Esam E. El-Fakahany

Marked induction of calcium-independent nitric oxide synthase activity after focal cerebral ischemia

We studied the effect of focal cerebral ischemia on inducible (iNOS) and constitutive (cNOS) nitric oxide synthase enzymatic activities in the affected brain. The middle cerebral artery (MCA) was occluded in spontaneously hypertensive rats. Animals were killed 1, 2, 4, and 7 days later. cNOS and iNOS enzymatic activities were determined in the infarcted cortex using the assay of Bredt and Snyder. cNOS was assayed in the presence of calcium, whereas iNOS was assayed in the absence of calcium and in the presence of tetrahydrobiopterin. The validity of the iNOS assay was verified in rats treated with bacterial lipopolysaccharide. In these animals, the magnitude of the induction of iNOS enzymatic activity in lung, spleen, and brain paralleled the expression of iNOS mRNA, assessed by reverse-transcription polymerase chain reaction. After MCA occlusion, calcium-dependent (cNOS) activity was markedly reduced only in lesioned cerebral cortex at days 1–7 (p < 0.001; analysis of variance and Tukeys test). In contrast to cNOS, calcium-independent (iNOS) activity was induced substantially in the infarct (p < 0.005) but not in the contralateral intact cortex (p > 0.05). iNOS activity peaked at day 2 and was not different from baseline at day 7 (p > 0.05). No NADPH diaphorase-positive neurons were observed in the area of the lesion at days 1–7. Macrophages appeared at day 2 and invaded the infarcted tissue by day 7. At this time, numerous glial fibrillary acidic protein-positive astrocytes were observed within the lesion. The results suggest that the decline in calcium-dependent (cNOS) activity reflects loss of NOS neurons within the lesion. The induction of calcium-independent activity is likely to reflect induction of iNOS in nonneuronal cells. Sustained nitric oxide production by iNOS may contribute to the late phase of tissue damage in focal cerebral ischemia.

ALLOSTERIC ANTAGONISTS OF THE MUSCARINIC ACETYLCHOLINE RECEPTOR

For the most part, the interaction of these selective antagonists with muscarinic receptors has been interpreted in the context of simple competitive bimolecular reaction which obeys the law of mass action. In fact, the use of selective antagonists as a pharmacological tool to identify receptor types and subtypes is based on this premise. However, the complex binding behavior exhibited by other compounds has pointed to the existence of an allosteric (secondary) binding site on the muscarinic receptor

Prolonged morphine treatment increases rat brain dihydropyridine binding sites: possible involvement in development of morphine dependence

Regulation of L-type Ca2+ channels by morphine in rat brain was determined by the binding of [3H]nimodipine. Morphine, administered by subcutaneous pellet implantation, increased the density of [3H]nimodipine binding sites in a time- and dose-dependent manner and this effect was reversible upon removal of the pellets. Increases in these dihydropyridine sites were localized to the cortex, hippocampus, hypothalamus and brainstem but not to the cerebellum and striatum. Additional experiments were performed to test the ability of different Ca2+ channel antagonists to affect naloxone-precipitated withdrawal in morphine-dependent mice and rats. These drugs effectively reduced the incidence of naloxone-induced jumping in mice and several of the withdrawal signs in rats. Taken together, our study underscores the plasticity of brain L-type Ca2+ channels and suggests that their upregulation might contribute to morphine dependence.

Prolonged inhibition of brain nitric oxide synthase by short-term systemic administration of nitro-l-arginine methyl ester

We studied the dose-response characteristics and the temporal profile of inhibition of brain nitric oxide (NO) synthase (NOS) elicited by i.v. administration of the NOS inhibitor nitro-l-arginine methyl ester (L-NAME). L-NAME was administered i.v. in awake rats equipped with a venous cannula. L-NAME was injected in cumulative doses of 5, 10, 20 and 40 mg/kg and rats were sacrificed 30 min after the last dose. NOS catalytic activity was assayed in forebrain cytosol as the conversion of [3H]l-arginine into [3H]l-citrulline. L-NAME attenuated brain NOS activity in a dose-dependent manner but enzyme activity could not be inhibited by more than ≈50%. After a single 20 mg/kg injection of L-NAME the inhibition of brain NOS activity was time dependent and reached a stable level at 2 hrs (52% of vehicle). Inhibition after a single injection was still present at 96 hrs, albeit to a lower magnitude. We conclude that intravenous administration of L-NAME in rats at concentrations commonly used in physiological experiments leads to a dose and time-dependent but partial inhibition of brain NOS catalytic activity. The finding that the inhibition persists for several days after a single administration is consistent with the hypothesis that nitro-L-arginine, the active principle of L-NAME, binds to NOS irreversibly.

