Nitric Oxide Activates Leak K⁺ Currents in the Presumed Cholinergic Neuron of Basal Forebrain

Abstract

T. Nitric oxide activates leak K ϩ currents in the presumed cholinergic neuron of basal forebrain. J Neurophysiol 98: [3397][3398][3399][3400][3401][3402][3403][3404][3405][3406][3407][3408][3409][3410] 2007. First published October 10, 2007; doi:10.1152/jn.00536.2007. Learning and memory are critically dependent on basal forebrain cholinergic (BFC) neuron excitability, which is modulated profoundly by leak K ϩ channels. Many neuromodulators closing leak K ϩ channels have been reported, whereas their endogenous opener remained unknown. We here demonstrate that nitric oxide (NO) can be the endogenous opener of leak K ϩ channels in the presumed BFC neurons. Bath application of 1 mM S-nitroso-N-acetylpenicillamine (SNAP), an NO donor, induced a long-lasting hyperpolarization, which was often interrupted by a transient depolarization. Soluble guanylyl cyclase inhibitors prevented SNAP from inducing hyperpolarization but allowed SNAP to cause depolarization, whereas bath application of 0.2 mM 8-bromoguanosine-3Ј,5Ј-cyclomonophosphate (8-Br-cGMP) induced a similar long-lasting hyperpolarization alone. These observations indicate that the SNAP-induced hyperpolarization and depolarization are mediated by the cGMP-dependent and -independent processes, respectively. When examined with the ramp command pulse applied at -70 mV under the voltage-clamp condition, 8-Br-cGMP application induced the outward current that reversed at K ϩ equilibrium potential (E K ) and displayed Goldman-Hodgkin-Katz rectification, indicating the involvement of voltage-independent K ϩ current. By contrast, SNAP application in the presumed BFC neurons either dialyzed with the GTP-free internal solution or in the presence of 10 M Rp-8-bromo-␤-phenyl-1,N 2 -ethenoguanosine 3Ј,5Ј-cyclic monophosphorothioate sodium salt, a protein kinase G (PKG) inhibitor, induced the inward current that reversed at potentials much more negative than E K and close to the reversal potential of Na ϩ -K ϩ pump current. These observations strongly suggest that NO activates leak K ϩ channels through cGMP-PKG-dependent pathway to markedly decrease the excitability in BFC neurons, while NO simultaneously causes depolarization by the inhibition of Na ϩ -K ϩ pump through ATP depletion.

Protein Kinase G Dynamically Modulates TASK1-Mediated Leak K+Currents in Cholinergic Neurons of the Basal Forebrain

“…The I-K leak enhanced by NO-cGMP-PKG activation in the type II BF neurons was very similar to TASK1 current in its pH sensitivity (Kang et al, 2007;Toyoda et al, 2008). In the present study, the single-cell RT-PCR revealed that the type II neurons invariably express both TASK1 and ChAT mRNAs (Fig.…”

“…The procedure for slice preparation was the same as that in our previous studies (Kang et al, 2007;Toyoda et al, 2008). Using 12-16 d postnatal Wistar rats of both sexes (Charles River Breeders), slices of 300 m thickness were cut coronally from the most rostral portion of BF containing the nuclei of medial septum/diagonal band (MS/DB).…”

“…The electrophysiological studies were performed on HEK cells and neurons identified as type II based on the characteristic late-spiking pattern due to A-like K ϩ current in the MS/DB nuclei (Griffith and Matthews, 1986;Markram and Segal, 1990;Kang et al, 2007). Axopatch 200B (MDS Analytical Technologies) was used for the whole-cell patch-clamp recording.…”

“…We previously presumed the type II neuron as cholinergic (Kang et al, 2007), based on its late-spiking pattern due to A-like K ϩ current in response to depolarizing current pulses applied at a hyperpolarized holding potential (Fig. 5A), which is characteristic to cholinergic BF neurons (Griffith and Matthews, 1986;Markram and Segal, 1990;Sim and Allen, 1998).…”

“…Recently, we have reported that the activation of NO-cGMP-PKG (protein kinase G) pathway upregulates the K leak current (I-K leak ), which is similar to TASK1 currents in its extracellular pH sensitivity, in the presumed cholinergic neurons of the basal forebrain (BF) (Kang et al, 2007;Toyoda et al, 2008). The enhancement of the I-K leak by PKG activation was seen at pH 7.3 but not at pH 6.3 and 8.3, as if PKG changes the pH sensitivity of the I-K leak from the one similar to that of TASK1 to the other similar to that of TASK3 current (Toyoda et al, 2008).…”