

Pharmacological and immunological identification of native alpha7 nicotinic receptors: evidence for homomeric and heteromeric alpha7 receptors

Controversy surrounds the expression of alpha7 nicotinic acetylcholine receptors (nAChRs) in adrenal chromaffin cells. In these studies, alpha7 nAChRs expressed in bovine adrenal chromaffin cells are investigated. Using radiolabeled ligand binding techniques, [(125)I]alpha-bungarotoxin (alphaBGT) binding reaches equilibrium within 4 h and is saturable with a K(d) value of 4.2 nM. Using homologous competition experiments, the K(i) for binding of alphaBGT was 1.9 nM. These data are consistent with the expression of homomeric alpha7 nAChRs. Methyllycaconatine (MLA), which binds alpha7 nAChRs with high affinity, inhibits [(125)I]alphaBGT binding in a concentration-dependent manner with a K(i) of 30.6 nM; this value is approximately 10 fold higher than the reported affinity of MLA for alpha7 nAChRs. We also document the ability of bromoacetylcholine (brACh) to alkylate alpha7 nAChRs, as has been previous demonstrated for bovine adrenal alpha3beta4 nAChRs. When adrenal nAChRs are immunoprecipated with mAb319, an antibody which recognizes alpha7 nAChR protein, and then probed with mAb319 using Western blot analysis, a single band of approximately 53 kDa is identified. When adrenal nAChRs are immunoprecipated with mAb35, an antibody which recognizes alpha3 and alpha5 nAChR proteins, and then probed with mAb319 using Western blot analysis, a single band of approximately 53 kDa is identified. Together, these results support the expression of alpha7 nAChRs in bovine adrenal chromaffin cells. However, these data suggest that the subunit composition of some of these receptors may include heteromeric alpha7 nAChRs.

Analysis of specificity of antibodies against synthetic fragments of different neuronal nicotinic acetylcholine receptor subunits

We have compared specificity of a panel of polyclonal antibodies against synthetic fragments of the alpha7 subunit of homooligomeric acetylcholine receptor (AChR) and some subunits of heteromeric AChRs. The antibody interaction with extracellular domain of alpha7 subunit of rat AChR (residues 7-208) produced by heterologous expression in E. coli and rat adrenal membranes was investigated by the ELISA method. For comparison, membranes from the Torpedo californica ray electric organ enriched in muscle-type AChR and polyclonal antibodies raised against the extracellular domain (residues 1-209) of the T. californica AChR alpha1 subunit were also used. Antibody specificity was also characterized by Western blot analysis using rat AChR extracellular domain alpha7 (7-208) and the membrane-bound T. californica AChR. Epitope localization was analyzed within the framework of AChR extracellular domain model based on the crystal structure of acetylcholine-binding protein available in the literature. According to this analysis, the 179-190 epitope is located on loop C, which is exposed and mobile. Use of antibodies against alpha7 (179-190) revealed the presence of alpha7 AChR in rat adrenal membranes.

Nicotinic Acetylcholine Receptor-Mediated [3H]Dopamine Release from Hippocampus

The mechanism of nicotinic acetylcholine receptor (nAChR)-induced hippocampal dopamine (DA) release was investigated using rat hippocampal slices. nAChRs involved in hippocampal DA and norepinephrine (NE) release were investigated using prototypical agonists and antagonists and several relatively novel compounds: ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine], (+/-)-UB-165 [(2-chloro-5-pyridyl)-9-azabicyclo [4.2.1]non2-ene], and MG 624 [N,N,N-triethyl-2-[4-(2 phenylethenyl)phenoxy]-ethanaminium iodine]. (+/-)-Epibatidine, (+/-)-UB-165, anatoxin-a, ABT-594, (-)-nicotine, 1,1-dimethyl-4-phenyl-piperazinium iodide, and (-)-cytisine (in decreasing order of potency) evoked [(3)H]DA release in a mecamylamine-sensitive manner. Aside from (+/-)-UB-165, all the agonists displayed full efficacy relative to 100 microM (-)-nicotine in [(3)H]DA release. In contrast, (+/-)-UB-165 was a partial agonist, evoking 58% of 100 microM (-)-nicotine response. Mecamylamine, MG 624, hexamethonium, d-tubocurare, and dihydro-beta-erythroidine (in decreasing order of potency), but not alpha-conotoxin-MII, methyllycaconitine, alpha-conotoxin-ImI, or alpha-bungarotoxin, attenuated 100 microM (-)-nicotine-evoked [(3)H]DA release in a concentration-dependent manner. (+/-)-UB-165, ABT-594, and MG 624 exhibited different pharmacologic profiles in the [(3)H]NE release assay when compared with their effect on [(3)H]DA release. ABT-594 was 4.5-fold more potent, and (+/-)-UB-165 was a full agonist in contrast to its partial agonism in [(3)H]DA release. MG 624 potently and completely blocked NE release evoked by 100 microM (-)-nicotine and 10 microM (+/-)-UB-165, whereas it only partially inhibited (-)-nicotine-evoked [(3)H]DA release. In conclusion, we provide evidence that [(3)H]DA can be evoked from the hippocampus and that the pharmacologic profile for nAChR-evoked hippocampal [(3)H]DA release suggests the involvement of alpha3beta4(*) and at least one other nAChR subtype, thus distinguishing it from that of nAChR-evoked hippocampal [(3)H]NE release.

