1. Inward rectifying currents were recorded, using tight‐seal, whole‐cell voltage‐clamp methods, from motoneurones visually identified in thin slices of neonatal rat spinal cord. 2. When motoneurones were hyperpolarized from holding potentials near the resting potential (‐60 to ‐70 mV), a slow inward‐going current was recorded. After the hyperpolarizing command pulses, inward tail currents were recorded. Amplitudes of the inward current at the end of hyperpolarizing pulses as well as those of the tail current increased non‐linearly with the membrane hyperpolarization, showing an inward rectification in the current‐voltage relation. 3. Neither the amplitude nor the kinetics of the inward rectifying current (IIR) was appreciably affected by replacement of extracellularly Ca2+ with Mg2+ combined with the application of tetrodotoxin (1 microM), tetraethylammonium (30 mM), and 4‐aminopyridine (4 mM). The current was relatively resistant to Ba2+, being only slightly suppressed at 2 but not at 0.2 mM. However, it was completely and reversibly abolished by Cs+ (2 mM). 4. When the external K+ concentration was raised, IIR was augmented. However, the activation curve of IIR constructed from relative tail current amplitudes in high K+ solutions was indistinguishable from that in normal solution. The chord conductance of IIR at various membrane potentials was similar for both normal and high K+ solutions. Thus the whole‐cell conductance of inward rectification in motoneurones depends on the membrane potential but not appreciably on the external K+ concentration ([K+]o). 5. The reversal potential of IIR was estimated by measuring the tail currents. In standard solution ([K+]o = 3 mM), the reversal potential was about ‐44 mV. Increasing [K+]o shifted the reversal potential toward positive potentials by 22 mV for a tenfold change in potassium concentration. 6. A fivefold reduction in the external Na+ concentration shifted the reversal potential of IIR in a negative direction by about 7 mV, suggesting that Na+ may carry part of IIR. A fivefold reduction in external Cl‐ concentration shifted the reversal potential by about 2 mV but in a negative direction, the opposite of the expected shift in the Cl‐ equilibrium potential. 7. When external Cl‐ was substituted with isethionate or gluconate, IIR was markedly and reversibly suppressed. 8. It is concluded that in spinal motoneurones, IIR is carried by both K+ and Na+ ions and that external Cl‐ might be required to maintain the inward rectifier current. © 1990 The Physiological Society
