Trapping blockage of muscle nicotinic cholinoreceptors by mecamilamine

Abstract

Nicotinic cholinergic receptors can be blocked by both competitive and noncompetitive blocking agents of cholinoreceptors interacting with different regions of ionic channels [1]. Noncompetitive blocking agents may be divided into two subgroups: simple blocking agents of open channels and trapping blocking agents, which were originally described for glutamate receptors [2][3][4][5]. Earlier, we showed in the study on chromaffin rat cells that mecamilamine, a well-known neuronal blocking agent of channels, blocked neuronal cholinoreceptors by the trapping mechanism [6]. This blockade is characterized by unblocking during membrane depolarization and activation of receptors by an agonist [4,6].The neuromuscular junction is a common model for studying the action of ionic-channel blocking agents [7,8]. Earlier [9], it was hypothesized that mecamilamine can block channels in a neuromuscular synapse as a simple blocking agent; however, the trapping mechanism has not been studied yet. Hence, we assessed the mechanism of the mecamilamine effect on the receptor-channel complex of muscle cholinoreceptors.Experiments were carried out using an isolated neuromuscular preparation (sciatic nerve-sartorius muscle) of Rana ridibunda frogs using the standard technique of two-electrode potential fixation described earlier [10]. After infrequent nerve stimulations ( f = 0.05 Hz), mecamilamine at concentrations of 5-20 µ M induced dose-dependent decrease in the amplitude and the decay time constant of evoked endplate currents (EECs). These effects were washed out slowly. A typical experiment with 20 µ M mecamilamine is shown on Fig. 1. It was estimated after approximating averaged dose-effect relationships with Hill's equation that, at -70 mV, I C 50 = 7.8 µ M and n H = 1.2 ( n = 9) . Hill's constant was practically the same as for neuronal cholinoreceptors ( n H = 1.1 [6]), which indicated that the channel was blocked by a single molecule of the blocking agent, whereas the concentration at which the blocking-agent effect was half the maximum ( I C 50 ) for muscle cholinoreceptors was considerably higher than that for neuronal receptors (0.34 µ M [6]). The latter suggests that mecamilamine has a smaller blocking effect on muscle cholinoreceptors.With respect to its EEC time-amplitude characteristics, mecamilamine is analogous to a desensitization promoter or a slow blocking agent of open channels, whose characteristic property is the enhancement of the inhibiting effect upon increasing the frequency of nerve stimulation [10][11][12]. However, our experiments with mecamilamine showed that the increase in the frequency of nerve stimulation to 10 Hz did not induce additional changes in current amplitudes, which indicates another action mechanism of this compound.The mecamilamine effect in the neuromuscular synapse appeared to be potential-sensitive, which is confirmed by a decrease in I C 50 during membrane hyperpolarization (Fig. 2). On the basis of this relationship, we calculated the relative depth of the mecamilamine binding site in ...

Mecamylamine inhibits muscle nicotinic acetylcholine receptors by competitive and noncompetitive mechanisms

“…Based on these results, (±)-mecamylamine binds better to the recently activated AChR, and consequently it might inhibit ion flux by maintaining the receptor in this blockade state for longer time (decreasing the fraction of activated ion channels), or alternatively by inducing receptor desensitisation. The former possibility is consistent with the trapping blocking mechanism described for (±)-mecamylamine in different AChR subtypes 8,9 , whereas the latter possibility is ruled out based on the fact that (±)-mecamylamine does not increase [ 3 H]cytisine binding to Torpedo AChRs in the resting but activatable state ( Figure 4A), opposite to that observed for other desensitising NCAs 14,16 .…”

“…The involved acetylcholine receptor (AChR) subtypes and mechanisms underlying these pharmacologic activities have not been clearly demonstrated yet. Additional results indicate that the pharmacological and clinical activities of mecamylamine are more complex than previously thought [2][3][4][5][6][7][8][9] .…”

Effect of a Ganglioblocker, Mecamylamine, on Muscle Ionotropic Cholinoreceptors of Rats

“…The phenomenon of partial reversible deblocking of the channels is typical of trap-type blockers of open channels under conditions where activation of the receptors is combined with depolarization of the membrane [13,18,19]. In our study, we initially measured the amplitude of current responses of С2С12 cells to periodic (0.5 min -1 ) application of 1 μM ACh against the background of the action of 10 μM MA at a membrane potential of -70 mV (first signal in Fig.…”

“…In studies of cells of the submandibular ganglion with the use of a trap-type blocker, hexamethonium, it was first demonstrated that the phenomenon of partial reversible deblocking under conditions of agonist-induced activation of receptors combined with depolarization of the membrane does exist, and this phenomenon is typical of such a mode of blockade [13]. Studies on rat chromaffin cells [18] and neuromuscular junctions of the frog [19] demonstrated that it is exactly a trap-type mechanism that underlies the MA-induced blockade of cholinoreceptors of neuronal and muscle types.…”

Potential therapeutic uses of mecamylamine and its stereoisomers

“…As a result, mecamylamine is trapped within the channel. In order for mecamylamine to exit the channel, the agonist must bind to the agonist recognition site in an orthosteric manner to produce a conformational change, returning the receptor to the open channel state (Gurney and Rang, 1984; Lingle, 1983; Skorinkin et al, 2004). A caveat of these studies is that they were performed in either non-neuronal (chromaffin cells) or neuromuscular models, and the interpretation of the results extrapolated to suggest a trapping mechanism for mecamylamine at nAChRs in the CNS.…”