Inhibition of [3H]diazepam binding in primary cultures of astrocytes by atrial natriuretic peptide and by a cyclic GMP analog

Mechanisms of ammonia-induced astrocyte swelling

Astrocyte swelling represents the major factor responsible for the brain edema associated with fulminant hepatic failure (FHF). The edema may be of such magnitude as to increase intracranial pressure leading to brain herniation and death. Of the various agents implicated in the generation of astrocyte swelling, ammonia has had the greatest amount of experimental support. This article reviews mechanisms of ammonia neurotoxicity that contribute to astrocyte swelling. These include oxidative stress and the mitochondrial permeability transition (MPT). The involvement of glutamine in the production of cell swelling will be highlighted. Evidence will be provided that glutamine induces oxidative stress as well as the MPT, and that these events are critical in the development of astrocyte swelling in hyperammonemia.

Effect of benzodiazepines and neurosteroids on ammonia-induced swelling in cultured astrocytes

Astroglial swelling occurs in acute hyperammonemic states, including acute hepatic encephalopathy. In these conditions, the peripheral-type benzodiazepine receptor (PBR), a receptor associated with neurosteroidogenesis, is up-regulated. This study examined the potential involvement of PBRs and neurosteroids in ammonia-induced astrocyte swelling in culture. At low micromolar concentrations, the PBR antagonist PK 11195, atrial natriuretic peptide, and protoporhyrin IX, which are known to interact with the PBR, attenuated (16-100%) the effects of ammonia, whereas the PBR agonists Ro5-4864, diazepam binding inhibitor (DBI51-70), and octadecaneuropeptide exacerbated (10-15%) the effects of ammonia. At micromolar concentrations, diazepam, which interacts with both the PBR and the central-type benzodiazepine receptor (CBR), increased swelling by 11%, whereas flumazenil, a CBR antagonist, had no effect. However, at 100 nM diazepam and flumazenil abrogated ammonia-induced swelling. The neurosteroids dehydroepiandrosterone sulfate, tetrahydroprogesterone, pregnenolone sulfate, and tetrahydrodeoxycorticosterone (THDOC), products of PBR stimulation, at micromolar concentrations significantly enhanced (70%) ammonia-induced swelling. However, at nanomolar concentrations, these neurosteroids, with exception of THDOC, blocked ammonia-induced swelling. We conclude that neurosteroids and agents that interact with the PBR influence ammonia-induced swelling. These agents may represent novel therapies for acute hyperammonemic syndromes and other conditions associated with brain edema and astrocyte swelling.

Effects of N omega-nitro-L-arginine methyl ester on benzodiazepine binding in some limbic areas of hyperlipidaemic rats

Quantitative autoradiography techniques were used to evaluate the chronic effects of the potent nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester, on the binding pattern of [3H]flunitrazepam (benzodiazepine agonist) in some behaviorally key limbic areas of the genetic hyperlipidaemic Pittsburg Yoshida rat. Administration of this potent synthase inhibitor was capable of supplying higher and moderately higher binding levels in the basolateral amygdala nucleus (+52%) and in the oriens-pyramidalis CA1 hippocampus layer (+38%), respectively. When we tested for the binding changes in the presence of GABA (principal benzodiazepine modulator) we noticed that a physiological concentration (20 microM) of this inhibitory neurotransmitter was sufficient to induce notable changes in other limbic areas. In fact, lower binding values (-65%) were reported for the bed nucleus of stria terminalis whereas moderately higher values (+38%) were obtained for the radiatum-lacunosum molecular CA1 hippocampus layer. From the saturation studies, it was possible to observe that the major receptor variations provoked by the potent synthase inhibitor were not only due to changes in the total number of binding sites because there were variations, as in the case of the basolateral amygdala nucleus, that were instead due to differences in the affinity binding state. These results provide evidences of a GABAergic-nitric oxide synthase inhibitor interaction that might also be involved in the regulation of convulsive, anxiolytic, and aggressive behaviors that are modulated at the benzodiazepine site.

Effect of hypoosmotic stress on peripheral-type benzodiazepine receptors in cultured astrocytes

Astrocytes appear to be the primary source of peripheral benzodiazepine (PBZD) receptors in brain. The function of this receptor is not well understood. Since there is evidence that this receptor may be involved in cell volume control, we examined the effect of hypoosmotic stress on the regulation of the PBZD receptors in homogenates of cultured astrocytes derived from neonatal rat cerebral cortex. Exposure of astrocytes that were maintained in the presence of dibutyryl cAMP (dBcAMP) to hypoosmotic medium (200 mOsm) for 24 h resulted in 27 and 57% increased in the number of [3H]PK 11195 and [3H]Ro5-4864-binding sites, respectively, as compared with isoosmotic media (320 mOsm). This receptor upregulation is osmolarity- and time-dependent. However, hypoosmotic stress had no effect on PBZD receptor-binding in astrocytes that were maintained in the absence of dBcAMP. Under isoosmotic conditions, dBcAMP appears to regulate [3H]Ro5-4864 but not [3H]PK 11195-binding sites, a finding which further supports a partial distinction between the binding sites labeled with these ligands. The modulation of PBZD receptors by hypoosmotic stress suggests a possible role for these receptor sites in astrocyte volume control.

