The non‐specific ion channel in Torpedo ocellata fused synaptic vesicles.

Abstract

1. Synaptic vesicles were isolated and fused into large structures with a diameter of more than 20 #sm 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 frequently observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conducting state and a substate. The main conducting state has a slope conductance of 246 + 15 pS (± S.E.M., n = 15), in the presence of different combinations of KCl and potassium glutamate. 4. From the reversal potentials of the current-voltage (I-V) relation, it was concluded that this channel conducts both Cl-and K+.5. The non-specific channel is highly voltage dependent: under steady-state voltages it has a high open probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decreases dramatically. 6. Voltage pulses showed that upon hyperpolarization (from holding potentials between -20 and +20 mV), the channels deactivated; 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 probability 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 roles 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 polarization. After fusion with the surface membrane, it might constitute an additional conductance pathway, taking part in frequency modulation of synaptic transmission.

Control of neurotransmitter release by an internal gel matrix in synaptic vesicles

“…Using low ionic concentration of the extracellular solution (2 mM KCl) in the frog neuromuscular junction, Van der Kloot (52) and Khanin et al (53) found that miniature end-plate currents changed polarity but were not abolished, suggesting that ions do not flow through the fusion pore. On the other hand, molecules that could generate ionic currents, such as synaptophysin (54), the proton pump, or other vesicular ionic channels (55,56), have been described in synaptic vesicles. The conjunction of an intravesicular matrix and ionic channels in synaptic vesicles would result in an adjustable molecular mechanism of neurotransmitter release after vesicle fusion, in which the activation of a single channel would lead to the release of part or all of the content.…”

Exocytotic catecholamine release is not associated with cation flux through channels in the vesicle membrane but Na+ influx through the fusion pore

“…Evidence for the presence of K + permeable channels in the membrane of synaptic vesicles and other neurosecretory vesicles including chromaffin granules [10][11][12][13][14][15] led to the hypothesis that during release entry of cations may occur through cation channels in the vesicle membrane 16 . If this were the dominant mechanism mediating ion exchange, the catecholamine molecules released through the fusion pore would be replaced by cations from the cytosol, giving rise to a net outward current.…”

Observations of calcium dynamics in cortical secretory vesicles

“…For such movement to occur, it is necessary that the lumen of the vesicle be positively charged as shown for a number of secretory vesicles [63, 64], and that there is an anion permeating channel in the vesicle membrane. Such channels have been described for a number of secretory vesicles [65–67]. The presence of pathways capable of altering the Ca 2+ -matrix and Ca 2+ transport properties supports coupled biochemistry as a factor in the observed super-Poisson behavior.…”