Figure 1.

GIRK channel direct activation with ML297 causes hyperpolarization and a decrease in input resistance in hypothalamic hypocretin neuron and hippocampal CA1 neurons. (A) ML297 (5 µM) caused a long-lasting hyperpolarization, decreased input resistance, and blocked spontaneous firing of action potentials of a hypocretin neuron in the hypothalamic brain slice. Lower inserts indicated by arrows show time-expanded traces. (B) The concentration-dependent response curve of hyperpolarization by ML297 reveals an EC₅₀ of 5.4 µM (each data point represents n = 3–5 cells). (C) In the presence of tetrodotoxin (0.3 µM), ML297 (5 µM) induced postsynaptic hyperpolarization and a decrease in input resistance. Lower inserts indicated by arrows show time-expanded traces. (D) Membrane potential changes in response to a series of current injections revealed a current-voltage relationship where the intersection point of the two current–voltage curves in the presence and absence of ML297 indicated a reversal potential of −84 mV (n = 11), which is close to the K⁺ equilibrium potential and the experimental condition. (E) R-baclofen (10 µM) hyperpolarized a hippocampal pyramidal CA1 neuron by −12 mV, decreased input resistance to 40% of control level, and blocked spontaneous firing of action potentials. (F) ML297 (10 µM) induced similar postsynaptic responses as baclofen in another hippocampal CA1 neuron, which was reversibly blocked by BaCl₂ (10 mM), a general K⁺ channel blocker. (G) In another CA1 neuron, in the presence of the GABA_B antagonist CGP52432 and tetrodotoxin (TTX, 0.3 µM), R-baclofen (10 µM) did not induce any postsynaptic responses. Subsequent application of 10 µM ML297 induced responses of hyperpolarization and an increase in input resistance. Hyperpolarizing current pulses (−0.3 nA, 500 ms) were delivered every 5 s throughout the experiment in all recorded neurons which had resting potentials between −50 and −65 mV.