Histamine stimulation of cyclic AMP accumulation in astrocyte-enriched and neuronal primary cultures from rat brain

Decrease of cocaine, but not heroin, self-administration and relapse by the tyrosine kinase inhibitor masitinib in male Sprague Dawley rats

RATIONALE

Accumulating evidence shows that cocaine, and also heroin, influence several tyrosine kinases, expressed in neurons and in non-neuronal populations such as microglia, astrocytes and mast-cells. Drug-induced activation of mast cells both triggers inflammatory processes in the brain mediated by the glial cells they activate, and facilitates histamine release which may directly influence the dopamine system. Thus, by triggering the activation and degranulation of mast cells dependent on the tyrosine kinase c-kit and Fyn, the latter being also involved in NMDA-dependent synaptic plasticity, cocaine and heroin may indirectly influence the neural mechanisms that mediate their reinforcing properties. Masitinib, a novel tyrosine kinase inhibitor with high selectivity for c-Kit, Fyn and Lyn, may alter the aberrant consequences of the activation of these tyrosine kinases by cocaine and heroin.

OBJECTIVE

We investigated in rats the effect of a chronic oral treatment with masitinib (20 mg/kg) on the reinforcing and motivational properties of self-administered cocaine (250 μg/infusion) and heroin (40 μg/infusion).

METHODS

Three different cohorts of rats were trained instrumentally to respond for cocaine, heroin or food under continuous reinforcement. In each group, we assessed the influence of chronic daily treatment with masitinib on the maintenance of instrumental responding and intake and the motivation for the reinforcer. Thus, masitinib and vehicle-treated rats were challenged to adapt to high behavioural demand, to respond under a progressive ratio schedule of reinforcement and to reinstate instrumental responding after extinction and/or abstinence.

RESULTS

Masitinib selectively decreased cocaine intake, the motivation for cocaine and the subsequent propensity to respond for cocaine under extinction, while having no effect on instrumental responding for heroin or food.

CONCLUSION

The present findings suggest masitinib, a drug with proven efficacy in CNS disorders, could represent a novel treatment for cocaine addiction provided its influence on the reinforcing and incentive properties of the drug is confirmed.

Histidine provides long-term neuroprotection after cerebral ischemia through promoting astrocyte migration

The formation of glial scar impedes the neurogenesis and neural functional recovery following cerebral ischemia. Histamine showed neuroprotection at early stage after cerebral ischemia, however, its long-term effect, especially on glial scar formation, hasn’t been characterized. With various administration regimens constructed for histidine, a precursor of histamine, we found that histidine treatment at a high dose at early stage and a low dose at late stage demonstrated the most remarkable long-term neuroprotection with decreased infarct volume and improved neurological function. Notably, this treatment regimen also robustly reduced the glial scar area and facilitated the astrocyte migration towards the infarct core. In wound-healing assay and transwell test, histamine significantly promoted astrocyte migration. H2 receptor antagonists reversed the promotion of astrocyte migration and the neuroprotection provided by histidine. Moreover, histamine upregulated the GTP-bound small GTPase Rac1, while a Rac1 inhibitor, NSC23766, abrogated the neuroprotection of histidine and its promotion of astrocyte migration. Our data indicated that a dose/stage-dependent histidine treatment, mediated by H2 receptor, promoted astrocyte migration towards the infarct core, which benefited long-term post-cerebral ischemia neurological recovery. Therefore, targeting histaminergic system may be an effective therapeutic strategy for long-term cerebral ischemia injury through its actions on astrocytes.

