Molecular mechanisms of homologous desensitization and internalization of muscarinic receptors in primary cultures of neonatal corticostriatal neurons

Homologous desensitization of muscarinic acetylcholine receptors (mAChR) was studied using primary cultures of corticostriatal neurons from neonatal rats. Prolonged incubation with carbachol attenuated phospholipase C responsiveness to muscarinic agonists and decreased the number of cell surface mAChR, as measured by binding of N-[3H] methylscopolamine to neuronal monolayers. When neurons were exposed to carbachol for 15 min, 40% of the mAChR lost from the membrane domain was recovered in the cytosol; a decrease of the total neuronal receptors was detected following an incubation with the agonist lasting longer than 15 min. Both 8-Br-cyclic AMP and forskolin neither affected N-[3H]methylscopolamine binding to cell monolayers or did they prevent the agonist-mediated mAChR desensitization. 8-Br-cyclic GMP also failed to decrease mAChR number. Pertussis toxin failed to prevent the homologous desensitization of mAChR under conditions that blocked the agonist-mediated inhibition of forskolin-stimulated cyclic AMP formation. The phorbol ester 12-O-tetradecanoyl-phorbol-12, 13-acetate induced a concentration-dependent decrease of N-[3H]methylscopolamine binding to neuronal monolayers. However, the protein kinase C inhibitors sphingosine and the ganglioside monosialosyl-gangliotetraglicosylceramide inhibited the 12-O-tetradecanoyl-phorbol-12,13-acetate-induced but not the agonist-induced desensitization of mAChRs. Furthermore, incubation with muscarinic agonists failed to translocate protein kinase C from cytosol to plasma membranes, as measured by binding of the phorbol ester [3H]-4-beta-phorbol-12,13-dibutyrate to neuronal monolayers. In corticostriatal neurons the agonist-induced desensitization and internalization of mAChR involves neither protein kinase C and protein kinase A activation nor changes in cyclic GMP and cyclic AMP content.

The peripheral-type benzodiazepine receptor is functionally linked to Leydig cell steroidogenesis

Testicular mitochondria were previously shown to contain an abundance of peripheral-type benzodiazepine recognition site(s)/receptor(s) (PBR). We have previously purified, cloned, and expressed an Mr 18,000 PBR protein (Antkiewicz-Michaluk, Mukhin, A. G., Guidotti, A., and Krueger, K. E. (1988) J. Biol. Chem. 263, 17317-17321; (Sprengel, R., Werner, P., Seeburg, P. H., Mukhin, A. G., Santi, M. R., Grayson, D. R., Guidotti, A., and Krueger, K. E. (1989) J. Biol. Chem. 264, 20415-20421); and in this report, we present evidence that PBR are functionally linked to Leydig cell steroid biosynthesis. A spectrum of nine different ligands covering a range of over 4 orders of magnitude in their affinities for PBR were tested for their potencies to modulate steroidogenesis in the MA-10 mouse Leydig tumor cell line. The Ki for inhibition of [3H]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide binding and the EC50 for steroid biosynthesis for this series of compounds showed a correlation coefficient of r = 0.95. The most potent ligands stimulated steroid production by approximately 4-fold in these cells. This stimulation was not inhibited by cycloheximide, unlike human chorionic gonadotropin- or cyclic AMP-activated steroidogenesis. The action of PBR ligands was not additive to stimulation by human chorionic gonadotropin or cyclic AMP, but was additive to that of epidermal growth factor, another regulator of MA-10 Leydig cell steroidogenesis. Moreover, PBR ligands stimulated, in a dose-dependent manner, pregnenolone biosynthesis by isolated mitochondria when supplied with exogenous cholesterol. This effect was not observed with mitoplasts (mitochondria devoid of the outer membrane). Cytochrome P-450 side chain cleavage activity, as measured by metabolism of (22R)-hydroxycholesterol, was not affected by PBR ligands in intact cells. Similar results were also obtained with purified rat Leydig cells. In conclusion, PBR are implicated in the acute stimulation of Leydig cell steroidogenesis possibly by mediating the entry, distribution, and/or availability of cholesterol within mitochondria.

