Desensitization at the frog neuromuscular junction: a biphasic process

Insights Into the Differential Desensitization of α4β2 Nicotinic Acetylcholine Receptor Isoforms Obtained With Positive Allosteric Modulation of Mutant Receptors

The development of highly efficacious positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (nAChR) has proven useful in defining the ligand dependence of the conformational dynamics of α7 receptors. No such effective modulators are known to exist for the α4β2 nAChR of the brain, limiting our ability to understand the importance of desensitization for the activity profile of specific ligands. In this study, we used mutant β2 subunits that allowed the use of the α7 PAM 3a,4,5,9b-tetrahydro-4-(1-naphthalenyl)-3H-cyclopentan[c]quinoline-8-sulfonamide (TQS) to probe the desensitizing effects of nicotinic ligands on the two forms of α4β2 receptors; high sensitivity (HS) (two α4 and three β2 subunits) and low sensitivity (LS) (three α4 and two β2 subunits). A total of 28 different ligands of 8 different categories, based on activity and selectivity, were tested for their ability to induce TQS-sensitive desensitization of HS and LS α4β2 receptors. Results confirm that HS α4β2 receptor responses are strongly limited by desensitization, by at least an order of magnitude more so than the responses of LS receptors. The activation of α4β2 receptors by the smoking-cessation drugs cytisine and varenicline is strongly limited by desensitization, as is the activation of LS receptors by the HS-selective agonists 6-[5-[(2S)-2-Azetidinylmethoxy]-3-pyridinyl]-5-hexyn-1-ol dihydrochloride and 4-(5-ethoxy-3-pyridinyl)-N-methyl-(3E)-3-buten-1-amine difumarate. The evaluation of drugs previously identified as α7-selective agonists revealed varying patterns of α4β2 cross-desensitization that were predictive of the effects of these drugs on the activation of wild-type α4β2 receptors by acetylcholine, supporting the utility of TQS-sensitive receptors for the development of focused therapeutics. SIGNIFICANCE STATEMENT: To varying degrees, ligands regulate the balance of active and desensitized states of the two forms of the primary nAChR subtypes in brain. Using mutant beta subunits, an allosteric modulator can reverse ligand-induced desensitization, revealing the differential desensitization of the receptors by specific ligands. This study shows that drugs believed to be selective for therapeutic targets may cross-desensitize other targets and that, within a class of drugs, improved specificity can be achieved by using agents that reduce such cross-desensitization.

Adolescent nicotine administration increases nicotinic acetylcholine receptor binding and functional connectivity in specific cortico-striatal-thalamic circuits

Nicotine exposure is associated with regional changes in brain nicotinic acetylcholine receptors subtype expression patterns as a function of dose and age at the time of exposure. Moreover, nicotine dependence is associated with changes in brain circuit functional connectivity, but the relationship between such connectivity and concomitant regional distribution changes in nicotinic acetylcholine receptor subtypes following nicotine exposure is not understood. Although smoking typically begins in adolescence, developmental changes in brain circuits and nicotinic acetylcholine receptors following chronic nicotine exposure remain minimally investigated. Here, we combined in vitro nicotinic acetylcholine receptor autoradiography with resting state functional magnetic resonance imaging to measure changes in [3H]nicotine binding and α4ß2 subtype nicotinic acetylcholine receptor binding and circuit connectivity across the brain in adolescent (postnatal Day 33) and adult (postnatal Day 68) rats exposed to 6 weeks of nicotine administration (0, 1.2 and 4.8 mg/kg/day). Chronic nicotine exposure increased nicotinic acetylcholine receptor levels and induced discrete, developmental stage changes in regional nicotinic acetylcholine receptor subtype distribution. These effects were most pronounced in striatal, thalamic and cortical regions when nicotine was administered during adolescence but not in adults. Using these regional receptor changes as seeds, resting state functional magnetic resonance imaging identified dysregulations in cortico-striatal-thalamic-cortical circuits that were also dysregulated following adolescent nicotine exposure. Thus, nicotine-induced increases in cortical, striatal and thalamic nicotinic acetylcholine receptors during adolescence modifies processing and brain circuits within cortico-striatal-thalamic-cortical loops, which are known to be crucial for multisensory integration, action selection and motor output, and may alter the developmental trajectory of the adolescent brain. This unique multimodal study significantly advances our understanding of nicotine dependence and its effects on the adolescent brain.

Fast desensitization of acetylcholine receptors induced by a spider toxin

Nicotinic acetylcholine receptors (nAChRs) are members of the “cys-loop” ligand-gated ion channel superfamily that play important roles in both the peripheral and central system. At the neuromuscular junction, the endplate current is induced by ACh binding and nAChR activation, and then, the current declines to a small steady state, even though ACh is still bound to the receptors. The kinetics of nAChRs with high affinity for ACh but no measurable ion conductance is called desensitization. This adopted desensitization of nAChR channel currents might be an important mechanism for protecting cells against uncontrolled excitation. This study aimed to show that Grammostola spatulata toxin (GsMTx4), which was first purified and characterized from the venom of the tarantula Grammostola spatulata (now genus Phixotricus), can facilitate the desensitization of nAChRs in murine C2C12 myotubes. To examine the details, muscle-type nAChRs, which are expressed heterologously in HEK293T cells, were studied. A single channel current was recorded under the cell-attached configuration, and the channel activity (NP_o) decayed much faster after the addition of GsMTx-4 to the pipette solution. The channel kinetics were further analyzed, and GsMTx-4 affected the channel activity of nAChRs by prolonging the closing time without affecting channel conductance or opening activity. The interaction between nAChRs embedded in the lipid membrane and toxin inserted into the membrane may contribute to the conformational change in the receptor and thus change the channel activity. This new property of GsMTx-4 may lead to a better understanding of the desensitization of ligand-gated channels and disease therapy.

The desensitization pathway of GABAA receptors, one subunit at a time

GABAA receptors mediate most inhibitory synaptic transmission in the brain of vertebrates. Following GABA binding and fast activation, these receptors undergo a slower desensitization, the conformational pathway of which remains largely elusive. To explore the mechanism of desensitization, we used concatemeric α1β2γ2 GABAA receptors to selectively introduce gain-of-desensitization mutations one subunit at a time. A library of twenty-six mutant combinations was generated and their bi-exponential macroscopic desensitization rates measured. Introducing mutations at the different subunits shows a strongly asymmetric pattern with a key contribution of the γ2 subunit, and combining mutations results in marked synergistic effects indicating a non-concerted mechanism. Kinetic modelling indeed suggests a pathway where subunits move independently, the desensitization of two subunits being required to occlude the pore. Our work thus hints towards a very diverse and labile conformational landscape during desensitization, with potential implications in physiology and pharmacology.