Stimulation of histamine H2 receptors activates TRPC3 channels through both phospholipase C and phospholipase D

Histamine plays an important role in many physiological functions; and a change in cytosolic Ca(2+) ([Ca(2+)](i)) may be an early signal in these processes. In the present study, we investigated the activation mechanism of TRPC3, the Canonical Transient Receptors Potential 3 Channels, by histamine via a non-capacitative Ca(2+) entry pathway. TRPC3 was transfected into HEK293 cells and the cells were treated with thapsigargin to deplete the intracellular Ca(2+) stores; re-addition of Ca(2+) initiated a capacitative Ca(2+) entry (CCE). A subsequent application of histamine evoked another Ca(2+) influx on top of the CCE signal only in the TRPC3-transfected HEK293 cells, indicating that histamine can activate TRPC3 via a non-capacitative Ca(2+) entry pathway (non-CCE). This histamine-induced non-CCE was abolished by cimitidine, a histamine H(2) receptors antagonist, but not by histamine H(1) receptor antagonists pyrilamine and diphenhydramine. KT5720, a protein kinase A (PKA) inhibitor, had no effect on the histamine-induced non-CCE. This histamine-induced non-CCE was partially reduced by U73122, a phospholipase C (PLC) inhibitor, and by butan-1-ol, a phospholipase D (PLD) inhibitor. When both PLC and PLD inhibitors were simultaneously applied, the non-CCE signal was completely abolished. Taken together, our results showed, for the first time, that histamine could activate TRPC3 via histamine H(2) receptors, and both PLC and PLD participated in this process.

Analysis of FK506-mediated protection in an organotypic model of spinal cord damage: heat shock protein 70 levels are modulated in microglial cells

Functional loss after spinal cord injuries is originated by primary and secondary injury phases whose underlying mechanisms include massive release of excitatory amino acids to cytotoxic levels that contribute to neural death. Attenuation of this excitotoxicity is a key point for improving the functional outcome after injury. One of the drugs with potential neuroprotective actions is FK506, a molecule widely used as an immunosuppressant. FK506 may exert neuroprotection via inhibition of calcineurin by binding the FKBP12, or by binding other immunophilins such as FKBP52, leading to modulation of heat shock proteins (Hsp) 90 and 70. In the present study, we used an in vitro model of organotypic culture of rat spinal cord slices to assess whether FK506 is able to protect them against glutamate excitotoxicity. The results showed that FK506 promoted a significant protective effect on the spinal cord tissue at concentrations of 50 and 100 nM. Hsp70 induction was restricted to microglial cells in spinal cord slices treated with either glutamate or FK506. In contrast, the combination of both agents led to a transient reduction in Hsp70 levels in parallel to a marked reduction in IL-1beta precursor production by glial cells. The use of geldanamycin, which promotes persistent induction of Hsp70 in these cells as well as in motoneurons, did not produce tissue neuroprotection. These observations suggest that FK506 might protect spinal cord tissue by targeting on microglial cells and that transient downregulation of Hsp70 on these cells after excitotoxicity is a relevant mechanism of action of FK506.

Histamine Excites Neonatal Rat Sympathetic Preganglionic Neurons In Vitro Via Activation of H1 Receptors

The role of histamine in regulating excitability of sympathetic preganglionic neurons (SPNs) and the expression of histamine receptor mRNA in SPNs was investigated using whole-cell patch-clamp electrophysiological recording techniques combined with single-cell reverse transcriptase polymerase chain reaction (RT-PCR) in transverse neonatal rat spinal cord slices. Bath application of histamine (100 μM) or the H1 receptor agonist histamine trifluoromethyl toluidide dimaleate (HTMT; 10 μM) induced membrane depolarization associated with a decrease in membrane conductance in the majority (70%) of SPNs tested, via activation of postsynaptic H1 receptors negatively coupled to one or more unidentified K+ conductances. Histamine and HTMT application also induced or increased the amplitude and/or frequency of membrane potential oscillations in electrotonically coupled SPNs. The H2 receptor agonist dimaprit (10 μM) or the H3 receptor agonist imetit (100 nM) were without significant effect on the membrane properties of SPNs. Histamine responses were sensitive to the H1 receptor antagonist triprolidine (10 μM) and the nonselective potassium channel blocker barium (1 mM) but were unaffected by the H2 receptor antagonist tiotidine (10 μM) and the H3 receptor antagonist, clobenpropit (5 μM). Single cell RT-PCR revealed mRNA expression for H1 receptors in 75% of SPNs tested, with no expression of mRNA for H2, H3, or H4 receptors. These data represent the first demonstration of H1 receptor expression in SPNs and suggest that histamine acts to regulate excitability of these neurons via a direct postsynaptic effect on H1 receptors.