CSF diazepam-binding inhibitor in alcoholics and normal controls

Diazepam-binding-inhibitor (DBI) and gamma-aminobutyric acid (GABA) are colocalized in neurons in the brain. This system has been implicated in anxiety and in the regulation of corticotropin-releasing hormone (CRH) secretion. Alcohol has direct and indirect effects on the functioning of GABAA receptors. Abstinent alcoholics are, on the average, more anxious than controls. In tests of animal behavior, DBI has anxiogenic, and alcohol has anxiolytic potency. Therefore, we compared alcoholic patients and healthy controls for cerebrospinal fluid (CSF) levels of DBI, and looked for a correlation between CSF levels of DBI and CRH. There was no significant difference in CSF concentrations of DBI between the two groups and no significant correlation between CSF DBI and our measure of anxiety. However, there was a significant positive correlation between CSF levels of DBI and CRH in both the alcoholic and control groups.

Down-regulation of protein kinase C protects cerebellar granule neurons in primary culture from glutamate-induced neuronal death

Exposing primary cultures of cerebellar granule neurons to 100 nM phorbol 12-myristate 13-acetate (PMA) for 24 hr decreases the Ca2+/phosphatidylserine/diolein-dependent protein kinase C (PKC; ATP:protein phosphotransferase, EC 2.7.1.37) by approximately 90% in the 100,000 x g supernatant and pellet fractions of neuronal culture homogenates. Immunoblot analysis of the homogenates with polyclonal antibodies raised against either the beta-type PKC peptide or total rat brain PKC reveals a virtual loss of 78-kDa PKC immunoreactivity in the supernatant and a marked decrease of PKC immunoreactivity in the pellet. Exposure of the cultures to 50 microM glutamate for 15 min (no Mg2+) induces the translocation of supernatant PKC immunoreactivity to the pellet. Such translocation persists after glutamate withdrawal and is followed by a progressive increase in neuronal death, which begins 2 hr later. Neuronal death approaches completion in about 24 hr. PMA-induced down-regulation of PKC decreases glutamate-elicited neurotoxicity. Yet, the culture exposure to 100 nM PMA fails to decrease the high-affinity binding of [3H]glutamate to neuronal membranes and does not reduce glutamate-induced activation of ionotropic or metabolotropic receptors (assayed as total membrane current measured in whole-cell voltage-clamped neurons, 45Ca2+ uptake in intact monolayers, inositolphospholipid hydrolysis, and transcriptional activation and translation of c-fos mRNA). Moreover, the immediate cell-body swelling and activation of spectrin proteolysis elicited by glutamate remain unchanged. On the other hand, PMA-induced PKC down-regulation reduces any increase in 45Ca2+ uptake or Ca2(+)-dependent proteolysis (measured as spectrin degradation) after glutamate withdrawal. These results support the view that PKC translocation is operative in glutamate-induced destabilization of cytosolic ionized Ca2+ homeostasis and neuronal death.

Serum and depolarizing agents cause acute neurotoxicity in cultured cerebellar granule cells: role of the glutamate receptor responsive to N-methyl-D-aspartate

The life span of neonatal rat cerebellar granule cells, grown in basal minimal Eagle's medium containing 10% (vol/vol) fetal calf serum, was extended to 21-30 days by weekly supplementation with glucose. Addition of 1% fetal calf serum to the culture at 14 days killed 85% of the cells within 1 hr. This lethal effect could be prevented by the N-methyl-D-aspartate (NMDA) receptor antagonists dibenzocyclohepteneimine (MK-801) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP). These findings suggested that the glutamate in the serum caused the dramatic neuronal death through action on the NMDA receptor. Indeed, a 5-min incubation in a Locke physiological salt solution containing 20 microM glutamate and 5 microM glycine killed 55-90% of the cells. This acute toxicity could be prevented by a lyso-GM1 ganglioside with N-acetylated sphingosine. The relatively low glutamate content of the sera analyzed suggests that factors in addition to glycine potentiate serum neurotoxicity. The above noted antagonists of the NMDA receptor also greatly reduced the lethal effect of depolarization by 90 mM KCl or 10 microM veratridine. Therefore, it is likely that the toxicity of the depolarizing agents is mediated by glutamate released from the cells. It is concluded that survival of cerebellar neurons in primary culture may be strongly affected by unsuspected neurotoxic phenomena elicited by brief action of a rather low glutamate concentration.