Progress in nicotinic receptor structural biology

Here we begin by briefly reviewing landmark structural studies on the nicotinic acetylcholine receptor. We highlight challenges that had to be overcome to push through resolution barriers, then focus on what has been gleaned in the past few years from crystallographic and single particle cryo-EM studies of different nicotinic receptor subunit assemblies and ligand complexes. We discuss insights into ligand recognition, ion permeation, and allosteric gating. We then highlight some foundational aspects of nicotinic receptor structural biology that remain unresolved and are areas ripe for future exploration. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

The dual-gate model for pentameric ligand-gated ion channels activation and desensitization

Pentameric ligand-gated ion channels (pLGICs) mediate fast neurotransmission in the nervous system. Their dysfunction is associated with psychiatric, neurological and neurodegenerative disorders such as schizophrenia, epilepsy and Alzheimer's disease. Understanding their biophysical and pharmacological properties, at both the functional and the structural level, thus holds many therapeutic promises. In addition to their agonist-elicited activation, most pLGICs display another key allosteric property, namely desensitization, in which they enter a shut state refractory to activation upon sustained agonist binding. While the activation mechanisms of several pLGICs have been revealed at near-atomic resolution, the structural foundation of desensitization has long remained elusive. Recent structural and functional data now suggest that the activation and desensitization gates are distinct, and are located at both sides of the ion channel. Such a 'dual gate mechanism' accounts for the marked allosteric effects of channel blockers, a feature illustrated herein by theoretical kinetics simulations. Comparison with other classes of ligand- and voltage-gated ion channels shows that this dual gate mechanism emerges as a common theme for the desensitization and inactivation properties of structurally unrelated ion channels.

Assessment of the expression and role of the α1-nAChR subunit in efferent cholinergic function during the development of the mammalian cochlea

Hair cell (HC) activity in the mammalian cochlea is modulated by cholinergic efferent inputs from the brainstem. These inhibitory inputs are mediated by calcium-permeable nicotinic acetylcholine receptors (nAChRs) containing α9- and α10-subunits and by subsequent activation of calcium-dependent potassium channels. Intriguingly, mRNAs of α1- and γ-nAChRs, subunits of the "muscle-type" nAChR have also been found in developing HCs (Cai T, Jen HI, Kang H, Klisch TJ, Zoghbi HY, Groves AK. J Neurosci 35: 5870-5883, 2015; Scheffer D, Sage C, Plazas PV, Huang M, Wedemeyer C, Zhang DS, Chen ZY, Elgoyhen AB, Corey DP, Pingault V. J Neurochem 103: 2651-2664, 2007; Sinkkonen ST, Chai R, Jan TA, Hartman BH, Laske RD, Gahlen F, Sinkkonen W, Cheng AG, Oshima K, Heller S. Sci Rep 1: 26, 2011) prompting proposals that another type of nAChR is present and may be critical during early synaptic development. Mouse genetics, histochemistry, pharmacology, and whole cell recording approaches were combined to test the role of α1-nAChR subunit in HC efferent synapse formation and cholinergic function. The onset of α1-mRNA expression in mouse HCs was found to coincide with the onset of the ACh response and efferent synaptic function. However, in mouse inner hair cells (IHCs) no response to the muscle-type nAChR agonists (±)-anatoxin A, (±)-epibatidine, (-)-nicotine, or 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) was detected, arguing against the presence of an independent functional α1-containing muscle-type nAChR in IHCs. In α1-deficient mice, no obvious change of IHC efferent innervation was detected at embryonic day 18, contrary to the hyperinnervation observed at the neuromuscular junction. Additionally, ACh response and efferent synaptic activity were detectable in α1-deficient IHCs, suggesting that α1 is not necessary for assembly and membrane targeting of nAChRs or for efferent synapse formation in IHCs.

Inside-out neuropharmacology of nicotinic drugs

Upregulation of neuronal nicotinic acetylcholine receptors (AChRs) is a venerable result of chronic exposure to nicotine; but it is one of several consequences of pharmacological chaperoning by nicotine and by some other nicotinic ligands, especially agonists. Nicotinic ligands permeate through cell membranes, bind to immature AChR oligomers, elicit incompletely understood conformational reorganizations, increase the interaction between adjacent AChR subunits, and enhance the maturation process toward stable AChR pentamers. These changes and stabilizations in turn lead to increases in both anterograde and retrograde traffic within the early secretory pathway. In addition to the eventual upregulation of AChRs at the plasma membrane, other effects of pharmacological chaperoning include modifications to endoplasmic reticulum stress and to the unfolded protein response. Because these processes depend on pharmacological chaperoning within intracellular organelles, we group them as "inside-out pharmacology". This term contrasts with the better-known, acute, "outside-in" effects of activating and desensitizing plasma membrane AChRs. We review current knowledge concerning the mechanisms and consequences of inside-out pharmacology. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Desensitization mechanism in prokaryotic ligand-gated ion channel

Crystal structures of Gloeobacter violaceus ligand-gated ion channel (GLIC), a proton-gated prokaryotic homologue of pentameric ligand-gated ion channel (LGIC) from G. violaceus, have provided high-resolution models of the channel architecture and its role in selective ion conduction and drug binding. However, it is still unclear which functional states of the LGIC gating scheme these crystal structures represent. Much of this uncertainty arises from a lack of thorough understanding of the functional properties of these prokaryotic channels. To elucidate the molecular events that constitute gating, we have carried out an extensive characterization of GLIC function and dynamics in reconstituted proteoliposomes by patch clamp measurements and EPR spectroscopy. We find that GLIC channels show rapid activation upon jumps to acidic pH followed by a time-dependent loss of conductance because of desensitization. GLIC desensitization is strongly coupled to activation and is modulated by voltage, permeant ions, pore-blocking drugs, and membrane cholesterol. Many of these properties are parallel to functions observed in members of eukaryotic LGIC. Conformational changes in loop C, measured by site-directed spin labeling and EPR spectroscopy, reveal immobilization during desensitization analogous to changes in LGIC and acetylcholine binding protein. Together, our studies suggest conservation of mechanistic aspects of desensitization among LGICs of prokaryotic and eukaryotic origin.

Ligand activation of the prokaryotic pentameric ligand-gated ion channel ELIC

While the pentameric ligand-gated ion channel ELIC has recently provided first insight into the architecture of the family at high resolution, its detailed investigation was so far prevented by the fact that activating ligands were unknown. Here we describe a study on the functional characterization of ELIC by electrophysiology and X-ray crystallography. ELIC is activated by a class of primary amines that include the neurotransmitter GABA at high micro- to millimolar concentrations. The ligands bind to a conserved site and evoke currents that slowly desensitize over time. The protein forms cation selective channels with properties that resemble the nicotinic acetylcholine receptor. The high single channel conductance and the comparably simple functional behavior make ELIC an attractive model system to study general mechanisms of ion conduction and gating in this important family of neurotransmitter receptors.

