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.

Living motor nerve terminals from several species can be stained in an activity-dependent fashion by certain styryl dyes, such as RH414, RH795, and a new dye, FM1-43, which can be imaged independently of the others. The dyes evidently become trapped within recycled synaptic vesicles. In frog cutaneus pectoris muscle, bright fluorescent spots spaced regularly along the length of the nerve terminals appear after stimulation in the presence of the dye. The spots align well with postsynaptic ACh receptors and are persistent for many hours, unless further stimulation is given, in which case the spots disappear. Destaining, like staining, requires transmitter release and proceeds gradually over several minutes at high stimulus frequencies (e.g., 30 Hz), and fluorescent spots in the same terminal disappear at about the same rate. We suggest that each spot is a cluster of hundred of synaptic vesicles and that the mechanism of staining involves the ability of the dyes to partition reversibly into the outer leaflet of surface membranes, without being able to penetrate the entire membrane thickness. Then, during endocytosis following transmitter release, dye molecules become trapped in recycled synaptic vesicle membranes. The dyes therefore make it possible optically to study vesicle exocytosis and recycling in living nerve terminals in real time, and should be useful for marking terminals in a variety of preparations according to their level of activity.

However, excitation by glutamate was abolished by (R,S)-3,5-dihydroxyphenylglycine (DHPG), and rather more effectively by (2R,1 -S,2 -R,3 -S)-2-(2 -carboxy-3 -phenylcyclopropyl)glycine (PCCG-13). PCCG-13 also significantly reduced stretch-activated excitability in the absence of exogenous glutamate. These data indicate that SLVs recycle at rest, releasing glutamate, and that mechanical activity increases this process. The blockade with DHPG and PCCG-13 suggests that endogenous glutamate release acts, at least in part, through the recently described phospholipase D-linked metabotropic Glu receptor to maintain the excitability of the sensory endings.

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00424-014-1536-9) contains supplementary material, which is available to authorized users.

We have investigated whether rat motor nerve terminals with different in vivo activity 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 their in vivo activity patterns by alterations in QC and VP size but not recycle time.