THE NONSPECIFIC ION-CHANNEL IN TORPEDO-OCELLATA FUSED SYNAPTIC VESICLES

1. Synaptic vesicles were isolated and fused into large structures wit h a diameter of more than 20 mu m to characterize their ionic channels . The 'cell'-attached and inside-out configurations of the patch clamp technique were used. 2. Two types of ion channels were most frequentl y observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conductin g state and a substate. The main conducting state has a slope conducta nce of 246 +/- 15 pS (+/- S.E.M., n = 15), in the presence of differen t combinations of KCl and potassium glutamate.4. From the reversal pot entials of the current-voltage (I-V) relation, it was concluded that t his channel conducts both Cl- and K+. 5. The non-specific channel is h ighly voltage dependent: under steady-state voltages it has a high ope n probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decr eases dramatically. 6. Voltage pulses showed that upon hyperpolarizati on (from holding potentials between -20 and +20 mV), the channels deac tivated; when the membrane was stepped back to the holding potential, the channels reactivated rapidly. 7. In a. number of experiments, when the pipette side was made more negative than the bath, the open proba bility also decreased. 8. Frequently, a substate with a conductance of about 44 +/- 4% (+/- S.E.M., n = 3) of the main state was detected. 9 . We speculate that this non-specific ion channel may have different r oles at the various stages of the life cycle of the synaptic vesicle. When the synaptic vesicle is an intracellular structure, it might help its transmitter-concentrating capacity by dissipating the polarizatio n. After fusion with the surface membrane, it might constitute an addi tional conductance pathway, taking part in frequency modulation of syn aptic transmission.