Voltage-Activated Potassium Outward Currents in Two Types of Spider Mechanoreceptor Neurons

Abstract

We studied the properties of voltage-activated outward currents in two types of spider cuticular mechanoreceptor neurons to learn if these currents contribute to the differences in their adaptation properties. Both types of neurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts lasting several hundred milliseconds. We found that both neurons had two outward current components, 1) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current could be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 microM quinidine, but these blockers also reduced the amplitude of the noninactivating outward current. Charybdotoxin or apamin did not have any effect on the outward currents, indicating that Ca2+-activated K+ currents were not present or not inhibited by these toxins. The only significant differences between type A and type B neurons were found in the half-maximal activation (V50) values of both currents. The transient current had a V50 value of 9. 6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We conclude that, although differences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

“…Therefore the remaining inward current in the test potentials used here (Ϫ50 to 0 mV) was most likely the LVA Ca 2ϩ current previously described in these neurons (Sekizawa et al 2000). The sustained outward current cannot be blocked by K-channel blockers or conventional inhibitors of I K(Ca) (Sekizawa et al 1999). However, this current was partially inhibited by Ni 2ϩ , which blocks LVA Ca channels (Sekizawa et al 2000), suggesting that it may be I K(Ca) that is not sensitive to the conventional blockers of this currents.…”

“…1) (Juusola et al 1994). For current-and voltage-clamp experiments a "hypodermis preparation" was used: the hypodermis membrane was detached from the cuticle with the VS-3 neurons attached and placed on a poly-L-lysine (1 mg/ml) coated coverslip in a recording chamber as described previously (Sekizawa et al 1999). Preparations were continuously superfused with spider saline [in mM: 223 NaCl, 6.8 KCl, 8 CaCl 2 , 5.1 MgCl 2 , 5 sucrose, and 10 HEPES, pH 7.8 (Höger et al 1997)].…”

“…In Ca 2ϩ -current and Ca 2ϩ -spike experiments the electrode-filling solution was 3 M CsCl. Discontinuous single-electrode current-or voltage-clamp methods with an SEC-10 L amplifier (npi electronic, Tamm, Germany) were used as described before (Sekizawa et al 1999). Switching frequencies of 20 -25 kHz and a duty cycle of 1/4 (current passing/voltage recording) were used in all experiments.…”

“…This Na ϩ current and action potentials can be completely blocked by tetrodotoxin (TTX). The repolarization phase is produced by a K ϩ outward current with transient and noninactivating components (Sekizawa et al 1999). Both components are sensitive to blockers of voltage-gated K channels, tetraethylammonium chloride (TEA) and 4-aminopyridine (4-AP).…”

“…Both components are sensitive to blockers of voltage-gated K channels, tetraethylammonium chloride (TEA) and 4-aminopyridine (4-AP). However, Յ50% of the noninactivating component was not blocked even with a high concentration of combination of these drugs or other K-channel blockers (Sekizawa et al 1999). The only Ca 2ϩ current that has been described in the VS-3 neurons is the transient low-voltage-activated (LVA) current, that can be completely blocked with Ni 2ϩ or Cd 2ϩ (Sekizawa et al 2000).…”