Two types of ACh receptors contribute to fast channel gating on mouse skeletal muscle

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

Under normal conditions in situ, muscle fibers and motoneurons, the main partners of motor units, are strongly dependent on each other. This interdependence hinders ex vivo studies of neuromuscular disorders where nervous or muscular components are considered separately. To allow in vitro access to complex nerve-muscle relationships, we developed a novel nerve-muscle co-culture system where mouse muscle innervation is assured by rat spinal cord explants. The degree of muscular maturation during co-culture was evaluated using the distribution of nicotinic acetylcholine receptors (AChRs) and their electrophysiological characteristics before and after innervation. In myotubes from non-innervated cultures, AChRs were diffusely distributed over the entire myotube surface. Their single-channel conductance (33.5+/-0.6 pS) and mean open time (8.1+/-0.7 ms) are characteristic of AChRs described in embryonic or denervated skeletal muscles. In innervated muscle fibers from co-cultures, AChRs appear as discrete aggregates and co-localize with synaptotagmin. In addition to the embryonic type currents, in innervated fibers AChR currents having high conductance (53.3+/-5.9 pS) and short mean open time (2.6+/-0.1 ms), characteristic of AChRs at mature neuromuscular junctions, were observed. Our data support the use of this new nerve-muscle co-culture system as a reliable model for the study of murine muscular differentiation and function.

Quantal endplate currents from newborn to adult mice and the switch from embryonic to adult channel type

Quantal endplate currents (qEPCs) were recorded extracellularly by a macropatch electrode from excised diaphragms of mice. During the first 3 days after birth, the mean rise time t(r) was 0.5 ms (0.1-0.9 peak, 20 degrees C). Double-exponential, amplitude-weighted fits of the decay discerned almost equally abundant components tau(1)' approximately equal to 6 ms and tau(2)' approximately equal to 9 ms. Beginning on day 3, on days 4 and 5 after birth both t(r) and the tau' dropped. Further decreasing slowly, adult values were reached at day 8, with t(r) approximately equal to 0.3 ms, tau(1)' approximately equal to 2 ms and a very weak tau(2)' approximately equal to 6 ms component. When compared to the kinetics of fetal channels, the tau(1)' and tau(2)' of up to 3 day qEPCs could correspond to the short and long splice variants of the fetal channel type. The tau(1)' of adult muscles of 2 ms agrees well with the burst durations of adult channels while a weak longer tau(2)' component may represent 'extrasynaptic' channels. The long t(r) of very young mice may correspond to the relatively slow rise of channel currents elicited by ACh pulses in mouse myoballs.

Maturation of neuromuscular transmission during early development in zebrafish

We have examined the rapid development of synaptic transmission at the neuromuscular junction (NMJ) in zebrafish embryos and larvae by patch-clamp recording of spontaneous miniature endplate currents (mEPCs) and single acetylcholine receptor (AChR) channels. Embryonic (24-36 h) mEPCs recorded in vivo were small in amplitude (<50 pA). The rate of mEPCs increased in larvae (3.5-fold increase measured by 6 days), and these mEPCs were mostly of larger amplitude (10-fold on average) with (</=5-fold) faster kinetics. Intracellular labeling with Lucifer yellow indicated extensive coupling between muscle cells in both embryos and larvae (</=10 days). Blocking acetylcholinesterase (AChE) with eserine had no effect on mEPC kinetics in embryos at 1 day and only partially slowed (by approximately 1/2) the decay rate in larvae at 6 days. In acutely dissociated muscle cells, we observed the same two types of AChR with conductances of 45 and 60 pS and with similar, brief (<0.5 ms) mean open times in both embryos and larvae. We conclude that AChR properties are set early during development at these early stages; functional maturation of the NMJ is only partly shaped by expression of AChE and may also depend on postsynaptic AChR clustering and presynaptic maturation.