Three conductance classes of nicotinic acetylcholine receptors are expressed in developing amphibian skeletal muscle

Abstract

Two previously described classes of nicotinic AChRs in vertebrate skeletal muscle have conductances of 40 and 60 pS. In addition, a third conductance class of AChR channels is present in developing Xenopus muscle. This class appears to represent an independent channel type, rather than a subconductance state of the larger conductance channels. The channel has a slope conductance of 25 pS and a reversal potential of about 0 mV membrane potential. Its kinetic properties resemble those of the 40 pS channels present in early embryonic myotomal muscle. The channel has a mean open time of about 6 msec (at 40 mV applied potential). The open time is dependent on membrane potential and increases e-fold for every 60 mV of hyperpolarization. Consecutive openings were often separated by brief closures of about 0.4 msec in duration. The identity of the channel as a nicotinic AChR was established by blocking the channel openings with alpha-BTX and by demonstrating bursting and desensitization in the presence of high agonist concentrations. In some muscles (e.g., extraocular), this channel may be a predominant form at early developmental stages and could therefore be important to the function of developing synapses in those muscles.

Multiple conductance classes of mouse nicotinic acetylcholine receptors expressed in Xenopus oocytes.

“…Recordings from BC3H-1 cells (27) have not revealed multiple conductance levels of AcChoRs such as we observed in oocytes; however, AcChoR channels with conductances lower than 40 pS have been reported in Xenopus muscle in vivo (7,8,28) and in denervated mouse muscle (29). One of these, a 25-pS channel, has been well characterized in developing Xenopus muscle (8).…”

“…Although both kinds of receptors assemble less efficiently and the 8-less receptors have reduced agonist affinity (5), the weak responses to AcCho might also reflect a reduced conductance or a briefer open time. We were interested in examining the properties ofthese receptors because they might offer an explanation for the small conductance channels (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) pS) observed in developing amphibian muscle (7,8). We injected mRNAs encoding the mouse a, 3, y, and 8 subunits into Xenopus oocytes and found that multiple conductance classes of channels were expressed on the oocyte membrane, and one of these, the smallest, is due to deletion of the 8 subunit.…”

“…One of these, a 25-pS channel, has been well characterized in developing Xenopus muscle (8). In some recordings, this is the predominant channel type, suggesting that it may play a significant role in neurotransmission.…”

Nicotinic acetylcholine receptors (nAChRs) at zebrafish red and white muscle show different properties during development

“…Outside out patches were primarily taken from the middle of the muscle and it is possible that we recorded from membrane that contained junctional and extrajunctional receptors. But we do not have any evidence to indicate that synaptic receptors are different from extrasynaptic ones, and findings from rats and Xenopus have indicated that junctional and extra‐junctional channels are identical (Hamill and Sakmann, ; Kullberg et al, ; Brehm et al, ; Brehm and Kullberg, ; Owens and Kullberg, ).…”

One GABA and two acetylcholine receptors function at the C. elegans neuromuscular junction

“…Why are there two classes of acetylcholine receptors in body wall muscles? First, the multiple acetylcholine receptors could provide kinetically and pharmacologically diverse responses [26][27][28] . For example, in chicken sympathetic neurons, multiple acetylcholine receptor subtypes with distinct single-channel conductances and kinetics 29 form homogeneous clusters, which are spatially segregated in the same neuron 26 .…”