Characterization of ouabain-induced phosphoinositide hydrolysis in brain slices of the neonatal rat

Effect of ouabain on calcium signaling in rodent brain: A systematic review of in vitro studies

The Na⁺/K⁺-ATPase is an integral membrane ion pump, essential to maintaining osmotic balance in cells in the presence of cardiotonic steroids; more specifically, ouabain can be an endogenous modulator of the Na⁺/K⁺-ATPase. Here, we conducted a systematic review of the in vitro effects of cardiotonic steroids on Ca²⁺ in the brain of rats and mice. Methods: The review was carried out using the PubMed, Virtual Health Library, and EMBASE databases (between 12 June 2020 and 30 June 2020) and followed the guidelines described in the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA). Results: in total, 829 references were identified in the electronic databases; however, only 20 articles were considered, on the basis of the inclusion criteria. The studies demonstrated the effects of ouabain on Ca²⁺ signaling in synaptosomes, brain slices, and cultures of rat and mouse cells. In addition to the well-known cytotoxic effects of high doses of ouabain, resulting from indirect stimulation of the reverse mode of the Na⁺/Ca²⁺ exchanger and increased intracellular Ca²⁺, other effects have been reported. Ouabain-mediated Ca²⁺ signaling was able to act increasing cholinergic, noradrenergic and glutamatergic neurotransmission. Furthermore, ouabain significantly increased intracellular signaling molecules such as InsPs, IP3 and cAMP. Moreover treatment with low doses of ouabain stimulated myelin basic protein synthesis. Ouabain-induced intracellular Ca²⁺ increase may promote the activation of important cell signaling pathways involved in cellular homeostasis and function. Thus, the study of the application of ouabain in low doses being promising for application in neurological diseases.

Systematic Review Registration:https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42020204498, identifier CRD42020204498.

Na+/K+-pump and neurotransmitter membrane receptors

Na⁺/K⁺-pump is an electrogenic transmembrane ATPase located in the outer plasma membrane of cells. The Na⁺/K⁺-ATPase pumps 3 sodium ions out of cells while pumping 2 potassium ions into cells. Both cations move against their concentration gradients. This enzyme's electrogenic nature means that it has a chronic role in stabilizing the resting membrane potential of the cell, in regulating the cell volume and in the signal transduction of the cell. This review will mainly consider the role of the Na⁺/K⁺-pump in neurons, with an emphasis on its role in modulating neurotransmitter receptor. Most of the literature on the modulation of neurotransmitter receptors refers to the situation in the mammalian nervous system, but the position is likely to be similar in most, if not all, invertebrate nervous systems.

High-affinity neurotensin receptor is involved in phosphoinositide turnover increase by inhibition of sodium pump in neonatal rat brain

Phosphoinositide (PI) metabolism is enhanced in neonatal brain by activation of neurotransmitter receptors and by inhibition of the sodium pump with ouabain or endogenous inhibitor termed endobain E. Peptide neurotensin inhibits synaptosomal membrane Na(+), K(+)-ATPase activity, an effect blocked by SR 48692, a selective antagonist for high-affinity neurotensin receptor (NTS1). The purpose of this study was to evaluate potential participation of NTS1 receptor on PI hydrolysis enhancement by sodium pump inhibition. Cerebral cortex miniprisms from neonatal Wistar rats were preloaded with [(3)H]myoinositol in buffer during 60 min and further preincubated for 0 min or 30 min in the absence or presence of SR 48692. Then, ouabain or endobain E were added and incubation proceeded during 20 or 60 min. Reaction was stopped with chloroform/methanol and [(3)H]inositol-phosphates (IPs) accumulation was quantified in the water phase. After 60-min incubation with ouabain, IPs accumulation values reached roughly 500% or 860% in comparison with basal values (100%), if the preincubation was omitted or lasted 30 min, respectively. Values were reduced 50% in the presence of SR 48692. In 20-min incubation experiments, IPs accumulation by ouabain versus basal was 300% or 410% if preincubation was 0 min or 30 min, respectively, an effect blocked 23% or 32% with SR 48692. PI hydrolysis enhancement by endobain E was similarly blocked by SR 48692, being this effect higher when sample incubation with the endogenous inhibitor lasted 60 min versus 20 min. Present results indicate that PI hydrolysis increase by sodium pump inhibition with ouabain or endobain E is partially diminished by SR 48692. It is therefore suggested that NTS1 receptor may be involved in cell signaling system mediated by PI turnover.

