Role of Potassium Channels in Facilitation of Transmitter Release from Frog Motor Nerve Ending (Electrophysiology and Mathematical Simulation)

In experiments on neuromuscular junctions in the frog m. thoraco-cutaneous, we studied changes in the transmitter release and shape of the nerve ending (NE) response related to high-frequency (10 or 50 sec^-1) rhythmic stimulation of the motor nerve; an extracellular recording technique was used. At a low extracellular Ca²⁺ concentration, rhythmic stimulation resulted in a gradual increase in the quantum content of end-plate currents, i.e., in facilitation. Simultaneously, the third (positive) phase of the NE response became smaller, the amplitude of the second (negative) phase of this response also decreased, while the duration of this phase increased. Modifications of the NE response upon stimulation with a 10 sec^-1 frequency were more clearly expressed than those at 50 sec^-1 stimulation. In Ca²⁺-free solutions, rhythmic stimulation was accompanied by analogous modifications of the shape of NE responses, and the dynamics of these changes were the same at both the above-mentioned stimulation frequencies. When 0.5-1.0 mM tetraethylammonium was applied, 10 sec^-1 stimulation was accompanied by no facilitation of transmitter release; at 50 sec^-1 stimulation, this phenomenon was observed but was weaker than in the control, and the shape of NE responses underwent only mild changes. Simulation of electrogenesis in the studied structure showed that modifications of the NE response shape related to rhythmic 10 sec^-1 stimulation can develop in the case of a gradual decrease in the voltage-dependent potassium membrane conductivity, which results in prolongation of the de- and repolarization phases of action potentials and increases in the amplitude and duration of the inward calcium current. At higher stimulation frequencies (50 sec^-1), this mechanism is accompanied by a gradual increase in the Ca²⁺-dependent potassium conductivity, due to an increase in the intracellular Ca²⁺ concentration. These data allow us to conclude that the intensity of facilitation of transmitter release from the frog motor NE is related not only to accumulation of residual calcium, but also to changes of presynaptic calcium current due to modification of the kinetics of functioning of the potassium channels.     

Changes in Electrogenesis in the Motor Nerve Terminal with Long-Lasting High-Frequency Activation as a Factor of the Control of Transmitter Release

Using the technique of extracellular recording from the region of the neuromuscular junction in the cutaneous-sternal muscle in the frog under conditions of a reduced concentration of Ca²⁺ in the surrounding milieu, we demonstrated that long-lasting (10 min) rhythmic stimulation of the motor nerve with a frequency of 10 sec^{− 1} leads to a gradual increase in the evoked transmitter release. These changes are accompanied by a decrease in the amplitude of electrical responses of the nerve terminal (NT) and by a retardation of its second phase, as well as by a diminution of the third phase. Under conditions of long-lasting (5 min) stimulation with a frequency of 50 sec⁻¹, we observed a two-phase change in the intensity of transmitter release: on the 2nd min, the initial rise was replaced by inhibition. Modifications of the response of the NT with different stimulation frequencies were qualitatively similar, but with a frequency of 10 sec⁻¹ they were clearly expressed. Mathematical simulation of ion currents in the NT demonstrated that voltage-dependent potassium and sodium channels are inactivated in the course of long-lasting high-frequency excitation; the shape of the action potential is modified with changes in the rate of such inactivation. This leads to either an increase or a decrease of the inward calcium current. We conclude that the change in electrogenesis in the NT with long-lasting high-frequency activation of neuromuscular junctions exerts a significant influence on the dynamics of transmitter release. Neirofiziologiya/Neurophysiology, Vol. 37, No. 2, pp. 108–115, March–April, 2005.