Distribution of the histamine H(2) receptor in monkey brain and its mRNA localization in monkey and human brain

The distribution of histamine H(2) receptor mRNA was determined by in situ hybridization histochemistry in human and monkey brain. In the case of monkey brain, we combined this technique with receptor ligand autoradiography to compare the distribution of mRNA and receptor binding sites. [(125)I]Iodoaminopotentidine ([(125)I]-APT), a reversible, high specific activity antagonist with high affinity and selectivity for the H(2) receptor, was used for receptor autoradiography. Radiolabeled oligonucleotides derived from the human mRNA sequence encoding this receptor were used as hybridization probes. The highest density of the H(2) receptor mRNA in human and monkey brain was found in caudate and putamen nuclei and external layers of cerebral cortex. Moderate levels were seen in the hippocampal formation and lower densities in the dentate nucleus of cerebellum. Areas such as globus pallidus, amygdaloid complex, cerebellar cortex, and substantia nigra were devoid of hybridization signal. The distribution of H(2) receptor mRNA in monkey brain is generally in good agreement with that of the corresponding binding sites: prominent in caudate, putamen, accumbens nuclei, and cortical areas. The hippocampus showed lower densities of receptors and low levels were detected in the globus pallidus pars lateralis. No binding sites were seen in amygdaloid complex and substantia nigra. The distribution of histaminergic innervation is in good correlation with the areas of high density for H(2) receptors: caudate, putamen, and external layers of cerebral cortex in monkey and human brain. The presence of mRNA in caudate and putamen nuclei, together with its absence from substantia nigra, suggests that the H(2) receptors found in the striatum are synthesized by intrinsic cells and not by nigral dopaminergic cells. These striatal H(2) receptors may be located on short circuit striatal interneurons or somatodendritically on striatal projection neurons which project to the globus pallidus pars lateralis. In conclusion, the present results, which constitute, to our knowledge, the first report of the regional distribution of mRNA encoding H(2) receptors detected by in situ hybridization, define the sites of synthesis of H(2) receptors and are the basis for future, more detailed studies that should result in a better understanding of H(2) receptor function.

Differences in the stimulation of the phosphoinositide cycle by amine neurotransmitters in cultured rat forebrain neurones and astrocytes

In this study, we compared the stimulation by carbachol (CCh), noradrenaline (NA), and histamine (HA) of phosphoinositide hydrolysis in rat forebrain neuronal and glial cultures. When Ca2+ was omitted from the stimulation buffer (low microM extracellular Ca2+), amine-induced [3H]inositol phosphate accumulation was reduced to a higher extent in astrocytes (70-80% for CCh and NA and 100% for HA) than in neurones (around 50-60% for all the amines). Furthermore, guanosine 5'-[gamma-thio]trisphosphate (GTP[S]) stimulation of phosphoinositidase C (PIC) in membranes was 5-fold higher in neurones than in astrocytes. These results indicate differences in the mechanism of PIC stimulation in the two cell types. After 30 min stimulation in the presence of 10 mM Li+, a higher accumulation of [3H]inositol 4-monophosphate and [3H]inositol 1,4-bisphosphate than of [3H]inositol 1/3-monophosphate occurred for all agonists in neurones, whereas the opposite was observed in astrocytes. Moreover, in these cells stimulation for 5 min in the absence of Li+ produced a 2-3-fold accumulation of all metabolites of the 3-kinase pathway of inositol-1,4,5-trisphosphate metabolism but not of those of the 5-phosphatase pathway. Thus, regardless of the amine receptor stimulated, the 3-kinase route appeared to prevail in astrocytes and the 5-phosphatase pathway in neurones. The histamine response in neurones differed from that of the other agonists in that it rapidly declined. Taken together these results indicate that the heterogeneity in amine stimulation of the phosphoinositide cycle previously observed in brain slices could arise to a great extent from the cellular diversity of this preparation and be related to the differential contribution of the amine receptors located in neurones and astrocytes.