Involvement of Na,K-pump in SEPYLRFamide-mediated reduction of cholinosensitivity in Helix neurons

SEPYLRFamide acts as an inhibitory modulator of acetylcholine (ACh) receptors in Helix lucorum neurones. Ouabain, a specific inhibitor of Na,K-pump, (0.1 mM, bath application) decreased the ACh-induced inward current (ACh-current) and increased the leak current. Ouabain decreased the modulatory SEPYLRFamide effect on the ACh-current. There was a correlation between the effects of ouabain on the amplitude of the ACh-current and on the modulatory peptide effect. Ouabain and SEPYLRFamide inhibited the activity of Helix aspersa brain Na,K-ATPase. Activation of Na,K-pump by intracellular injection of 3 M Na acetate or 3 M NaCl reduced the modulatory peptide effect on the ACh-current. An inhibitor of Na/Ca-exchange, benzamil (25 muM, bath application), and an inhibitor of Ca(2+)-pump in the endoplasmic reticulum, thapsigargin (TG, applied intracellularly), both prevented the effect of ouabain on SEPYLRFamide-mediated modulatory effect. Another inhibitor of Ca(2+)-pump in the endoplasmic reticulum, cyclopiazonic acid (applied intracellularly), did not prevent the effect of ouabain on SEPYLRFamide-mediated modulatory effect. These results indicate that Na,K-pump is responsible for the SEPYLRFamide-mediated inhibition of ACh receptors in Helix neurons. Na/Ca-exchange and intracellular Ca(2+) released from internal pools containing TG-sensitive Ca(2+)-pump are involved in the Na,K-pump pathway for the SEPYLRFamide-mediated inhibition of ACh receptors.

Inositol triphosphate and ryanodine receptors in the control of the cholinosensitivity of common snail neurons by the Na,K pump during habituation

The effects of the Na,K pump inhibitor ouabain on habituation of the common snail to tactile stimulation were identical to the ouabain-induced modification of the decrease in the cholinosensitivity of defensive behavior command neurons in the common snail in a cellular model of habituation. Studies addressed the effects of intracellularly delivered ligands of two types of Ca2+ depot receptors--inositol triphosphate (IP3) receptors and ryanodine receptors--on the action of ouabain in the cellular analog of habituation. The IP3 receptor antagonist heparin (0.1 mM), the IP3 receptor agonist inositol triphosphate (0.1 mM), and the ryanodine-dependent Ca2+ mobilization inhibitor dantrolene (0.1 mM) prevented ouabain from modifying the depression of the evoked acetylcholine current. The ryanodine agonist/antagonist ryanodine was used at two concentrations (0.1 and 1 mM) and neither had any effect on the action of ouabain. It is concluded that Ca2+ mobilized from intracellular Ca2+ depots via IP3 receptors is involved in the neuronal mechanism of regulation of the habitation of the common snail to tactile stimulation by the Na,K pump.

Involvement of Na/Ca exchange and intracellular mobilized Ca2+ in Na,K-pump-mediated control of depression of the cholinosensitivy of common snail neurons [correction of neorons] using a cellular analog of habituation