Binding of atrial and brain natriuretic peptides to cultured mouse astrocytes from different brain regions and effect on cyclic GMP production

We prepared primary cultures of mouse astrocytes from the cerebral cortex, hypothalamus, and cerebellum to examine the possibility of regional disparity in binding of human atrial and porcine brain natriuretic peptides (hANP, pBNP) and their effect on cyclic guanosine monophosphate (cGMP) production. 125I-hANP and 125I-pBNP bound in a specific and saturable manner to all three regions. For both peptides, Scatchard analysis suggested a single population of binding sites on astrocytes from all three regions. No significant differences were observed in the maximal binding capacities (Bmax) or binding dissociation constants (KD) between the two peptides in the astrocyte preparations from different regions. ANP and BNP also evoked cGMP stimulation in a similar, dose-dependent fashion in astrocytes from all three regions, with maximal responses to both peptides reached at a concentration above 1 microM. While BNP elicited a greater maximal cGMP accumulation than ANP, no difference could be demonstrated in the cGMP responses to either peptide between brain regions. Thus we have been unable to demonstrate regional heterogeneity in the responsiveness of astrocytes to ANP and BNP.

Morphology of astroglia in colony cultures following transient exposure to potassium ion, hypoösmolarity and vasopressin

Brain swelling is the major cause of delayed neuronal damage following injury to the central nervous system. Swelling of mouse astroglial cells was studied in colony cultures by light and electron microscopy. Swelling of suspended astroglial cells was studied by flow cytometry. Swelling caused by hypoösmolarity solution was more pronounced than that caused by 15 or 60 mM K+. Under both conditions swelling in both immature and mature astroglia was followed by a regulatory volume decrease. Arginine vasopressin caused mild astroglial swelling and atrial natriuretic peptide did not significantly affect cell volume. All changes in extracellular environment were associated with changes in the morphology of microvilli and varying amounts of membrane ruffling. Immature cells exhibited a delayed response to the application of atrial natriuretic peptide and less membrane ruffling following exposure to 60 mM K+ than mature astroglia. These nonspecific morphological changes are likely associated with changes in membrane ion pump activity.

Characteristics and specific localization of receptors for atrial natriuretic peptides at non-neuronal cells in cultured mouse spinal cord cells

Characteristics of atrial natriuretic peptide receptors were determined in cultured mouse spinal cord cells. Saturation and competition experiments demonstrated the presence of a single class of atrial natriuretic peptide binding sites with high affinity (KD = 0.054 nM) and a density of 1.92 fmoles/10(6) cells. A similar affinity (KD = 0.070 nM) was observed in rat spinal cord membrane preparations. These atrial natriuretic peptide binding sites were functional receptors since the treatment of cells with atrial natriuretic peptide increased cyclic guanosine monophosphate levels within these cells in a classical time-dependent manner. When atrial natriuretic peptide was applied onto the cell body of intracellularly recorded spinal cord neurons, this peptide did not evoke a change of the input resistance or of the resting membrane potential value. Light-microscopic autoradiography studies showed that no atrial natriuretic peptide binding could be detected on typical birefringent neurons but it could be located on astroglial and epithelial cells as identified by immunocytochemical markers. These results show that functional atrial natriuretic peptide receptors with high affinity exist in cultured mouse spinal cord cells and are not located on neurons. The presence of atrial natriuretic peptide receptors on astrocytes suggests that this neuropeptide might be a good candidate for neuron-glial communication. As the atrial natriuretic peptide binding sites previously shown in epithelia responsible for maintaining fluid and electrolyte gradients, the atrial natriuretic peptide receptors on epithelial cells in these spinal cord cultures may be involved in vivo in the control of water balance in the central nervous system.

Evidence for involvement of the astrocytic benzodiazepine receptor in the mechanism of action of convulsant and anticonvulsant drugs

The anticonvulsant drugs carbamazepine, phenobarbital, trimethadione, valproic acid and ethosuximide at pharmacologically relevant concentrations inhibit [3H]diazepam binding to astrocytes in primary cultures but have much less effect on a corresponding preparation of neurons. Phenytoin as well as pentobarbital (which is not used chronically as an anticonvulsant) are equipotent in the two cell types. The convulsants picrotoxinin and pentylenetetrazol, the convulsant benzodiazepine RO 5-3663 and the two convulsant barbiturates DMBB and CHEB similarly inhibit diazepam binding to astrocytes but have little effect on neurons. On the basis of these findings it is suggested that these convulsants and anticonvulsants owe at least part of their effect to an interaction with the astrocytic benzodiazepine receptor, perhaps by interference with a calcium channel.