Desensitization of neurotransmitter-gated ion channels during high-frequency stimulation: a comparative study of Cys-loop, AMPA and purinergic receptors

Changes in synaptic strength allow synapses to regulate the flow of information in the neural circuits in which they operate. In particular, changes lasting from milliseconds to minutes (‘short-term changes') underlie a variety of computational operations and, ultimately, behaviours. Most studies thus far have attributed the short-term type of plasticity to activity-dependent changes in the dynamics of neurotransmitter release (a presynaptic mechanism) while largely dismissing the role of the loss of responsiveness of postsynaptic receptor channels to neurotransmitter owing to entry into desensitization. To better define the response of the different neurotransmitter-gated ion channels (NGICs) to repetitive stimulation without interference from presynaptic variables, we studied eight representative members of all three known superfamilies of NGICs in fast-perfused outside-out patches of membrane. We found that the responsiveness of all tested channels (two nicotinic acetylcholine receptors, two glycine receptors, one GABA receptor, two AMPA-type glutamate receptors and one purinergic receptor) declines along trains of brief neurotransmitter pulses delivered at physiologically relevant frequencies to an extent that suggests that the role of desensitization in the synaptic control of action-potential transmission may be more general than previously thought. Furthermore, our results indicate that a sizable fraction (and, for some NGICs, most) of this desensitization occurs during the neurotransmitter-free interpulse intervals. Clearly, an incomplete clearance of neurotransmitter from the synaptic cleft between vesicle-fusion events need not be invoked to account for NGIC desensitization upon repetitive stimulation.

Kinetics of desensitization and recovery from desensitization for human alpha4beta2-nicotinic acetylcholine receptors stably expressed in SH-EP1 cells

AIM

Studies were conducted to define the kinetics of the onset of and recovery from desensitization for human alpha4beta2-nicotinic acetylcholine receptors (nAChR) heterologously expressed in the SH-EP1 human epithelial cell line.

METHODS

Whole-cell patch clamp recordings were performed to evaluate alpha4beta2-nAChR currents.

RESULTS

Application of 0.1 micromol/L nicotine or 1 mmol/L acetylcholine (ACh) for 1 s or longer induced two phases, with time constants of approximately 70 and approximately 700 ms, for the onset of alpha4beta2-nAChR desensitization. For a given duration of agonist exposure, recovery from desensitization induced by nicotine was slower than recovery from ACh-induced desensitization. Comparisons with published reports indicate that time constants for the recovery of alpha4beta2-nAChRs from desensitization are smaller than those for the recovery of human muscle-type nAChRs(1) from desensitization produced by the same concentrations and durations of exposure to an agonist. Moreover, the extent of human alpha4beta2-nAChR desensitization and rate of recovery are the same, regardless of whether they are measured using whole-cell recording or based on published findings(2) using isotopic ion flux assays; this equality demonstrates the equivalent legitimacy of these techniques in the evaluation of nAChR desensitization. Perhaps most significantly, recovery from desensitization also was best fit to a biphasic process. Regardless of whether it was fit to single or double exponentials, however, half-times for recovery from desensitization grew progressively longer with an increased duration of agonist exposure during the desensitizing pulse.

CONCLUSION

These findings indicate the existence of alpha4beta2-nAChRs in many distinctive states of desensitization, as well as the induction of progressively deeper states of desensitization with the increased duration of agonist exposure.

Behavioural pharmacology of nicotine: implications for multiple brain nicotinic receptors

Behavioural studies can contribute to the characterization of receptors for psychoactive drugs, and attempts have been made to link behavioural effects of nicotinic agonists with the high affinity binding site for [3H] nicotine. Cueing (discriminative stimulus) effects of drugs enable trained humans or animals to recognize when a specific drug is administered. There was a correlation between the potencies of some compounds in the binding procedure and their ability to produce the nicotine discriminative stimulus in rats, supporting the view that the high affinity binding site was a functional receptor. Nicotine also produced complex changes in locomotor activity of rats, characterized acutely by transient depression and chronically by persistent stimulation. The abilities of nicotinic compounds to produce these locomotor effects were not always consistent with the studies on binding and the nicotine discriminative stimulus. Some compounds were relatively more potent in producing locomotor depression or stimulation than the discriminative effect. Some compounds also failed to produce chronic locomotor activation at doses that produced discriminative and acute depressant effects. These findings may be interpreted as preliminary evidence that different behavioural effects of nicotine may be mediated through different mechanisms, possibly involving multiple subtypes of nicotinic receptors.

Regulation of Nicotinic Acetylcholine Receptor Desensitization by Ca2+

The relationship between the concentration of intracellular Ca2+ ([Ca2+]i) and recovery from desensitization of nicotinic acetylcholine receptors (nAChRs) in rat medial habenula (MHb) neurons was investigated using the whole cell patch-clamp techniques in combination with microfluoresecent [Ca2+]i measurements. Recovery from desensitization was assessed with a paired-pulse agonist application protocol. Application of 100 μM nicotine (5 s) caused pronounced desensitization of nAChRs, after which recovery proceeded with two components. The relative weight of the two phases of recovery was sensitive to the nature of the intracellular Ca2+ chelator, with a greater fraction of channels recovering during the fast phase in the presence of BAPTA than EGTA. Recovery was affected by differential Ca2+ buffering only when Ca2+ was present in the extracellular solution, implying that Ca2+ influx through nAChRs was responsible for slowing the recovery. Simultaneous [Ca2+]i measurements showed that recovery from desensitization was inversely correlated with the instantaneous [Ca2+]i, further supporting the suggestion that elevation of [Ca2+]i limits the return of nAChRs to the resting state. In a separate set of experiments, activation of voltage-gated Ca2+ channels during the recovery phase produced a sufficiently large increase in [Ca2+]i to reduce recovery from desensitization even in the absence of Ca2+ influx through nAChRs. Overall, it is suggested that Ca2+ entry through both nAChRs and voltage-gated Ca2+ channels exerts a negative feedback on nAChR activity through stabilization of desensitized states. The interaction of these two Ca2+ sources could form the basis of a coincidence detector under specific circumstances.