The role of Na/Ca exchange and intracellular mobilized Ca2+ in modifying the depression of defensive behavior command neuron cholinosensitivity induced by the the Na,K pump inhibitor ouabain was studied in common snails using a cellular analog of habituation. Integral transmembrane acetylcholine-evoked currents (ACh currents) were recorded using a two-electrode membrane potential clamping technique. Decreases in neuron cholinosensitivity in the cellular analog of habituation were assessed in terms of the depth of depression of the amplitude of ACh currents during rhythmic local application of acetylcholine (with interstimulus intervals of 2-4 min) to the somatic membrane. The Na/Ca exchange inhibitor benzamyl (applied extracellularly, 15-35 microM) and two specific endoplasmic reticulum Ca-ATPase inhibitors, cyclopiazonic acid and thapsigargin (applied intracellularly. 0.1 mM) prevented modification of depression of the ACh current by ouabain (100 microM). It is concluded that Na/Ca exchange and the release of mobilized Ca2+ from intracellular calcium depots are involved in the mechanism by which the Na,K pump controls the depression of neuron cholinosensitivity in the cellular analog of habituation.

Mechanisms underlying the effects of amphetamine on particulate PKC activity

Amphetamine stimulates particulate protein kinase C (PKC) activity that is associated with the outward-transport of dopamine (DA) (Giambalvo [2003] Synapse 49:125-133). This stimulatory effect requires intracellular calcium ([Ca](i)) and endogenous DA and when DA release is diminished, the inward-transport of amphetamine inhibits PKC activity. This study examines the mechanisms involved. It was found that synaptoneurosomes incubated with amphetamine showed a dose-dependent increase in phospholipase C and A(2) activities. Furthermore, pretreatments with the phospholipase C inhibitor D609 or the phospholipase A(2) inhibitors quinacrine or p-bromophenacylbromide attenuated the amphetamine-induced increase in PKC activity. This suggests that both phospholipases were essential for the amphetamine-induced increase in PKC activity. The Na/Ca antiporter was also involved, since pretreatment with amiloride or benzamil attenuated the amphetamine-induced increase in PKC activity. Since these drugs by themselves increased PKC activity, the return to basal activity after addition of amphetamine suggests that, in the absence of Na/Ca exchange, amphetamine had an inhibitory effect on PKC activity. This inhibitory effect might be due to the activation of phospholipase A(2) through an increase in intracellular pH induced by amphetamine. This was supported by the finding that pretreatment with dimethylamiloride, an inhibitor of the Na/H antiporter that increases intracellular [H(+)], attenuated the effects of amphetamine on PKC activity. Other drugs that decrease intracellular [H(+)] (ammonia, monensin) also inhibited PKC activity without Ca. In contrast to amphetamine, monensin had no effect on PKC activity with Ca. This could be related to its large differential effects on phospholipase A(2) vs. phospholipase C activity. Thus, the monensin-mediated decrease in PKC activity seen without Ca was partially attenuated by pretreatment with quinacrine. Furthermore, when Na/Ca antiporter was inhibited with benzamil, monensin inhibited PKC activity. These results suggest that amphetamine, as well as monensin, may have dual effects on PKC activity, a Ca-dependent stimulatory effect via phospholipase C, and an inhibitory effect via phospholipase A(2).

Metabotropic glutamate receptor involvement in phosphoinositide hydrolysis stimulation by an endogenous Na(+), K(+)-ATPase inhibitor and ouabain in neonatal rat brain

The mechanism of action of an endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, on phosphoinositide hydrolysis was studied in neonatal rat brain cortex and compared with that of ouabain. Lack of additivity for endobain E and glutamate paired stimulation on inositol phosphates accumulation suggested that they share at least a common step on inositol phosphate metabolism, as previously advanced for ouabain. In addition, Cd(2+) sensitivity of endobain E and ouabain effects strengthened the involvement of glutamate receptors. The participation of ionotropic glutamate receptors on endobain E- and ouabain-induced phosphoinositide hydrolysis seems untenable, since antagonists dizocilpine and CNQX proved unable to inhibit these effects. However, the endobain E effect was blocked by 2 x 10 (-4) M L-AP3 (an antagonist for group I mGluRs) when at least a 15-min preincubation protocol was employed. Maximal inhibition of endobain E effect (42%) occurred when L-AP3 preincubation was extended to 60 min, as already shown with glutamate, but only a trend to decrease was recorded with ouabain. At variance, the ouabain effect was reduced to 50% employing 5 x 10 (-4) M MCPG (a competitive antagonist for group I mGluRs), whereas no blockade was observed with endobain E or glutamate. In addition, MPEP (a selective mGluR5 antagonist) partially reduced ouabain, endobain E and glutamate responses and the selective mGluR1 antagonist LY367385 showed no activity at all. To sum up, the present findings support the involvement of mGluR5 in both endobain E and ouabain phosphoinositide hydrolysis stimulation in neonatal rat brain, in spite of dissimilar response to tested antagonists.