Glutamate-induced neuronal death in primary cultures of cerebellar granule cells: protection by synthetic derivatives of endogenous sphingolipids

The delayed neuronal death induced by a brief (15 min) application of glutamate to primary cultures of cerebellar granule cells can be prevented by pretreating the cultures with the natural ganglioside monosialoglycosylceramide (GM1), the semisynthetic GM1 with N-acetyl sphingosine (LIGA4), GM1 with N-dichloroacetyl sphingosine (LIGA20) and d-eritro 1,3-dihydroxy-2-dichloroacetylamide-4-trans-octadecene (PKS3). The semisynthetic lipids LIGA4, LIGA20 and PKS3 are more potent than the parent natural compounds. The rank order of potency for the protection against glutamate-induced neuronal death is: LIGA20 greater than or equal to LIGA4 greater than PKS3 greater than GM1; the corresponding EC50 values are 4.5 microM for LIGA20, 5 microM for LIGA4, 30 microM for PKS3 and 55 microM for GM1. The effect of the semisynthetic lipids is faster (maximal protection after a 5-min preincubation) and lasts longer (up to 24 hr) than that of the natural compound GM1. The protection from glutamate-induced neuronal death by the semisynthetic sphingolipids persists after thorough washout of free sphingolipid from the incubation medium. Therefore, LIGA4, LIGA20 and PKS3 are potent and efficacious antagonists of glutamate-induced neuronal death with a good separation between the doses needed for pharmacological action and the intrinsic neurotoxic activity. The natural (GM1) and semisynthetic (LIGA4, LIGA20 and PKS3) sphingolipids block neuronal death without affecting the function of glutamate-operated cationic channels. The protective action of sphingolipids appears to be associated with their insertion into membranes where they inhibit specific second messenger-mediated responses triggered by persistent stimulation of glutamate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]D-serine labels strychnine-insensitive glycine recognition sites of rat central nervous system

In the central nervous system, glycine binds to two recognition sites; one of them (G2), associated with the glutamate receptor, is insensitive to strychnine. Strychnine-insensitive sites were predominant in the forebrain areas and bound D-serine and D-alanine better than the respective L stereoisomers. [3H]D-serine was a more selective radioligand than [3H]glycine for the strychnine-insensitive sites. In the forebrain, the binding of both ligands was inhibited by the putative G2 receptor antagonists, 7-chlorokynurenate and 3-amino-1-hydroxy-2-pyrrolidone, while in pons and in spinal cord only the latter drug was effective. This may indicate the heterogeneity of strychnine-insensitive glycine recognition sites.

Different modes of action of 3-amino-1-hydroxy-2-pyrrolidone (HA-966) and 7-chlorokynurenic acid in the modulation of N-methyl-D-aspartate-sensitive glutamate receptors

The N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors are known to be inhibited by 3-amino-1-hydroxy-2-pyrrolidone (HA-966) and 7-chlorokynurenic acid (Cl-KYN), which act at the glycine-regulated allosteric modulatory center. In this work we show that, in synaptic membranes prepared from rat brain, Cl-KYN and HA-966 inhibit the binding of [3H]glycine. Moreover, Cl-KYN can also completely inhibit the binding of [3H]glutamate to the primary transmitter recognition site for the NMDA receptor, whereas HA-966 only partially reduces this binding. Cl-KYN also abolishes the binding of the NMDA receptor antagonist [3H]3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). In contrast, HA-966 increases [3H]CPP binding, affecting the affinity but not the maximal number of binding sites. This increase is inhibited by glycine and Cl-KYN. The binding of [3H] (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801), used as an index of NMDA receptor activation, is completely inhibited by Cl-KYN but only partially by HA-966. In addition, HA-966, but not Cl-KYN, increases the potency of CPP in inhibiting [3H]MK-801 binding. Our results demonstrate that Cl-KYN and HA-966 differ in their ability to modulate the NMDA receptor, perhaps acting at distinct but overlapping recognition sites. Furthermore, our results suggest that agonist and antagonist recognition sites of the NMDA receptor may be independently regulated by glycine and HA-966, which would result, respectively, in a positive and negative allosteric modulation of the NMDA receptor complex.

