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

Abstract

The M-current (I(K(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 I(K(M)) is inhibited. To test whether this is the case, the perforated patch technique was used to investigate the capacity of I(K(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 I(K(M)) when the membrane potential was held at -30 mV (IC(50) 2.56 and 0.26 microM, 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, I(K(M)) could be inhibited at these hyperpolarized voltages by other inhibitors such as oxotremorine-methiodide and barium. Under current-clamp conditions, neither linopirdine (10 microM) nor XE991 (3 microM) 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 microM) depolarized mouse superior cervical ganglion neurons by about 10 mV and reduced the SFA. In contrast to classical I(K(M)) inhibitors, the activity of linopirdine and XE991 on the I(K(M)) is voltage dependent and, thus, these newly developed I(K(M)) blockers do not reduce the SFA. These results may shed light on the mode of action of these putative cognition enhancers in vivo.

Activation of TREK Currents by the Neuroprotective Agent Riluzole in Mouse Sympathetic Neurons

“…Hence, they could not only contribute to set the resting membrane potential by stabilization but also to act as a "brake" on action potential firing and to contribute to adaptation in SCG neurons, as comprehensively demonstrated for M-channels (Brown and Constanti, 1980;Lamas, 1998;. This could explain the strong reduction in spike frequency adaptation induced by barium (which inhibits both KCNQ and TREK channels) in mSCG neurons when compared with other more specific M-current inhibitors (Romero et al, 2004). Our results using low concentrations of fluoxetine suggest that, although adaptation is mainly under M-current control, TREK channels induce a delay in the latency of the first action potential evoked by depolarizing stimuli.…”

“…It is noteworthy that barium inhibits heterologously expressed TREK-1 , TREK-2 (Bang et al, 2000), and TRAAK channels. As shown in Figure 3, both barium and TEA (but not other blockers) strongly inhibited the steady-state outward current at Ϫ30 mV, such inhibition representing the blockage of the non-inactivating potassium M-current characteristic of SCG neurons (Hadley et al, 2003;Romero et al, 2004).…”

“…1Cc, 3E) ( p Ͼ 0.05). The hyperpolarizationactivated cation current (I h ) fulfils an important role in stabilizing the resting membrane potential in SCG cells (Lamas, 1998;Romero et al, 2004). Nevertheless, the inhibition of this current with 1 mM cesium did not affect the riluzole-activated current (78.9 Ϯ 15.7 pA; n ϭ 4) (Figs.…”

Contribution of KCNQ2 and KCNQ3 to the medium and slow afterhyperpolarization currents

“…This might suggest that members of the KCNQ channel family are not the sole mediators of the IsAHP or that their contribution is small. Alternatively, the lack of complete XE991 block may be due to the presence of an auxillary subunit (7), holding membrane potential (27), or subunit composition (28).…”

Kv7/KCNQ Channels Control Action Potential Phasing of Pyramidal Neurons during Hippocampal Gamma OscillationsIn Vitro

“…For example, linopirdine, an I M blocker and a neurotransmitter release enhancer, appeared to be a promising therapeutic agent for memory deficits a decade ago (Lamas et al, 1997) but failed to deliver meaningful results in clinical trials (Rockwood et al, 1997;Börjesson et al, 1999). Another more potent I M blocker and neurotransmitter release enhancer, XE991 [10,10-bis(4-pyridinylmethyl)-9(10 H)-anthracenone], is currently undergoing clinical trials, with no significant results obtained so far (Romero et al, 2004). To understand the discrepancy between the known effects of I M on the cellular level and the apparent ineffectiveness of therapies targeting I M it is fundamen-tal to address its role in cognition-relevant activity in local networks.…”