Formation of cholinergic synapse-like specializations at developing murine muscle spindles

Muscle spindles are complex stretch-sensitive mechanoreceptors. They consist of specialized skeletal muscle fibers, called intrafusal fibers, which are innervated in the central (equatorial) region by afferent sensory axons and in both polar regions by efferent gamma-motoneurons. We show that AChRs are concentrated at the gamma-motoneuron endplate as well as in the equatorial region where they colocalize with the sensory nerve ending. In addition to the AChRs, the contact site between sensory nerve ending and intrafusal muscle fiber contains a high concentration of choline acetyltransferase, vesicular acetylcholine transporter and the AChR-associated protein rapsyn. Moreover, bassoon, a component of the presynaptic cytomatrix involved in synaptic vesicle exocytosis, is present in gamma-motoneuron endplates but also in the sensory nerve terminal. Finally, we demonstrate that during postnatal development of the gamma-motoneuron endplate, the AChR subunit stoichiometry changes from the gamma-subunit-containing fetal AChRs to the epsilon-subunit-containing adult AChRs, similar and approximately in parallel to the postnatal subunit maturation at the neuromuscular junction. In contrast, despite the onset of epsilon-subunit expression during postnatal development the gamma-subunit remains detectable in the equatorial region by subunit-specific antibodies as well as by analysis of muscle spindles from mice with genetically-labeled AChR gamma-subunits. These results demonstrate an unusual maturation of the AChR subunit composition at the annulospiral endings and suggest that in addition to the recently described glutamatergic secretory system, the sensory nerve terminals are also specialized for cholinergic synaptic transmission, synaptic vesicle storage and exocytosis. (C) 2014 Elsevier Inc. All rights reserved.