Reduced Striatal [3H]Inositol 1,4,5-Trisphosphate Binding in Huntington''s Disease

Specific [3H]inositol 1,4,5-trisphosphate [( 3H]InsP3) binding was studied in regions of postmortem brain from 15 patients with Huntington's disease (HD) and 13 nonneurological controls. Single-point binding analyses, using 5.0 nM InsP3, showed statistically significant reductions in specific [3H]InsP3 binding in the caudate (-71%) and putamen (-75%) of HD patients compared with controls. Frontal and occipital cortical [3H]InsP3 binding was not significantly different between HD and controls, a finding suggesting that the reduced [3H]InsP3 binding parallels the brain regional specificity of the neuropathological changes in HD. Scatchard analyses of data from [3H]InsP3 competition binding assays performed on caudate nucleus revealed that the reductions found using single-point binding assays were due to a decrease in both binding density (-57%) and affinity (-50%) in HD brain compared with controls. The concomitant changes in InsP3 receptor density and affinity in HD brain suggest that these alterations may be produced by processes in addition to cell loss. These results suggest the possibility that disturbances in InsP3 receptor function, possibly resulting in altered intracellular calcium flux and homeostasis, occur in HD and may participate in the pathogenesis of this neurodegenerative disorder.     

Formation of [3H]Inositol Metabolites in Rat Hippocampal Formation Slices Prelabelled with [3H]Inositol and Stimulated with Carbachol

Rat hippocampal formation slices were prelabelled with [3H]inositol and stimulated with carbachol for times between 7 s and 3 min. The [3H]inositol metabolites in an acid extract of the slices were resolved with anion-exchange HPLC. Carbachol dramatically increased the concentration of [3H]inositol monophosphate, [3H]inositol bisphosphate (two isomers), [3H]inositol 1,3,4-trisphosphate, [3H]inositol 1,4,5-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate. The levels of [3H]inositol 1,4,5-trisphosphate rose most rapidly; they were maximally elevated after only 7 s and declined toward control levels in 1 min followed by a more sustained elevation in levels for up to 3 min. When [3H]inositol 1,4,5-trisphosphate was incubated with hippocampal formation homogenates in an ATP-containing buffer it was very rapidly metabolised. After 5 min [3H]inositol 1,4-bisphosphate, [3H]inositol 1,3,4-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate could be detected in the homogenates. Under similar experimental conditions [3H]inositol 1,3,4,5-tetrakisphosphate is metabolised to [3H]inositol 1,3,4-trisphosphate and an inositol bisphosphate isomer that is not [3H]inositol 1,4-bisphosphate. We conclude that like other tissues the primary event in the hippocampus following carbachol stimulation is the activation of phosphatidylinositol 4,5-bisphosphate selective phospholipase C.     

Depolarization and Agonist-Stimulated Changes in Inositol 1,4,5-Trisphosphate and Inositol 1,3,4,5-Tetrakisphosphate Mass Accumulation in Rat Cerebral Cortex

Muscarinic receptor stimulation or depolarization with elevated extracellular K+ induced rapid and sustained increases in mass accumulations of myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] in cerebral cortex slices. Synergistic but transient responses of both inositol polyphosphate second messengers were observed when slices were stimulated with carbachol under depolarizing conditions; this synergy was observed as an increase in the maximal responsiveness, with no significant change in EC50 values for carbachol. Omission of buffer Ca2+ ([Ca2+]e 10-20 microM) reduced basal Ins(1,4,5)P3 and Ins(1,3,4,5)P4 concentrations; the relative stimulatory effects of muscarinic receptor stimulation were maintained, but the effects of depolarization were markedly attenuated under these conditions. A component of the response to depolarization appeared to be indirectly mediated by the release of acetylcholine, because the K(+)-evoked increase in Ins(1,3,4,5)P4 was enhanced by the cholinesterase inhibitor physostigmine, and was partially attenuated by atropine. An additive suppression by nitrendipine suggests that entry of Ca2+ through L-type Ca2+ channels may serve to accelerate phosphorylation of Ins(1,4,5)P3 by 3-kinase. Norepinephrine did not significantly increase Ins(1,4,5)P3 or Ins(1,3,4,5)P4 accumulation; however, in the presence of depolarizing K+, norepinephrine caused a dramatic increase in Ins(1,3,4,5)P4 mass accumulation. In contrast, the excitatory amino acid quisqualate caused significant increases in the mass accumulations of both inositol polyphosphates measured, with no further increase being observed under depolarizing conditions. The results are discussed with respect to the interactive effects of agonist and depolarization stimuli on inositol polyphosphate accumulation which might more accurately reflect the conditions pertaining in vivo.     

