Whole-cell voltage-clamp recording techniques were used to investigate the blockade of voltage-dependent K+ channels by phencyclidine (PCP) in cultured rat hippocampal neurons. All recordings were carried out in the presence of tetrodotoxin (1-2 μM) to eliminate Na+ currents. Step depolarization from a holding potential of -40 mV activated a slowly rising, minimally inactivating K+ current (I(K)). PCP (0.5-1000 μM) caused a reduction in the maximum conductance of I(K) [IC50(+30 mV), 22 μM] without altering its voltage dependency. The PCP block of I(K) diminished at depolarized potentials. Analysis according to the scheme of Woodhull (1973) suggested that block occurs via binding to an acceptor site (presumably within the channel pore) that senses 40-50% of the transmembrane electrostatic field. PCP had no effect on the kinetic properties of I(K) and the block failed to show use dependency, suggesting that PCP may bind to the I(K) channel via a hydrophobic mechanism not requiring open channels. For comparison, we also investigated the effect of PCP on the transient K+ current, I(A), activated by step depolarization following a 200 msec prepulse to -90 mV (20 mM tetraethylammonium was present in the bathing solution to reduce I(K)). In contrast to the potent blocking action of PCP on I(K), the drug only affected I(A) at high concentrations [IC50(+30 mV), 224 μM]. At concentrations causing substantial block (300-500 μM), PCP produced an acceleration in the I(A) inactivation rate, and, for brief (5-6 msec) depolarizing steps, the suppression of I(A) was use dependent. These observations suggest that PCP block of I(A) requires open channels. PCP reduced inward current responses induced by the excitatory amino acid agonist N-methyl-D-aspartate (NMDA) at substantially lower concentrations than those required for its effect on K+ channels [IC50(-60 mV), 0.45 μM]. The PCP-like dioxadrol stereoisomer dexoxadrol (10 μM) blocked NMDA-evoked inward current responses, while its behaviorally inactive enantiomer levoxadrol did not. Dexoxadrol and levoxadrol also blocked I(K) in a stereoselective fashion (IC50's, 73 and 260 μM, respectively), whereas the sigma ligands (+)- and (-)-SKF 10,047 and (+)-3-[3-hydroxyphenyl]-N-(1-propyl)piperidine [(+)-3-PPP] had little effect on the current (IC50's >300-500 μM). We conclude that PCP causes a selective, voltage-dependent block of I(K) in hippocampal neurons via a PCP- and not a sigma-type acceptor site. The affinity of PCP for its acceptor site on I(K) channels (corrected for the voltage dependence) is 13 times lower than its affinity for NMDA-receptor-associated channels. Therefore, at low doses, the behavioral effects of the drug are more likely to result from an interaction with NMDA receptor channels than voltage-dependent K+ channels.
|Original language||English (US)|
|Number of pages||11|
|Journal||Journal of Neuroscience|
|State||Published - 1989|
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