Modulation of fetal and adult acetylcholine receptors by Ca2+ and Mg2+ at developing mouse end-plates

Abstract

It has long been known that extracellular Ca2+ and Mg2+ modulate synaptic transmission at the neuromuscular junction, acting both pre- and post-synaptically. Relevant questions concerning the modulation of acetylcholine (ACh) receptors (AChRs) are however still open: are the fetal (gamma-AChR) and adult (epsilon-AChR) receptors modulated differently? Does the ACh concentration influence the effect of divalent cations? Is the effect on channel open duration dependent on type and concentration of divalent cation? These questions were addressed by studying the modulation of the single-channel behaviour of gamma- and epsilon-AChRs by Ca2+ and Mg2+ at the endplate of muscle fibres acutely dissociated from 12- to 14-day-old mice. Ca2+ reduced the conductances of the two receptor channels comparably. Mg2+ had a stronger effect than Ca2+ and reduced the conductance of epsilon-AChR significantly more than that of gamma-AChR. With 0.1 microM ACh, Ca2+ and Mg2+ increased the mean open duration of gamma- and epsilon-AChR channels comparably. At 100 microM ACh, gamma- and epsilon-AChR channels opened in bursts of strikingly similar duration, which was unaffected by divalent cations. These findings indicate that Ca2+, and even more so Mg2+, may regulate synaptic transmission by modulating the function of AChRs in addition to the well-established effects on transmitter release.

Functional maturation of nicotinic acetylcholine receptors as an indicator of murine muscular differentiation in a new nerve-muscle co-culture system

“…Thus, in this new co-culture model, nerve-muscle The development of mature neuromuscular junctions in this co-culture system is also supported by the electrophysiological characterization of AChRs. Indeed, single-channel currents recorded from non-innervated myotube cultures had characteristics (small single-channel conductance, long mean open time) consistent with expression of AChRs of the embryonic form [9,10,16,25,29]. As AChR activity with similar characteristics was observed in myotubes from 10-day-old co-cultures, we conclude that fibers from young co-cultures express the fetal form of AChR.…”

“…4B), the single-channel slope conductance was 33.5€0.6 pS (n=20). This value is typical of the embryonic type of AChR channel containing the g subunit [4,9,10,16,25,28]. The extrapolated zero-current potential was 72.1€2.6 mV (Fig.…”

“…4). The larger currents had a significantly greater single-channel conductance of 53.3€5.9 pS and a significantly briefer mean channel open time of 2.6€0.1 ms, characteristic of eAChR (adult) channel currents [9,10,16,25]. The extrapolated zero-current potentials of 60.8€5.9 mV for the smaller current and 62.9€2.8 mV for the larger current are not significantly different, again close to an absolute value of 0 mV for the nAChR reversal potential if the muscle fiber resting potential is assumed to be about 60 mV.…”

Magnesium sulfate enhances non-depolarizing muscle relaxant vecuronium action at adult muscle-type nicotinic acetylcholine receptor in vitro

“…Krendal [7] suggested that magnesium sulfate has a pre-synaptic effect by inhibiting acetylcholine release at motor nerve terminals, effects that may be responsible for the interaction with vecuronium. Furthermore, some studies have shown that increasing extracellular magnesium reduces the size of acetylcholine-evoked responses and the single-channel conductance of nicotinic acetylcholine receptors at the mouse end-plate [8][9][10] .…”

Modulation of intercellular calcium signaling in astrocytes by extracellular calcium and magnesium

“…Significant fluctuations in [Ca 2ϩ ] o and [Mg 2ϩ ] o may occur in association with normal brain activity (Massimini and Amzica, 2001), as well as with a variety of pathological phenomena including ischemia, hypoglycemia, and seizures (Harris and Symon, 1984;Silver and Erecinska, 1990;Kristian et al, 1993;Altura et al, 1997). The effects of extracellular divalent cations may be mediated by multiple mechanisms, including modulation of Ca 2ϩ , K ϩ , and Na ϩ channels (Delpiano and Altura, 1996;Su et al, 2001;Yamaoka et al, 2002), cyclic nucleotide-gated channels (Colamartino et al, 1991;Frings et al, 2000;Lee et al, 2001), and ligand-gated channels (Grassi and Degasperi, 2000;Negulyaev and Markwardt, 2000;Arvanian and Mendell, 2001), as well as by direct activation of G-protein-coupled receptors (Riccardi, 1999;Brown, 2000). Until recently, attention has been focused primarily on the effects of extracellular divalent cations on neuronal signaling.…”