Effect of catecholamines on the hyperpolarization‐activated cationic Ih and the inwardly rectifying potassium IKir currents in the rat substantia nigra pars compacta

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Abstract
Whole-cell ruptured-patch and perforated-patch recordings were used in principal neurons of the rat substantia nigra pars compacta (SNc) to study the effect of catecholamines both on the hyperpolarization-activated cationic (Ih) and the inwardly rectifying potassium (IKir) currents. In internal potassium, a 2 min bath application of noradrenaline (NA; 50 μm) or dopamine (DA; 50 μm) both inhibited Ih and induced an outward current associated with an increase in IKir conductance. These two effects recovered poorly after wash-out. Protein kinase A (PKA), protein kinase C (PKC) and phosphatases 1 and 2A inhibitors did not modify the NA and DA effects on the amplitude of Ih and IKir currents. They also had no effect on the recovery of the catecholamine responses. In perforated-patch experiments, NA and DA also induced an inhibition of Ih and revealed an outward current associated with an increase in conductance. However, both effects recovered in less than 5 min following the wash-out. These results indicate that neither PKA, PKC, nor phosphatases 1 or 2A were required in the NA and DA modulation of these two currents and that an intracellular factor, that could be either washed-out or inversely up-regulated in the ruptured-patch configuration, was implicated in the recovery of both effects. In the presence of external barium (300 μm) or internal caesium which both blocked the outward current and the increase in conductance, neither NA nor DA affected Ih, suggesting that the effect on Ih observed is secondary to the activation of the IKir channels. Increasing chloride conductance of the cell by activation of GABAA receptors also induced an inhibition of Ih. All together these results suggest that the NA or DA induced inhibition of Ih could result from an occlusion of Ih by a space-clamp effect.