An endogenous Na+, K+-ATPase inhibitor enhances phosphoinositide hydrolysis in neonatal but not in adult rat brain cortex

The effect of an endogenous Na+, K+-ATPase inhibitor, termed endobain E, on phosphoinositide hydrolysis was studied in rat brain cortical prisms and compared with that of ouabain. As already shown for ouabain, a transient effect was obtained with endobain E; maximal accumulation of inositol phosphates induced by endobain E was 604 +/- 138% and 186 +/- 48% of basal values in neonatal and adult rats, respectively. The concentration-response plot for the interaction between endobain E and phosphoinositide turnover differed from that of ouabain, thus suggesting the involvement of distinct mechanisms. In the presence of endobain E plus ouabain at saturating concentrations, no additive effect was recorded, suggesting that both substances share at least a common step in their activation mechanism of inositol phosphates metabolism or that they enhance phosphatidylinositol 4,5-biphosphate breakdown from the same membrane precursor pool, until its exhaustion. Experiments with benzamil, a potent blocker of Na+/Ca2+ exchanger, showed that it partially and dose-dependently inhibited endobain E effect. These results indicate that the endogenous Na+, K+-ATPase inhibitor endobain E, like ouabain, is able to stimulate phosphoinositide turnover transiently during postnatal brain development.

How many endobains are there?

Oxidative metabolism is very active in brain, where large amounts of chemical energy as ATP molecules are consumed, mostly required to maintain cellular Na+/K+ gradients through the participation of the sodium pump (Na+,K+-ATPase), whose activity is selectively and potently inhibited by the alkaloid ouabain. Na+/K+ gradients are involved in nerve impulse propagation, in neurotransmitter release and cation homeostasis in the nervous system. Likewise, enzyme activity modulation is crucial for maintaining normal blood pressure and cardiovascular contractility as well as renal sodium excretion. The present article reviews the progress in disclosing putative ouabain-like substances, examines their denomination according to different research teams, tissue or biological fluid sources, extraction and purification, assays, biological properties and chemical and biophysical features. When data is available, comparison with ouabain itself is mentioned. Likewise, their potential action in normal physiology as well as in experimental and human pathology is summarized.

Regulation of phosphoinositide cycle by intracellular sodium in the blood-brain barrier

In the present study of cerebral microvessels, we report that monensin, a Na+ ionophore, elicits a decrease in 32P radioactivity incorporation into phosphoinositides in cerebral microvessels. In addition, monensin evokes enhanced production of inositol-1-monophosphate (IP) and inositol-1,4-bisphosphate (IP2), together with an increase in the diacylglycerol (DAG) mass. These results indicate that monensin evokes a phosphoinositide hydrolysis by phospholipase C (PLC). The absence of inositol-1,4,5-trisphosphate (IP3) production leads us to think that although phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis occurs in this process, there is a very rapid disappearance of IP3. The net decrease in 32P radioactivity incorporated into phosphoinositides suggests that a partial inhibition of their re-synthesis is also evoked. Experimental evidence with pharmacological tools suggests that: (1) these effects are secondary to an increase in Ca2+ through the Na+/Ca2+ exchanger; and (2) the intracellular Ca2+ release is not involved in these effects of monensin. Since some neuropeptide receptors in cerebral microvessels have been reported to be coupled to either the Na+/H+ exchanger or to PLC, we discuss the possibility that cross-talk exists between these intracellular signalling pathways (phosphoinositide metabolism and Na+ transport) in the blood-brain barrier (BBB).

