Role of G Protein βγ Subunits in Muscarinic Receptor-Induced Stimulation and Inhibition of Adenylyl Cyclase Activity in Rat Olfactory Bulb

Abstract: In the olfactory bulb, muscarinic receptors exert a bimodal control on cyclic AMP, enhancing basal and G_s-stimulated adenylyl cyclase activities and inhibiting the Ca²⁺/calmodulin- and forskolin-stimulated enzyme activities. In the present study, we investigated the involvement of G protein βγ subunits by examining whether the muscarinic responses were reproduced by the addition of βγ subunits of transducin (βγ_t) and blocked by putative βγ scavengers. Membrane incubation with βγ_t caused a stimulation of basal adenylyl cyclase activity that was not additive with that produced by carbachol. Like carbachol, βγ_t potentiated the enzyme stimulations elicited by vasoactive intestinal peptide and corticotropin-releasing hormone. RT-PCR analysis revealed the expression of mRNAs encoding both type II and type IV adenylyl cyclase, two isoforms stimulated by βγ synergistically with activated G_s. In addition, βγ_t inhibited the Ca²⁺/calmodulin- and forskolin-stimulated enzyme activities, and this effect was not additive with that elicited by carbachol. Membrane incubation with either one of two βγ scavengers, the GDP-bound form of the α subunit of transducin and the QEHA fragment of type II adenylyl cyclase, reduced both the stimulatory and inhibitory effects of carbachol. These data provide evidence that in rat olfactory bulb the dual regulation of cyclic AMP by muscarinic receptors is mediated by βγ subunits likely acting on distinct isoforms of adenylyl cyclase.     

Formation of γ-Amino-β-Hydroxybutyric Acid from 2-Hydroxyputrescine in Rat Brain

gamma-Amino-beta-[3H]hydroxybutyric acid ([3H]-GABOB) was formed in rat brain from 2-[3H]-hydroxyputrescine that had been chemically synthesized from 2-oxoputrescine and [3H]sodium borohydride. After the injection of 2-[3H]hydroxyputrescine into the lateral ventricle of a rat brain, the rat was killed and then the brain was removed. [3H]GABOB in the brain was identified by a combination of ion-exchange chromatography, high-voltage paper electrophoresis, and recrystallization of the radioactive compound with authentic GABOB.     

Differential Localization of the γ3 and γ12 Subunits of G Proteins in the Mammalian Brain

Abstract: The localization of two forms of the γ subunit of G proteins, γ₃ and γ₁₂, was examined in the mammalian brain. Concentrations of these two γ subunits increased markedly, as did those of glial fibrillary acidic protein, during postnatal development in the rat cerebral cortex. In aged human brains, by contrast, the concentration of γ₃ tended to decrease with age, whereas that of γ₁₂ in the temporal cortex increased slightly. An immunohistochemical study of human brains revealed that γ₃ was abundant in the neuropil, whereas γ₁₂ was localized in glial cells. In the hippocampal formation of aged human brains, levels of γ₁₂-positive cells, as well as levels of glial fibrillary acidic protein- and vimentin-positive astrocytes, increased, in particular in the CA1 subfield and the prosubiculum, in which there was a decrease in the number of pyramidal cells. The appearance of γ₁₂-positive cells associated with the loss of pyramidal cells was also observed in the hippocampus of rats that had been treated with kainic acid. These results indicate that γ₁₂ is strongly expressed in reactive astrocytes. In a study of cultured neural cells, we found that γ₁₂ was predominant in glioma cells, such as C6 and GA-1 cells, in contrast with the specific localization of γ₃ in PC12 pheochromocytoma cells, which are neuron-like cells. Taken together, the results indicate that γ₃ and γ₁₂ are selectively expressed in neuronal and glial cells, respectively, and that concentrations of γ₃ and γ₁₂ in the brain are related to the numbers and/or extent of maturation of these cells.     

