Voltage-gated whole-cell K+ currents have been investigated in olfactory bulb (OB) output (mitral/tufted) neurones from neonatal rats, which were retrogradely labelled by rhodamine or Fast Blue and identified after enzymatic dissociation. Forty-five per cent of labelled neurones exhibited either phasic or non-phasic spontaneous firing in cell-attached configuration. 2. Four outward K+ currents have been identified in all such identified OB output neurones. They are the transient (IA), the delayed rectifier (IDK), and two Ca(2+)-dependent (IK(Ca)) currents. No inward rectifier was detected. 3. The IA was activated at around -45 mV and reached its peak within 3-10 ms. The decay phase could be described by single exponential distribution with the time constant of 45.2 +/- 3.8 ms at depolarizations 10-60 mV from a holding potential of -70 mV. Its activation and steady-state inactivation processes could be fitted with Boltzmann equations yielding half-maximal activation potentials of 7.6 +/- 0.4 and -47.4 +/- 0.2 mV, respectively. It was sensitive to block by 4-AP (1 mM) and by Zn2+ (1 mM). 4. The IDK was activated at potentials more positive than -30 mV, with half-maximal activation at 21 mV. It was sustained during 1 s test pulses without apparent decay. It was blocked by TEA at a concentration of 20 mM. About 8% of the sustained current, in 11/24 cells tested, was found to resist block by a combination of all pharmacological agents tested. 5. Apamin at 100 nM blocked a TEA-insensitive component which accounted for about 23% of the maximal sustained currents. Iberiotoxin (IbTX), which has been found to block maxi K+ currents more selectively than does charybdotoxin, reversibly blocked Ca(2+)-activated K+ current, with a half-maximal dose of about 100 nM in 8/13 OB output neurones tested. This accounted for 20% of the maximal sustained K+ current. The effect of IbTX was not observed in the presence of 20 mM external TEA. 6. Direct evidence is provided in this study regarding kinetic and pharmacological properties of four types of outward K+ channels in OB output neurones.