Profile of phosphatidylinositol metabolism stimulated by carbachol and glutamate in primary cultures of rat cerebellar neurons

The formation of inositol phosphates, after stimulation of primary cultures of cerebellar neurons of the neonatal rat, in the presence of lithium chloride, by glutamate, carbachol, norepinephrine, histamine and Mg2+-free conditions, was measured by anion exchange high-pressure liquid chromatography (HPLC) with on-line radioactivity detection. All of the above agents caused a persistent, dose-dependent and calcium-sensitive preferential accumulation of inositol-4-phosphate, while the levels of inositol-1-phosphate were virtually unaffected. Agonist stimulation produced also a transient increase of a second peak which co-eluted with the standard for inositol 1,4-bisphosphate. However, no significant accumulation of inositol-1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate was detected, possibly due to the fast kinetics of the metabolism of inositol phosphate. The results indicate that receptor-stimulated metabolism of inositol phosphate, in cultures of cerebellar granule cells, is due to a preferential hydrolysis of polyphosphoinositides and leads to the formation of inositol-4-phosphate through several calcium- and lithium-sensitive enzymatic steps.

Cellular and subcellular localization of an octadecaneuropeptide derived from diazepam binding inhibitor: immunohistochemical studies in the rat brain

Immunocytochemical methods, both light and electron microscopic, were used to identify the cellular and subcellular locations of octadecaneuropeptide-like immunoreactivity (ODN-LI) in rat brains serially sectioned in total. ODN-LI includes a newly discovered family of rat brain neuropeptides that are processing products of a common endogenous neuropeptide precursor, diazepam binding inhibitor (DBI). The members of this neuropeptide family have been shown to displace benzodiazepines and beta carbolines from their specific recognition sites located on the allosteric modulatory centers of GABAA receptors. We have previously examined the distribution of DBI-LI in rat brain. The anti-ODN antiserum used in this study does not cross-react with rat DBI, and thus allows a distinct analysis of ODN-LI as opposed to DBI-LI, in rat brain. Neuronal perikarya with ODN-LI were located in many brain nuclei, such as the pontine n., reticular thalamic n., subgeniculate n., supraoptic n. and suprachiasmatic n., and also in brain areas such as cerebral and cerebellar cortex, hippocampus, inferior colliculus, olfactory bulb and subiculum. In addition to perikaryal labelling, a punctate or diffuse immunostaining with ODN antibodies was detected in many brain regions such as cerebellum, hippocampus, amygdaloid area, olfactory tubercle, some of the deep cerebellar nuclei and some circumventricular organs. At the electron microscopic level ODN-LI was identified in neuronal perikarya, processes and terminals. In the axon terminals, ODN-LI appears to be associated with synaptic vesicles. Whenever ODN-LI was detected within neurons, DBI-LI was also found in identical cells. In addition to neurons, DBI-LI was found in glia or glial-like cells, while ODN-LI was not found in these cells. Our findings are consistent with the hypothesis that ODN may be a neuron-specific processing product of DBI and that ODN-like peptides may act as putative endogenous allosteric modulators of various GABAA receptor subtypes.

Isolation and characterization of a rat brain triakontatetraneuropeptide, a posttranslational product of diazepam binding inhibitor: specific action at the Ro 5-4864 recognition site

This report describes the purification and characterization from rat brain of triakontatetraneuropeptide (TTN, DBI 17-50), a major biologically active processing product of diazepam binding inhibitor (DBI). Brain TTN was purified by immunoaffinity chromatography with polyclonal octadecaneuropeptide, DBI 33-50) antibodies coupled to CNBr-Sepharose 4B followed by two reverse-phase HPLC steps. The amino acid sequence of the purified peptide is: Thr-Gln-Pro-Thr-Asp-Glu-Glu-Met-Leu-Phe-Ile-Tyr-Ser-His-Phe-Lys-Gln-Ala-Thr-Val - Gly-Asp-Val-Asn-Thr-Asp-Arg-Pro-Gly-Leu-Leu-Asp-Leu-Lys. Synthetic TTN injected intracerebroventricularly into rats induces a proconflict activity (IC50 0.8 nmol/rat) that is prevented by the specific "peripheral" benzodiazepine (BZ) receptor antagonist isoquinoline carboxamide, PK 11195, but not by the "central" BZ receptor antagonist imidazobenzodiazepine, flumazenil. TTN displaces [3H]Ro 5-4864 from synaptic membranes of olfactory bulb with a Ki of approximately 5 microM. TTN also enhances picrotoxinin inhibition of gamma-aminobutyric acid (GABA)-stimulated [3H]flunitrazepam binding. These data suggest that TTN, a natural DBI processing product acting at "Ro 5-4864 preferring" BZ binding site subtypes, might function as a putative neuromodulator of specific GABAA receptor-mediated effects.