Characteristics of nitric oxide synthase type I of rat cerebellar astrocytes

We have previously reported that stimulation of astrocyte cultures by particular agonists and calcium ionophores induces cyclic GMP formation through activation of a constitutive nitric oxide synthase (NOS) and that astrocytes from cerebellum show the largest response. In the present work we have used rat cerebellar astrocyteenriched primary cultures to identify and characterise the isoform of NOS expressed in these cells. The specific NOS activity in astrocyte homogenates, determined by conversion of [3H]arginine to [3H]citrulline, was ten times lower than in homogenates from cerebellar granule neurons. Upon centrifugation at 100,000 g, the astroglial activity was recovered in the supernatant, whereas in neurons around 30% of the activity remained particulate. The cytosolic NOS activities of both astrocytes and granule neurons displayed the same Km for L-arginine, dependency of calcium, and sensitivity to NOS inhibitors. Expression of NOS-I in astrocyte cytosolic fractions was revealed by Western blot with a specific polyclonal antiserum against recombinant NOS-I. Double immunofluorescence labelling using anti-glial fibrillary acidic protein (GFAP) and anti-NOS-I antibodies revealed that a minor population of the GFAP-positive cells, usually in clusters, presented a strong NOS-I immunostaining that was predominantly located around the nuclei and had a granular appearance, indicating association with the endoplasmic reticulum-Golgi system. Astrocytes of stellate morphology also showed immunoreactivity in the processes. Similar staining was observed with the avidin-biotin-peroxidase complex using different anti-NOS-I antisera. With this method the majority of cells showed a weak NOS-I immunoreactivity around the nuclei and cytosol. A similar pattern was observed with the NADPH-diaphorase reaction. These results demonstrate that the NOS-I expressed in astrocytes presents the same biochemical characteristics as the predominant neuronal isoform but may differ in intracellular location.

Effects of exogenous linoleic acid on fatty acid composition, receptor-mediated cAMP formation, and transport functions in rat astrocytes in primary culture

We have examined the effects of culturing neonatal rat-brain astrocytes in medium containing delipidated serum, with or without added linoleic acid (LA, 18:2 omega 6), on membrane fatty-acid composition and functions. After 18-21 days in culture, polyunsaturated fatty acids (PUFA) constituted approximately equal to 24 mol% of the total fatty acids in the astrocytes grown in delipidated media ("controls'); these proportions were increased by 35-40% to approximately equal to 33 mol% when the cells were supplemented with 35 microM LA. Notable differences in the PUFA profiles of the cells cultured with or without added LA included: (a) higher proportions of omega 6 PUFA in the LA-supplemented astrocytes (approximately equal to 25%, relative to approximately equal to 10% in controls) that were accompanied by an increase in the ratio of omega 6/omega 3 PUFA (from < 2 in controls to approximately equal to 5), and (b) higher proportions of 20:3 omega 9 and 22:3 omega 9 in the control astrocytes (> 5%) relative to the LA-supplemented cells (approximately equal to 1%). The major metabolites in the omega 6 PUFA-enriched cells were arachidonic (20:4 omega 6), adrenic (22:4 omega 6) and docosapentaenoic (22:5 omega 6) acids (15, 5 & 3 mol%, respectively). Enrichment of the astrocytes in omega 6 PUFA did not alter basal levels of cAMP, nor did it affect the amounts of cAMP formed in response to forskolin, isoproterenol, adenosine or histamine. However, dopamine-dependent increases in cAMP formation in the presence of the phosphodiesterase inhibitor, Ro 20-1724, were reduced by approximately equal to 25% relative to those in controls. LA supplementation modified uptake of [3H]adenosine into the astrocytes; values for Kt for a high affinity transport were increased relative to controls, and maximum capacity of a lower affinity process was reduced. Uptake of [3H]glutamate was not altered in the omega 6 PUFA-enriched astrocytes. This study demonstrated that cultured astrocytes take up exogenous linoleic acid and incorporate its metabolites into phospholipid, and that the resulting changes in membrane PUFA composition modify only specific cell functional properties.

