Guanosine 3',5'-cyclic monophosphate regulates calcium channels in neurones of rabbit vesical pelvic ganglia

Ammonia inhibits the C-type natriuretic peptide-dependent cyclic GMP synthesis and calcium accumulation in a rat brain endothelial cell line

Recently we reported a decrease of C-type natriuretic peptide (CNP)-dependent, natriuretic peptide receptor 2 (NPR2)-mediated cyclic GMP (cGMP) synthesis in a non-neuronal compartment of cerebral cortical slices of hyperammonemic rats [Zielińska, M., Fresko, I., Konopacka, A., Felipo, V., Albrecht, J., 2007. Hyperammonemia inhibits the natriuretic peptide receptor 2 (NPR2)-mediated cyclic GMP synthesis in the astrocytic compartment of rat cerebral cortex slices. Neurotoxicology 28, 1260-1263]. Here we accounted for the possible involvement of cerebral capillary endothelial cells in this response by measuring the effect of ammonia on the CNP-mediated cGMP formation and intracellular calcium ([Ca2+]i) accumulation in a rat cerebral endothelial cell line (RBE-4). We first established that stimulation of cGMP synthesis in RBE-4 cells was coupled to protein kinase G (PKG)-mediated Ca2+ influx from the medium which was inhibited by an L-type channel blocker nimodipine. Ammonia treatment (1h, 5mM NH4Cl) evoked a substantial decrease of CNP-stimulated cGMP synthesis which was related to a decreased binding of CNP to NPR2 receptors, and depressed the CNP-dependent [Ca2+]i accumulation in these cells. Ammonia also abolished the CNP-dependent Ca2+ accumulation in the absence of Na+. In cells incubated with ammonia in the absence of Ca2+ a slight CNP-dependent increase of [Ca2+]i was observed, most likely representing Ca2+ release from intracellular stores. Depression of CNP-dependent cGMP-mediated [Ca2+]i accumulation may contribute to cerebral vascular endothelial dysfunction associated with hyperammonemia or hepatic encephalopathy.

cGMP-induced presynaptic depression and postsynaptic facilitation at glutamatergic synapses in visual cortex

The mechanisms by which the intracellular messenger cGMP can modulate synaptic efficacy remain poorly understood. Here we report that cGMP, acting through cGMP-dependent protein kinase (PKG), has multiple rapid and reversible effects on synaptic transmission in slices and cultures of rodent visual cortex. Extracellular application of the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and the PKG specific activator beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate sp-isomer (Sp-8-Br-PET-cGMPS) reduced stimulus-evoked EPSPs in slices. In cortical cultures, both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. In both slices and cultures, intracellular perfusion of the postsynaptic neurons with a pseudosubstrate inhibitory peptide specific for PKG had no effect on the reduction in EPSPs and EPSCs, indicating that the inhibition occurred at presynaptic sites. Whole-cell calcium currents in cultured cortical neurons were also reduced by both analogs, which may account for the effect on synaptic release. To determine whether cGMP was also acting at postsynaptic sites, we applied exogenous kainate/AMPA and NMDA to the recorded cells directly. cGMP and its analogs showed little effect on the postsynaptic kainate/AMPA responses but produced a dramatic enhancement of NMDA responses. cGMP-induced NMDA potentiation was prevented by the specific PKG inhibitory peptide infused into the postsynaptic cell. In summary, cGMP, acting through PKG, had depressive presynaptic and facilitatory postsynaptic actions at excitatory synapses in the visual cortex. We suggest that these opposing actions may be useful for altering the balance of synaptic inputs to cortical neurons in ways that enhance signals important for synaptic facilitation and neuronal plasticity.

Distribution of cGMP in guinea pig autonomic ganglia after stimulation with sodium nitroprusside

Nitric oxide (NO) is an intercellular messenger molecule in the nervous system and exerts its action in many regions by generating cyclic GMP (cGMP) via soluble guanylyl cyclase. In this study, on the male guinea pig, we have analyzed the localization of cGMP in some autonomic ganglia with immunohistochemistry after stimulation with sodium nitroprusside (SNP) as NO donor, and made correlations with the NO synthesizing enzyme NO synthase (NOS), tyrosine hydroxylase (TH) and some neuropeptides. The putative target neurons for NO were examined in the anterior pelvic ganglia (APGs), as well as some pre- and paravertebral sympathetic ganglia. The results show that cGMP-like immunoreactivity (LI) in the APG was in most cases observed in the TH-positive, NOS-negative neuron population after SNP stimulation, whereas the NOS-expressing cholinergic population mostly lacked detectable cGMP-LI. In the pre- and paravertebral ganglia, SNP stimulation increased cGMP levels to a much lesser extent than in the APGs. cGMP was also observed in blood vessels, in the ganglion capsule, and in some cases. possibly in satellite cells. We propose, as one alternative, that there is a functional, intraganglionic regulatory loop between the parasympathetic and sympathetic divisions of the APG, using the NO/cGMP pathway.