Kinetic evidence that desensitized nAChR may promote transitions of active nAChR to desensitized states during sustained exposure to agonists in skeletal muscle

During prolonged exposure of postjunctional nicotinic acetylcholine receptors (nAChR) of skeletal muscle to acetylcholine (ACh), agonist-activated nAChR (nAChRa) gradually fall into a refractory "desensitized" state (nAChRd), which no longer supports the high-conductance channel openings characteristic of the initially active nAChRa. In the present study, the possibility was examined that nAChRd, rather than simply constituting a passive "trap" for nAChRa, may actively promote further conversions of nAChRa to nAChRd in a formally autocatalytic manner. Single-ion whole-cell voltage-clamp currents (Na+ and Li+ in separate trials) were measured using two KCl-filled capillary electrodes (5-10 MOmega) implanted at the postjunctional locus of single frog skeletal muscle fibers (Rana pipiens) equilibrated in 30 mM K+ bath media to eliminate mechanical responses. Various nAChR agonists (carbamylcholine, acetylcholine, suberyldicholine) at different concentrations were delivered focally by positive pressure microjet. It was found that the decline of postmaximal agonist-induced currents under these different conditions (driven by the growth of the subpool of nAChRd) consistently followed an autocatalytic logistic rule modified for population growth of fixed units in a planar array: [Formula: see text] (where y represents the remaining agonist-induced current at time t, A=initial maximum current, and n is a constant). Some further experimental features that might result from a self-promoting growth of nAChRd were also tested, namely, (1) the effect of increased nAChRa and (2) the effect of increased nAChRd. Increase in agonist concentration of the superfusate, by increasing the planar density of active nAChRa at the outset, should enhance the probability of autocatalytic interactions with emerging nAChRd, hence, the rate of decline of agonist-induced current, and this was a consistent finding under all conditions tested. Raising the initial level of desensitized nAChRd by pretreatment of fibers with very low concentrations of agonist would be another way to increase autocatalytic interactions with active nAChRa, and this was also found to produce increased rates of decline of agonist-induced currents when tested in additional trials. It is concluded that several kinetic features of nAChR desensitization in skeletal muscle are consistent with an action of nAChRd to promote further transitions of nAChRa to desensitized forms. This could occur by a direct effect of nAChRd on contiguous nAChRa or perhaps through some intermediary membrane component or local intracellular pathway.

Desensitization of nicotine acetylcholine receptors: modulation by kinase activation and phosphatase inhibition

The desensitization of alpha-bungarotoxin-insensitive native neuronal nicotinic receptors was studied in rat cortical cell cultures using the patch clamp technique. Thirty-minute perfusions of nicotine reduced currents evoked by short test pulses of 300 microM acetylcholine over a range of 3 to 300 nM, with an IC50 of 51 nM. The time course of desensitization onset was fit by a biexponential function consisting of a fast time constant of about 1 min and a slower component of 6-10 min. The desensitization recovery process was also biexponential and was dominated by a slow time constant of 12-20 min, as well as a minor component of about 1 min. The intracellular dialysis of either the protein kinase C activator phorbol-12-myristate-13 acetate or the phosphatase inhibitor cyclosporin A accelerated the desensitization recovery rate by 2-fold. The data imply that endogenous cortical nicotinic receptor channels may enter one of two desensitization states. The first state (D1) is characterized by rapid entry and recovery, whereas transitions into and out of the second state (D2) occur at slower rates. The D2 receptor state may arise by a sequential transition from the D1 conformation. Protein kinase C activation or phosphatase 2B inhibition may favor the D1 receptor state over that of D2 to promote faster overall rates of desensitization recovery.

Desensitized nicotinic receptors in brain

Desensitization is an intriguing characteristic of ligand-gated channels, whereby a decrease or loss of biological response occurs following prolonged or repetitive stimulation. Nicotinic acetylcholine receptors (nAChRs), as a member of transmitter gated ion channels family, also can be desensitized by continuous or repeated exposure to agonist. Desensitization of nicotinic receptors can occur as a result of extended nicotine exposure during smoking or prolonged acetylcholine when treatment of Alzheimer's disease (AD) with cholinesterase inhibitors, or anticholinesterase agent poisoning. Studies from our lab have shown that nAChRs desensitization is not a nonfunctional state and we proposed that desensitized nAChRs could increase sensitivity of brain muscarinic receptor to its agonists. Here, we will review the regulation of nicotinic receptor desensitization and discuss the important biological function of desensitized nicotinic receptors in light of our previous studies. These studies provide the critical information for understanding the importance of nicotinic receptors desensitization in both normal physiological processing and in various disease states.

Nicotine is highly effective at producing desensitization of rat alpha4beta2 neuronal nicotinic receptors

We examined desensitization by acetylcholine (ACh) and nicotine at the rat alpha4beta2 neuronal nicotinic receptor stably expressed in HEK cells. For both agonists, the decay in response due to desensitization ('onset') was best fitted by the sum of two exponentials with the fast component dominant at concentrations > 1 microM. The time constants for onset were similar for both agonists, and showed little concentration dependence over the range of 0.1-100 microM. Recovery from desensitization also showed two exponential components. In contrast to the similarity in onset, nicotine produced longer lasting desensitization, resulting from an increase in the proportion of receptors in the slowly recovering population and from an increase in the time constant for the slow recovery process. The proportion of receptors in the slowly recovering population increased as the duration of the desensitizing pulse increased. Desensitization was also induced by low concentrations of agonist, with no apparent macroscopic response. A 100 s application of 10 nM nicotine desensitized 70 % of the peak response, while 100 s of 10 nM ACh desensitized only 15 %. At higher concentrations of agonist, which result in a macroscopic response, desensitization in the absence of activation also can occur. Nicotine is a very potent and efficacious desensitizing agent at this neuronal nicotinic receptor.

Studies of NMDA receptor function and stoichiometry with truncated and tandem subunits

The subunits that compose eukaryotic glutamate ion channel receptors have three transmembrane domains (TMs) and terminate with intracellular tails that are important for controlling channel expression and localization. Truncation of NMDA receptor subunits before the final TM showed that this TM and intracellular tail region are necessary to form functional channels. However, it is shown here that these truncated subunits may be partially rescued by coexpressing the final TM and tail as a separate protein. The whole-cell currents so produced are somewhat lower than with full-length subunits, and they do not show the sag characteristic of currents from channels containing NR1 and NR2A subunits in the continued presence of an agonist. In addition, these truncated subunits were joined to full-length subunits to generate tandems. The functional expression of these tandems confirmed the tetrameric structure of NMDA receptors and also suggested that the subunits making up NMDA receptors are arranged as a dimer of dimers in the receptors with a 1-1-2-2 orientation of the subunits in the channel, and not in an alternating pattern of subunits around the pore. These results may redirect future studies into the mechanism of binding and gating in these receptors toward schemes including dimers, and may also be relevant to studies of glutamate receptor ion channels in general.