Localization and Developmental Changes of τ Protein Kinase I/Glycogen Synthase Kinase-3β in Rat Brain

Abstract: τ protein kinase I (TPKI) purified from bovine brain extract has been shown to phosphorylate τ and to form paired helical filament (PHF) epitopes and was found recently to be identical to glycogen synthase kinase-3β (GSK-3β). Before elucidating a role of TPKI/GSK-3β in PHF formation, it is necessary to investigate the normal function of the enzyme. To study the distribution and developmental changes of the enzyme, specific polyclonal antibodies were prepared against TPKI and GSK-3α. Immunoblot analysis demonstrated that TPKI was nearly specifically localized in the brain of adult rats. The level of TPKI in the rat brain was high at gestational day 18, peaked on postnatal day 8, and then decreased rapidly to a low level, which was sustained up to 2 years. Immunohistochemistry indicated primarily neuronal localization of TPKI. Growing axons were stained most intensely in the developing cerebellum, but the immunoreactivity became restricted to the gray matter in the mature tissue. Parallel fibers had a high level of TPKI and also stained intensely for τ. These findings indicate that τ is one of the physiological substrates of TPKI and suggest that the enzyme plays an important role in the growth of axons during development of the brain.     

γ-Aminobutyric AcidB Receptor Activation Modifies Agonist Binding to β-Adrenergic Receptors in Rat Brain Cerebral Cortex

The interaction of isoproterenol with beta-adrenergic receptor (beta AR) binding sites was measured in membranes prepared from rat brain cerebral cortical slices previously incubated in the presence or absence of gamma-aminobutyric acid (GABA) receptor agonists. Both GABA and baclofen, but not isoguvacine, altered beta AR agonist binding by increasing the affinity of both the low- and high-affinity binding sites and by increasing the proportion of low-affinity receptors. The response to baclofen was stereoselective, and the effect of GABA was not inhibited by bicuculline. The results suggest that GABAB, but not GABAA, receptor activation modifies the coupling between beta AR and stimulatory guanine nucleotide-binding protein, which may in part explain the ability of baclofen to augment isoproterenol-stimulated cyclic AMP accumulation in brain slices.     

β-Glucosidase and β-Galactosidase in Primary Cultures of Rat Astrocytes: Comparison to the Brain Enzymes

In primary astrocyte cultures beta-glucosidase (EC 3.2.1.21) and beta-galactosidase (EC 3.2.1.23) showed pH optima and Km values identical to rat brain enzymes, using methylumbelliferyl glycosides and labeled gluco- and galactocerebrosides as substrates. The activities of both glycosidases increased in culture up to 3-4 weeks. In rat brain only galactosidase increased; glucosidase activity declined between 12-20 days after birth. The specific activities were two- to sixfold higher in astrocyte cultures than in rat brain. These activities were not due to uptake of enzymes from the growth medium. Secretion of beta-galactosidase, but not beta-glucosidase nor acid phosphatase could be demonstrated. These results support the suggestion of a degradative function for astrocytes in the brain.     

Properties of a Leu-Phe-Cleaving Endopeptidase Activity Putatively Involved in β-Endorphin Metabolism in Rat Brain

Incubation of beta-endorphin with cytosolic and particulate fractions of rat brain resulted in the formation of several peptides, including gamma-endorphin [beta-endorphin-(1-17)] and beta-endorphin-(18-31), indicating the presence of enzyme activity cleaving the Leu17-Phe18 bond of beta-endorphin. An assay for this Leu-Phe cleaving activity, based on the cleavage of the 14C-labeled substrate acetyl-Val-Thr-Leu-Phe-[epsilon-([14C]CH3)2]Lys-NHCH3, was used to examine the properties of this enzyme activity. beta-Endorphin-(1-31) competitively inhibited the Leu-Phe-cleaving enzyme activity on the pentapeptide substrate. Over 90% of activity was recovered in the cytosolic fraction. Leu-Phe-cleaving activity behaved like a thiol endopeptidase because it was inhibited by low concentrations of N-ethylmaleimide, p-chloromercuribenzoate, p-chloromercuribenzoyl sulfate, and low concentrations of Hg2+. Low concentrations of sulfhydryl compounds stimulated Leu-Phe-cleaving activity. The activity was optimal between pH 8.5 and 9.0. The Km of Leu-Phe-cleaving activity in the cytosolic fraction was 35 microM and in the particulate fraction 88 microM with Vmax values of 193 and 15 nmol mg protein-1 h-1, respectively. The apparent molecular mass of the Leu-Phe-cleaving enzyme was estimated by gel filtration to be approximately 200 kilodaltons. These properties of Leu-Phe-cleaving activity indicate that the Leu-Phe-cleaving enzyme is distinct from any known brain endopeptidase.     

