CONICET | Buscador de Institutos y Recursos Humanos

At mouse neuromuscular junction (NMJ) there are two functionally distinct synaptic vesicle pools according to FM dyes loading/ unloading patterns: a fast destaining vesicle pool which is rapidly recycled during high frequency stimulation and is modulated through L channel, and a slow destaining vesicle pool which is recycled during prolonged stimulation and keeps on refilling after end of stimulation. It has been reported that presynaptic receptors play a role in adjusting the pattern of neuromuscular transmission. Modulation of Ca2+ currents at mammalian NMJ occurs through hydrolysis of ATP to adenosine, followed by the activation of adenosine inhibitory (A1 and A3) and excitatory (A2A and A2B) receptors. However, whether adenosine may regulate vesicle recycling is a question which has not been addressed until now. We used fluorescence microscopy of FM2-10-labeled synaptic vesicles and electrophysiological recordings to examine whether adenosine has a role on vesicle recycling. We found that the quantal content significantly decreased by 40 in presence of adenosine and increased by 25 in presence of DPCPX (A1 antagonist) at high (50 Hz) frequency stimulation, suggesting an inhibitory effect of either exogenously applied or endogenous adenosine. We studied the effect of adenosine and DPCPX on the amount of FM2-10 loaded during a stimulation protocol that preferentially load the fast destaining pool (5 s at 50 Hz). In both cases, we found that loading was lower than control experiments. However, dye unloading during a second round of stimulation was faster when the dye was loaded in presence of DPCPX but not in presence of adenosine. These results showed that: 1) exogenously applied and endogenous adenosine has an inhibitory effect on transmitter release and reduces the size of the recycling pool, 2) the antagonist of the adenosine inhibitory receptor, DPCPX, increases the neurotransmitter release recycling vesicles towards a fast release pool and 3) in presence or absence of adenosine, there are activity-dependent differences in endocytosic mechanisms. Adenosine, as L channel, modulates vesicle recycling.