Data from: Functional limb muscle innervation prior to cholinergic transmitter specification during early metamorphosis in Xenopus

In vertebrates, functional motoneurons are defined as differentiated
neurons that are connected to a central premotor network and activate
peripheral muscle using acetylcholine. Generally, motoneurons and muscles
develop simultaneously during embryogenesis. However, during Xenopus
metamorphosis, developing limb motoneurons must reach their target muscles
through the already established larval cholinergic axial neuromuscular
system. Here, we demonstrate that at metamorphosis onset, spinal neurons
retrogradely labeled from the emerging hindlimbs initially express neither
choline acetyltransferase nor vesicular acetylcholine transporter.
Nevertheless, they are positive for the motoneuronal transcription factor
Islet1/2 and exhibit intrinsic and axial locomotor-driven
electrophysiological activity. Moreover, the early appendicular
motoneurons activate developing limb muscles via nicotinic
antagonist-resistant, glutamate antagonist-sensitive, neuromuscular
synapses. Coincidently, the hindlimb muscles transiently express
glutamate, but not nicotinic receptors. Subsequently, both pre- and
postsynaptic neuromuscular partners switch definitively to typical
cholinergic transmitter signaling. Thus, our results demonstrate a novel
context-dependent respecification of neurotransmitter phenotype during
neuromuscular system development.