Effects of Cerebral Ischemia on [3H]Inositol Lipids and [3H]Inositol Phosphates of Gerbil Brain and Subcellular Fractions

Abstract: Intracerebral injection of [³H]inositoi into gerbil brain resulted in labeling of phosphoinositides and inositolphosphates in various subcellular membrane fractions. Phosphatidylinositol (PI) comprised >90% of the radioactivity of inositol lipids. However, the level of labeled poly-PI (with respect to PI) was higher in synaptosomes than in other membrane fractions. Ischemia induced in gerbils by ligation of the common carotid arteries resulted in a 30% decrease in labeled poly-PI in brain homogenates and this decrease was largely attributed to the poly-PI in synaptosomes (50% decrease). Among the inositol phosphates, the ischemia induction resulted in a decrease in labeling of inositol trisphosphate (63%) and inositol bisphosphate (38%), but labeling of inositol phosphate (IP) was increased by 59%. The results suggested a rapid turnover of the inositol phosphates in the gerbil brain. In general, changes in inositol lipids and inositol phosphates due to ischemia were attenuated after pretreatment with lithium (3 meq/kg) injected intraperitoneally 5 h prior to ligation. Surprisingly, lithium treatment alone did not cause an increase in IP labeling in the gerbil brain.     

Lithium Reduqes the Accumulation or Inositol Polyphosphate Second Messengers Following Cholinergic Stimulation of Cerebral Cortex Slices

Abstract: The ability of lithium to interfere with the metabolism of inositol phosphates in brain may underlie its therapeutic action in manic-depressive illness. In these experiments, lithium, at therapeutic concentrations, enhanced the accumulation of [³H]inpsitol monophosphate but suppressed the accumulation of the putative second messengers [³H]inositol 1,4,5-trisphosphate ([³H]Ins(1,4,5)P₃) and f³H]inositol 1,3,4,5-tetrakisphosphate following stimulation of cerebral cortex slices with carbachol. Mass measurements of Ins(1,4,5)P₃showed similar inhibitory effects, which could be prevented by preincubation with myo -inositol. These data may reveal the mechanism by which lithium can reduce polyphosphoinositide-midiated neurotransmission in brain.     

Mobilization of Inositol 1,4,5-Trisphosphate-Sensitive Ca2+ Stores Supports Bradykinin- and Muscarinic-Evoked Release of [3H]Noradrenaline from SH-SY5Y Cells

Abstract: The human neuroblastoma cell line SH-SY5Y, maintained at confluence for 14 days, released [³H]-noradrenaline ([³H]NA) when stimulated with either the muscarinic receptor agonist methacholine or bradykinin. The major fraction of release was rapid, occurring in <10 s, whereas nicotine-evoked release was slower. When the extracellular [Ca²⁺] ([Ca²⁺]_e) was buffered to ～50–100 nM, release evoked by nicotine was abolished, whereas that in response to methacholine or bradykinin was reduced by ～50% with EC₅₀ values of ?5.46 ± 0.05 M and ?7.46 ± 0.06 M (log₁₀), respectively. Methacholine and bradykinin also produced rapid elevations of both inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] and intracellular free [Ca²⁺] ([Ca²⁺]_i). These elevations were reduced at low [Ca²⁺]_e and under these conditions the EC₅₀ values for peak elevation of [Ca²⁺]_i were ?6.00 ± 0.14 M for methacholine and ?7.95 ± 0.34 M for bradykinin (n = 3 for all EC₅₀ determinations). At low [Ca²⁺]_e, depletion of nonmitochondrial intracellular Ca²⁺ stores with the Ca²⁺-ATPase inhibitor thapsigargin produced a transient small elevation of [Ca²⁺]_i and a minor release of [³H]NA. At low [Ca²⁺]_e, thapsigargin abolished elevation of [Ca²⁺]_i in response to methacholine and bradykinin and completely inhibited their stimulation of [³H]NA release. It is proposed, therefore, that Ca²⁺ release from Ins(1,4,5)P₃-sensitive stores is a major trigger of methacholine- and bradykinin-evoked [³H]NA release in SH-SY5Y cells.     