Studies of NMDA receptor function and stoichiometry with truncated and tandem subunits

The subunits that compose eukaryotic glutamate ion channel receptors have three transmembrane domains (TMs) and terminate with intracellular tails that are important for controlling channel expression and localization. Truncation of NMDA receptor subunits before the final TM showed that this TM and intracellular tail region are necessary to form functional channels. However, it is shown here that these truncated subunits may be partially rescued by coexpressing the final TM and tail as a separate protein. The whole-cell currents so produced are somewhat lower than with full-length subunits, and they do not show the sag characteristic of currents from channels containing NR1 and NR2A subunits in the continued presence of an agonist. In addition, these truncated subunits were joined to full-length subunits to generate tandems. The functional expression of these tandems confirmed the tetrameric structure of NMDA receptors and also suggested that the subunits making up NMDA receptors are arranged as a dimer of dimers in the receptors with a 1-1-2-2 orientation of the subunits in the channel, and not in an alternating pattern of subunits around the pore. These results may redirect future studies into the mechanism of binding and gating in these receptors toward schemes including dimers, and may also be relevant to studies of glutamate receptor ion channels in general.

Desensitization of neuronal nicotinic receptors

The loss of functional response upon continuous or repeated exposure to agonist, desensitization, is an intriguing phenomenon if not as yet a well-defined physiological mechanism. However, detailed evaluation of the properties of desensitization, especially for the superfamily of ligand-gated ion channels, reveals how the nervous system could make important use of this process that goes far beyond simply curtailing excessive receptor stimulation and the prevention of excitotoxicity. Here we will review the mechanistic basis of desensitization and discuss how the subunit-dependent properties and regulation of nicotinic acetylcholine receptor (nAChR) desensitization contribute to the functional diversity of these channels. These studies provide the essential framework for understanding how the physiological regulation of desensitization could be a major determinant of synaptic efficacy by controlling, in both the short and long term, the number of functional receptors. This type of mechanism can be extended to explain how the continuous occupation of desensitized receptors during chronic nicotine exposure contributes to drug addiction, and highlights the potential significance of prolonged nAChR desensitization that would also occur as a result of extended acetylcholine lifetime during treatment of Alzheimer's disease with cholinesterase inhibitors. Thus, a clearer picture of the importance of nAChR desensitization in both normal information processing and in various diseased states is beginning to emerge.

Analysis of a slow desensitized state of recombinant adult-type nicotinic acetylcholine receptor channels

A characteristic feature of the kinetics of nicotinic acetylcholine receptor (nAChR) channels is fast and nearly complete desensitization with a time course between 10 and 100 ms and recovery from desensitization in the range of some hundred ms. In the present study we used a piezo-driven system for ultra-fast solution exchange, analysed the recovery from the fast desensitized state of mouse recombinant adult-type nAChR channels and found no difference to that of embryonic-type channels. By double pulse experiments with application of pulses with a saturating concentration of 1 mm acetylcholine (ACh) with increasing duration of the first pulse and a constant interval between pulses we detected a second slow desensitized state which was entered with a time constant of 2835 ms. Recovery from the slow desensitized state proceeded with a single exponential with a time constant of 16134 ms. The experimental data were interpreted by the addition of a transition from the desensitized state with two bound ACh molecules to a slow desensitized state to the well known circular kinetic scheme of activation and desensitization of nAChR channels. This slow desensitized state might play a role in muscle fatigue or in pathological states like myasthenic syndromes.

Desensitization of diliganded mouse muscle nicotinic acetylcholine receptor channels

Nicotinic ACh receptor channels (AChRs) exposed to high concentrations of ACh adopt 'desensitized' conformations that have a high affinity for the transmitter and no measurable ion conductance. Single-channel currents elicited by 0.1 or 1 mM ACh were recorded from human embryonic kidney (HEK) cells that had been transiently transfected with mouse alpha, beta, delta, and epsilon subunits. On the time scale of approximately 0.1 ms to approximately 1 h, apparent open intervals are described by a single exponential component, and shut intervals associated with desensitization are described by the sum of four or five exponential components. The kinetic behaviour appeared to be stationary and homogeneous. Desensitization rate constants were estimated by kinetic modelling of currents from cell-attached and outside-out patches (where the number of channels in the patch was measured). A single AChR recovered from the longest-lived desensitized state only after approximately 5 min. The occupancy of an AChR for each of the desensitized states was calculated as a function of time after the continuous application of a pulse of saturating ACh. The longest-lived desensitized state accounted for 90 % of the total only after several seconds. The fractional recovery from desensitization (during a 200 ms wash period) decreased as the duration of the desensitizing pulse increased, suggesting that recovery is slower from the longer-lived desensitized states. The free energy landscape for the AChR desensitization reaction in cell-attached patches exhibited an initial destabilization, followed by a plateau region of gradually increasing stability, followed by a deep well.

Agonists cause endocytosis of nicotinic acetylcholine receptors on cultured myotubes

Regulated trafficking of neurotransmitter receptors in excitable cells may play an important role in synaptic plasticity. In addition, agonist-induced endocytosis of nicotinic acetylcholine receptors (nAChRs) in particular might be involved in nicotine tolerance and addiction. The existing evidence concerning regulated internalization of cell-surface nAChRs is indirect and equivocal, however. In the present study, radioligand binding and fluorescence microscopy were used to show that agonists cause substantial endocytosis of nAChRs on cultured myotubes. Exposure to carbachol or nicotine caused a decrease in the intensity of fluorescent labeling of clusters of cell-surface nAChRs that was blocked by low temperature. Overall, myotubes exposed to carbachol or nicotine bound 50-70% less [(125)I]-alpha-bungarotoxin on the cell surface than untreated cells. The effect of carbachol was significant within 5 min, increased progressively for at least 4 h, and had a sensitivity of 100 nM or less. Exposure to carbachol caused the appearance or dramatic expansion of an intracellular pool of nAChRs, which were localized to discrete, largely perinuclear structures. A pulse-chase labeling protocol allowed the selective labeling and localization of nAChRs that had been internalized from the cell surface. In untreated cells, very little internalization of nAChRs occurred over a period of 3 h, indicating that constitutive endocytosis of receptors over this period was minimal. Exposure to carbachol, however, caused a dramatic increase in the endocytosis of nAChRs. These results provide direct evidence that agonists, including the tobacco alkaloid nicotine, can cause substantial endocytosis of cell-surface nAChRs.