Activation of calcium-dependent chloride channels causes post-tetanic depolarization in rabbit parasympathetic neurons

Intracellular recordings were made from neurons in rabbit and feline vesical parasympathetic ganglia in vitro. In response to cathodal current injection (0.1-1 nA for 2-20 ms) the majority of rabbit neurons (229 out of 250) exhibited a single action potential that was followed by a fast and slow after-hyperpolarization (sAHP neuron). The remainder of the cells exhibited an action potential followed by only a fast after-hyperpolarization (fAHP neuron). fAHP neurons did not exhibit anomalous rectification and a spontaneous rhythmic hyperpolarization, which were common membrane properties in sAHP neurons. In response to a train of cathodal current pulses (5-20 Hz for 0.1-10 s), fAHP neurons exhibited action potentials followed by a post-tetanic depolarization (PTD). The PTD was associated with a decrease in membrane input resistance. The amplitude and duration of the PTD were a function of the number of action potentials in the train. The amplitude of the PTD was increased by membrane hyperpolarization and its estimated reversal potential was approximately -30 mV. Low-chloride solution and intracellular injection of chloride ions augmented the amplitude and duration of the PTD, whereas low-sodium, high-potassium and low-potassium solutions did not affect them. Tetraethylammonium (5-10 mM) and barium (0.5-1 mM) increased the amplitude and duration of the PTD. Nominal calcium-free solutions and omega-conotoxin (500 nM) abolished the PTD. The data suggest that activation of chloride channels by calcium influx through omega-conotoxin-sensitive calcium channels mediates the PTD. Repetitive stimulation of the pelvic nerve evoked a train of orthodromic action potentials followed by the PTD of fAHP neurons. (+)-Tubocurarine (10 microM) and hexamethonium (200 microM), but not atropine (1 microM), abolished orthodromic action potentials and the PTD, whereas these cholinergic antagonists did not depress the PTD evoked by direct action potentials. In summary, the data suggest that the PTD may function as a slow synaptic potential in fAHP neurons. This appears likely because neither slow excitatory nor inhibitory postsynaptic potentials are present in neurons of rabbit vesical parasympathetic ganglia. In contrast, slow inhibitory and excitatory postsynaptic potentials were recorded from neurons in feline vesical parasympathetic ganglia.

Dual modulation of the L-type calcium current of rat osteoblastic cells by parathyroid hormone: opposite effects of protein kinase C and cyclic nucleotides

Using whole-cell voltage-clamp recording of rat osteoblastic cells, we show that PTH-(1-34), known to stimulate protein kinase C (PKC) and adenylate cyclase, has a dual effect on the L-type calcium current. It induces a long-lasting increase and a superimposed reversible decrease, which can be separated by repeating hormone applications. The stimulatory effect is the only effect induced by the (3-34) fragment, able to stimulate PKC but unable to stimulate adenylate cyclase. The L current is stimulated by an active phorbol ester and is reduced by permeable analogues of cyclic AMP. Thus, the effect of PTH-(1-34) can be explained by the opposite effects of PKC and cyclic AMP. Dibutyryl cyclic GMP reduces the L current even more potently than dibutyryl cyclic AMP. The above modulations are all voltage-insensitive. These results led us to reinvestigate the effects of some vitamin D3 metabolites known to stimulate PKC and/or guanylate cyclase, and previously reported to affect the voltage-sensitivity of the L current. We only detected voltage-insensitive effects.

13-HODE increases intracellular calcium in vascular smooth muscle cells

13-Hydroxyoctadecadienoic acid (HODE) (2 microM) consistently increased porcine aortic and pulmonary artery smooth muscle cell calcium concentrations ([Ca2+]i), whereas 9-HODE and linoleic acid had no significant effect in the aortic cells and a much lesser effect in the pulmonary artery cells. A transient increase in [Ca2+]i occurred with as little as 50 nM 13-HODE. Structural specificity for elevation of [Ca2+]i also was seen with the monohydroxyeicosatetraenoic acids (HETEs), with 12-HETE but not 5- or 15-HETE increasing [Ca2+]i. 13-HODE, but not 9-HODE, increased smooth muscle cell guanosine 3',5'-cyclic monophosphate (cGMP) levels. The [Ca2+]i increase produced by 13-HODE was dependent on extracellular calcium and was inhibited by the calcium channel blockers verapamil and nifedipine and by KT-5823, a cGMP-dependent kinase inhibitor. A similar increase in [Ca2+]i was produced by 8-bromo-cGMP. These results suggest that 13-HODE, a 15-lipoxygenase product formed from linoleic acid, can act as a lipid mediator in vascular smooth muscle. It raises smooth muscle cGMP, causing a secondary increase in [Ca2+]i due to Ca2+ influx through a cGMP kinase-dependent L-type channel.