γ-Hydroxybutyric Acid in Subcellular Fractions of Rat Brain

The presence of gamma-hydroxybutyric acid (GHB) in synaptosome-enriched fractions of rat brain was ascertained using a GLC technique. The stability of GHB in synaptosomes was evaluated by addition of various gamma-aminobutyric acid (GABA) transaminase (GABA-T) inhibitors, GHB, or ethosuximide to the homogenizing medium. Furthermore, changes in whole brain GHB levels were compared with those in the synaptosomal fraction in animals treated with GABA-T inhibitors, GABA, or ethosuximide. GHB was present in synaptosome-enriched fractions in concentrations ranging from 40 to 70 pmol/mg of protein. There was no evidence for redistribution, leakage, or metabolism of GHB during the preparation of synaptosomes. The elevations of whole brain GHB level associated with GABA-T or ethosuximide treatment were reflected by a parallel increase in synaptosomal GHB content. These data add to the growing evidence that GHB may have neurotransmitter or neuromodulator function.     

Allosteric Modulation of Flunitrazepam Binding to Rat Brain Benzodiazepine Receptors by Methyl β-Carboline-3-Carboxylate

The inhibition of flunitrazepam (FNP) binding to rat brain benzodiazepine (BZ) receptors by methyl beta-carboline-3-carboxylate (MCC) was studied. Biphasic dissociation was observed for [3H]FNP and [3H]MCC in cerebral cortex, cerebellum, and hippocampus, although the dissociation of [3H]MCC was much faster. The dissociation rate of [3H]FNP was increased by MCC in the cerebellum, but was not altered in cerebral cortex or hippocampus. [3H]FNP binding stimulated by gamma-aminobutyric acid was enhanced in the presence of MCC in all three regions examined. These results indicate that MCC exerts these effects by interacting with allosteric sites that are different from the FNP recognition sites on the BZ receptors.     

Localization of Various Forms of the γ Subunit of G Protein in Neural and Nonneural Tissues

Abstract: For a study of the localization of various forms of the γ subunit of G proteins, antibodies were raised in rabbits against peptides that corresponded to partial amino acid sequences of bovine γ₂, γ₃, γ₅, and γ₇. Affinity-purified antibodies against γ₂, γ₃, and γ₅ reacted specifically with γ₂, γ₃, and γ₅, respectively, but the antibody against γ₇ reacted with γ₂, γ₃, and a novel γ subunit, designated γ_s1, as well as with γ₇. Because these antibodies reacted with the respective forms of the γ subunit from rat brain, we investigated the localization of γ subunits in the rat. γ₂ and γ₃ were abundant in all regions in the brain, whereas the concentration of γ₅ and γ₇ was relatively low with the single exception being a high concentration of γ₇ in the striatum. The concentration of γ₂ was consistently high during ontogenic development in the rat brain, whereas γ₃ appeared about a week after birth and their concentrations then increased until a month after birth. In tissues other than the brain, γ₃ was observed only in the pituitary gland, whereas γ₂, γ₅, and γ₇ were found in a variety of tissues. In addition, most tissues contained relatively high concentrations of some other γ subunit, which was detected with an antibody against a γ₇-derived peptide and appeared to be γ_s1. Among cloned cells tested, γ₃ was detected only in PC12 pheochromocytoma cells. Taken together, the results indicated that γ₃ was expressed specifically in neuronal cells, and γ_s1 was the major γ subunit in most nonneural cells. γ₂, γ₅, and γ₇ were distributed in a variety of tissues, but γ₂ was dominant in the brain.     

