Newly developed blockers of the M‐current do not reduce spike frequency adaptation in cultured mouse sympathetic neurons

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Abstract
The M‐current (IK(M)) is believed to modulate neuronal excitability by producing spike frequency adaptation (SFA). Inhibitors of M‐channels, such as linopirdine and 10,10‐bis(4‐pyridinylmethyl)‐9(10H)‐anthracenone (XE991), enhance depolarization‐induced transmitter release and improve learning performance in animal models. As such, they are currently being tested for their therapeutic potential for treating Alzheimer's disease. The activity of these blockers has been associated with the reduction of SFA and the depolarization of the membrane observed when IK(M) is inhibited. To test whether this is the case, the perforated patch technique was used to investigate the capacity of IK(M) inhibitors to alter the resting membrane potential and to reduce SFA in mouse superior cervical ganglion neurons in culture. Linopirdine and XE991 both proved to be potent blockers of IK(M) when the membrane potential was held at −30 mV (IC50 2.56 and 0.26 µm, respectively). However, their potency gradually declined upon membrane hyperpolarization and was almost null when the membrane potential was kept at −70 mV, indicating that their blocking activity was voltage dependent. Nevertheless, IK(M) could be inhibited at these hyperpolarized voltages by other inhibitors such as oxotremorine‐methiodide and barium. Under current‐clamp conditions, neither linopirdine (10 µm) nor XE991 (3 µm) was effective in reducing the SFA and both provoked only a small slowly developed depolarization of the membrane (2.27 and 3.0 mV, respectively). In contrast, both barium (1 mm) and oxotremorine‐methiodide (10 µm) depolarized mouse superior cervical ganglion neurons by about 10 mV and reduced the SFA. In contrast to classical IK(M) inhibitors, the activity of linopirdine and XE991 on the IK(M) is voltage dependent and, thus, these newly developed IK(M) blockers do not reduce the SFA. These results may shed light on the mode of action of these putative cognition enhancers in vivo.