Desensitization of junctional and extrajunctional nicotinic ACh receptors expressed in Xenopus oocytes

Mechanisms Underlying the Strong Inhibition of Muscle-Type Nicotinic Receptors by Tetracaine

Nicotinic acetylcholine (ACh) receptors (nAChRs) are included among the targets of a variety of local anesthetics, although the molecular mechanisms of blockade are still poorly understood. Some local anesthetics, such as lidocaine, act on nAChRs by different means through their ability to present as both charged and uncharged molecules. Thus, we explored the mechanisms of nAChR blockade by tetracaine, which at physiological pH is almost exclusively present as a positively charged local anesthetic. The nAChRs from Torpedo electroplaques were transplanted to Xenopus oocytes and the currents elicited by ACh (I_ACh s), either alone or co-applied with tetracaine, were recorded. Tetracaine reversibly blocked I_ACh , with an IC₅₀ (i.e., the concentration required to inhibit half the maximum I_ACh ) in the submicromolar range. Notably, at very low concentrations (0.1 μM), tetracaine reduced I_ACh in a voltage-dependent manner, the more negative potentials produced greater inhibition, indicating open-channel blockade. When the tetracaine concentration was increased to 0.7 μM or above, voltage-independent inhibition was also observed, indicating closed-channel blockade. The I_ACh inhibition by pre-application of just 0.7 μM tetracaine before superfusion of ACh also corroborated the notion of tetracaine blockade of resting nAChRs. Furthermore, tetracaine markedly increased nAChR desensitization, mainly at concentrations equal or higher than 0.5 μM. Interestingly, tetracaine did not modify desensitization when its binding within the channel pore was prevented by holding the membrane at positive potentials. Tetracaine-nAChR interactions were assessed by virtual docking assays, using nAChR models in the closed and open states. These assays revealed that tetracaine binds at different sites of the nAChR located at the extracellular and transmembrane domains, in both open and closed conformations. Extracellular binding sites seem to be associated with closed-channel blockade; whereas two sites within the pore, with different affinities for tetracaine, contribute to open-channel blockade and the enhancement of desensitization, respectively. These results demonstrate a concentration-dependent heterogeneity of tetracaine actions on nAChRs, and contribute to a better understanding of the complex modulation of muscle-type nAChRs by local anesthetics. Furthermore, the combination of functional and virtual assays to decipher nAChR-tetracaine interactions has allowed us to tentatively assign the main nAChR residues involved in these modulating actions.

Microtransplantation of acetylcholine receptors from normal or denervated rat skeletal muscles to frog oocytes

Cell membranes, carrying neurotransmitter receptors and ion channels, can be 'microtransplanted' into frog oocytes. This technique allows a direct functional characterization of the original membrane proteins, together with any associated molecules they may have, still embedded in their natural lipid environment. This approach has been previously demonstrated to be very useful to study neurotransmitter receptors and ion channels contained in cell membranes isolated from human brains. Here, we examined the possibility of using the microtransplantation method to study acetylcholine receptors from normal and denervated rat skeletal muscles. We found that the muscle membranes, carrying their fetal or adult acetylcholine receptor isoforms, could be efficiently microtransplanted to the oocyte membrane, making the oocytes become sensitive to acetylcholine. These results show that oocytes injected with skeletal muscle membranes efficiently incorporate functional acetylcholine receptors, thus making the microtransplantation approach a valuable tool to further investigate receptors and ion channels of human muscle diseases.

Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

Modulation of muscle acetylcholine (AcCho) receptors (AcChoRs) by serotonin [5-hydroxytryptamine (5HT)] and other serotonergic compounds was studied in Xenopus laevis oocytes. Various combinations of alpha, beta, gamma, and delta subunit RNAs were injected into oocytes, and membrane currents elicited by AcCho were recorded under voltage clamp. Judging by the amplitudes of AcCho currents generated, the levels of functional receptor expression were: alpha beta gamma delta > alpha beta delta > alpha beta gamma > alpha gamma delta. The alpha beta gamma delta and alpha beta delta AcChoR Subtypes were strongly blocked by 5HT, whereas the alpha beta gamma receptor was blocked only slightly. The order of blocking potency of AcChoRs by 5HT was: alpha beta delta > alpha beta gamma delta > alpha beta gamma. 5HT receptor antagonists, such as methysergide and spiperone, were even more potent blockers of AcChoRs than 5HT but did not show much subunit selectivity. Blockage of alpha beta gamma delta and alpha beta delta receptors by 5HT was voltage-dependent, and the voltage dependence was abolished when the delta subunit was omitted. These findings may need to be taken into consideration when trying to elucidate the mode of action of many clinically important serotonergic compounds.

The desensitization of the embryonic mouse muscle acetylcholine receptor depends on the cellular environment

The rate of desensitization of nicotinic acetylcholine (ACh) receptor (nAChR), an important characteristic of nAChR function, was studied in myotubes of the mouse C2C12 cell line at different times after fusion, by measuring the decay of ACh-evoked currents (IACh) under various patch-clamp configurations. We observed a progressive slowing of IACh decay rate (half-decay time rose from about 0.5 s to over 5 s) in myotubes of increasing size (i.e. age) under all experimental conditions, except in outside-out patches, when IACh decayed as fast as in the smallest myotubes. Single-channel conductance (about 35 pS) and open time (about 3.5 ms), measured in outside-out and cell-attached patches, were independent of myotube size. In Xenopus oocytes injected with poly(A+)RNA extracted from C2C12 myoblasts or mature myotubes, IACh decay was about 50 times slower than in myotubes. Neither cAMP-dependent nor diacylglycerol-dependent protein kinases, actin nor microtubule polymerization state influenced IACh decay. Our data indicate that the cellular environment, but not readily dialysable cytosolic factors, markedly influences the functional behaviour of nAChR.

Incorporation of reconstituted acetylcholine receptors from Torpedo into the Xenopus oocyte membrane

Xenopus oocytes are a valuable aid for studying the molecular structure and function of ionic channels and neurotransmitter receptors. Their use has recently been extended by the demonstration that oocytes can incorporate foreign membranes carrying preassembled receptors and channels. Here we show that when reconstituted in an artificial lipid matrix and injected into Xenopus oocytes, purified nicotinic acetylcholine receptors are efficiently inserted into the plasma membrane, where they form "clusters" of receptors that retain their native properties. This constitutes an innovative approach that, besides allowing the analyses of membrane fusion processes, is also a powerful technique for studying the characteristics and regulation of many membrane proteins (with their native stoichiometry and configuration) upon reinsertion into the membrane of a very convenient host cell system.

Differential desensitization properties of rat neuronal nicotinic acetylcholine receptor subunit combinations expressed in Xenopus laevis oocytes

1. Chronic administration of nicotine up-regulates mammalian neuronal nicotinic acetylcholine receptors (nAChRs). A key hypothesis that explains up-regulation assumes that nicotine induces desensitization of receptor function. This is correlated with behaviorally expressed tolerance to the drug. 2. The present experiments were conducted to: (a) obtain information on the nicotine-induced desensitization of neuronal nAChR function, a less understood phenomenon as compared to that of the muscle and electric fish receptor counterparts; (b) test the hypothesis that different receptor subunit combinations exhibit distinct desensitization patterns. 3. Xenopus laevis oocytes were injected with mRNAs encoding rat receptor subunits alpha 2, alpha 3, or alpha 4 in pairwise combination with the beta 2 subunit. The responses to various concentrations of acetylcholine (ACh) or nicotine were analyzed by the two electrode voltage clamp technique. 4. Concentration-effect curves showed that nicotine was more potent than ACh for all the receptor subunit combinations tested. Only the alpha 4 beta 2 combination exhibited a depression of the maximum effect at concentrations higher than 20 microM nicotine. 5. After a single nicotine pulse, receptor desensitization (calculated as a single exponential decay) was significantly slower for alpha 4 beta 2 than for either alpha 3 beta 2 or alpha 2 beta 2. 6. Concentrations of nicotine that attained a near maximum effect were applied, washed, and re-applied in four minute cycles. The responses were calculated as percentages of the current evoked by the initial application. Following 16 minutes of this protocol, the alpha 4 beta 2 combination showed a greater reduction of the original response as compared to the alpha 2 beta 2 and alpha 3 beta 2 subunit combinations. Taking points 5 and 6 together, these experiments suggest that the alpha 4 beta 2 receptor subtype desensitizes at a slower rate and remains longer in the desensitized state. 7. Because alpha 4 beta 2 is the main receptor subunit combination within the brain and is up-regulated by nicotine, our data may be important for understanding the molecular basis of tolerance to this drug.