β-Amyloid Precursor Protein Isoforms in Various Rat Brain Regions and During Brain Development

To address the question of the possible functions of different Alzheimer's disease beta-amyloid precursor protein (beta-APP) isoforms in the brain, we studied their expression at different times during postnatal rat brain development and in various regions of the adult rat brain. Polyclonal antibodies directed to two peptide antigens were used. The majority of all beta-APP forms was found to be soluble as revealed by western blot analysis. The highest level of most beta-APP forms was reached in the second postnatal week, which is the time of brain maturation and completion of synaptic connections. Strikingly high concentrations of the Kunitz protease inhibitor-containing beta-APP were present in the adult olfactory bulb, where continuous synaptogenesis occurs in the adult animal. These findings support the idea of an involvement of beta-APPs in the processes of cell differentiation and, probably, in the establishment of synaptic contacts.     

α-N-Acetyl β-Endorphins in the Pituitary: Immunohistochemical Localization Using Antibodies Raised Against Dynorphin(1-13)

Abstract: Intense immunohistochemical staining of the intermediate lobe of the pituitary was observed by using an antiserum raised against synthetic dynorphin(1-13) treated with a water-soluble carbodiimide (CDI). Subsequent studies showed that the immunostaining was blocked by preincubation of the antiserum with acetylated derivatives of both β-endorphin and dynorphin(1-13) as well as by CDI-treated dynorphin(1-13), but only weakly by authentic dynorphin(1-13). Neither nonacetylated β-endorphin nor any other fragments of the ACTH/endorphin precursor blocked the immunostaining of the intermediate lobe. Analysis of the CDI-treated dynorphin(1-13) used as an antigen showed that most of the peptide was acetylated at primary amino groups. CDI treatment of dynorphin(1-13) results in the formation of an acetyl derivative because the commercially available peptide is supplied as the acetate salt. The antibodies responsible for the intermediate lobe staining were isolated by affinity chromatography, using a column containing partially purified intermediate lobe extract linked to an affinity resin and a radioimmunoassay (RIA) was developed with CDI-treated dynorphin(1-13) used as a trace and as a standard. Competition studies showed 0.5-1% cross-reactivity with α-N-acetyl β-endorphin(1-31), α-N-acetyl β-endorphin(1-27), and totally acetylated β-endorphin(1-31). Nonacetylated β-endorphins did not cross-react. Posterior-intermediate lobe extracts from rat and beef were fractionated by gel filtration. Rat posterior-intermediate lobe extracts were also fractionated by cation-exchange chromatography. Fractionated extracts were analyzed by RIAs for β-endorphin, CDI-treated dynorphin(1-13), and authentic dynorphin(1-13). The results suggested that the peptides responsible for the intermediate lobe staining were mainly four different derivatives of β-endorphin bearing an acetyl group at the amino terminus. No immunostaining was seen in the posterior and anterior lobes of the pituitary. This suggests that the intermediate lobe is the main source of acetylated β-endorphins in the pituitary.     

Islet-Activating Protein Inhibits the β-Adrenergic Receptor Facilitation Elicited by γ-Aminobutyric AcidB Receptors

gamma-Aminobutyric acidB (GABAB) receptor recognition sites that inhibit cyclic AMP formation, open potassium channels, and close calcium channels are coupled to these effector systems by guanine nucleotide binding proteins (G proteins). These G proteins are ADP-ribosylated by islet-activating protein (IAP), also known as pertussis toxin. This process prevents receptor coupling to these G proteins. In slices of cerebral cortex and hippocampus from rat, stimulation of GABAB receptors with baclofen, a receptor agonist, also potentiates the accumulation of cyclic AMP stimulated by beta-adrenergic agonists. It was unknown whether those GABAB receptors that potentiate the beta-adrenergic response were also sensitive to IAP. IAP was injected intracerebroventricularly into rats to ADP-ribosylate IAP-sensitive G proteins. Four days after the IAP injection, 38% and 52% of these G proteins from cerebral cortex and hippocampus, respectively, were ADP-ribosylated by the IAP injection. In slices of both structures prepared from IAP-treated rats, the GABAB receptor-mediated potentiation of the beta-adrenergic receptor response was attenuated. Thus, many GABAB receptor-mediated responses are coupled to IAP-sensitive G proteins.     

