cGMP/Protein Kinase G-Dependent Potentiation of Glutamatergic Transmission Induced by Nitric Oxide in Immature Rat Rostral Ventrolateral Medulla Neurons in Vitro

Abstract

Although both nitric oxide (NO) and glutamate within the rostral ventrolateral medulla (RVLM) are important mediators of the central cardiovascular regulation, little is known about the functional interactions between these two mediators. Herein, we investigated the possible role of NO on the glutamatergic transmission of RVLM neurons. Whole-cell patch-clamp recordings were performed on visualized RVLM neurons in the brainstem slice preparation of rats. We found that bath application of Larginine, the substrate for NO production, significantly increased the amplitude of excitatory postsynaptic currents (EPSCs). This enhancement was completely abolished by coadministration of the NO synthase inhibitor 7-nitroindazole and mimicked by the NO donors 3-morpholinylsydnoneimine and spermine NONOate. Bath application of a NO-sensitive guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, or a protein kinase G (PKG) inhibitor, Rp-8-bromo-guanosine 3Ј,5Ј-cyclic monophosphorothioate, fully prevented the L-arginine-, 3-morpholinylsydnoneimine-, and N-[4-[1-(3-(spermine NONOate)-induced synaptic potentiation. Direct activation of PKG with 8-(4-chlorophenylthio)-cGMP mimicked the action of NO donors. Furthermore, the augmentation by spermine NONOate of EPSC was accompanied by a reduction of the paired-pulse facilitation and synaptic failure rate of EPSCs. Spermine NONOate also significantly increased the frequency of both spontaneous and miniature EPSCs without altering their amplitude distribution. Pretreatment with the N-type Ca 2ϩ channel blocker -conotoxin GVIA selectively blocked the spermine NONOate-induced synaptic potentiation. These results suggest that NO acts presynaptically to elicit a synaptic potentiation on the RVLM neurons through an enhancement of presynaptic N-type Ca 2ϩ channel activity leading to facilitating glutamate release. The presynaptic action of NO is mediated by a cGMP/PKG-coupled signaling pathway.It is well known that rostral ventrolateral medulla (RVLM) neurons play an important role in cardiovascular, respiratory, and nociceptive functions (Dampney, 1994). Both electrophysiological and anatomical studies showed that a subset of RVLM neurons directly innervates preganglionic vasomotor neurons in the intermediolateral cell columns of the spinal cord, which are involved in the regulation of vasomotor and cardiac tone (Morrison et al., 1991;Deuchars et al., 1995). In addition, the RVLM neurons are tonically active in vivo and receive convergent excitatory and inhibitory inputs from multiple sources (Mtui et al., 1995;Guyenet et al., 1996). However, little is known regarding the physiological regulation of the tonic excitation of these neurons.Nitric oxide (NO), an unstable diatomic radical synthesized from L-arginine by nitric-oxide synthase (NOS), is recognized as an important signaling molecule mediating a variety of physiological processes, including synaptic transmission and synaptic plasticity (Garthwaite, 1991). In addition, considerable evidence suggests that NO i...

Endogenous Nitric Oxide Is a Key Promoting Factor for Initiation of Seizure-Like Events in Hippocampal and Entorhinal Cortex Slices

“…This is in line with previous reports demonstrating that NO interferes with both glutamatergic (Prast et al, 1998) and GABAergic (Wall, 2003) neurotransmission and influences neurotransmitter release depending on the type of synapse (Meffert et al, 1996;Mironov and Langohr, 2007;Wang et al, 2007). NO increases the release of glutamate by modulating presynaptic N-type Ca 2ϩ channels (Huang et al, 2003) and the release of GABA by an elevation of the intracellular Ca 2ϩ concentration via cyclic adenosine diphosphate ribose/ryanodine-sensitive stores (Wang et al, 2006). The increase in transmitter release might also be related to NO-mediated acceleration of recycling of synaptic vesicles (Micheva et al, 2003).…”

Immunohistochemical assessment of cyclic guanosine monophosphate (cGMP) and soluble guanylate cyclase (sGC) within the rostral ventrolateral medulla

“…A number of physiological studies indicate that NO can modulate pressor and reflex responses in the RVLM via an sGC/cGMP dependent mechanism [6,7,44,45]. While we were unable to detect cGMP-IR in the neurons of the C1 region, either in the resting state, when stimulated by NMDA or NO donor, or after PDE inhibition, our sGC-IR results certainly indicate the capacity for cGMP synthesis.…”

“…Work by Chan et al [19] suggests that the relative balance of functional nNOS versus inducible (i)NOS activity determines RVLM output, and further, that the different NOS isoforms activate different downstream signals, with nNOS driving sGC/cGMPdependent sympathoexcitatory responses, and NO produced by iNOS driving peroxynitrite formation and sympathoinhibitory responses [20]. Glutamate-induced pressor responses in the RVLM are in general proposed to be modulated by NO-sGC/cGMP [5,6,21] and this is supported by electrophysiological studies showing cGMPdependent potentiation of glutamate currents in RVLM slice preparations [7].…”

“…Glutamate is the major neurotransmitter driving the activity of these neurons [3,4], but the gaseous neurotransmitter nitric oxide (NO) is thought to play a key role in regulating glutamate induced pressor responses, and therefore modulating homeostatic blood pressure mechanisms [5][6][7]. Altered NO signalling in the RVLM is associated with increased sympathetic outflow in animal models of hypertension [8], however the mechanism of action is unclear, with a range of effects consistent with NO-mediated pressor, depressor or neutral effects being reported [9][10][11].…”

3-Morpholinylsydnonimine Inhibits Glutamatergic Transmission in Rat Rostral Ventrolateral Medulla via Peroxynitrite Formation and Adenosine Release

“…It is generally believed that different levels of NO may exert opposite effects on RVLM neurons (Chan et al, 2001) and, in turn, regulates vascular vasomotor tone via activation of different intracellular signaling pathways (Morimoto et al, 2000). In a recent study (Huang et al, 2003), we also reported an NO donor-mediated bidirectional modulation of glutamatergic transmission on RVLM neurons. Our results show that low doses of NO donors act presynaptically to elicit a reversible potentiation of synaptic transmission through NO-sensitive soluble guanosine cyclase (sGC)-cGMP signaling pathway.…”