A structural model of agonist binding to the alpha3beta4 neuronal nicotinic receptor

(alpha3)2(beta4)3 is the most abundant type of neuronal nicotinic ACh receptor (nAChR) mediating cholinergic actions on the autonomic nervous system. Studies to refine or devise drugs selectively acting on (alpha3)2(beta4)3 receptors would benefit from a detailed description of the hitherto unclear agonist-binding domain. The present study reports a three-dimensional model for the ligand-binding domain (LBD) of this receptor either in its unoccupied or agonist-bound conformation. The receptor model was based on the structure of the acetylcholine-binding protein (AChBP), and was obtained using molecular modelling techniques. ACh, nicotine and cytisine (full agonists), muscarine (a selective agonist of muscarinic ACh receptors) and the allosteric modulator eserine were docked into the binding pockets of the receptor model. Simulated agonist-receptor complexes were compared with the agonist-binding complex of the AChBP, as well as of the (alpha4)2(beta2)3 type of nAChR, which is the commonest in the brain. Agonist docking identified discrete amino-acid residues of the beta subunits important for pharmacological selectivity of nAChRs. The predicted affinity of muscarine for the nAChR was low, suggesting the present model to be suitable for effective discrimination of nicotinic agonist binding versus nonselective cholinergic binding. Furthermore, the current model outlined a potential binding site for the allosteric modulator eserine, the site of action of which has remained elusive. The present LBD model of the receptor in its free state provides a novel structural framework to interpret experimental observations and a useful template for future investigations to develop (alpha3)2(beta4)3-selective ligands.

Modulation of neuronal nicotinic receptor function by the neuropeptides CGRP and substance P on autonomic nerve cells

1. One classical example of how neuropeptides can affect the function of ligand-gated receptors is the modulation of neuronal nicotinic receptors (nAChRs) by substance P. The present review updates current understanding of this action by substance P and compares it with other neuropeptides more recently found to modulate nAChRs in the autonomic nervous system. 2. Calcitonin gene-related peptide (CGRP) and its N-terminal fragments have been shown to exert complex inhibitory as well facilitatory actions on nAChRs. Fragments such as CGRP(1-4), CGRP(1-5) and CGRP(1-6) rapidly and reversibly enhance agonist sensitivity of nAChRs without directly activating those receptors. Longer fragments or the full-length peptide potently inhibit responses mediated by nAChRs via an apparently competitive-type antagonism. This phenomenon differs from the substance P-induced block, which is agonist use-dependent and preferential towards large nicotinic responses. 3. It is argued that the full-length peptides CGRP and substance P might play distinct roles in the activity-dependent modulation of cholinergic neurotransmission, by inhibiting background noise in the case of CGRP or by reducing excessive excitation in the case of substance P. Hence, multiple neuropeptide mechanisms may represent a wide array of fine-tuning processes to regulate nicotinic synaptic transmission. 4. The availability of novel CGRP derivatives with a strong enhancing action on nAChRs may offer new leads for the drug design targeted for potentiation of nAChRs in the autonomic nervous system as well as in the brain, a subject of interest to counteract the deficit of the nAChR function associated with neurodegenerative diseases like Alzheimer's and Parkinson's diseases.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Pharmacological characterization of recombinant bovine alpha3beta4 neuronal nicotinic receptors stably expressed in HEK 293 cells

