(-)-BACLOFEN-INDUCED AND CONSTITUTIVELY ACTIVE INWARDLY RECTIFYING POTASSIUM CONDUCTANCES IN CULTURED RAT MIDBRAIN NEURONS

Biophysical and pharmacological properties, and development of the (-) -baclofen-induced potassium (K-Bac) conductance and the constitutively active inwardly rectifying potassium (K-IR) conductance were characte rised using the patch-clamp technique in cultured embryonic rat midbra in neurons. K-Bac conductance was induced by (-)-baclofen acting on ga mma-aminobutyric acid B (GABA(B)) receptors, and displayed a high degr ee of se lectivity for potassium ions, an approximate square-root depe ndence of conductance on extracellular potassium concentration and str ongly voltage-dependent activation. Ba2+ blocked the conductance in a voltage-independent manner, whereas Cs+ produced a voltage-dependent b lock. In the same preparation, the K-IR conductance displayed biophysi cal properties indistinguishable from those of the K-Bac conductance. Block of K-IR currents by Ba2+ was voltage independent (K-I, 4 mu M), whereas Cs+ produced a voltage-dependent block (K-I, 370 mu M at -100 mV, equivalent valence, z', 1.67). The K-Bac and K-IR conductances add itionally displayed a strikingly similar pattern of development in cul ture; the specific conductance (nS/pF) of both conductances increased two- to three-fold between the first and second week in vitro, and rem ained constant thereafter.