Activation and inactivation of the bursting potassium channel from fused Torpedo synaptosomes.

Abstract

SUMMARY1. The voltage dependence of the bursting potassium channel in fused synaptosomes from Torpedo electric organ was studied in vitro, using the inside-out and the cell-attached configurations of the patch clamp technique.2. The patch of membrane was held at various holding potentials (-140 to -50 mV) and then stepped to test potentials (-50 to +40 mV) for periods ranging from 5 to 300 ms. Each potential step was repeated 200-600 times. After subtraction of the capacitative transients and the leakage currents, an ensemble-averaged current was obtained. This ensemble current showed a marked activation upon depolarization, followed by an inactivation.3. The activation of the bursting potassium channel is markedly dependent on the voltage step. Activation was detected at voltages positive to -50 mV. The peak of the ensemble current increases with the degree of depolarization, while the time to the peak decreases. With progressively larger depolarization, there is a shortening in the delay between the onset of the voltage step and the opening of the bursting potassium channels.4. The inactivation phase of the ensemble current could be described adequately in most of the experiments, as a single exponential decay to a steady-state inactivation level. The time constant of inactivation was not markedly voltage dependent. by 5 ms pulses produces a progressive decline in the peak ensemble current amplitude. The decline is larger at higher stimulation frequencies.9. The voltage-and time-dependent activation and inactivation properties of the bursting potassium channel make it a possible candidate for participating in frequency modulation of transmitter release and thus of synaptic transmission. We propose the potassium inactivation hypothesis for frequency modulation, which states that the inactivation of the potassium channel by previous stimulation could cause a broadening of the subsequent action potential and hence augmentation of calcium entry and transmitter release.

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

“…Since we cannot record from the native preparation, it is hard to assess whether this is a major problem. But the similarity of other channels that were found in the fused synaptic vesicles (Yakir & Rahamimoff, 1993 b) or in the fused synaptosomes (Edry-Schiller, Ginsburg & Rahamimoff, 1991;Edry-Schiller & Rahamimoff, 1993) to channels characterized in preparations that were not subjected to fusion procedures (Miller & Richard, 1990;Zagotta and Aldrich, 1990) makes us believe that the alterations are probably small. The third drawback is the loss of the intravesicular content which is greatly diluted by the fusion procedure.…”

“…Since we cannot record from the native preparation, it is hard to assess whether this is a major problem. But the similarity of other channels that were found in the fused synaptic vesicles (Yakir & Rahamimoff, 1993 b) or in the fused synaptosomes (Edry-Schiller, Ginsburg & Rahamimoff, 1991;Edry-Schiller & Rahamimoff, 1993) to channels characterized in preparations that were not subjected to fusion procedures (Miller & Richard, 1990;Zagotta and Aldrich, 1990) One advantage of the fused preparation used here over other fusion procedures is that no foreign lipids are added. In some other methods, developed to study organelles too small for direct patch clamp recordings, foreign lipids were added; changes in the lipid composition of membranes are known to affect the behaviour of channels in some cases (Bell & Miller, 1984;Moczydlowski, Alvarez, Vergara & Latorre, 1985;Coronado & Affolter, 1986;Cukierman, Zinkand, French & Krueger, 1988).…”