Inhibition of Resting Potassium Conductances by Long-Term Activation of the NO/cGMP/Protein Kinase G Pathway: A New Mechanism Regulating Neuronal Excitability

Abstract

Glutamate-induced excitotoxicity, the most common pathological mechanism leading to neuronal death, may occur even with normal levels of glutamate if it coincides with a persistent enhancement of neuronal excitability. Neurons expressing nitric oxide (NO) synthase (NOS-I), which is upregulated in many human chronic neurodegenerative diseases, are highly susceptible to neurodegeneration. We hypothesized that chronic production of NO in damaged neurons may increase their intrinsic excitability via modulation of resting or "leak" K ϩ currents. Peripheral XIIth nerve injury in adult rats induced de novo NOS-I expression and an increased incidence of lowthreshold motor units, the latter being prevented by chronic inhibition of the neuronal NO/cGMP pathway. Accordingly, sustained synthesis of NO maintained an enhanced basal activity in injured motoneurons that was slowly reverted (over the course of 2-3 h) by NOS-I inhibitors. In slice preparations, persistent, but not acute, activation of the NO/cGMP pathway evoked a robust augment in motoneuron excitability independent of synaptic activity. Furthermore, chronic activation of the NO/cGMP pathway fully suppressed TWIK-related acid-sensitive K ϩ (TASK) currents through a protein kinase G (PKG)-dependent mechanism. Finally, we found evidence for the involvement of this long-term mechanism in regulating membrane excitability of motoneurons, because their pH-sensitive currents were drastically reduced by nerve injury. This NO/cGMP/PKG-mediated modulation of TASK conductances might represent a new pathological mechanism that leads to hyperexcitability and sensitizes neurons to excitotoxic damage. It could explain why de novo expression of NOS-I and/or its overexpression makes them susceptible to neurodegeneration under pathological conditions.

Endogenous Rho-Kinase Signaling Maintains Synaptic Strength by Stabilizing the Size of the Readily Releasable Pool of Synaptic Vesicles

“…Whole-cell patch-clamp experiments were performed on HMNs from transverse brainstem slices of 6-to 9-d-old Wistar rats as previously described (González-Forero et al, 2007;Sunico et al, 2010). Recordings were performed at 31°C on slices superfused (rate ϳ3-4 ml/min) with aCSF solution equilibrated with 95% O 2 and 5% CO 2 .…”

Nitric Oxide Induces Pathological Synapse Loss by a Protein Kinase G-, Rho Kinase-Dependent Mechanism Preceded by Myosin Light Chain Phosphorylation

“…Whole-cell recordings were obtained from somata of HMNs visually identified based on location and characteristic size and shape (González-Forero et al, 2007) using a Nikon Eclipse CFI60 microscope equipped with infrared-differential interference contrast (IR-DIC), a 40ϫ water-immersion objective, and an IR camera system (TILL Photonics). eGFP-transfected HMNs were identified by epifluorescence.…”

“…trol, 3.7 Ϯ 0.1 ms; DETA/NO, 4.6 Ϯ 0.5 ms), these differences did not reach statistical significance. We recently reported that longterm incubation with DETA/NO had a direct effect on the intrinsic excitability of HMNs through inhibition of leak potassium currents, which leads to increases in their R N (González-Forero et al, 2007). This purely postsynaptic action on R N was also evidenced in the present study (50.5 Ϯ 5.4 M⍀ in control compared with 90.1 Ϯ 7.0 M⍀ in DETA/NO-treated HMNs; p Ͻ 0.001, unpaired two-tailed Student's t test).…”

“…The molecular machinery to support a sGC/PKG-mediated action of NO has been reported to be present in motoneurons and surrounding fibers (Furuyama et al, 1993;de Vente et al, 2001;Lin and Talman, 2005;González-Forero et al, 2007); this agrees with the increase in cGMP levels that occurs in motoneurons, fibers, and bouton-like structures in the HN by perfusing adult rats with a NO donor (Montero et al, 2008). These data agree with previous studies that implicated cGMP/PKG in mediating collapse responses in retinal and ganglionar growth cones to ephrin-B1 and Semaphorin 3A, respectively (Dontchev and Letourneau, 2002;Mann et al, 2003).…”

Disruption of Neurosynaptic Physiology and Neuron Network Dysfunction in Brain Disorders: An Environmental and Occupational Health Perspective

“…Chronic activation of NO/cGMP pathway suppresses TWIK-related Acid Sensitive K + (TASK) currents, through a Protein Kinase G (PKG) dependant mechanism. The NO/cGMP/PKG mediated modulation of TASK conductance leads to hyper excitability and sensitizes neurons to excitotoxic damage [24]. The commonest mechanism leading to neuron loss is glutamate induced excitotoxicity.…”