Calcium-dependent nitric oxide formation in glial cells

We have previously demonstrated nitric oxide (NO)-dependent cyclic GMP (cGMP) formation in response to noradrenaline (NA) and glutamate (GLU) in astrocyte-enriched cultures from rat cerebrum. In the present work we show heterogeneity in agonist responses in astrocyte cultures from cerebellum, hippocampus and cortex. The response to NA was higher in cells from cerebellum, intermediate in cultures from hippocampus and low in cortical astrocytes. GLU had no significant effect in cortical and cerebellar cultures and presented lower effects than NA in cells from hippocampus. The NO donor sodium nitroprusside (SNP) produced much higher cGMP levels than agonists and the order of efficacies was cerebellum > cortex > hippocampus. Responses to NA and SNP in cerebellar astrocytes were sensitive to culture conditions decreasing when cells were seeded at low density or subcultured. Microglial cells were the main contaminants of the cerebellar astrocyte cultures but did not contribute to the NA or the SNP responses. No soluble guanylyl cyclase or calcium-dependent NO synthase (cNOS) activities were detected in microglial cultures. The effect of NA in cerebellar astrocytes was blocked by L-arginine analogues and by the alpha 1-adrenoceptor antagonist prazosin. The calcium ionophore A23187 mimicked the effect of NA and omission of calcium from the medium prevented both responses. NA did not elicit cGMP formation in granule cell cultures. These results support an astroglial location of the alpha 1-adrenoceptors and the cNOS that mediate NA stimulation of cGMP formation in cerebellum.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Receptors on astrocytes--what possible functions?

Receptors for transmitters, as varied as those expressed by neurons, have been described on primary astrocyte cultures prepared from new-born rats and mice. A variety of functional effects and considerable cell-to-cell and regional heterogeneity have been observed for such receptors in vitro. The various systems available for studying the presence and properties of receptors on astrocytes in situ, and the results from these studies, are discussed. Much fewer studies using these more difficult systems have been done. So far, some resemblances and differences between in situ and in vitro work have been observed. More of these in situ studies, to supplement the ongoing in vitro work, are needed to enable us to determine unequivocally which receptors are present on astrocytes, and their functions in vivo. If there is cell-to-cell and CNS regional heterogeneity in vivo comparable to that seen in vitro, these analyses will be very complex. To illustrate the importance and variety of receptor-linked functions, a number of suggestions are made in this commentary, based on current proposals for the roles of astrocytes. However, it is argued that we need to have a more complete understanding of astrocyte functions in vivo, before we can really understand the functional significance of astrocyte receptors.

Effect of zinc, copper and glucocorticoids on metallothionein levels of cultured neurons and astrocytes from rat brain

The knowledge of brain metallothionein (MT) regulation and especially of MT presence in specific cell types is scarce. Therefore, the effect of several well-known MT inducers, measured by radioimmunoassays using antibodies that cross-react with MT-I and MT-II or specific for MT-I and which do not cross-react with human growth inhibitory factor (GIF or MT-III), has been studied in primary cultures of neurons or astrocytes obtained from rat cerebrum. MT-I levels in glial cells were about ten times higher than those in neuronal cells (538 +/- 194 vs. 49 +/- 16 pg MT-I/micrograms protein, mean +/- S.D. from three separate cell preparations). Increasing the concentration of Zn in the bovine serum albumin (BSA)-containing culture medium up to 50 microM significantly increased MT-I levels by up to 3.5-fold in neurons and 2.5-fold in astrocytes. In contrast, Cu up to 50 microM increased MT-I levels in a saturable manner in both neurons (up to 5-fold) and astrocytes (up to 1.5-fold), the maximum effect occurring at 5 microM Cu. In general, the combination of Zn and Cu further increased MT-I levels. The effect of the metals on MT-I appeared to reflect metal uptake, since MT-I induction was less marked when the BSA concentration in the medium was increased from 2 to 10 mg/ml. Dexamethasone increased MT-I levels in both neurons and astrocytes in vitro in a concentration-dependent manner. Endotoxin, IL-1 and IL-6 did not have a significant effect on glial MT levels at the concentrations studied. The administration of dexamethasone to rats increased MT-I levels in non-frontal cortex, cerebellum, pons+medulla, midbrain and hippocampus, but not in hypothalamus, frontal cortex and striatum. Endotoxin increased liver but not brain MT-I levels. Immunocytochemical studies in adult rat brain preparations with a polyclonal antibody that cross-reacts with MT-I and MT-II indicated that immunostaining was always nuclear in glial cells, whereas in neurons it was nuclear in the cerebral cortex, hippocampus and the granular layer of the cerebellum, and nuclear plus cytoplasmic in Purkinje cells in the cerebellum, hypothalamic nuclei and gigantocellular reticular nucleus in the brain stem. Meninges, choroidal plexus, ependymal and endothelial cells were also MT-immunoreactive.