Reduced [3H]Cyclic AMP Binding in Postmortem Brain from Subjects with Bipolar Affective Disorder

Abstract: Findings of increased G_sα levels and forskolin-stimulated adenylyl cyclase activity in selective cerebral cortical postmortem brain regions in bipolar affective disorder (BD) implicate increased cyclic AMP (cAMP)-mediated signaling in this illness. Accumulating evidence suggests that intracellular levels of cAMP modulate the abundance and disposition of the regulatory subunits of cAMP-dependent protein kinase (cAMP-dPK). Thus, in the present study, we tested further whether hyperfunctional G_sα-linked cAMP signaling occurs in BD by determining [³H]cAMP binding, a measure of the levels of regulatory subunits of cAMP-dPK, in cytosolic and membrane fractions from discrete brain regions of postmortem BD brain. Specific [³H]cAMP (5 n M ) binding was determined in autopsied brain obtained from 10 patients with DSM-III-R diagnoses of BD compared with age- and postmortem delay-matched controls. [³H]cAMP binding was significantly reduced across all brain regions in cytosolic fractions of BD frontal (−22%), temporal (−23%), occipital (−22%) and parietal (−15%) cortex, cerebellum (−36%), and thalamus (−13%) compared with controls, but there were no differences in [³H]cAMP binding in the membrane fractions from these same regions. These results suggest that changes occur in the cAMP-dPK regulatory subunits in BD brain, possibly resulting from increased cAMP signaling. The possibility that antemortem lithium and/or other mood stabilizer treatment may contribute to the above changes, however, cannot be ruled out.     

Comparison of Type 2 Inositol 1,4,5-Trisphosphate Receptor Distribution and Subcellular Ca2+ Release Sites that Support Ca2+ Waves in Cultured Astrocytes

Abstract: We have examined the mechanisms that underlie Ca²⁺ wave propagation in cultured cortical astrocytes. Norepinephrine evoked Ca²⁺ waves in astrocytes that began at discrete initiation loci and propagated throughout the cell by regenerative amplification at a number of cellular sites, as shown by very high Ca²⁺ release rates at these regions. We have hypothesized previously that domains displaying elevated Ca²⁺ release kinetics in astrocytes may correspond to sites of high inositol 1,4,5-trisphosphate receptor (InsP₃R) density. To examine this possibility, we compared the distribution pattern of endoplasmic reticulum (ER) and InsP₃Rs with Ca²⁺ release kinetics in subcellular regions during propagation of norepinephrine-evoked waves. 3,3'-Dihexyloxacarbocyanine iodide staining revealed that the ER in astrocytes exists as a meshwork of membranes extending throughout the cells, including fine processes. A specific antibody directed against type 2 InsP₃Rs (InsP₃R2) detected a 260-kDa band in western blotting of astrocyte membranes. Immunocytochemistry using this antibody stained the entire ER system in a punctate, variegated manner. When Ca²⁺ responses and InsP₃R2 immunofluorescence were compared in the same cell, domains of elevated Ca²⁺ response kinetics (high amplitude and rapid rate of rise) showed significant positive correlation with high local intensity of InsP₃R2 staining. It appears, therefore, that specializations in the ER responsible for discrete local Ca²⁺ release sites that support regenerative wave propagation include increased levels of InsP₃R2 expression.     

Ca2+ Release from Inositol 1,4,5-Trisphosphate-Sensitive Ca2+ Store by Antidepressant Drugs in Cultured Neurons of Rat Frontal Cortex

