Dystrophin is a cytoskeletal protein of muscle fibers; its loss in humans leads to Duchenne muscular dystrophy, an inevitably fatal wasting of skeletal and cardiac muscle. mdx mice also lack dystrophin, but are only mildly dystrophic. Utrophin, a homolog of dystrophin, is confined to the postsynaptic membrane at skeletal neuromuscular junctions and has been implicated in synaptic development. However, mice lacking utrophin show only subtle neuromuscular defects. Here, we asked whether the mild phenotypes of the two single mutants reflect compensation between the two proteins. Synaptic development was qualitatively normal in double mutants, but dystrophy was severe and closely resembled that seen in Duchenne. Thus, utrophin attenuates the effects of dystrophin deficiency, and the double mutant may provide a useful model for studies of pathogenesis and therapy.

We have used the activity-dependent probe FM1-43 with electron microscopy (EM) to examine endocytosis at the vertebrate nerve-muscle synapse. Preparations were fixed after very brief neural stimulation at reduced temperature, and internalized FM1-43 was photoconverted into an electron-dense reaction product. To locate the reaction product, we reconstructed computer renderings of individual terminal boutons from serial EM sections. Most of the reaction product was seen in 40-60 nm vesicles. All of the labeled vesicles were clathrin-coated, and 92% of them were located within 300 nm of the plasma membrane, suggesting that they had undergone little processing after retrieval from their endocytic sites. The vesicles (and by inference the sites) were not dispersed randomly near the plane of the membrane but instead were clustered significantly near active zones. Additional reaction product was found within putative macropinosomes; these appeared to form from deep membrane invaginations near active zones. Thus two mechanisms of endocytosis were evident after brief stimulation. Endocytosis near active zones is consistent with the existence of local exo/endocytic cycling pools. This mechanism also might serve to maintain alignment of active zones with postsynaptic folds during periods of activity when vesicular and plasma membranes are interchanged. Key words: clathrin; endocytosis; nerve terminal; neuromuscular junction; neurosecretion; optical probes; vesicle processingThe cycle of transmitter exocytosis and subsequent endocytosis of spent vesicular membranes first was described nearly three decades ago (Couteaux and Pecot-Dechavassine, 1970;Heuser and Miledi, 1971;Ceccarelli et al., 1973Ceccarelli et al., , 1979Heuser and Reese, 1973). Although exocytosis was associated with active zones (AZs; Pecot-Dechavassine, 1970, 1974), the endocytic process was unclear in two respects: the type of endocytosis that was used under physiological conditions and the location at which, relative to the AZ, endocytosis took place (see Heuser, 1989). Additional information about exocytosis, endocytosis, and their coupling has become available from a variety of recent techniques (for review, see Angleson and Betz, 1997). However, the questions first debated by Heuser, Ceccarelli, and their colleagues have not been answered completely.Three endocytic mechanisms have been proposed. Clathrinmediated endocytosis, evidenced by coated pits and coated vesicles (Heuser and Reese, 1973), is the standard model. Macropinocytosis (Kadota et al., 1994;Takei et al., 1996), or bulk endocytosis of plasma membrane, is probably responsible for the formation of large "cisternae" within terminals; clathrin-mediated budding from the cisternae (and perhaps also from noninternalized membrane invaginations; Takei et al., 1996) then produces vesicles similar or identical to those endocytosed directly from the plasma membrane. "Kiss and run" transmitter release (Fesce et al., 1994;Ales et al., 1999; Daly et al., 2000) refers to the putative process whereby exo...

We have used a sensitive activity-dependent probe, sulforhodamine 101 (SR101), to view endocytic events within snake motor nerve terminals. After very brief neural stimulation at reduced temperature, SR101 is visualized exclusively at punctate sites located just inside the presynaptic membrane of each terminal bouton. The number of sites (approximately 26 sites/bouton) and their location (in register with postsynaptic folds) are similar to the number and location of active zones in snake motor terminals, suggesting a spatial association between exocytosis and endocytosis under these stimulus conditions. With more prolonged stimulation, larger SR101-containing structures appear at the bouton margins. Thus endocytosis occurs initially at distinct sites, which we call "endocytic active zones," whereas further stimulation recruits a second endocytic paradigm.

A B S T R A C T The effect of changing the ionic composition of bathing fluid on the receptor potential of primary endings has been examined in isolated mammalian spindles whose capsule was removed in the sensory region. After impulse activity is blocked by tetrodotoxin, ramp-and-hold stretch evokes a characteristic pattern of potential change consisting of a greater dynamic depolarization during the ramp phase and a smaller static depolarization during the hold phase. After a high-velocity ramp there is a transient post-dynamic undershoot to below the static level. On release from hold stretch, the potential shows a postrelease undershoot relative to base line. The depolarization produced by stretch is rapidly decreased by the removal of Na + and Ca 2+. Addition of normal Ca 2+ partly restores the response. Stretch appears to increase the conductance to Na + and Ca 2+ in the sensory terminals. The postdynamic undershoot is diminished by raising external K + and blocked by tetraethylammonium (TEA). It apparently results from a voltage-dependent potassium conductance. The postrelease undershoot is decreased by raising external K +, but is not blocked by TEA. It is presumably caused by a relative increase in potassium conductance on release. Substitution of isethionate for CI-or the addition of ouabain does not alter the postdynamic and postrelease undershoots.

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