Fast inward-rectifying current accounts for anomalous rectification in olfactory cortex neurones

Abstract

The somatic membrane of guinea-pig olfactory cortex neurones in vitro (23 degrees C) was voltage clamped by means of a single-micro-electrode sample-and-hold technique. In most cells the current-voltage (I-V) relationship showed inward (anomalous) rectification with increasing hyperpolarization beyond the resting potential (ca. -80 mV). Under current-clamp conditions a time-dependent 'sag' of the hyperpolarizing electrotonic potentials was observed following an initial overshoot. No depolarizing after-potential was seen on return to the resting potential. Inward rectification was activated between -100 and -110 mV (irrespective of pre-set resting potential) and increased the membrane input conductance by up to three-fold. The rectification was unaffected by tetrodotoxin or Cd2+. Under somatic voltage clamp, hyperpolarization beyond -110 mV activated a rapid inward relaxation fitted by a single exponential. The relaxation time constant (tau on) decreased e-fold for a 40 mV hyperpolarization. (Typical values: 28 ms at -110 mV declining to 13 ms at -140 mV; external K+ concentration 3 mM, 23 degrees C). More extreme hyperpolarizations evoked a slower 'inactivation' phase (tau = 40-60 ms). A transient outward-decaying 'tail' current reflecting deactivation of inward rectification was seen on stepping from -140 mV to more positive potentials. tau off became slower with hyperpolarization. The tail current disappeared at a potential close to the expected VK but was rarely inverted to an inward-decaying tail. It is proposed that the fast inward-rectifying current of olfactory neurones (If.i.r.) is a K+ current analogous to the anomalous K+ rectifier of marine egg and frog muscle membranes. The behaviour of the inward rectifier was dependent on external K+ concentration in accordance with the unique 'V--VK' dependence of classical anomalous rectification; however, of several agents tested (external Cs+, Ba2+, Rb+, Tl+ or tetraethylammonium), only Cs+ and Ba2+ blocked If.i.r. in a time- and voltage-dependent manner. The effect of tetraethylammonium resembled that of an increase in external K+. The possible contribution of the inward rectifier to the passive cell membrane properties is discussed.