Regulation of metallothionein-I+II levels in specific brain areas and liver in the rat: role of catecholamines

The role of the catecholamines noradrenaline, adrenaline and dopamine on metallothionein (MT) levels of specific areas of the rat brain has been studied. MT-I or MT-I + II levels were measured by radioimmunoassay using specific antibodies that cross-react only slightly with human MT-III (growth inhibitory factor, GIF). The inhibition of tyrosine hydroxylase with alpha-methyl-p-tyrosine (MPT), which depletes brain dopamine, noradrenaline, and adrenaline, increased MT levels in all brain areas studied (frontal cortex, cortex, medulla oblongata plus pons, midbrain, striatum, hippocampus, hypothalamus, and cerebellum) when considering the results of two separate experiments. The alpha- and beta-receptor blockers, phentolamine, and propranolol, alone or together, did not increase brain MT levels in any area of the brain, suggesting that the effect of MPT in vivo is related to inhibition of the synthesis of dopamine rather than of noradrenaline and adrenaline. Dopamine, noradrenaline, and serotonin increased MT-I levels in primary cultures of neurons, whereas decreased them in astrocyte-enriched primary cultures. Since MT-I levels are about ten times higher in astrocytes than in neurons, the increased brain MT levels induced by MPT may reflect the suppression of the normal inhibitory effect of dopamine on astrocyte MT levels. The increase in MT concentrations induced in most parts of the brain by immobilization stress was not prevented by MPT, phentolamine, or propranolol, suggesting that it was not mediated by the central monoamines.

Histamine and prostanoid receptors on glial cells

Glial cells in vitro express at least two types (H1 and H2) of histamine receptors and three types (EP, FP, and TP) of prostanoid receptors. The receptors expressed by glial cells differ according to the cell type and source in the brain. Furthermore primary astrocytes of same type derived from the same brain region are composed of heterogeneous subpopulations expressing different subsets of receptors. Fura-2 based Ca2+ microscopy revealed that astrocyte processes are important sites for histamine-induced Ca2+ signalling. Histamine and prostanoid receptors on glial cells may play important roles in the actions of histamine and prostanoids in the central nervous system.

Rat ciliary neurotrophic factor (CNTF): gene structure and regulation of mRNA levels in glial cell cultures

The structure of the rat ciliary neurotrophic factor (CNTF) gene and the regulation of CNTF mRNA levels in cultured glial cells were investigated. The rat mRNA is encoded by a simple two-exon transcription unit. Sequence analysis of the region upstream of the transcription start-site did not reveal a typical TATA-box consensus sequence. Low levels of CNTF mRNA were detected in cultured Schwann cells, and CNTF mRNA was not increased by a variety of treatments. Three-week-old astrocyte-enriched cell cultures from new-born rat brain contained easily detectable CNTF mRNA. In astrocyte-enriched cultures, upregulation of CNTF mRNA levels was observed after treatment with IFN-gamma. CNTF mRNA levels were down-regulated in these cells by treatments that elevate intracellular cyclic AMP and by members of the fibroblast growth factor (FGF) family. The implications of these results for potential in vivo functions of CNTF are discussed.

