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 (ISlow) relaxations whose amplitude and exponential rate of onset increased with increasing hyperpolarization (n=40); the ISlow activation time constant (tauon) 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 (IIR) with either rapid (n=9) or 'instantaneous' onset kinetics (n=2). ISlow activation threshold was at approximately -60 mV, with full activation at -120 mV; the half-maximal voltage (V0.5) and slope factor (k) of activation were: -85+/-0.4 mV and 11+/-0.5, respectively (n=13). The estimated reversal potential for ISlow was -28+/-2 mV (n=5). No obvious age-dependent changes in maximal ISlow current amplitude or density (at -120 mV) or in the proportion of cells showing IIR were found between P10 and P22. Islow was blocked by Cs+ (5 mM, n=6) or the specific h-current blocker ZD 7288 (50 microM, n=11) but not Ba2+ (500 microM, n=7); in contrast, IIR 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 ISlow component, resembled the h-current (Ih) previously identified in other central neurones.