The fluorescent dyes FM1-43 and RH414 label motor nerve terminals in an activity-dependent fashion that involves dye uptake by synaptic vesicles that are recycling. This allows optical monitoring of vesicle recycling in living nerve terminals to determine how recycled vesicles reenter the vesicle pool. The results suggest that recycled vesicles mix with the pool morphologically and functionally. One complete cycle of release of transmitter, recycling of a vesicle, and rerelease of transmitter appears to take about 1 minute.

Acid-sensing ion channel 3 (ASIC3) is involved in acid nociception, but its possible role in neurosensory mechanotransduction is disputed. We report here the generation ofAsic3-knockout/eGFPf-knockinmice and subsequent characterization of heterogeneous expression of ASIC3 in the dorsal root ganglion (DRG). ASIC3 is expressed in parvalbumin (Pv+) proprioceptor axons innervating muscle spindles. We further generate a floxed allele ofAsic3(Asic3^f/f) and probe the role of ASIC3 in mechanotransduction in neurite-bearing Pv+ DRG neurons through localized elastic matrix movements and electrophysiology. Targeted knockout ofAsic3disrupts spindle afferent sensitivity to dynamic stimuli and impairs mechanotransduction in Pv+ DRG neurons because of substrate deformation-induced neurite stretching, but not to direct neurite indentation. In behavioural tasks, global knockout (Asic3^−/−) andPv-Cre::Asic3^f/fmice produce similar deficits in grid and balance beam walking tasks. We conclude that, at least in mouse, ASIC3 is a molecular determinant contributing to dynamic mechanosensitivity in proprioceptors.

The focus of this review is on the principal sensory ending of the mammalian muscle spindle, known as the primary ending. The process of mechanosensory transduction in the primary ending is examined under five headings: (i) action potential responses to defined mechanical stimuli—representing the ending's input–output properties; (ii) the receptor potential—including the currents giving rise to it; (iii) sensory-terminal deformation—measurable changes in the shape of the primary-ending terminals correlated with intrafusal sarcomere length, and what may cause them; (iv) putative stretch-sensitive channels—pharmacological and immunocytochemical clues to their identity; and (v) synaptic-like vesicles—the physiology and pharmacology of an intrinsic glutamatergic system in the primary and other mechanosensory endings, with some thoughts on the possible role of the system. Thus, the review highlights spindle stretch-evoked output is the product of multi-ionic receptor currents plus complex and sophisticated regulatory gain controls, both positive and negative in nature, as befits its status as the most complex sensory organ after the special senses.

The distributions of dystrophin, 'dystrophin-related protein' (DRP) and beta-spectrin were compared with that of acetylcholine receptors (AChRs) at rat nerve-muscle junctions (NMJs) using immunofluorescence techniques. In sections, monoclonal antibodies (MAbs) to dystrophin and beta-spectrin labelled the entire sarcolemma but were concentrated at the NMJs while those to DRP labelled only NMJs. In permeabilized muscle fibres, DRP was precisely co-localized with the AChRs, whereas the zone of high density labelling of dystrophin and beta-spectrin extended 0.3-0.4 microns beyond the AChRs. Within the NMJ, the labelling of DRP appeared as a series of interconnecting lines similar to that of AChRs. However, labelling of dystrophin and beta-spectrin was consistently more punctate. These data suggest DRP is more closely associated with AChRs than are dystrophin or beta-spectrin.

We have investigated whether rat motor nerve terminals with differentin vivoactivity patterns also have different vesicle trafficking characteristics. To do this, we monitored, using combined optical and electrical techniques, the rate of exocytosis (during different frequencies and patterns of activity), the releasable pool size, and the recycle time of synaptic vesicles in terminals on soleus (slow-twitch) and extensor digitorum longus [(EDL); fast-twitch] muscle fibers. EDL terminals had a higher initial quantal content (QC) than soleus, but during tonic or phasic stimulation at 20–80 Hz, EDL QC ran down to a greater extent than soleus QC. By recording loss of fluorescence from exocytosing vesicles labeled with the dye FM1–43, EDL terminals were found to destain faster than those in soleus. Simultaneous intracellular recording of end plate potentials, to count the number of vesicles released, permitted estimation of the total vesicle pool (VP) size and the recycle time by combining the optical and electrophysiological data. Soleus vesicle pool was larger than EDL, but recycle time was not significantly different. These terminals, therefore, are adapted to theirin vivoactivity patterns by alterations in QC and VP size but not recycle time.