The ionic mechanism of the slow outward current in Aplysia neurons (original) (raw)
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Three types of early transient potassium currents in Aplysia neurons
The Journal of Neuroscience, 1992
In an attempt to categorize the various early transient K+ currents (A-type K+ currents) present in mature neurons, we have explored these currents in the identified neurons in the abdominal ganglion of the mollusk, Ap/ysia californica. Three distinct types of A-type K+ currents (I
1996
The R20 neurons of Aplysia exhibit frequency-dependent spike broadening. Previously, we had used two-electrode voltage clamp to examine the mechanisms of this spike broadening (Ma and Koester, 1995). We identified three K ϩ currents that mediate action-potential repolarization: a transient A-type K ϩ current (I Adepol), a delayed rectifier current (I K-V), and a Ca 2ϩsensitive K ϩ current (I K-Ca). A major constraint in that study was the lack of completely selective blockers for I Adepol and I K-V , resulting in an inability to assess directly the effects of their activation and inactivation on spike broadening. In the present study, the dynamic-clamp technique, which employs computer simulation to inject biologically realistic currents into a cell under current-clamp conditions (Sharp et al., 1993a,b), was used either to block I Adepol or I K-V or to modify their inactivation properties. The data in this paper, together with earlier results, lead to the following hypothesis for the mechanism of spike broadening in the R20 cells. As the spike train progresses, the primary responsibility for spike repolarization gradually shifts from I Adepol to I K-V to I K-Ca. This sequence can be explained on the basis of the relative rates of activation and inactivation of each current with respect to the constantly changing spike durations, the cumulative inactivation of I Adepol and I K-V , and the progressive potentiation of I K-Ca. Positive feedback interactions between spike broadening and inactivation contribute to the cumulative inactivation of both I Adepol and I K-V. The data also illustrate that when two or more currents have similar driving forces and partially overlapping activation characteristics, selectively blocking one current under current-clamp conditions can lead to a significant underestimate of its normal physiological importance.
The Journal of Neuroscience, 1996
The R20 neurons of Aplysia exhibit frequency-dependent spike broadening. Previously, we had used two-electrode voltage clamp to examine the mechanisms of this spike broadening (Ma and Koester, 1995). We identified three K ϩ currents that mediate action-potential repolarization: a transient A-type K ϩ current (I Adepol), a delayed rectifier current (I K-V), and a Ca 2ϩsensitive K ϩ current (I K-Ca). A major constraint in that study was the lack of completely selective blockers for I Adepol and I K-V , resulting in an inability to assess directly the effects of their activation and inactivation on spike broadening. In the present study, the dynamic-clamp technique, which employs computer simulation to inject biologically realistic currents into a cell under current-clamp conditions (Sharp et al., 1993a,b), was used either to block I Adepol or I K-V or to modify their inactivation properties. The data in this paper, together with earlier results, lead to the following hypothesis for the mechanism of spike broadening in the R20 cells. As the spike train progresses, the primary responsibility for spike repolarization gradually shifts from I Adepol to I K-V to I K-Ca. This sequence can be explained on the basis of the relative rates of activation and inactivation of each current with respect to the constantly changing spike durations, the cumulative inactivation of I Adepol and I K-V , and the progressive potentiation of I K-Ca. Positive feedback interactions between spike broadening and inactivation contribute to the cumulative inactivation of both I Adepol and I K-V. The data also illustrate that when two or more currents have similar driving forces and partially overlapping activation characteristics, selectively blocking one current under current-clamp conditions can lead to a significant underestimate of its normal physiological importance.
Properties of voltage dependent potassium currents in acutely isolated rat oculomotor neurons
Neuroscience Research, 2002
Voltage-dependent potassium currents in oculomotor neurons (OMNs) were studied with whole-cell patch clamp recordings. Fast inactivating outward currents (I fast) had half activation voltage (Vh) of − 37.1 mV with slope factor (Vc) of 10.9 mV. I fast had half inactivation voltage (Vh) of − 66.5 mV and Vc of 11.4 mV. I fast decayed with a time constant(~) of 5.1 ms at + 10 mV. I fast were sensitive to 4-aminopyridine, showing 50% inhibitory concentration (IC 50) of 0.97 mM. Slowly inactivating outward currents (I slow) had two components. The low-concentration-TEA-sensitive currents had Vh of − 3.7 mV with Vc of 9.7 mV in activation and had Vh of −54.7 mV with Vc of 23.8 mV in inactivation. The persistent currents had Vh of 7.4 mV and Vc of 11.8 mV in activation and Vh of −54.4 mV and Vc of 21.2 mV in inactivation. Decay of I slow (+10 mV) followed a double exponential time course (~215, 1165.6 ms). Low-concentration-TEA-sensitive currents were blocked completely by tetraethylammonium (TEA) of 3 mM with an IC 50 of 1.52 mM. Higher concentrations (3-20 mM) of TEA blocked the persistent currents, with an IC 50 of 6.9 mM.
