Relationship of calcium and adenylate cyclase messenger systems in rat brain synaptosomes

Brain Bioenergetics in Chronic Hypertension: Risk Factor for Acute Ischemic Stroke

Chronic hypertension is one of the key modifiable risk factors for acute ischemic stroke, also contributing to determine greater neurological deficits and worse functional outcome when an acute cerebrovascular event would occur. A tight relationship exists between cerebrovascular autoregulation, neuronal activity and brain bioenergetics. In chronic hypertension, progressive adaptations of these processes occur as an attempt to cope with the demanding necessity of brain functions, creating a new steady-state homeostatic condition. However, these adaptive modifications are insufficient to grant an adequate response to possible pathological perturbations of the established fragile hemodynamic and metabolic homeostasis. In this narrative review, we will discuss the main mechanisms by which alterations in brain bioenergetics and mitochondrial function in chronic hypertension could lead to increased risk of acute ischemic stroke, stressing the interconnections between hemodynamic factors (i.e. cerebral autoregulation and neurovascular coupling) and metabolic processes. Both experimental and clinical pieces of evidence will be discussed. Moreover, the potential role of mitochondrial dysfunction in determining, or at least sustaining, the pathogenesis and progression of chronic neurogenic hypertension will be considered. In the perspective of novel therapeutic strategies aiming at improving brain bioenergetics, we propose some determinant factors to consider in future studies focused on the cause-effect relationships between chronic hypertension and brain bioenergetic abnormalities (and vice versa), so to help translational research in this so-far unfilled gap.

Effect of in vivo treatment of clonidine on ATP-ase's enzyme systems of synaptic plasma membranes from rat cerebral cortex

The effects on energy-consuming ATP-ases were studied in two types of synaptic plasma membranes from rat cerebral cortex after in vivo injection of clonidine. To study the mechanism of action of clonidine at subcellular level, the enzyme activities of Na+, K+-ATP-ase, Ca2+, Mg2+-ATP-ase, Low- and High-affinity Ca2+-ATP-ase, and Mg2+-ATP-ase were evaluated on synaptic plasma membranes of control and treated animals with clonidine (5 microg x kg(-1); i.p. 30 minutes). Acute treatment with clonidine decreased the catalytic activity of Ca2+, Mg2+-ATP-ase and of low-affinity Ca2+-ATP-ase only in synaptic plasma membranes of II type, that is the fraction enriched in synaptic plasma membranes. The decreases of these enzymatic activities are related to the interference of the drug on Ca2+ homeostasis in synaptoplasm. The reductions of these enzyme-consuming ATP-ases give further evidence that clonidine has not only neuroreceptorial effects, but that the drug also affects the energy metabolism of cerebral tissue, improving the knowledges about the pharmacology of clonidine. Because the elevation of [Ca2+]i, during ischemia/hypoxia contributes to cellular injury, these findings may suggest that the prevention of calcium overload may be the key mechanism of protection by alpha2-agonist.

Modulation of free intracellular calcium and cAMP by morphine and cannabinoids, alone and in combination in mouse brain and spinal cord synaptosomes

Changes in [Ca++]i and cAMP were evaluated as possible mechanisms by which the cannabinoids enhance the antinociception of morphine. The addition of subactive concentrations of delta 9-(THC) and morphine in combination to brain synaptosomes did not result in an enhanced decrease in [Ca++]i; however, this drug combination enhanced decreases in [Ca++]i in spinal cord synaptosomes. The combination of CP55,940 and morphine produced enhanced decreases in [Ca++]i in both brain and spinal cord synaptosomes. In brain synaptosomes, the combination of delta 9-THC and morphine produced an additive decrease in cAMP accumulation, whereas no significant change was observed with this combination in the spinal cord. Thus, the difference in the modulation of [Ca++]i but not cAMP in the brain in vitro may be a predictor of the greater-than-additive antinociceptive effects observe in vivo.