Regional and Subcellular Localization in Rat Brain of the Enzymes That Can Synthesize γ-Hydroxybutyric Acid

Abstract: Rat brain contains two major NADPH-linked aldehyde reductases that can reduce succinate semialdehyde to 4-hydroxybutyrate. One of these enzymes appears to be fairly specific for succinate semialdehyde and is not significantly inhibited by classic aldehyde reductase inhibitors such as barbiturates. The other enzyme can reduce several aromatic aldehydes and is strongly inhibited by barbiturates and branched-chain fatty acids. Using one such inhibitor, it was possible to distinguish between and measure the two enzyme activities separately in various rat brain regions and in subcellular fractions. Both enzymes are mainly cytoplasmic but there is some activity in the synaptosomal fraction. The activity of the specific succinic semialdehyde reductase is highest in the cerebellum, where it represents 21% of the total activity, and lowest in the cortex, where it represents about 11% of the total activity.     

αγ-Enolase in the Rat: Ontogeny and Tissue Distribution

Abstract: The rat brain enolases are dimers composed of α and γ subunits. At pH 8.6 αγ-enolase seemed to be stable, and no evidence was found for the possible formation of αγ-enolase from αα-enolase and γγ-enolase in the course of rat brain homogenization. During ontogeny of the rat forebrain, αγ-enolase was formed before γγ-enolase. The half-maximal specific concentrations were reached at postnatal days 14 and 23, respectively. The distribution of αγ- and γγ-enolase in various rat brain areas was also investigated. In all areas both forms were present. In neuroendocrine tissues αγ-enolase was present at a much higher concentration than γγ-enolase. The ratio between γγ-enolase and αγ-enolase may be indicative of the degree of neuronal maturation, a conclusion further substantiated by the high ratio observed in cerebellum and the low ratio observed in olfactory bulbs, both compared with the ratio in forebrain.     

Heterogeneity of γ-Aminobutyric Acid/Benzodiazepine/β-Carboline Receptor Complex in Rat Spinal Cord

The properties of muscimol, beta-carboline (BC), and benzodiazepine (BZD) binding to crude synaptic membranes were studied in the spinal cord and cerebellum of rats. In cerebellar membranes, the density of high-affinity [3H]muscimol and [3H]6,7-dimethoxy-4-ethyl-beta-carboline ([3H]BCCM) binding sites is almost identical to that of [3H]flunitrazepam ([3H]FLU) or [3H]flumazenil (Ro 15-1788; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a] [1-4]benzodiazepine-3-carboxylate). In contrast to the cerebellum, the number of muscimol and BC binding sites in rat spinal cord is approximately 20-25% of the number of FLU or flumazenil binding sites. Moreover, in spinal cord membranes, BC recognition site ligands displace [3H]-flumazenil bound to those sites, with low affinity and a Hill slope significantly less than 1; the potency of the different BCs in displacing [3H]flumazenil is 20-50-fold lower in the spinal cord than in the cerebellum. [3H]Flumazenil is not displaced from spinal cord membranes by the peripheral BZD ligand Ro 5-4864 (4'-chlorodiazepam), whereas it is displaced with low affinity and a Hill slope of less than 1 (nH = 0.4) by CL 218,872 (3-methyl-6-(3-trifluoromethylphenyl)-1,2,4-triazolol[4,3-b] pyridazine). These data suggest that a large number of BZD binding sites in spinal cord (approximately 80%) are of the central-type, BZD2 subclass, whereas the BZD binding sites in cerebellum are predominantly of the central-type, BZD1 subclass.(ABSTRACT TRUNCATED AT 250 WORDS)     

Repeated Administration of γ-VinylGABA Reduces Rat Brain Glutamine Synthetase Activity