Deactivation and desensitization of mouse embryonic- and adult-type nicotinic receptor channel currents

Recombinant nicotinic acetylcholine receptor (nAChR) channels transiently expressed in HEK293 cells were investigated using the patch-clamp technique in the cell-attached and outside-out modes for single-channel analysis and ultra-fast agonist application to multiple channels. Deactivation (current decay after removal of agonist) and desensitization (current decay in the presence of agonist) were analyzed at embryonic- (gamma) and adult-type (epsilon) nAChR channels. Time constants of desensitization were similar for both receptor types (epsilon: 53.1+/-16.9 ms; gamma: 49.2+/-15.7 ms) and corresponded to the mean duration of clusters of single channel openings activated by pulses of 1 mM ACh. Deactivation showed distinct characteristics. Time constants were 1.76+/-0.16 ms for epsilon- and 3.19+/-0.18 ms for gamma-nAChR channels, corresponding to mean burst duration analyzed from single channels in the same preparation (epsilon: 1.85+/-1.2 ms, gamma: 3.85+/-2.1 ms). It is assumed that differences in deactivation are of functional relevance at the muscle endplate.

Synaptically Released Neurotransmitter Fails to Desensitize Postsynaptic GABAA Receptors in Cerebellar Cultures

GABA concentration jump experiments performed on membrane patches predict that postsynaptic GABAAreceptors will become desensitized following the release of the contents of a single GABA-containing synaptic vesicle. To examine this we used a single synaptic bouton stimulation technique to directly examine whether postsynaptic GABAA receptors in cultured cerebellar granule cells exhibit transmitter-induced desensitization. In a large number of recordings, no evidence was found for desensitization of postsynaptic GABAAreceptors by vesicularly released transmitter. This was the case even when as many as 40 vesicles were released from a single bouton within 1.5 s. In addition, postsynaptic depolarization and application of the benzodiazepine flunitrazepam, manipulations previously shown to enhance desensitization of GABAA receptors, failed to unmask transmitter-induced desensitization. In contrast, a single 2- to 3-s application of a high concentration of exogenous GABA was able to depress synaptic responsiveness for up to 70 s. Furthermore, pharmacological depletion of GABA eliminated inhibitory synaptic communication, suggesting that GABA is the transmitter and the desensitization-resistant inhibitory postsynaptic currents are not mediated by a “nondesensitizing” ligand such as β-alanine. Overall our data indicate that a specific desensitization-resistant population of GABAA receptors are present at postsynaptic sites on cultured cerebellar granule cells.

Desensitization of neuronal nicotinic acetylcholine receptors conferred by N-terminal segments of the beta 2 subunit

Desensitization is a general property of ligand-gated ion channels. Because of a wide array of available subunit combinations, it generates different time constants for channel closure, thereby modulating the processing of information in the brain. Within the family of neuronal nicotinic acetylcholine receptors (nAChRs), alpha 3 beta 2 and alpha 3 beta 4 receptors display contrasting properties of desensitization. When measured using two-electrode voltage-clamp in Xenopus oocytes, desensitization results in current decreases 2 s after initiation of acetylcholine application by 94% for alpha 3 beta 2 receptors, but only by 6% in the case of alpha 3 beta 4 receptors. Desensitization was analyzed by inserting different portions of the beta2 into the beta 4 subunit. Residues 1--212 of the beta2 subunit were able to confer 78% desensitization in 2 s, while smaller chimeras revealed desensitization in 2 s conferred by residues 1--42 alone to a level of 50%, by residues 72--89 to a level of 74%, and by residues 96--212 to a level of 77%. Some long-term (25 min) effects of desensitization driven by acetylcholine were found to rely partially on the same elements, including an enhancement mediated by residues 1--95 and 96--212 of the beta 2 subunit individually. Our results reveal that desensitization relies independently on diverse portions of the extracellular domain of the beta 2 subunit. Phenotype of alpha 3 beta 4 involves, in contrast, complex structural requirements involving residues dispersed throughout the entire N-terminal domain of the beta 4 subunit.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

Roles of agonist-binding sites in nicotinic acetylcholine receptor function

Under equilibrium conditions, the nicotinic acetylcholine receptor from Torpedo electroplax carries two high affinity-binding sites for agonists. It is generally assumed that these are the only agonist sites on the receptor and that their occupancy results in rapid channel activation followed by slower conformational transitions that lead to the high affinity equilibrium state. These slow transitions are thought to reflect the physiological process of desensitization. Here we show that preequilibration of the high affinity sites with saturating concentrations of carbamylcholine does not diminish the ion flux response to subsequent exposure to higher (activating) concentrations of this agonist. This finding has profound implications with respect to receptor function: (1) occupancy of the high affinity sites per se does not desensitize the receptor and (2) these sites cannot be directly involved in receptor activation. It is thus necessary to invoke the presence of additional binding sites in channel opening.

Regulation of the sensitivity of acetylcholine receptors to nicotine in rat habenula neurons

Time-dependent changes in nicotinic acetylcholine receptor (nAChR) function were studied in acutely isolated medial habenula neurons during whole-cell perfusion. The peak amplitude of inward currents induced by 1 s pulses of nicotinic agonists, applied at 30 s intervals, gradually increased over the first several minutes of whole-cell recording. The ratio of response amplitudes at 1 and 15 min (t15/t1) was 1.9. Run-up of responses occurred independently of channel activation and was specific to nAChRs. The channel blocker chlorisondamine (30 microM), co-applied with nicotine, was used to irreversibly block the majority (91 %) of the nAChRs that opened in the first 2 min of recording. Run-up in the remaining 9 % unblocked channels assessed at 15 min (t15/t2 = 3.4) was similar to that in control cells not exposed to nicotine and chlorisondamine simultaneously, implying that run-up is not due to the incorporation of new receptors. A marked alteration in the sensitivity of nAChRs to extracellular Ca2+ was also observed during whole-cell perfusion. The ratio of current amplitudes obtained in 0.2 and 4.0 mM Ca2+ changed from 0.54 (t = 5 min) to 0.82 (t = 30 min). Inward rectification of nicotine-induced responses was reduced during internal dialysis. Voltages for half-maximal conductance were -23.0 and -13.8 mV at 2 and 15 min, respectively. Inclusion of either free Mg2+ ( approximately 2 mM) or spermine (100 microM) in the internal solution counteracted the change in rectification, but did not prevent run-up. The period of run-up was followed by a use-dependent run-down phase. Little run-down in peak current amplitude was induced provided that agonist was applied infrequently (5 min intervals), whereas applications at 30 s intervals produced a loss of channel function after approximately 15 min whole-cell perfusion. The time at which run-down began ( approximately 5-30 min) was correlated with the initial rate of nAChR desensitization ( approximately 200-4000 ms); slowly desensitizing nicotinic currents demonstrated delayed run-down. We suggest that run-up of nAChR-mediated responses does not require receptor activation and may result from a change in channel open probability. We also hypothesize that channel run-down reflects accumulation of nAChRs in long-lived desensitized/inactivated states.