The control of intracellular calcium and neurotransmitter release in guinea pig-derived cerebral cortical synaptoneurosomes

Synaptoneurosomes are a simply derived brain vesicular preparation which are believed to contain elements of both presynaptic and postsynaptic material. Inositol phosphates production and neurotransmitter release in the synaptoneurosome have previously been shown to be under the control of a number of receptor agonists. However, there have been few investigations of the role of intracellular calcium ([Ca2+]i) in these events. In this study we report that potassium (K+; 50 mM) was able to increase [Ca2+]i and subsequently release [3H]noradrenaline in guinea pig cerebral cortical synaptoneurosomes via activation of dihydropyridine-insensitive, voltage-sensitive calcium channels. Veratridine (30 microM) produced similar effects but these involved activation of sodium channels which could be blocked by pre-incubation with tetrodotoxin (0.15 microM). A number of agonists were used to investigate possible modulation of these events and to look for agonist-stimulated mobilization of [Ca2+]i. No evidence was found for either receptor-mediated release of calcium from intracellular stores or for modulation of K(+)-induced neurotransmitter release. This might be related to the observed passive entry of calcium through the synaptoneurosomal membrane and the subsequently high levels of [Ca2+]i.

Effect of substance P on respiratory rhythm and pre-inspiratory neurons in the ventrolateral structure of rostral medulla oblongata: an in vitro study

The pre-inspiratory (Pre-I) neurons which fire in the pre- and usually also during the post-inspiratory phase are located in the ventrolateral structures of the rostral medulla. They are suggested as primary rhythm generating neurons for respiration. These have been studied in isolated brainstem-spinal cord preparations from newborn 0-5-day-old rats. We have found that application of substance P (SP) enhanced the respiratory rhythm as measured by C4 ventral root and pre-I neuronal activities. Furthermore, the effect of SP was dependent on basal respiratory rate. An increase of the Pre-I and C4 burst rate by SP was clearer when the basal respiratory rhythm was somewhat lower. Moreover, long lasting depression of respiratory rate after the application of the alpha 2-agonist clonidine was reversed by SP. On the other hand, an inhibitory effect appeared in preparations with a higher basal respiratory rate, while the Pre-I burst rate tended to increase during SP perfusion. During chemical synaptic transmission blockade by perfusion with low Ca2+, high Mg2+ solution, a pre-I burst retained or completely blocked was found to be enhanced or reactivated by SP perfusion. The results suggest a direct postsynaptic action of SP, which could strongly stimulate burst generating properties of Pre-I neurons.

Effects of the N-methyl-D-aspartate antagonists on the rise in [Ca2+]i following depolarization in aged rat brain synaptosomes

The effects of non-competitive NMDA antagonists, MK-801 and dextrorphan in relation to the rise in intracellular Ca2+ concentrations ([Ca2+]i) after stimulation with 15 mM K+ in whole brain synaptosomes from young (3 months old) and aged (24 months old) Fisher344 rats were examined. A fluorescent chelating agent, Rhod-2, was employed to monitor any alterations of K(+)-evoked [Ca2+]i. In young rats, the rise in [Ca2+]i following depolarization was affected by neither dextrorphan (1, 10, 100 microM) nor MK-801 (0.1, 1, 10 microM), while in aged rats, 1 microM dextrorphan and 0.1 microM MK-801 brought about a significant increase in [Ca2+]i following depolarization. In low Mg2+ medium, 10 microM MK-801 and 100 microM dextrorphan significantly inhibited the rise in [Ca2+]i after stimulation with 15 mM K+ in young rats, while neither dextrorphan nor MK-801 could affect the rise in [Ca2+]i significantly in aged rats. When 100 microM NMDA was applied in a medium containing 1.2 mM Mg2+, the rise in [Ca2+]i following depolarization was slightly inhibited by 1 microM MK-801 in young rats, but it was not inhibited significantly by dextrorphan. In aged rats, both 100 microM dextrorphan and 10 microM MK-801 strongly inhibited the rise in [Ca2+]i following depolarization in the presence of 100 microM NMDA. Instead of NMDA, when 100 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a non-NMDA receptor agonist, was applied, dextrorphan did not inhibit the rise in [Ca2+]i. In low Mg2+ medium, 100 microM NMDA potentiated the inhibitory effect of 10 microM dextrorphan in young rats, while 100 microM dextrorphan or MK-801 did not show any further inhibition by adding 100 microM NMDA. The addition of 100 microM AMPA did not affect the effect of dextrorphan in a low Mg2+ medium in young rats. These results suggest that NMDA antagonist-mediated [Ca2+]i homeostatic system may alter through aging. In addition, the findings that NMDA potentiated the inhibitory effect of NMDA antagonist, which being further potentiated by aging or lowered extrasynaptosomal Mg2+, indicate the possibility that the Mg2+ block to NMDA receptors might be attenuated through aging.