Prenatal viral infection causes alterations in nNOS expression in developing mouse brains.

&NA; Epidemiological evidence points to prenatal viral infection being responsible for some forms of schizophrenia and autism. We hypothesized that prenatal human influenza viral infection in day 9 pregnant mice may cause changes in the levels of neuronal nitric oxide synthase (nNOS), an important molecule involved in synaptogenesis and excitotoxicity, in neonatal brains. Brains from 35‐ and 56‐day‐old mice were prepared for SDS‐gel electrophoresis and Western blotting using polyclonal anti nNOS antibody. Quantification of nNOS showed time and region‐dependent changes in the levels of nNOS protein. Mean rostral brain area value from prenatally infected animals showed a significant (p = 0.067) increase of 147% in nNOS levels at 35 days postnatally, with an eventual 29% decrease on day 56. Middle and caudal brain areas showed reductions in nNOS in experimental mice at 35 and 56 days, with a significant 27% decrease in nNOS in the middle segment of day 56 brains (p = 0.016). Significant interactions were found between group membership and brain area (Wilks lambda = 0.440, F(2.9) = 5.72, p = 0.025); there was also a significant interaction between brain area, group and age (Wilks lambda = 0.437, F(2.9) = 5.79, p = 0.024). These results provide further support for the notion that prenatal viral infection affects brain development adversely via the pathological involvement of nNOS expression.

Constitutive activity of the M1–M4 subtypes of muscarinic receptors in transfected CHO cells and of muscarinic receptors in the heart cells revealed by negative antagonists

We investigated whether muscarinic receptors of the M1–M4 receptor subtypes are constitutively active. We have found that the synthesis of cyclic AMP was enhanced by the muscarinic antagonists atropine and N‐methylscopolamine (NMS) in Chinese hamster ovary (CHO) cells stably transfected with human m2 and m4 muscarinic receptor genes and in rat cardiomyocytes expressing the M2 receptor subtype, and that the production of inositol phosphates was inhibited by atropine and NMS in CHO cells stably transfected with human m1 and m3 and with rat m1 muscarinic receptor genes. The muscarinic antagonists quinuclidinyl benzilate and AF‐DX 116 had no effect in some cases and acted like atropine and NMS in others. We conclude that the M1–M4 subtypes of muscarinic receptors are constitutively active in the CHO cell lines expressing them and in cardiomyocytes and that atropine and NMS act as negative antagonists on these receptor subtypes by stabilizing them in the inactive conformation.

Morphine-induced opioid receptor down-regulation detected in intact adult rat brain cells

Intact brain cells dissociated from the brains of adult rats were used to study the regulation of opioid receptors by in vivo chronic morphine treatment. When the specific binding of [3H]naloxone in a physiological iso-osmotic buffer was compared in cells obtained from sham-operated rats and in those treated for 6 days with four (2 X 2) 75 mg morphine pellets, it was found that morphine treatment resulted in a significant reduction in the density of [3H]naloxone binding sites. This receptor down-regulation was not accompanied by a change in the receptor affinity for the ligand. This effect of morphine was reversed upon the removal of morphine pellets for 18 h after tolerance induction. When similar experiments were performed using brain homogenates prepared and assayed in the same physiological buffer, there was an increase in the number of [3H]naloxone binding sites in morphine-treated animals compared to controls. On the other hand, when the binding experiments were conducted in 50 mM Tris-HCl buffer, no difference in ligand binding was apparent between control and morphine-treated groups. The present results demonstrate opioid receptor down-regulation by chronic morphine treatment measured in intact brain cells, and suggest that different conclusions may be reached when other tissue preparations are used to assess the receptor density.

