VOLTAGE-CLAMP ANALYSIS AND COMPUTER-SIMULATION OF A NOVEL CESIUM-RESISTANT A-CURRENT IN GUINEA-PIG LATERODORSAL TEGMENTAL NEURONS

Increased firing of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) plays a critical role in generating the behavioral state s of arousal and rapid eye movement sleep. The majority of these neuro ns exhibit a prominent transient potassium current (I-A) that shapes f iring but the properties of which have not been examined in detail. Al though I-A has been reported to be blocked by intracellular cesium, th e I-A in LDT neurons appeared resistant to intracellular cesium. The p resent study compared the properties of this cesium-resistant current to those typically ascribed to I-A Whole cell recordings were obtained from LDT neurons (n = 67) in brain slices with potassium- or cesium-c ontaining pipette solutions. A transient current was observed in cells dialyzed with each solution (KGluc-85%; CsGluc-79%). However, in cesi um-dialyzed neurons, the transient current was inward at test potentia ls negative to about -35 mV. Extracellular 4-aminopyridine (4-AP; 2-5 mM) blocked both inward and outward current, suggesting the inward cur rent was reversed I-A rather than an unmasked transient calcium curren t as previously suggested. This conclusion was supported by increasing [K](o) from 5 to 15 mM, which shifted the reversal potential positive ly for both inward and outward current (+17.89 +/- 0.41 mV; mean +/- S E). Moreover, recovery from inactivation was rapid (tau = 15.5 +/- 4 m s; n = 4), as reported for I-A, and both inward and outward transient current persisted in calcium-free solution [0 calcium/4 mM ethylene gl ycol-bis (beta-aminoethyl ether) -N,N,N',N'-tetraacetic acid; n = 4] a nd during cadmium-blockade of calcium currents (n = 3). Finally, the t ransient current was blocked by intracellular 4-AP indicating that ade quate dialysis occurred during the recordings. Thus the Cs-resistant c urrent is a subthreshold IA We also estimated the voltage-dependence o f activation (V-1/2 = -45.8 +/- 2 mV, k = 5.21 +/- 0.62 mV, n = 6) and inactivation (V-1/2 = -59.0 +/- 2.38 mV, k = -5.4 +/- 0.49 mV, n = 3) of this current. Computer simulations using a morphologically accurat e model cell indicated that except for the extreme case of only distal A-channels and a high intracellular resistivity, our parameter estima tes were good approximations. In conclusion, guinea pig LDT neurons ex press subthreshold A-channels that are resistant to intracellular cesi um ions. This suggests that these channels differ fundamentally in the ir ion permeation mechanism from those previously studied. It remains to be determined if Cs+ resistance is common among brain A-channels or if this property is conferred by known A-channel subunits.