In these studies, [(3)H]epibatidine is used as the radioligand to characterize recombinant bovine alpha3beta4 nicotinic acetylcholine receptors (nAChRs) expressed in HEK 293 cells. Specific binding reaches equilibrium quickly and is saturable with a K(d) value of 0.66 nM. The affinities of the several cholinergic agents were determined, including nicotine (K(i), 0.5 microM), cytisine (K(i), 0.5 microM), carbachol (K(i), 4.1 microM), dihydro-(beta)-erythroidine (K(i), 43.5 microM), d-tubocurarine (K(i), 0.1 microM), 1,1-dimethyl-4-phenylpiperazinium (K(i), 0.5 microM), decamethonium (K(i), 175 microM) and methyllycaconitine (K(i), 0.4 microM). These studies show that the pharmacological characteristics of recombinant bovine alpha3beta4 nAChRs are similar to native bovine alpha3beta4* nAChRs, and indicate that the alpha5 subunit, if present in the native nAChRs, does not affect ligand affinity.

Molecular biology and electrophysiology of neuronal nicotinic receptors of rat chromaffin cells

Neuronal nicotinic acetylcholine receptors of chromaffin cells in the adrenal medulla are physiologically activated by acetylcholine to mediate catecholamine release into the bloodstream. The present study examined the subunit composition and functional properties of rat chromaffin cell neuronal nicotinic acetylcholine receptors using molecular biology, immunocytochemistry and whole-cell patch-clamp. Reverse transcription-polymerase chain reaction analysis indicated the presence of alpha2, alpha3, alpha4, alpha5, alpha7, beta2 and beta4 transcripts (alpha6 and beta3 could not be detected). Immunocytochemistry revealed most cells positive for alpha3, beta2, beta4 and alpha5 proteins. Few cells were immunoreactive for alpha2 and alpha4, while none was for alpha7. At single-cell level, colocalization could be demonstrated for alpha3alpha5 and alpha4beta2. Western blot analysis confirmed antibody specificity for alpha3, alpha4, alpha5, beta2 and beta4 subunits. Inward currents elicited by nicotine pulses were insensitive to alpha-bungarotoxin and low doses of methyllycaconitine, demonstrating lack of functional alpha7 receptors. Partial block of nicotine currents was observed with either AuIB alpha-conotoxin (selective against alpha3beta4 receptors) or MII alpha-conotoxin (selective against alpha3beta2 receptors). With high concentrations of co-applied toxins, antagonism occlusion developed, suggesting loss of subunit selectivity. Antagonism by dihydro-beta-erythroidine summated nonlinearly with AuIB and MII inhibition, confirming heterogeneity of neuronal nicotinic acetylcholine receptor block. The present results suggest that the most frequently encountered receptors of rat chromaffin cells should comprise alpha3beta4, alpha3beta2 with the addition of alpha5 subunits. Because of the prevailing subunit composition, rat chromaffin cell neuronal nicotinic acetylcholine receptors are suitable models, particularly for the alpha3beta4 subclasses of mammalian brain receptors recently demonstrated in discrete cerebral areas.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Enhancement of neuronal nicotinic receptor activity of rat chromaffin cells by a novel class of peptides

The N-terminal 1-7 fragment of the neuropeptide CGRP inhibits neuronal nicotinic acetylcholine receptors (nAChRs) of rat chromaffin cells. To identify the structural motif responsible for this action, we investigated the effects of shorter CGRP fragments on patch-clamped rat chromaffin cells in culture. CGRP(1-6) evoked no direct change in baseline current or input conductance, but it strongly potentiated inward currents induced by very fast, nondesensitizing applications of nicotine. Potentiation was use independent and present even when coapplied with nicotine. The action of CGRP(1-6) was voltage independent and agonist independent. Because equimolar concentrations of CGRP(1-6) and CGRP(1-7) left nicotine-induced submaximal currents unchanged, these peptides presumably acted via a similar site through which they generated opposite effects. This observation also suggests that a single amino acid deletion could transform a peptide antagonist into a potentiating one. Deleting one amino acid from the COO(-) end of the CGRP(1-6) sequence yielded CGRP(1-5), which retained smaller potentiating activity. Even the CGRP(1-4) fragment possessed slight potentiation, which was lost with CGRP(1-3). CGRP(1-6) preferentially potentiated small over large responses to nicotine. One possibility is that CGRP(1-6) interacted with nAChRs like an allosteric modulator (e.g., physostigmine). Coapplication of enhancing concentrations of physostigmine and CGRP(1-6) led to linear summation of the individual effects, while CGRP(1-6) could partly reverse the depression by a large concentration of physostigmine. These data indicate functionally distinct sites of action for CGRP(1-6) and physostigmine. Potentiation of nicotinic receptors by CGRP(1-6) and its derivatives suggests them to be a new class of molecules enhancing activity mediated by nAChRs.

