Inward-rectifying membrane currents activated by hyperpolarization in immature rat olfactory cortex neurones in vitro

The properties of inward-rectifying membrane currents in immature rat olfactory cortex neurones (postnatal day (P) 10-22) were analysed using whole-cell patch-clamp recordings. In 78% of cells (40/51), injection of hyperpolarizing current pulses elicited graded electrotonic potentials showing a slowly developing sag in the membrane potential. Under voltage clamp, negative commands from -50 mV activated slow inward current (I-Slow) relaxations whose amplitude and exponential rate of onset increased with increasing hyperpolarization (n = 40); the I-Slow activation time constant (tau(on)) ranged from 650 +/- 116 (mean +/- S.E.M.) ms at -70 mV to 177 +/- 18 ms at -120 mV; n = 34). By contrast, in 11/51 neurones, similar negative commands revealed only fast-type inward rectification (I-IR) with either rapid (n = 9) or 'instantaneous' onset kinetics (n = 2). I-Slow activation threshold was at similar to -60 mV, with full activation at -120 mV; the half-maximal voltage (V-0.5) and slope factor (k) of activation were: -85 +/- 0.4 mV and 11 +/- 0.5, respectively (n = 13). The estimated reversal potential for I-Slow was -28 +/- 2 mV (n = 5). No obvious age-dependent changes in maximal I-Slow current amplitude or density (at -120 mV) or in the proportion of cells showing I-IR were found between P10 and P22. I-slow was blocked by Cs+ (5 mM, n = 6) or the specific h-current blocker ZD 7288 (50 mu M, n = 11) but not Ba2+ (500 mu M, n = 7); in contrast, I-IR was blocked by Cs+ or Ba2+ but not ZD 7288. It is concluded that unlike adult olfactory cortical cells, immature olfactory neurones can exhibit both slow and fast-types of inward rectification: the more predominant I-Slow component, resembled the h-current (I-h) previously identified in other central neurones. (C) 1998 Elsevier Science B.V. All rights reserved.