Abstract: The glutamine cycle has been proposed as a pathway in which glutamine synthesized in glia provides substrate for synthesis of the neurotransmitters glutamate and GABA as they are lost from neurons. To test whether GABA may regulate this pathway, the effect of elevated GABA on the glial enzyme glutamine synthetase was examined in rat brain. Repeated subcutaneous injections of the antiepileptic GABA transaminase inhibitor γ-vinylGABA at a dose of 150 mg/kg per day for 21 days reduced glutamine synthetase activity by 36% in the cortex and 22% in the cerebellum. At 30 mg/kg per day, glutamine synthetase activity was reduced by 9.5% in the cortex but unchanged in the cerebellum. The reductions were brain specific because the skeletal muscle and liver enzymes were unaffected by γ-vinylGABA administration. Amino acid analysis of the cortex from γ-vinylGABA-treated rats demonstrated a 270% increase in GABA levels after 150 mg/kg but no change after 30 mg/kg. GABA levels and glutamine synthetase activity were inversely correlated. The 150 mg/kg dose significantly lowered cortical glutamine and glutamate levels. The decline in brain glutamine synthetase activity with chronic γ-vinylGABA administration developed gradually over time and may be due to the slow turnover of this enzyme in vivo.     

Lipopolysaccharide and Interleukin-1β Augmented Histidine Decarboxylase Activity in Cultured Cells of the Rat Embryonic Brain

Abstract: We investigated the effect of lipopolysaccharide (LPS) and various inflammatory cytokines on the histidine decarboxylase (HDC) activity in cultured cells of the rat embryonic brain. Histaminergic neuronal cell bodies were supposed to exist in cultured cells of the diencephalon but not in those of the cortex. The HDC activity was elevated by adding LPS and interleukin-1 β (IL-1β) but not by tumor necrosis factor-α (TNF-α) and IL-6 to the mixed primary cultures of diencephalon. In the adherent cell fraction of the cultured diencephalon cells, HDC activity was also enhanced by LPS and IL-1β. In a similar manner, LPS augmented HDC activity in the mixed primary culture of cerebral cortical cells and in its adherent cell fraction. The effects of IL-1β but not LPS in the mixed primary culture of diencephalon were canceled by a prior exposure to cytosine-β- d -arabinofuranoside. The changes in HDC activity after exposure to LPS for 12 h were not accompanied by increased mRNA levels. In these cell cultures, mast cells were not detected by Alcian Blue staining. These results indicated the presence of the third type of HDC-bearing cell besides neurons and mast cells in the brain. The increase of HDC activity by IL-1β might be due to cell proliferation.     

The Effect of γ-Aminobutyric Acid on Substrate-Level Phosphorylation in Brain Mitochondria

Abstract: A consequence of the metabolism of γ-aminobutyric acid (GABA) via the "GABA shunt" should be a decreased rate of substrate-level phos- phorylation of GDP to GTP. ³²P₁ labeling of nucleotides was, therefore, studied in uncoupled brain mitochondria with α-ketoglutarate or a-ketoglutarate + GABA as substrates. The addition of an equimolar amount of GABA resulted in an approximately 50% reduction of the final specific activity of all mitochondrial nucleotides. This effect was completely reversed by aminooxyacetic acid. GABA did not affect the time course of nucleotide labeling. Although delineation of the mechanism involved requires further study, these preliminary results suggest an important modulatory role of GABA in the intermediary metabolism of brain mitochondria.     

Effect of Early Iron Deficiency in Rat on the γ-Aminobutyric Acid Shunt in Brain

Early iron deficiency in rat does not affect the weight or the protein, DNA, and RNA content but results in a slight reduction in gamma-aminobutyric acid (GABA) (13%, p less than 0.01) and glutamic acid (20%, p less than 0.001) content of the brain. The activities of the two GABA shunt enzymes, glutamate dehydrogenase and GABA-transaminase, and of the NAD+-linked isocitrate dehydrogenase (ICDH) were inhibited whereas the glutamic acid decarboxylase, mitochondrial NADP+-linked ICDH, and succinic dehydrogenase activities remained unaltered in brain. On rehabilitation with the iron-supplemented diet for 1 week, these decreased enzyme activities in brain attained the corresponding control values. However, the hepatic nonheme iron content increased to about 80% of the control, after rehabilitation for 2 weeks. A prolonged iron deficiency resulting in decreased levels of glutamate and GABA may lead to endocrinological, neurological, and behavioral alterations.