Characterization of noradrenaline-stimulated cyclic GMP formation in brain astrocytes in culture

Cyclic GMP accumulation induced by noradrenaline in astrocyte-enriched primary cultures from rat cerebrum involves synthesis of NO, as evidenced by the competitive inhibition exerted by the NO synthase inhibitor NG-monomethyl-L-arginine (IC50 = 3 microM). Furthermore, the noradrenaline effect was potently inhibited by haemoglobin (IC50 = 25 nM) and potentiated by superoxide dismutase, indicating that NO synthesis and cyclic GMP formation may occur in different subsets of astrocytes. Investigation of the receptors implicated by using selective adrenoceptor agonists and antagonists indicates that about 75% of the NO-dependent noradrenaline response is mediated by alpha 1-adrenoceptors and the rest by beta-adrenoceptors, with no evidence for potentiating effects between the two receptor types. This noradrenaline effect appears to require Ca2+ entry, since it is strongly dependent on extracellular Ca2+ but is not affected by conditions that will abolish intracellular Ca2+ mobilization (incubation with neomycin or pretreatment with carbachol). Inhibition by pretreatment with pertussis toxin is in agreement with involvement of the alpha 1A-adrenoceptor subtype in this Ca(2+)-dependent effect. However, implication of an unknown alpha 1-adrenoceptor subtype cannot be disregarded, because a similar inhibition is exerted by the presumably selective alpha 1B- and alpha 1C-adrenoceptor blocking agent chloroethylclonidine. Treatment of the cultures with the protein kinase C activator phorbol 12-myristate 13-acetate inhibits to a great extent the noradrenaline-induced cyclic GMP formation.

Endothelin receptors on cultured fetal rat diencephalic glia

The recently described family of proteins, the endothelins, are produced in neurons and bind to extravascular sites in the CNS. To characterize these receptors, we carried out studies on cultures of fetal rat diencephalic glia. Scatchard analysis of saturation binding studies was done for astrocytes (greater than 95% glial fibrillary acidic protein positive). For endothelin 3 (ET-3) and ET-1, respectively, a single receptor class of KD 0.41 +/- 0.05 and 0.62 +/- 0.04 nM and a receptor density of 42 +/- 0.8 and 58 +/- 1.1 fmol/mg of glial protein was found. Bound and cross-linked 125I-ET-3 or ET-1 showed a single predominant receptor band at Mr 52,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis; a minor band at 50,000 was also seen. At concentrations equal to the receptor KD, the major brain form of ET, ET-3, stimulated a nearly 200% increase in the incorporation of tritiated thymidine into glia. ET-3 and ET-1 significantly impaired the ability of atrial natriuretic peptide (ANP) to generate cyclic GMP, and isoproterenol to generate cyclic AMP. The ability of ET to inhibit ANP-induced cyclic GMP generation was reversed by cycloheximide and actinomycin-D, whereas the inhibition of isoproterenol-induced cyclic AMP generation was partially and significantly blocked by inhibitors of calcium influx, protein kinase C action, or G protein activation, as well. Astrocytes from this part of the brain are a potential target cell for endothelin, assuming these findings are present in vivo. This neuropeptide may serve as a growth stimulator for astrocytes and modulator of the actions of catecholamines or ANP on glia by inhibiting second messenger generation.

Different receptors mediate stimulation of nitric oxide-dependent cyclic GMP formation in neurons and astrocytes in culture

The ability of various compounds to stimulate cyclic GMP accumulation was studied in neuronal and astrocyte-enriched primary cultures from rat cerebrum. Glutamate was the only agonist eliciting a response in neurons whereas several agonists had an effect in astrocytes but only those due to norepinephrine and glutamate were of considerable magnitude. The responses were markedly inhibited by the nitric oxide synthase inhibitor NG-monomethyl-L-arginine. The effect of glutamate appears to be mediated predominantly by NMDA receptors in neurons and by quisqualate AMPA-insensitive receptors in astrocytes.