Abstract: The ability of antidepressant drugs (ADs) to increase the concentration of intracellular Ca²⁺ ([Ca²⁺]_i) was examined in primary cultured neurons from rat frontal cortices using the Ca²⁺-sensitive fluorescent indicator fura-2. Amitriptyline, imipramine, desipramine, and mianserin elicited transient increases in [Ca²⁺]_i in a concentration-dependent manner (100 μM to 1 mM). These four AD-induced [Ca²⁺]_i increases were not altered by the absence of external Ca²⁺ or by the presence of La³⁺ (30 μM), suggesting that these ADs provoked intracellular Ca²⁺ mobilization rather than Ca²⁺ influx. All four ADs increased inositol 1,4,5-trisphosphate (IP₃) contents by 20–60% in the cultured cells. The potency of the IP₃ production by these ADs closely correlated with the AD-induced [Ca²⁺]_i responses. Pretreatment with neomycin, an inhibitor of IP₃ generation, significantly inhibited amitriptyline- and imipramine-induced [Ca²⁺]_i increases. In addition, by initially perfusing with bradykinin (10 μM) or acetylcholine (10 μM), which can stimulate the IP₃ generation and mobilize the intracellular Ca²⁺, the amitriptyline responses were decreased by 76% and 69%, respectively. The amitriptyline-induced [Ca²⁺]_i increases were unaffected by treatment with pertussis toxin. We conclude that high concentrations of amitriptyline and three other ADs mobilize Ca²⁺ from IP₃-sensitive Ca²⁺ stores and that the responses are pertussis toxin-insensitive. However, it seems unlikely that the effects requiring high concentrations of ADs are related to the therapeutic action.     

Inositol 1,4,5-Trisphosphate 3-Kinase mRNA: High Levels in the Rat Hippocampal CA1 Pyramidal and Dentate Gyrus Granule Cells and in Cerebellar Purkinje Cells

The distribution of inositol 1,4,5-trisphosphate (InsP3) 3-kinase mRNA in the rat brain is reported using oligonucleotides based on a cDNA clone sequence that encodes rat brain InsP3 3-kinase and the in situ hybridization technique. Moderate levels were found in CA2-4 pyramidal neurons, in the cortex, and in the striatum. The cerebellar granule cells, thalamus, hypothalamus, brainstem, spinal cord, and white matter tracts were almost negative. The levels of InsP3 3-kinase mRNA were highest in the hippocampal CA1 pyramidal neurons, granule cells of the dentate gyrus, and cerebellar Purkinje cells. These results contrast with the lower concentration of the InsP3 receptor already reported in the hippocampus versus the Purkinje cells and suggest a special role for inositol 1,3,4,5-tetrakisphosphate in Ammon's horn.     

Muscarinic Receptor-Mediated Inositol 1,4,5-Trisphosphate Formation in SH-SY5Y Neuroblastoma Cells Is Regulated Acutely by Cytosolic Ca2+ and by Rapid Desensitization

Abstract: Stimulation of muscarinic receptors expressed in SH-SY5Y human neuroblastoma cells resulted in a complex profile of inositol 1,4,5-trisphosphate (InsP₃) accumulation, with a dramatic increase (six- to eightfold) over the first 10 s (the “peak” phase) and subsequently, from ～60 s onward, maintained at a lower but sustained level (the “plateau” phase). Chelation of extracellular Ca²⁺ with EGTA or inhibition of Ca²⁺ channels with Ni²⁺ showed that the plateau phase was dependent upon Ca²⁺ entry. Furthermore, use of thapsigargin and EGTA to discharge and sequester Ca²⁺ from intracellular stores revealed that Ca²⁺ from this source was capable of supporting the peak phase of the InsP₃ response. Carbachol-stimulated phosphoinositidase C activity in permeabilized SH-SY5Y cells was also shown to be highly dependent on free Ca²⁺ concentration (20–100 nM) and suggests that under normal conditions, InsP₃ formation is enhanced by increases in cytosolic free Ca²⁺ concentration that accompany muscarinic receptor activation. Measurement of carbachol-stimulated total inositol phosphate accumulation in the presence of Li⁺ indicated that the initial rate of phosphoinositide hydrolysis (from 0 to 30 s) was about fivefold greater than that from 30 to 300 s. This rapid but partial desensitization of receptor-mediated phosphoinositide hydrolysis provides strong evidence for the mechanism underlying the changes in InsP₃ accumulation over this time. Because very similar data were obtained in Chinese hamster ovary cells transfected with human m3 receptor cDNA, we suggest that although increases in cytosolic free Ca²⁺ concentration amplify InsP₃ formation during stimulation of m3 muscarinic receptors, the primary factor that governs the profile of InsP₃ accumulation is rapid, but partial, desensitization. Such desensitization does not appear to be mediated by changes in cytosolic Ca²⁺ or protein kinase C activity.     