Interaction of ethanol and anoxia with muscarinic receptor--stimulated phosphoinositide metabolism during brain development

The mechanism(s) by which ethanol induces alterations in brain development may involve direct actions (e.g. changes in specific biochemical pathways), or indirect effects, such as cerebral hypoxia resulting from ethanol - induced circulatory changes. Since both ethanol and hypoxia are known to affect the metabolism of phosphoinositides, which has been suggested as a possible target for ethanol's developmental neurotoxicity, in the present study we have investigated the in vitro effects of both severe hypoxia (anoxia) and ethanol (alone or in combination) on muscarinic receptor-stimulated phosphoinositide metabolism in cerebral cortex slices from neonatal rats. Anoxia markedly inhibited carbachol - stimulated phosphoinositide metabolism in adult rats (67%), but only slightly (10%) in neonatal animals. Reoxygenation reversed the effect of anoxia at both ages. On the other hand, ethanol's inhibitory effect was pronounced in neonatal rats only, and was additive to that of anoxia. The presence of ethanol did not affect the recovery of carbachol - stimulated phosphoinositide metabolism following anoxia and reoxygenation. These results indicate that ethanol and anoxia differently and independently affect muscarinic receptor - stimulated phosphoinositide metabolism and may mutually contribute to the CNS effects observed following developmental ethanol exposure.

Carbachol, but not norepinephrine, NMDA, ionomycin, ouabain, or phorbol myristate acetate, increases inositol 1,3,4,5-tetrakisphosphate accumulation in rat brain cortical slices

Ionomycin, a Ca2+ ionophore, stimulated phosphoinositide breakdown in rat brain cortical slices incubated in the presence of 1.2 mM Ca2+, but, unlike muscarinic cholinergic stimulation, it had little effect on inositol 1,3,4,5-tetrakisphosphate accumulation. However, at 2 min, the increase in inositol 1,4,5-trisphosphate due to 10 microM ionomycin was equivalent to that seen with 1 mM carbachol. Phorbol 12-myristate 13-acetate or high K+ (30 mM) increased inositol 1,4,5-trisphosphate, but not inositol 1,3,4,5-tetrakisphosphate accumulation. The stimulation of inositol 1,4,5-trisphosphate accumulation due to ionomycin, unlike that seen with carbachol, was abolished in buffer containing 0.2 mM Ca2+. The increase in inositol 1,3,4,5-tetrakisphosphate accumulation in brain slices due to 1 mM carbachol ranged from 55 to 68% of that for inositol 1,4,5-trisphosphate. Norepinephrine, NMDA, veratridine, and ouabain also increased inositol 1,4,5-trisphosphate, but had minimal effects on inositol 1,3,4,5-tetrakisphosphate accumulation. These results suggest that there is something unique about the stimulation of inositol 1,3,4,5-tetrakisphosphate accumulation by carbachol, which is also the only one of these agents that is able to activate phosphoinositidase C beta 1 in isolated rat brain membranes.

The role of P-type calcium channels in the depolarization-induced activation of nitric oxide synthase in frontal cortex

In this study we demonstrate that 50 mM K+ stimulates the conversion of L-[3H]arginine to L-[3H]citrulline and that this effect is blocked by 10 microM N omega-nitro-L-arginine, a nitric oxide synthase inhibitor, and Ca(2+)-free conditions. Amiloride (1 mM) and low Na+ conditions were used to test the possible involvement of the Na(+)-Ca2+ exchanger. These treatments were without effect. The calcium channel blockers 10 mM Mg2+, 100 microM Cd2+, and 10 mM Co2+ also blocked the K+ response, suggesting the involvement of voltage-dependent calcium channels (VDCCs). The specific VDCC involved seems to be the P type, as funnel-web spider toxin blocked the response whereas 200 microM Ni2+, 10 microM nifedipine, and 100 nM omega-conotoxin did not.