Allosteric modulatory centers of transmitter amino acid receptors

Transmitter amino acid receptors (gamma-aminobutyric acid [GABA] and excitatory amino acids) include in their structure allosteric modulatory centers that regulate the probability of transmitter action. These are sites of action for drugs. In GABA receptors, benzodiazepines and beta-carbolines act as positive and negative modulators. Various subtypes of GABAA receptors exist that differ with regard to the structure of the receptor subunits and the characteristic of the allosteric modulatory centers. This brings up the possibility that classes of benzodiazepines exist that, by acting selectively on specific subtypes of GABAA receptors, may bring about selectivity of drug action in specific anxiety disorders. For instance, clonazepam appears to act better than diazepam on panic attacks and fails to bind to GABAA receptor subtypes located in spinal cord. Also, glutamate receptors and specifically the N-methyl-D-aspartate-sensitive subtype modulated by an allosteric center may include various molecular forms differing with respect to the properties of the allosteric modulatory center. This variability suggests that this center may be used as a target for discovery of drugs acting as specific allosteric modulators of glutamate receptors.

Differences in the negative allosteric modulation of gamma-aminobutyric acid receptors elicited by 4'-chlorodiazepam and by a beta-carboline-3-carboxylate ester: a study with natural and reconstituted receptors

Cl- currents elicited by gamma-aminobutyric acid (GABA) application were recorded with the whole-cell tight-seal technique from voltage-clamped cortical neurons of neonatal rats in primary culture. The peripheral benzodiazepine recognition site ligand 4'-chlorodiazepam [Ro 5-4864; 7-chloro-1,3-dihydro-1-methyl-5-(4-chlorophenyl)-2H-[1,4]-benzodiazep in-2- one] inhibited the GABA-generated currents in a dose-dependent manner. Also, a beta-carboline (DMCM; 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate methyl ester), acting as a negative allosteric modulator of GABAA receptors, reduced the intensity of GABA-generated currents with similar efficacy but greater potency. Flumazenil (Ro 15-1788; 8-fluro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-[1,5-a] [1,4]-benzodiazepine-3-carboxylate ethyl ester) antagonized DMCM inhibition but not that elicited by 4'-chlorodiazepam. The isoquinoline carboxamide PK 11195, an antagonist of 4'-chlorodiazepam effects in other systems, failed to antagonize the action of 4'-chlorodiazepam. The transient expression of various molecular forms of GABAA receptors in the human embryonic kidney cell line 293 allowed a study of the minimal structural requirements for the inhibition of GABA-induced Cl- currents by bicuculline, picrotoxin, 4'-chlorodiazepam, and DMCM. GABA-elicited Cl- currents in cells coexpressing alpha 1 and beta 1 subunits of GABAA receptors were inhibited by bicuculline and picrotoxin, but not by DMCM or 4'-chlorodiazepam. Conversely, the GABA currents in cells coexpressing alpha 1 beta 1 and gamma 2 subunits were inhibited by bicuculline, picrotoxin, 4'-chlorodiazepam, and DMCM. Since the Cl- currents generated by GABA in some molecular forms of GABAA receptors are inhibited by bicuculline and picrotoxin only, 4'-chlorodiazepam cannot be acting isosterically with picrotoxin.

Diazepam-binding inhibitor and corticotropin-releasing hormone in cerebrospinal fluid

Diazepam-binding inhibitor (DBI) is a neuromodulatory peptide for gamma-aminobutyric acid (GABA) neurotransmission. Cerebrospinal fluid (CSF) levels of DBI have been found to be elevated in depression. CSF levels of the peptide corticotropin-releasing hormone (CRH) have also been found to be elevated in depression. Therefore, we examined for a relationship between DBI and CRH in human CSF. We found significant positive correlations between CSF levels of DBI and CRH in depressed patients, pathological gamblers, and normal controls. These data, along with the elevated CSF levels of DBI in depression, suggest the possibility that DBI may have a role in coordinating responses to stress in humans in addition to its possible role in the pathophysiology of depression.

Islet-activating protein inhibits the beta-adrenergic receptor facilitation elicited by gamma-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.