Comparative Effects of Lithium on the Phosphoinositide Cycle in Rat Cerebral Cortex, Hippocampus, and Striatum

Abstract: The effects of lithium on muscarinic cholinoceptor-stimulated phosphoinositide turnover have been investigated in rat hippocampal, striatal, and cerebral cortical slices using [³H]inositol or [³H]cytidine prelabelling and inositol 1,4,5-trisphosphate [lns(1,4,5)P₃] and inositol 1,3,4,5-tetrakisphosphate [lns(1,3,4,5)P₄] mass determination methods. Carbachol addition resulted in maintained increases in lns(1,4,5)P₃ and lns(1,3,4,5)P₄ mass levels in hippocampus and cerebral cortex, whereas in striatal slices these responses declined significantly over a 30-min incubation period. Carbachol-stimulated lns(1,4,5)P₃ and lns(1,3,4,5)P₄ accumulations were inhibited by lithium in all brain regions studied in a time-and concentration-dependent manner. For example, in hippocampal slices significant inhibitory effects of LiCl were observed at times > 10 min after agonist challenge; IC₅₀ values for inhibition of agonist-stimulated lns(1,4,5)P₃ and lns(1,3,4,5)P₄ accumulations by lithium were 0.22 ± 0.09 and 0.33 ± 0.13 mM, respectively. [³H]CMP-phosphatidate accumulation increased in all brain regions when slices were stimulated by agonist and lithium. The ability of myo-inositol to reverse these effects, as well as lithium-suppressed lns(1,4,5)P₃ accumulation, implicates myo-inositol depletion in the action of lithium in the hippocampus and cortex at least. The results of this study suggest that although significant differences in the magnitude and time courses of changes in inositol (poly)phosphate metabolites occur in different brain regions, lithium evokes qualitatively similar enhancements of [³H]inositol monophosphate and [³H]CMP-phosphatidate levels and inhibitions of lns(1,4,5)P₃ and lns(1,3,4,5)P₄ accumulations. However, the inability of striatal slices to sustain carbachol-stimulated inositol polyphosphate accumulation in the absence of lithium and the inability to reverse effects with myo-inositol may indicate differences in phosphoinositide signalling in this brain region.     

High- and Low-Affinity Binding Components for [3H]Imipramine in Rat Cerebral Cortex

The present study demonstrates that [3H]imipramine binds to both high- and low-affinity imipramine binding components on membranes prepared from rat cerebral cortex. Scatchard and computer analyses of saturation experiments using a wide range of [3H]imipramine concentrations (0.5 nM-50 nM) revealed the presence of two binding components. Inhibition experiments in which membranes were incubated with [3H]imipramine and various concentrations of unlabelled imipramine gave shallow inhibition curves with a Hill coefficient of 0.60 +/- 0.04. When dissociation rates of imipramine were studied, biphasic dissociation curves were obtained with apparent half-times of dissociation of 2.5 +/- 0.4 min and 18.5 +/- 2.5 min. Thus analysis of saturation, competition, and dissociation experiments indicate that [3H]imipramine binds to low as well as high-affinity binding sites in rat cortex.     

Agonist-Evoked Ca2+ Mobilization from Stores Expressing Inositol 1,4,5-Trisphosphate Receptors and Ryanodine Receptors in Cerebellar Granule Neurones