Effect of haloperidol on cyclic AMP and inositol trisphosphate in rat striatum in vivo

To investigate the effect of haloperidol (HAL) on second messengers in the brain striatum, the concentrations of cAMP and inositol trisphosphate (IP-3) were measured in the striatum of rats in vivo after intravenous administration of HAL, and their concentrations were compared with the severity of catalepsy and changes in dopamine (DA) metabolism in the striatum. Catalepsy developed both in the animals treated with 5 mg/kg and those with 0.5 mg/kg of HAL, but it appeared earlier, and the period of severe catalepsy was longer in the former than in the latter. In the animals treated with 5 mg/kg of HAL, DOPAC and HVA began to increase at 20 min after administration, and their percent increases were correlated with the severity of catalepsy. In the 5 mg/kg animals, both cAMP and IP-3 increased. The IP-3 showed a delayed peak but a greater increase as compared with the cAMP. In the 0.5 mg/kg animals, only IP-3 increased. These findings suggest that HAL might affect not only the adenylate cyclase system but also the phosphoinositide response in the striatum. Moreover, the changes in the phosphoinositide response might be secondarily induced by the blocking of D-2 receptors by HAL.

Protein kinase C and phospholipase C mediate alpha 1- and beta-adrenoceptor intercommunication in rat hypothalamic slices

These experiments examined the mechanism by which phenylephrine enhances beta-adrenoceptor-stimulated cyclic AMP formation in rat hypothalamic and preoptic area slices. To this end we manipulated phospholipase C. phospholipase A2, and protein kinase C activity in slices and assessed the effects of these manipulations on phenylephrine augmentation of isoproterenol-stimulated cyclic AMP generation. Since previous work indicated that estrogen enhances the alpha 1-component of cyclic AMP formation, we examined slices from both gonadectomized and estrogen-treated animals. The alpha 1-antagonist prazosin eliminated phenylephrine augmentation of the beta-response, suggesting that alpha 1-adrenergic receptors mediate the potentiation of cyclic AMP formation. Inhibition of protein kinase C by H7 attenuated the alpha 1-augmentation of beta-stimulated cyclic AMP formation. Staurosporine, a more potent protein kinase C inhibitor, completely abolished the alpha 1-augmenting response. In addition, phenylephrine potentiation of the isoproterenol response was not observed if protein kinase C was first stimulated directly with a synthetic diacylglycerol (1-oleoyl-2-acetyl-sn-glycerol) or phorbol ester (phorbol 12,13-dibutyrate). Neomycin, an inhibitor of phospholipase C, decreased alpha 1-receptor enhancement of beta-stimulated cyclic AMP formation, whereas quinacrine, an inhibitor of phospholipase A2, did not. The data suggest that the postreceptor mechanism involved in alpha 1-adrenergic receptor potentiation of cyclic AMP generation in hypothalamic and preoptic area slices includes activation of phospholipase C and protein kinase C.