Molecular investigations on the nicotinic acetylcholine receptor: conformational mapping and dynamic exploration using photoaffinity labeling

The nicotinic acetylcholine receptor (nAChR) is a well-understood member of the ligand-gated ion channels superfamily. The members of this signaling proteins group, including 5HT3, GABA(A), glycine, and ionotropic glutamate receptors, are thought to share common secondary, tertiary, and quaternary structures on the basis of a very high degree of sequence similarity. Despite the absence of X-ray crystallographic data, considerable progress on structural analysis of nAChR was achieved from biochemical, mutational, and electron microscopy data allowing the emergence of a three-dimensional image. Photoaffinity labeling and site-directed mutagenesis gave information on the tertiary structure with respect to the agonist/antagonist binding sites, the ion channel, and its selectivity filter. nAChR is an allosterical protein that undergoes interconversion among several conformational states. Time-resolved photolabeling was used in an attempt to elucidate the structural changes that occur in nAChR on neurotransmitter activation. Tertiary and quaternary rearrangements were found in the cholinergic binding pocket and in the channel lumen, but the structural determinant and the functional link between the binding of agonist and the channel gating remain unknown. Time-resolved photolabeling of the functional activated A state using photosensitive agonists might help in understanding the dynamic process leading to the interconversion of the different states.

Upregulation of surface alpha4beta2 nicotinic receptors is initiated by receptor desensitization after chronic exposure to nicotine

It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upregulation of nAChR number. To test this hypothesis directly, oocytes expressing alpha4beta2 receptors were chronically incubated (24-48 hr) in nicotine, and the resulting changes in specific [3H]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensitization. Four lines of evidence strongly support the hypothesis. (1) The half-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The concentration of [3H]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical desensitization models. (3) Functional desensitization of alpha3beta4 receptors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha4beta2 receptors that do not recover fully from desensitization, but not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-maximal binding of alpha4beta2 receptors in total cell membrane homogenates was 20-fold lower than that measured for surface nAChRs in intact oocytes. These data suggest that cell homogenate binding assays may not accurately reflect the in vivo desensitization affinity of surface nAChRs and may account for some of the previously reported differences in the efficacy of nicotine for inducing nAChR desensitization and upregulation.

Upregulation of surface alpha4beta2 nicotinic receptors is initiated by receptor desensitization after chronic exposure to nicotine

It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upregulation of nAChR number. To test this hypothesis directly, oocytes expressing alpha4beta2 receptors were chronically incubated (24-48 hr) in nicotine, and the resulting changes in specific [3H]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensitization. Four lines of evidence strongly support the hypothesis. (1) The half-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The concentration of [3H]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical desensitization models. (3) Functional desensitization of alpha3beta4 receptors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha4beta2 receptors that do not recover fully from desensitization, but not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-maximal binding of alpha4beta2 receptors in total cell membrane homogenates was 20-fold lower than that measured for surface nAChRs in intact oocytes. These data suggest that cell homogenate binding assays may not accurately reflect the in vivo desensitization affinity of surface nAChRs and may account for some of the previously reported differences in the efficacy of nicotine for inducing nAChR desensitization and upregulation.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

Agonist binding and affinity state transitions in reconstituted nicotinic acetylcholine receptors revealed by single and sequential mixing stopped-flow fluorescence spectroscopies

The affinity state of nicotinic acetylcholine receptors (nAcChoRs) reconstituted into either dioleoylphosphatidylcholine (DOPC) or a mixture of dioleoylphosphatidylcholine, dioleoylphosphatidic acid, and cholesterol (DOPC/DOPA/cholesterol) has been determined using single and sequential mixing stopped-flow fluorescence spectroscopies. These techniques have millisecond temporal resolution, permitting low- and high-affinity conformational states of the nAcChoR to be resolved following mixing with the fluorescent partial agonist Dns-C6-Cho from their characteristic Dns-C6-Cho dissociation rates. Our studies reveal that prior to agonist-induced affinity state conversion, nAcChoRs reconstituted into either DOPC or DOPC/DOPA/cholesterol are predominantly in a conformational state that has a low affinity for agonist. Prolonged exposure to Dns-C6-Cho converts nearly all DOPC/DOPA/cholesterol-reconstituted nAcChoRs to the high-affinity state. In contrast, Dns-C6-Cho converts only half of all DOPC-reconstituted nAcChoRs to the high-affinity state. The other half persists in a low-affinity state characterized by a Kd for Dns-C6-Cho of 0.61+/-0.07 microM. This Kd is similar to that previously reported for Dns-C6-Cho binding to low-affinity, resting-state nAcChoRs in native membranes. However, affinity state conversion of DOPC-reconstituted nAcChoRs may be facilitated by re-reconstituting them into bilayers composed of DOPC/DOPA/cholesterol. These results indicate that the lipid bilayer composition modulates nAcChoR agonist-induced affinity state transitions.

Desensitization of mouse nicotinic acetylcholine receptor channels. A two-gate mechanism

The rate constants of acetylcholine receptor channels (AChR) desensitization and recovery were estimated from the durations and frequencies of clusters of single-channel currents. Diliganded-open AChR desensitize much faster than either unliganded- or diliganded-closed AChR, which indicates that the desensitization rate constant depends on the status of the activation gate rather than the occupancy of the transmitter binding sites. The desensitization rate constant does not change with the nature of the agonist, the membrane potential, the species of permeant cation, channel block by ACh, the subunit composition (epsilon or gamma), or several mutations that are near the transmitter binding sites. The results are discussed in terms of cyclic models of AChR activation, desensitization, and recovery. In particular, a mechanism by which activation and desensitization are mediated by two distinct, but interrelated, gates in the ion permeation pathway is proposed.

Long-lasting inactivation of nicotinic receptor function in vitro by treatment with high concentrations of nicotine

Exposure of nicotinic acetylcholine receptors (nAChRs) to brief pulses of nicotine results in the stimulation of dopamine release, whereas prolonged treatment with low concentrations of nicotine (approximately 10 nM) produces a reversible blockade of a subsequent nicotine challenge as a result of nAChR desensitization. We and others have observed that, following prolonged treatment with stimulating (microM) concentrations of nicotine, there is incomplete recovery from desensitization. In this study we investigated this nonrecoverable component by characterizing the ability of nicotine to stimulate [3H]dopamine release from rat striatal synaptosomes following recovery from nicotine-induced desensitization. Brief (12 s) exposure to 30 microM nicotine, or longer exposure (> or = 5 min) to 0.3 microM nicotine produced a long-lasting decrease in nAChR function with an apparent IC50 of 0.7 microM. The maximal inactivation achieved was approximately 50%. Recovery of nAChR function did not return even after 5 h, whereas recovery from desensitization occurred within 20 min. Determinations of the concentration of nicotine in the superfusate indicated that residual nicotine could not account for the observed decrease in response as a consequence of desensitization. These results indicate that high concentrations of nicotine can produce a long-lasting nAChR inactivation which can be distinguished from reversible nAChR desensitization.