Abstract: The mechanisms involved in Ca²⁺ mobilization evoked by the muscarinic cholinoceptor (mAChR) agonist carbachol (CCh) and N-methyl-d -aspartate (NMDA) in cerebellar granule cells have been investigated. An initial challenge with caffeine greatly reduced the subsequent intracellular Ca²⁺ concentration ([Ca²⁺]_i) response to CCh (to 45 ± 19% of the control), and, similarly, a much reduced caffeine response was detectable after prior stimulation with CCh (to 27 ± 6% of the control). CCh-evoked [Ca²⁺]_i responses were inhibited by preincubation with thapsigargin (10 µM), 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ; 25 µM), ryanodine (10 µM), or dantrolene (25 µM). BHQ pretreatment was found to have no effect on the sustained phase of the NMDA-evoked [Ca²⁺]_i response. Both CCh (1 mM) and 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD; 200 µM) evoked a much diminished increase in [Ca²⁺]_i in granule cells pretreated with CCh for 24 h compared with vehicle-treated control cells (CCh, 23 ± 14%; ACPD, 27 ± 1% of respective control values). In contrast, a 24-h CCh pretreatment decreased the subsequent inositol 1,4,5-trisphosphate (InsP₃) response to CCh to a much greater extent compared with responses evoked by metabotropic glutamate receptor (mGluR) agonists; this suggests that the former effect on Ca²⁺ mobilization represents a heterologous desensitization of the mGluR-mediated response distal to the pathway second messenger. Furthermore, [Ca²⁺]_i responses to caffeine and NMDA were unaffected by a 24-h pretreatment with CCh. This study indicates that ryanodine receptors, as well as InsP₃ receptors, appear to be crucial to the mAChR-mediated [Ca²⁺]_i response in granule cells. As BHQ apparently differentiates between the CCh- and NMDA-evoked responses, it is possible that the directly InsP₃-sensitive pool is physically different from the ryanodine receptor pool. Also, activation of InsP₃ receptors may not contribute significantly to NMDA-evoked elevation of [Ca²⁺]_i in cerebellar granule cells. A model for the topographic organization of cerebellar granule cell Ca²⁺ stores is proposed.     

Decreased Incorporation of [3H]Inositol and [3H]Glycerol into Glycerolipids of Sciatic Nerve from the Streptozotocin Diabetic Rat

The incorporation of [3H]myo-inositol into individual phosphoinositides and of [3H]glycerol into glycerolipids was determined in sciatic nerve obtained from normal and streptozotocin diabetic rats and incubated in vitro. The uptake of inositol into lipid was approximately linear with time. More than 80% of the label was present in phosphatidylinositol with the remainder divided about equally between phosphatidylinositol phosphate and phosphatidylinositol-4,5-bisphosphate. Labeling was unchanged 2 weeks after induction of diabetes, but was reduced by 32% after 20 weeks of the disease. Glycerol incorporation occurred primarily into phosphatidylcholine and triacylglycerol and was depressed up to 45% into major phosphoglycerides in nerves from both 2- and 20-week diabetic animals. Triacylglycerol labeling was also substantially decreased, and the reduction was comparable in intact and epineurium free nerve, suggesting that a metabolically active pool of this compound, which is sensitive to hyperglycemia and/or insulin deficiency, is located in or immediately adjacent to the nerve fibers. The considerable decline in incorporation of these lipid precursors in diabetic nerve may be related to impaired inositol transport and to decrease overall energy utilization by the tissue.     

Low [3H]Cytochalasin B Binding in the Cerebral Cortex of Newborn Rat

The concentrations of glucose transporter in the cerebral cortex and brainstem of neonatal (4–7 days old) and adult rats were measured using [³H]cytochalasin B binding. There was significantly lower binding in neonatal cortex (1.9 ± 0.7 pmol/mg protein) compared to adult (8.9 ± 2.5 pmol/mg protein). Scatchard analysis indicates this difference is due to a lower B_max (neonate, 9.7 pmol/mg protein; adult, 18.6 ± 1.3 pmol/mg protein). Measurement of [³H]cytochalasin B binding in microvessels prepared from cortex of adult (28.1 ± 3.5 pmol/mg protein) and neonate (12.8 ± 1.9 pmol/mg protein) indicates a lower binding in the microvasculature of neonates, whereas no such difference was seen in the binding in microvessels prepared from adult and neonatal brainstem (adult, 11.8 ± 2.3 pmol/mg protein; neonate, 9.4 ± 2.7 pmol/mg protein). In both adult and neonate brain, there is an enrichment of glucose transporters in the microvasculature.     

Binding Sites for [3H]Glutamate and [3H]Aspartate in Human Cerebellum

Abstract The binding of [³H]aspartate and [³H]glutamate to membranes prepared from frozen human cerebellar cortex was studied. The binding sites differed in their relative proportions, their inhibition by amino acids and analogues, and by the effects of cations. A proportion (about 30%) of [³H]glutamate binding was to sites similar to those labelled by [³H]aspartate. An additional component of [³H]gluta-mate binding (about 50%) was displaced by quisqualate and aL-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and may represent a “quisqualate-preferring” receptor. Neither N-methyl-d-aspartic acid-sensitive nor dl-2-amino-4-phosphonobutyric acid-sensitive [³H]glutamate binding was detected.     