Dystrophin stabilizes alpha 3- but not alpha 7-containing nicotinic acetylcholine receptor subtypes at the postsynaptic apparatus in the mouse superior cervical ganglion

The nicotinic acetylcholine receptor (nAChR) subtypes were characterized in the superior cervical ganglion (SCG) of wild-type and dystrophin-lacking mdx mice. The binding of Epibatidine and alphaBungarotoxin, ligands for alpha3- and alpha7-containing receptors, respectively, revealed, for each ligand, a single class of high-affinity binding sites, with similar affinity in both wild-type and mdx mice. The Epibatidine-labeled receptors were immunoprecipitated by antibodies against the alpha3, beta2, and beta4 subunits. Immunocytochemistry showed that the percentage of alpha3-, beta2-, and beta4- but not of alpha7-immunopositive postsynaptic specializations was significantly lower in mdx than in wild-type mouse SCG. These observations suggest that the mouse SCG contains nAChRs, stabilized by dystrophin, in which the alpha3 subunit is associated with the beta2 and/or beta4 subunits. Conversely, dystrophin is not involved in the stabilization of the alpha7-containing nAChRs, as the percentage of alpha7-immunopositive synapses is similar in both wild-type and mdx mouse SCG.

[3H]Epibatidine binding to bovine adrenal medulla: evidence for alpha3beta4* nicotinic receptors

In these studies, [3H]epibatidine is used as the radioligand to characterize nicotinic acetylcholine receptors (nAChRs) from bovine adrenal medulla. Specific binding reaches equilibrium within 30 min, and is saturable with a Kd value of 0.5 nM. The affinities of several cholinergic agents were determined, including nicotine (Ki, 0.2 microM), cytisine (Ki, 0.4 microM), carbachol (Ki, 4.7 microM), dihydro-beta-erythrodine (Ki, 33.6 microM), D-tubocurarine (Ki, 0.4 microM), 1,1-dimethyl-4-phenyl-piperazinium (Ki, 0.8 microM), decamethonium (Ki, 234 microM) and methyllycaconitine (Ki, 1.3 microM). These values are similar to reported values for recombinant alpha3beta4 nAChRs in transfected cell lines. These studies demonstrate [3H]epibatidine binding to an easily obtainable adrenal membrane preparation and support the characterization of adrenal nAChRs as alpha3beta4* nAChRs.

A novel class of peptides with facilitating action on neuronal nicotinic receptors of rat chromaffin cells in vitro: functional and molecular dynamics studies

Peptides related to the N-terminal region of calcitonin gene-related peptide (CGRP) were tested for their ability to modulate neuronal nicotinic acetylcholine receptors (nAChRs) of rat cultured chromaffin cells under whole cell patch-clamp conditions. Although CGRP(1-7) and CGRP(2-7) depressed responses mediated by nAChRs, CGRP(1-6), CGRP(1-5), or CGRP(1-4) rapidly and reversibly potentiated submaximal nicotine currents while sparing maximal currents. CGRP(1-3) was inactive. The threshold concentration for the enhancing effect of CGRP(1-6) was 0.1 microM. CGRP(1-5) or CGRP(1-4) were less effective than CGRP(1-6). Coapplication of CGRP(1-6) and of the allosteric potentiator physostigmine (0.5 microM) gave additive effects on nicotine currents. CGRP(1-6) did not enhance responses generated by muscle-type nicotinic receptors of cultured myoblasts or by gamma-aminobutyric acid(A) receptors expressed by human embryonic kidney cells. Molecular dynamics (MD) simulations suggested that CGRP(1-7) exhibited a relatively rigid ring structure imparted by the disulfide bridge between Cys(2) and Cys(7). The circular dichroism (CD) spectrum recorded from the same peptide was in agreement with this result. Shorter peptides, missing such a bridge, exhibited propensity for alpha-helix configuration. Replacing Cys(7) with Ala yielded CGRP(1-7A), a fragment with partial alpha-helix structure and ability to enhance nicotine currents. CD measurements on CGRP(1-6) were compatible with these MD structural findings. Short terminal fragments of CGRP represent a novel class of substances with selective, rapid, and reversible potentiation of nAChRs.