Delayed increase of Ca2+ influx elicited by glutamate: role in neuronal death

The mechanism of delayed neurotoxicity, triggered by glutamate, was studied in 7-8-day-old primary cultures of rat cerebellar granule cells. Treatment of cultures for 15 min with 50 microM glutamate in Mg2+ -free medium, followed by removal of the excitoxin, resulted in neuronal death, which started to appear 2-3 hr after the termination of glutamate treatment. The number of dead neurons increased gradually in the next few hours and 80-85% of neurons were found dead 24 hr later. Antagonists of N-methyl-D-aspartate-sensitive glutamate receptors (phencyclidine) or 1.2 mM MgCl2, but not the antagonist of N-methyl-D-asparatate-insensitive glutamate receptors (6-cyano-7-nitroquinoxaline-2,3-dione), abolished the neurotoxic effect of kainate. Development of glutamate-induced neuronal death depends strongly on Ca2+. Removal of extracellular Ca2+ (with 1mM ethyleneglycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid) immediately after the termination of glutamate exposure and before the appearance of the early signs of neuronal death (post-glutamate period) dramatically reduced neuronal degeneration. Neurotoxic concentrations of glutamate induced sustained increase of 45Ca2+ uptake in the post-glutamate period. The delayed increase of 45Ca2+ uptake, as well as the delayed neurotoxicity, were not affected by post-glutamate treatment with phencyclidine, dibenzocyclohepteneimine; DL-2-amino-5-phosphonovalerate, or MgCl2 or with voltage-dependent Ca2+ channel blockers (nitrendipine, verapamil, diltiazem). Neurotoxic concentrations of glutamate also induced a delayed sustained increase of [3H]phorbol-12,13-dibutyrate binding, reflecting an increased translocation of protein kinase C (PKC) from cytosol to the cell membrane during the post-glutamate period. Pretreatment of neurons with the ganglioside GT1b (trisialosylgangliotetraglycosylceramide), followed by removal of free GT1b from the incubation medium, prevented PKC translocation, the sustained increase of 45Ca2+ uptake in the post-glutamate period, and the delayed neuronal death. We suggest that the sustained activation and translocation of PKC primed by glutamate receptor stimulation may be the triggering event causing the protracted increase of neuronal Ca2+ influx. This influx is insensitive to voltage-dependent Ca2+ channel blockers and glutamate receptor antagonists. It appears that this delayed increase of Ca2+ influx may be important in causing neuronal death.

[Coronary angioplasty in elderly patients. A therapeutic option]

Of 304 consecutive percutaneous transluminal coronary angioplasties (PTCA) performed between March 86 and March 88, 61 patients were 65 years or older. The indications for PTCA were: stable angina, unstable angina and acute myocardial infarction with suitable anatomy. In elderly patients, the female sex, and the calcific deposits, were more frequent than in the younger group (p less than 0.0001). Primary success was achieved in 82%, vs 88% in patients younger than 65 years; complications and mortality were also statistically irrelevant between the two groups. Late clinical follow-up ranging from 3 to 36 months (mean 10) showed that symptomatic improvement was achieved in 86% of elderly patients in whom PTCA was successful (vs 80% in the younger group). These data support the safety and clinical effectiveness of PTCA in elderly symptomatic patients with suitable anatomy.

Cerebrospinal fluid content of diazepam binding inhibitor in chronic hepatic encephalopathy

The neuropeptide diazepam binding inhibitor (DBI) is an endogeneous allosteric modulator of gamma-aminobutyric acid (GABA) receptors at the benzodiazepine recognition site. Recent theories on the neurochemical cause for hepatic encephalopathy have implicated activation of inhibitory neurotransmitter GABA systems. In 20 patients with hepatic disease, blood and cerebrospinal fluid (CSF) levels of ammonia and amino acids were measured. As in previous studies there was a selective elevation of CSF amino acids as well as a correlation between CSF glutamine levels and encephalopathy. CSF DBI levels were maximally elevated 5-fold in patients with hepatic encephalopathy, but they were normal in those patients with liver disease not associated with changes in mental status and in patients with nonhepatic encephalopathy. Levels of DBI correlated with the clinical staging of hepatic encephalopathy. These data suggest that DBI may participate in the modulation of cerebral function in hepatic encephalopathy.

Glycine and D-serine act as positive modulators of signal transduction at N-methyl-D-aspartate sensitive glutamate receptors in cultured cerebellar granule cells

In cultures of rat neonatal cerebellar granule cells the signal transduction at ionotropic NMDA-sensitive glutamate receptors (GC1) was measured as an increase of influx of 45Ca2+. This transmitter-mediated influx of Ca2+ was enhanced by glycine and D-serine in a dose-dependent manner. D-Alanine was less active than glycine and D-serine, while L-alanine and L-serine were inactive. These amino acids failed to activate basal influx of Ca2+. Activation of calcium influx at GC2 receptors by kainate was unchanged by the amino acids mentioned above. Glycine and D-serine increased the potency but failed to change the efficacy of GC1 agonists. This action was not changed by strychnine. The enhancement of aspartate signal transduction by glycine and D-serine was inhibited by the noncompetitive GC1 receptor antagonist, phencyclidine, but was even more evident in presence of Mg2+ ions. Hence, glycine and D-serine may function as positive allosteric modulators of signal transduction at NMDA-sensitive (GC1) glutamate receptors.