Neurotransmitter and Depolarization-Stimulated Accumulation of Inositol 1,3,4,5-Tetrakisphosphate Mass in Rat Cerebral Cortex Slices

[32P]Inositol 1,3,4,5-tetrakisphosphate ([32P]Ins(1,3,4,5)P4) binds to a rat cerebellar membrane site with high affinity (KD = 2.8 +/- 0.6 nM) and low capacity (Bmax = 176 +/- 34 fmol/mg of protein). Evidence for a low-affinity site (KD = 164 +/- 48 nM) was also apparent. The high-affinity site displayed marked specificity for the Ins(1,3,4,5)P4 isomer, compared with several other inositol polyphosphates, and has been used as the basis of a radioreceptor assay for Ins(1,3,4,5)P4 in extracts of rat cerebral cortex slices. The resting Ins(1,3,4,5)P4 concentration (1.89 +/- 0.11 pmol/mg of protein) in the slices was rapidly and dramatically increased by carbachol and quisqualate. K+ depolarization of cerebral cortex slices also stimulated Ins(1,3,4,5)P4 accumulation, with at least 50% of the response being sensitive to atropine, a result indicating that muscarinic receptor stimulation by released acetylcholine contributes significantly to the K+ effect.     

Increased Intracellular Calcium Stimulates 3H-Inositol Polyphosphate Accumulation in Rat Cerebral Cortical Slices

Agents that increase the intracellular Ca2+ concentration have been examined for their ability to stimulate 3H-inositol polyphosphate accumulation in rat cerebral cortex slices. Elevated extracellular K+ levels, the alkaloid sodium channel activator veratrine, the calcium ionophore ionomycin, and the marine toxin maitotoxin were all able to stimulate phosphoinositide metabolism. Certain features appear common to the agents studied. Thus, although [3H]inositol monophosphate, [3H]inositol bisphosphate ([3H]InsP2), and [3H]inositol trisphosphate were all stimulated, a proportionally greater effect was observed on [3H]InsP2 in comparison to stimulation by the muscarinic receptor agonist carbachol. However, only an elevated K+ level stimulated [3H]inositol tetrakisphosphate ([3H]InsP4) accumulation alone or produced marked synergy with carbachol on the formation of this polyphosphate. The results suggest that agents that elevate the cytoplasmic Ca2+ concentration in cerebral cells can increase the hydrolysis of membrane polyphosphoinositides. The pattern of the response differs from that produced by muscarinic receptor agonists and indicate that Ca2(+)-dependent hydrolysis may involve different pools of lipids, phosphoinositidase C enzymes, or both. However, clear differences in the ability of these agents to stimulate InsP4, alone or in the presence of muscarinic agonist, suggest that factors other than a simple elevated intracellular Ca2+ concentration are implicated.     

Characteristics of [3H]GBR 12935 Binding in the Human and Rat Frontal Cortex

Binding characteristics of the selective dopamine uptake inhibitor [3H]GBR 12935 have been described for the striatum but not for the frontal cortex. We have developed assay conditions for quantifying [3H]GBR 12935 binding in the frontal cortex. In both the rat and human frontal cortex, the assay required four times more tissue (8 mg/ml) than in the striatum (2 mg/ml). [3H]GBR 12935 binding in the frontal is complex, as it involves multiple binding sites. The high-affinity binding site is sodium dependent and is inhibited by sodium. In human but not in rat frontal cortex, addition of K+ reversed the sodium inhibition. The pharmacological profile of the high-affinity [3H]GBR 12935 binding site is consistent with that of the dopamine transporter, because drugs with the most selective dopamine reuptake blocking activities are the most potent displacers of [3H]GBR 12935 binding. There is a positive correlation between the rat and human inhibitory constants, a finding indicating that there are similar pharmacological profiles across at least these two species. Rats with a 6-hydroxydopamine lesion had a 47% decrease in number of [3H]GBR 12935 binding sites, a result indicating that at least a portion of these sites had been on presynaptic dopamine terminals.