Calibration of agonist concentrations applied by pressure pulses or via rapid solution exchanger

Desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) can develop so rapidly to preclude functional studies when agonists are applied under steady-state conditions. To minimize receptor desensitization, we recorded from rat cultured chromaffin cells currents evoked by nicotine applied by pressure (via a fine tipped pipette) or through a multibarreled rapid solution exchanger. By comparing equi-amplitude, non-fading responses on the same cells, we constructed log 'dose'-response curves with the amount of nicotine expressed as either pulse duration (ms) or initial concentration (microM) inside the perfusion tubes. The linear part of these plots (10-50 ms or 20-100 microM) allowed extrapolating the initial nicotine concentration for a certain pulse duration within these limits. Potential dilution of nicotine in the bath was calculated with diffusion equations for a continuous point source: the maximal concentration of nicotine attainable by pressure application from a 100 microM containing pipette was 92 microM. We also calculated how diluted the same nicotine solution became following rapid superfusion and found it to be 46%. These results indicate that the amount of agonist applied by pressure to a relatively close cell could be calibrated in terms of effective concentration at membrane level and, with the present experimental arrangement, underwent limited dilution in the extracellular microenvironment.

An α4β4 Nicotinic Receptor Subtype Is Present in Chick Retina: Identification, Characterization and Pharmacological Comparison with the Transfected α4β4 and α6β4 Subtypes

Retina from 1-day-old chicks is a valuable tissue model for studying neuronal nicotinic receptors because it expresses a large number of the developmentally regulated high affinity [(3)H]epibatidine labeled nicotinic receptors. Most of these receptors contain the beta4 subunit associated with different alpha subunits. Using a sequential immunodepletion procedure with anti-alpha6, anti-beta3, anti-beta2, and anti-beta4 antibodies, we purified an alpha4beta4 nicotinic receptor subtype that accounts for approximately 20 to 25% of the high affinity [(3)H]epibatidine labeled receptors present in retina at that developmental time. Immunoprecipitation and Western blotting experiments confirmed that the purified subtype contains only the alpha4 and beta4 subunits. This receptor binds a number of agonists and the antagonist dihydro-beta-erythroidine with nanomolar affinity, whereas it has micromolar affinity for the alpha-conotoxin MII and methyllycaconitine toxins and other nicotinic antagonists. Comparison of the pharmacological profile of this purified native subtype with that of the same subtype transiently expressed in human BOSC23 cells showed that they have very similar rank orders and absolute Ki values for several nicotinic drugs. Finally, because chick retina expresses an alpha6beta4-containing subtype with a high affinity for the alpha-conotoxin MII, we used native and transfected alpha4beta4 and alpha6beta4 subtypes to investigate the relative contributions of the alpha and beta subunits to this binding, and found that the alpha6 subunit determines the high affinity for this toxin.

Drugs selective for nicotinic receptor subtypes: a real possibility or a dream?

Nicotine exerts a number of different effects on the nervous system by interacting with neuronal nicotinic acetylcholine receptors (nAChRs). These effects are mediated by its interaction with different nAChR subtypes, and this has led to the finding of subtype specific agonists and antagonists. In the search for subtype-selective drugs, we have synthesized some compounds derived from 4-oxystilbene, two of which (MG624 and F3) are selective ligands for the chick neuronal alphaBgtx receptors containing the alpha7 and/or alpha8 subunits. They have an antagonist action on oocyte-expressed chick and rat alpha7 subtypes. These compounds are selective toward the alpha7-containing receptors in chick, but, in mammals, although they still retain their potency toward alpha7-containing receptors, they are also active in non-alpha7-containing receptors.