Role of Intercellular and Intracellular Communication by Nitric Oxide in Coupling of Muscarinic Receptors to Activation of Guanylate Cyclase in Neuronal Cells

Abstract: Muscarinic receptor‐mediated cyclic GMP formation and release of nitric oxide (NO) (or a precursor thereof) were compared in mouse neuroblastoma N1E‐115 cells. [3H]Cyclic GMP was assayed in cells prelabeled with [3H]guanine. Release of NO upon the addition of muscarinic agonists to unlabeled neuroblastoma cells (NO donor cells) was quantitated indirectly by its ability to increase the [3H]cyclic GMP level in labeled cells whose muscarinic receptors were inactivated by irreversible alkylation (NO detector cells). Carbachol increased NO release in a concentration‐dependent manner, with half‐maximal stimulation at 173 μM (compared to 96 μM for direct activation of cyclic GMP formation). The maximal effect of carbachol in stimulating release of NO when measured indirectly was lower than that in elevating [3H]cyclic GMP directly in donor cells. Hemoglobin was more effective in blocking the actions of released NO than in attenuating direct stimulation of [3H]cyclic GMP synthesis. There was a good correlation between the ability of a series of muscarinic agonists to release NO or to activate [3H]cyclic GMP formation directly, and the potency of pirenzepine in inhibiting the two responses. Furthermore, there was a similar magnitude of desensitization of both responses by prolonged receptor activation or stimulation of protein kinase C. NO release was also regulated in relation to the cellular growth phase. A model is proposed in which a fraction of NO generated upon receptor activation does not diffuse extracellularly and stimulates cyclic GMP synthesis within the same cell where it is formed (locally acting NO). The remainder of NO that is extruded extracellularly might travel to neighboring cells (neurotransmitter NO) or might be taken back into the cells of origin (homing NO).

High-affinity activation by paraoxon of a muscarinic receptor subtype in rat brain striatum

Abstract The mechanism of action of the anticholinesterase paraoxon on the function of a muscarinic receptor subtype in rat brain striatum was investigated. Paraoxon inhibited binding of the muscarinic agonist cis -[ 3 H]methyldioxolane, which binds to a high-affinity population of muscarinic receptors, with K 0.5 of 80 n M , compared to a K 0.5 of 7 μ M for parathion. The inhibition was competitive, suggesting that paraoxon and CD bind to a common site. When this muscarinic receptor (possibly an M 4 subtype) is activated it inhibits cAMP synthesis. Thus, function of the paraoxon-sensitive receptor was assayed by the inhibition of the forskolin-activated [ 3 H]cAMP synthesis. Paraoxon inhibited cAMP synthesis in a dose-dependent manner as did the muscarinic agonist carbachol, and this inhibition was completely blocked by the muscarinic antagonist atropine. When low concentrations of carbachol and paraoxon were used together, there was additive inhibition of cAMP synthesis. However, there was no further increase when both paraoxon and carbachol were present in concentrations that individually produced maximal inhibition. The data suggest that paraoxon acts like carbachol, causing activation of the muscarinic receptor subtype in brain striatum and leading to inhibition of cAMP synthesis. Considering the high affinity that this muscarinic receptor has for paraoxon, it is suggested that this direct reversible action of paraoxon on the muscarinic receptor could affects its toxicity, expecially early on before most of the acetylcholinesterase is phosphorylated.

Parainfluenza virus type 1 reduces the affinity of agonists for muscarinic receptors in guinea-pig lung and heart.

Membrane preparations of guinea-pig lung (containing multiple muscarinic receptor subtypes) and heart (containing M2 receptors only) were incubated with either neuraminidase, parainfluenza virus (which contains neuraminidase), or virus plus 2,3-dehydro-2-deoxy-N-acetylneuraminic acid, a neuraminidase inhibitor. None of these treatments affected [3H]quinuclidinyl benzilate [( 3H]QNB) binding. In the lung and heart, carbachol displaced 0.2 nM [3H]QNB from two sites. After treatment with either neuraminidase or virus the high affinity site was shifted to the right, and carbachol displaced QNB from one site with low affinity in the lung. In contrast, neuraminidase or virus decreased the affinity of carbachol for both sites in the heart. The neuraminidase inhibitor completely blocked virus-induced changes in carbachol affinity in both tissues. These results suggest that parainfluenza virus decreases the affinity of agonists for some of the muscarinic receptors in the lung, and for all of the muscarinic receptors in the heart due to its neuraminidase activity, which results in removal of sialic acid. The decreased agonist affinity in the lung may be responsible for the increased vagally induced bronchoconstriction seen in viral respiratory infections.