Characterization of desensitization in recombinant N-methyl-D-aspartate receptors: comparison with native receptors in cultured hippocampal neurons

In the present study we have characterized the effect of Ca2+, glycine, and agonist concentration on inactivation and desensitization in native and recombinant N-methyl-d-aspartate (NMDA) receptors. In agreement with earlier studies on neurons, we found that in the presence of saturating glycine concentrations, lowering [Ca2+]o, will decrease inactivation of NMDA receptors in cultured hippocampal neurons. However, unlike native NMDA receptors under the same recording conditions, recombinant receptors did not exhibit Ca2+-dependent inactivation. We also show that the glycine-insensitive desensitization observed in the recombinant receptors is subunit dependent, as NR1a2A and NR1a2B receptors significantly desensitized while the NR1a2C combination did not. Furthermore, we show this form of desensitization in NR1a2A receptors is due to classic agonist-induced desensitization. In addition, we demonstrate the presence of glycine-dependent desensitization in recombinant receptors. The ability of glycine to inhibit desensitization correlates to the rank order of glycine's affinity for potentiating the peak response for each subtype. Finally, using ifenprodil in the presence of high and low glycine concentrations, we present evidence that both 2A-like and 2B-like subtypes of receptors can independently coexist in single neurons.

Transient low-affinity agonist binding to Torpedo postsynaptic membranes resolved by using sequential mixing stopped-flow fluorescence spectroscopy

We have detected the binding of the fluorescent agonist Dns-C6-Cho to both low- and high-affinity states of the nicotinic acetylcholine receptor (nAcChoR) using sequential mixing stopped-flow fluorescence spectroscopy. Our approach to resolving low- and high-affinity binding was to first preincubate receptor membranes with the fluorescent partial agonist Dns-C6-Cho for 15 ms to 1000 s and then to follow the fluorescence decay upon chemical dilution into excess acetylcholine. The fast and slow decays, reflecting Dns-C6-Cho dissociation from low- and high-affinity receptors, had rates of 140 +/- 27 s-1 and 0.1 +/- 0.02 s-1, respectively. With increasing preincubation times, the number of low-affinity receptors decreased while the number of high-affinity receptors increased in a Dns-C6-Cho concentration-dependent manner consistent with current models for agonist-induced affinity state conversion. At receptor-activating concentrations of Dns-C6-Cho, the apparent rates with which high-affinity receptors formed approximated those of ion flux desensitization, implying that the fast desensitized state has an agonist dissociation rate that is indistinguishable from the equilibrium slow desensitized state. The KD for the low-affinity binding site was determined to be 1.1 microM from the increase in the amplitude of the fast decay with Dns-C6-Cho concentration with preincubation times that were sufficiently brief to minimize affinity state conversion. Assuming a bimolecular association rate of 10(8) M-1 s-1, a second estimate of 1.4 microM was made for low-affinity binding. We also detected a fluorescence enhancement consistent with a conformational isomerization of Dns-C6-Cho-inhibited nAcChoRs.

Kinked-helices model of the nicotinic acetylcholine receptor ion channel and its complexes with blockers: simulation by the Monte Carlo minimization method

A model of the nicotinic acetylcholine receptor ion channel was elaborated based on the data from electron microscopy, affinity labeling, cysteine scanning, mutagenesis studies, and channel blockade. A restrained Monte Carlo minimization method was used for the calculations. Five identical M2 segments (the sequence EKMTLSISVL10LALTVFLLVI20V) were arranged in five-helix bundles with various geometrical profiles of the pore. For each bundle, energy profiles for chlorpromazine, QX-222, pentamethonium, and other blocking drugs pulled through the pore were calculated. An optimal model obtained allows all of the blockers free access to the pore, but retards them at the rings of residues known to contribute to the corresponding binding sites. In this model, M2 helices are necessarily kinked. They come into contact with each other at the cytoplasmic end but diverge at the synaptic end, where N-termini of M1 segments may contribute to the pore. The kinks disengage alpha-helical H-bonds between Ala12 and Ser8. The uncoupled lone electron pairs of Ser8 carbonyl oxygens protrude into the pore, forming a hydrophilic ring that may be important for the permeation of cations. A split network of H-bonds provides a flexibility to the chains Val9-Ala12, the numerous conformations of which form only two or three intrasegment H-bonds. The cross-ectional dimensions of the interface between the flexible chains vary essentially at the level of Leu11. We suggest that conformational transitions in the chains Val9-Ala12 are responsible for the channel gating, whereas rotations of more stable alpha-helical parts of M2 segments may be necessary to transfer the channel in the desensitized state.

Imaging of intracellular calcium during desensitization of nicotinic acetylcholine receptors of rat chromaffin cells

1. The possible role of intracellular Ca2+ levels ([Ca2+]i) in desensitization of nicotinic acetylcholine receptors (AChRs) was investigated in rat cultured chromaffin cells by use of combined whole-cell patch clamping and confocal laser scanning microscopy with the fluorescent dye fluo-3. 2. On cells held at -70 mV, pressure-application of nicotine elicited inward currents with associated [Ca2+]i rises mainly due to influx through nicotinic AChRs. These responses were blocked by (+)-tubocurarine (10 microM) but were insensitive to alpha-bungarotoxin (1 microM) or Cd2+ (0.1 mM). 3. Pressure applications of 1 mM nicotine for 2 s (conditioning pulse) evoked inward currents which faded biexponentially to a steady state level due to receptor desensitization and were accompanied by a sustained increase in [Ca2+]i. Inward currents evoked by subsequent application of brief test pulses of nicotine were depressed but recovered with a time course reciprocal to the decay of the [Ca2+]i transient induced by the conditioning pulse. 4. Omission of intracellular Ca2+ chelators or use of high extracellular Ca2+ solution (10 mM) lengthened recovery of nicotinic AChRs from desensitization while adding BAPTA or EGTA intracellularly had the opposite effect. When the patch pipette contained fluo-3 or no chelators, after establishing whole cell conditions the rate of recovery became progressively longer presumably due to dialysis of endogenous Ca2+ buffers. None of these manipulations of external or internal Ca2+ had any effect on onset or steady state level of desensitization. 5. High spatial resolution imaging of [Ca2+]i in intact cells (in the presence of 0.1 mM Cd2+) showed that its level in the immediate submembrane area decayed at the same rate as in the rest of the cell, indicating that Ca2+ was in a strategic location to modulate (directly or indirectly) AChR desensitization. 6. The present data suggest that desensitized nicotinic AChRs are stabilized in their conformation by raised [Ca2+]i and that this phenomenon retards their recovery to full activity.