Kynurenic acid inhibits the activation of kainic and N-methyl-D-aspartic acid-sensitive ionotropic receptors by a different mechanism

The action of kynurenic acid on currents elicited by the activation of amino acid receptors was investigated in primary cultures of cortical neurons prepared from neonatal rats. Kynurenic acid was tested on currents elicited by both N-methyl-D-aspartic acid (NMDA) and kainate, using patch-clamp recording techniques in "outside-out" and "whole-cell" configurations. The inhibition by kynurenic acid was compared with that elicited by amino-phosphono-valeric acid (APV). Whole-cell currents, elicited by increasing doses of NMDA, were antagonized competitively by APV and non-competitively by kynurenic acid (ID50 70 microM); in contrast, kynurenic acid inhibited competitively the whole-cell currents elicited by kainic acid (ID50 500 microM). The non-competitive inhibition by kynurenic acid of the whole cell currents elicited by NMDA was antagonized competitively by glycine, a specific positive allosteric modulator of NMDA receptors; on the other hand glycine failed to change the inhibition by APV of the NMDA-elicited responses. Thus, kynurenic acid inhibits NMDA receptors allosterically (non-competitively) and kainic acid receptors isosterically (competitively).

Regulation of nerve growth factor biosynthesis by beta-adrenergic receptor activation in astrocytoma cells: a potential role of c-Fos protein

The chain of events that results in increased production of nerve growth factor (NGF) following beta-adrenergic receptor (BAR) stimulation has been investigated in the C6-2B rat astrocytoma cell line. Exposure of these cells to the BAR agonist isoproterenol elicits the following cascade of events: (i) increase of cAMP content; (ii) increase of c-Fos mRNA content; (iii) accumulation of c-Fos protein immunoreactivity in the nucleus; (iv) increase of NGF mRNA content. The increase in c-Fos mRNA and its translation product are early events (15 and 40 min, respectively) and precede the accumulation of NGF mRNA, which peaks at 3 hr. The increase in the two mRNAs appears interrelated because cycloheximide inhibits the accumulation of c-Fos protein and NGF mRNA elicited by isoproterenol. Moreover, the accumulation of nuclear c-Fos protein and NGF mRNA induced by BAR stimulation is reduced by 2-aminopurine, an inhibitor of c-Fos mRNA induction. These data suggest that, in C6-2B astrocytoma cells, the nuclear accumulation of c-Fos protein is required for the induction of NGF mRNA expression by BAR stimulation.

In primary cultures of cerebellar granule cells the activation of N-methyl-D-aspartate-sensitive glutamate receptors induces c-fos mRNA expression

L-Glutamate, the natural agonist of quisqualate- and N-methyl-D-aspartate (NMDA)-sensitive excitatory amino acid receptors, elicits a rapid, transient, dose-dependent increase of the steady state level of c-fos mRNA followed by an accumulation of c-fos protein immunostaining in cell nuclei. This induction is prevented by 2-amino-5-phosphonovalerate, an isosteric glutamate receptor antagonist, and by Mg2+ ion and phencyclidine, two noncompetitive allosteric antagonists of NMDA-sensitive glutamate receptors. Kainate and quisqualate (up to 150 microM) failed to alter the basal expression of c-fos mRNA. Furthermore, glycine, a positive allosteric modulator of NMDA-sensitive glutamate receptors, potentiated the glutamate response in a strychnine-insensitive manner. Activation of other transmitter receptors present in these cells (gamma-aminobutyric acid(A), gamma-aminobutyric acid(B), and muscarinic) failed to increase c-fos mRNA expression. Our results provide evidence that activation of NMDA-sensitive glutamate receptors plays an exclusive role in the induction of c-fos mRNA expression and translation in primary cultures of granule cells. It can be inferred that, by this mechanism, glutamate can initiate a transcriptional program that may result in changes in the simultaneous expression of a set of target genes involved in neuron-specific responses.