A, currents elicited by hyperpolarizing voltage steps (1 s duration, from −50 to −130 mV with 10 mV decrements) before (grey) and during (black) the bath application of 8Br-cAMP (1 mM). Note a slight increase in the amplitude of Ih induced by 8Br-cAMP at millimolar concentrations. B, activation curve of the conductance underlying Ih constructed with plots of the mean normalized tail currents as a function of the membrane potential before (○) and during (□) the application of 8Br-cAMP (n = 7 cells in each). Each of the resulting values was fitted to the Boltzmann equation and yielded an activation with V1/2 =−97.5 ± 0.8 and −94.6 ± 0.9 mV, k = 10.5 ± 0.6 and 11.6 ± 0.6, before and during the application of 8Br-cAMP, respectively.