Baclofen inhibition of the hyperpolarization-activated cation current, Ih, in rat substantia nigra zona compacta neurons may be secondary to potassium current activation

1. The properties of the hyperpolarization-activated cation current (Ih), and its modulation by gamma-aminobuturic acid-B (GABAB) receptor activation and protein kinase A, were investigated using whole cell voltage clamp of substantia nigra zona compacta principal neurons in rat midbrain slices in vitro. 2. At 30 degrees C, Ih activated between -75 and -155 mV, with a V1/2 of -115 mV. At 35 degrees C, the activation curve shifted positive by 10 mV. Ih had an estimated reversal potential of -27 mV. Ion substitution experiments showed that the current was carried by Na+ and K+. 3. Application of the GABAB receptor agonist baclofen (30 microM) induced an outward potassium current (GIRK), increased neuronal membrane conductance and inhibited Ih. The inhibition of Ih was voltage independent. Baclofen induced an 11-mV positive shift in the reversal potential of Ih. 4. Extracellular barium (300 microM) markedly reduced the baclofen-evoked outward current and associated increase in membrane conductance due to GIRK activation. There was also very little inhibition of Ih by baclofen in the presence of barium. When cesium was the major intracellular cation, both the increase in membrane conductance due to GIRK activation and the inhibition of Ih evoked by baclofen were reduced by a similar extent. 5. Neither forskolin (10 microM) nor the protein kinase A inhibitor, H89 (10 microM), had any effect on Ih or its inhibition by baclofen. 6. These data suggest that the inhibition of Ih by baclofen is secondary to the activation of GIRK, i.e., due directly to alteration of membrane conductance, rather than a distinct effect, and is not mediated by inhibition of adenylyl cyclase.