The Journal of neuroscience : the official journal of the Society for Neuroscience, 1986
The neurosecretory bag cell neurons of the mollusk, Aplysia, control egg-laying behavior in the animal. In these cells, elevation of cAMP greatly enhances the height and width of action potentials. A similar enhancement of action potentials is seen during the bag cell afterdischarge, a 30 min period of repetitive activity that may be triggered by peptides from the reproductive tract or by brief extracellular stimulation. The enhancement of action potentials during an afterdischarge is well correlated with the observed elevations of cAMP. In the present study, we have examined the effects of forskolin (an activator of adenylate cyclase) and theophylline (a phosphodiesterase inhibitor) on the delayed outward currents that are likely to control repolarization of the action potential. Isolated bag cell neurons, maintained in primary culture, were studied with a whole-cell patch clamp technique. High intracellular concentrations of EGTA were used to block potassium current activated by c...
A-type potassium currents active at subthreshold potentials in mouse cerebellar Purkinje cells
The Journal of physiology, 2002
Voltage-dependent and calcium-independent K+ currents were whole-cell recorded from cerebellar Purkinje cells in slices. Tetraethylammonium (TEA, 4 mM) application isolated an A-type K+ current (I(K(A))) with a peak amplitude, at +20 mV, of about one third of the total voltage-dependent and calcium-independent K+ current. The I(K(A)) activated at about -60 mV, had a V(0.5) of activation of -24.9 mV and a V(0.5) of inactivation of -69.2 mV. The deactivation time constant at -70 mV was 3.4 +/- 0.4 ms, while the activation time constant at +20 mV was 0.9 +/- 0.2 ms. The inactivation kinetics was weakly voltage dependent, with two time constants; those at +20 mV were 19.3 +/- 3.1 and 97.6 +/- 9.8 ms. The recovery from inactivation had two time constants of 60.8 ms (78.4 %) and 962.3 ms (21.6 %). The I(K(A)) was blocked by 4-aminopyridine with an IC50 of 67.6 microM. Agitoxin-2 (2 nM) blocked 17.4 +/- 2.1 % of the I(K(A)). Flecainide completely blocked the I(K(A)) with a biphasic effect ...
Voltage-clamp analysis of a transient potassium current in rat neostriatal neurons
Brain Research, 1988
Whole cell voltage-clamp recordings were made from cultured rat neostriatal neurons. Depolarizing voltage commands evoked transient and sustained outward K-currents. The transient K-current was activated by depolarizing commands beyond -50 mV; peak current was dependent upon holding potential. Bath application of 4-aminopyridine, but not inorganic calcium channel blockers (Cd, Co, Mn), attenuated the transient current. Reversal was near the K-equilibrium potential. These properties suggest that this transient K-current is similar to the A-current described in a number of other neurons.
A fast, transient K+ current in neurohypophysial nerve terminals of the rat
The Journal of physiology, 1991
1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Macroscopic terminal membrane currents and voltages were studied using the whole-cell patch clamp technique. 2. In physiological solutions, depolarizing voltage clamp steps, from a holding potential (-80 mV) similar to the normal terminal resting potential, elicited a fast, inward followed by a fast, transient, outward current. 3. The threshold of activation for the outward current was -60 mV. The outward current quickly reached a peak and then decayed more slowly. The decay was fitted by two exponentials with time constants of 21 +/- 2.9 and 143 +/- 36 ms. These decay constants did not show a dependence on voltage. The time to peak of the outward current decreased and the amplitude increased with increasingly depolarized potential steps. 4. The outward current was blocked by the substitution of K+ with Cs+ and its reversa...