Adenosine-dependent regulation of cyclic AMP accumulation in primary cultures of rat astrocytes and neurons

The regulation of intracellular cyclic AMP (cAMP) formation by adenosine (Ado) and its analogues has been examined in primary cultures of rat-brain astrocytes and neurons. In the presence of the phosphodiesterase inhibitor, Ro 20-1724, basal levels of cAMP ranged from 40-120 pmol/mg protein in both cell types. Levels were not altered by treating the cells with Ado deaminase, which suggested that they did not produce appreciable amounts of endogenous Ado under standard culture conditions. In the astrocytes, microM quantities of agonists increased cAMP up to 30-fold higher than basal values; the relative potencies were typical of an A2 Ado receptor (NECA greater than Ado greater than R-PIA). Neuron-enriched cultures exhibited a maximum fourfold increase in cAMP in response to NECA; this was decreased a further eightfold when the cultures had prolonged exposure to the antimitotic agent, c-Ara, to eliminate greater than 98% of the nonneuronal cells. Low (nM) amounts of the Ado agonists inhibited cAMP formation in both cell types. In the astrocytes, the order of potency of inhibition of isoproterenol-stimulated cAMP formation was typical of an A1 receptor (R-PIA greater than Ado greater than NECA); maximum inhibition was 55-65%. Isoproterenol did not increase cAMP in the neuronal cultures. However, forskolin-stimulated formation was effectively (approximately 50%) inhibited by A1 Ado agonists; inhibition was not affected by prolonged treatment with c-Ara. From this study we tentatively concluded that rat astrocytes and neurons both contain inhibitory A1 Ado receptors, but that the stimulatory "A2" subtype is localized mainly on astrocytes.

Histamine-induced cyclic AMP accumulation in type-1 and type-2 astrocytes in primary culture

Histamine-induced cyclic AMP (cAMP) accumulation was studied in purified primary cultures of type-1 and type-2 astrocytes from neonatal rat brain. Histamine induced remarkable cAMP accumulation in type-1 astrocytes in a dose-dependent manner (EC50 = 1.2 x 10(-5) M, Emax = 1100% of control). In contrast, histamine had no significant effect on cAMP accumulation in type-2 astrocytes. Famotidine, an H2-antagonist, dose-dependently inhibited histamine-induced cAMP accumulation in type-1 astrocytes (Ki = 3 x 10(-8) M), but mepyramine (10(-6) M), an H1-antagonist, had no effect. Dimaprit and impromidine, H2-agonists, stimulated cAMP accumulation, but 2-pyridylethylamine, an H1-agonist, did not stimulate it nor augment the H2-agonist-induced cAMP accumulation. These results indicate that (1) histamine induces cAMP accumulation in type-1 astrocytes but not in type-2 astrocytes, and that (2) histamine-induced cAMP accumulation in type-1 astrocytes is mediated by H2-receptors without significant augmentation via H1-receptors.

Is the histaminergic neuron system a regulatory center for whole-brain activity?

Recent immunocytochemical studies have demonstrated the existence of histaminergic neurons in the brain, which are concentrated in the tuberomammillary nucleus of the posterior hypothalamus, and which project efferent fibers to almost all parts of the brain. Three subtypes of histamine receptors are widely distributed in the brain, not only on neurons but also on astrocytes and blood vessels. Consistent with its wide-ranging output, the histaminergic neuron system regulates various activities of the brain, such as the arousal state, brain energy metabolism, locomotor activity, neuroendocrine, autonomic and vestibular functions, feeding, drinking, sexual behavior, and analgesia--this regulation is possibly achieved by the histaminergic system as a whole.

Norepinephrine increases cyclic GMP in astrocytes by a mechanism dependent on nitric oxide synthesis

Norepinephrine induces a rapid and concentration-dependent rise (EC50 = 1.21 +/- 0.33 microM) in cyclic GMP levels in astrocyte-enriched cultures from rat brain. The response is partially mediated by alpha 1-adrenoceptors since a marked inhibition is observed in the presence of prazosin while the beta-antagonist propranolol shows a smaller effect and the alpha 2-antagonist yohimbine is ineffective. L-NG-methylarginine, an inhibitor of nitric oxide synthesis from L-arginine, blocks the norepinephrine-induced cyclic GMP accumulation. This effect is reversed by L-arginine.