A cAMP-dependent Ca2+ signalling pathway at the endplate provided by the gamma to epsilon subunit switch in ACh receptors

During development of neuromuscular junctions there is a switch in the expression of acetylcholine receptors (AChR) from an embryonic form (gamma-AChR) to an adult form (epsilon-AChR). Studies with gamma- and epsilon-AChRs expressed in Xenopus oocytes showed that the gamma to epsilon subunit switch accelerates rates of desensitization and increases Ca2+ permeability. Site-directed mutagenesis of the gamma and epsilon subunits suggests that these changes are regulated by cAMP-dependent phosphorylation on the epsilon subunit. These results suggest that the gamma to epsilon subunit switch could provide for a cAMP-dependent Ca2+ signalling pathway at the endplate.

Tunicamycin alters channel gating characteristics of junctional and extrajunctional acetylcholine receptors expressed in Xenopus oocytes

The acute effects of tunicamycin on mouse junctional and extrajunctional acetylcholine receptors (AChRs) expressed in Xenopus oocytes, distinct from its ability to block N-glycosylation of the protein, was examined at the single channel level. Tunicamycin reduced the frequency of ACh-activated single channel openings and prolonged the mean channel open times of AChRs in a dose-dependent manner, without affecting inward conductances. Continuous application of ACh to both junctional and extrajunctional AChR channels very slowly decreased the number of channel opening events, and tunicamycin accelerated this process. These results suggest that tunicamycin alters channel gating kinetics and accelerates transition towards a desensitized state.

Differential interactions of gentamicin with mouse junctional and extrajunctional ACh receptors expressed in Xenopus oocytes

The nicotinic acetylcholine receptors (AChRs) from Torpedo electric organ and mouse muscles when expressed in Xenopus oocytes desensitize with different time courses. Initially, the role of cAMP-dependent phosphorylation on the gamma subunits in the different desensitization rates was investigated by expressing normal and mutant AChRs in the oocytes cultured in the presence of gentamicin. Mutant Torpedo AChRs lacking the potential cAMP-dependent phosphorylation sites in the gamma subunit appear to desensitize like normal Torpedo AChRs. Similarly, mutant mouse extrajunctional AChRs containing a newly created phosphorylation site in the gamma subunit appeared to desensitize like normal mouse AChRs, which lack the potential cAMP-dependent phosphorylation site in the gamma subunit. These results suggest that different rates of desensitization between the Torpedo and muscle extrajunctional AChRs are not attributable to differential cAMP-dependent phosphorylation of these AChRs. Subsequently, to determine whether gentamicin used in culturing oocytes differentially interacts with muscle junctional and extrajunctional AChRs, we analyzed rates of current decay following different gentamicin treatments. Both chronic and acute treatment with gentamicin profoundly accelerated the decay of whole-cell currents mediated by both types of AChR. The effect of prolonged gentamicin treatment on junctional AChRs was long lasting when compared to treatment on extrajunctional AChRs. Although the two types of AChR still desensitize differently in the absence of gentamicin, these results suggest that the characteristic desensitization of junctional and extrajunctional AChRs observed previously is largely due to differential interactions of gentamicin with the two types of AChR.