Purification and characterization of a low molecular weight endogenous glutamate binding inhibitor (LGBI) in porcine brain

One of the endogenous substances which modulate glutamate receptor binding was isolated and highly purified from porcine brain. The purification involved extraction of brain tissue with doubled distilled water, followed by gel filtration, anion exchange, cation exchange, and several steps of C18 reverse-phase high performance liquid chromatography (HPLC). A low molecular weight glutamate binding inhibitor (LGBI) was purified to apparent homogeneity as judged from the elution profile of an HPLC column, in which a symmetrical peak was obtained when the eluate was monitored at 220 nm. The LGBI appears to be a small molecule (< 2 kD) that is heat- and acid/base-stable. The highly purified LGBI has no effect on GABAA and benzodiazepine receptor binding. The LGBI is not L-glutamate, L-aspartate or other negatively charged endogenous substances, since they are clearly separated from the LGBI in anion exchange chromatography. The inhibitory effect of the LGBI on [3H]L-glutamate binding is reversible, and it only changes the Bmax while the Kd remains the same. Since the membrane preparations used for [3H]L-glutamate binding assays for the detection of LGBI activity were enriched with quisqualate (QA)-sensitive subtypes, it was suggested that the LGBI could be a modulator of the QA receptor. Some amino acids which produce significant inhibition of glutamate binding activity were also compared with the LGBI, and they all showed no resemblance to the LGBI. The chemical structure of the LGBI remains to be determined.

The crystal structure of recombinant human neutrophil-activating peptide-2 (M6L) at 1.9-A resolution

Neutrophil-activating peptide-2 (NAP-2) is a 70-residue carboxyl-terminal fragment of platelet basic protein, which is found in the alpha-granules of human platelets. NAP-2, which belongs to the CXC family of chemokines that includes interleukin-8 and platelet factor 4, binds to the interleukin-8 type II receptor and induces a rise in cytosolic calcium, chemotaxis of neutrophils, and exocytosis. Crystals of recombinant NAP-2 in which the single methionine at position 6 was replaced by leucine to facilitate expression belong to space group P1 (unit cell parameters a = 40.8, b = 43.8, and c = 44.7 A and alpha = 98.4 degrees, beta = 120.3 degrees, and gamma = 92.8 degrees), with 4 molecules of NAP-2 (Mr = 7600) in the asymmetric unit. The molecular replacement solution calculated with bovine platelet factor 4 as the starting model was refined using rigid body refinement, manual fitting in solvent-leveled electron density maps, simulated annealing, and restrained least squares to an R-factor of 0.188 for 2 sigma data between 7.0- and 1.9-A resolution. The final refined crystal structure includes 265 solvent molecules. The overall tertiary structure, which is similar to that of platelet factor 4 and interleukin-8, includes an extended amino-terminal loop, three strands of antiparallel beta-sheet arranged in a Greek key fold, and one alpha-helix at the carboxyl terminus. The Glu-Leu-Arg sequence that is critical for receptor binding is fully defined by electron density and exhibits multiple conformations.

Brain L-glutamate decarboxylase. Inhibition by phosphorylation and activation by dephosphorylation

Previously we showed that the activity of the gamma-aminobutyric acid-synthesizing enzyme L-glutamate decarboxylase (GAD) in crude brain extract is inhibited by ATP and protein phosphatase inhibitors. We suggested that GAD activity is regulated by protein phosphorylation. In this paper we further present evidence to support our hypothesis that protein kinase A and calcineurin may be involved in regulation of GAD activity through phosphorylation and dephosphorylation fo GAD, respectively. In addition, the effect of neuronal stimulation on GAD activity in cultured neurons is also included. A model to link neuronal excitation and activation of GAD by Ca(2+)-dependent phosphatase is proposed.

Immunopositive GABAergic neural sites display nitric oxide synthase-related NADPH diaphorase activity in the human colon

In the enteric nervous system, gamma-aminobutyric acid (GABA) is a transmitter of interneurons which are proposed to innervate excitatory and inhibitory motor neurons. Nitric oxide (NO) is a putative transmitter of enteric inhibitory motor nerves targeted by GABA. In addition, NO is synthesized by a variety of enteric nerves throughout the gut wall indicative of its potential to be a transmitter of other nerve types, including interneurons. We sought to determine if some populations of nitrergic neurons are interneurons in human infant colon. As enteric neural GABA is exclusive to interneurons, colocalization with NO synthase-related NADPH diaphorase was examined. GABA-transaminase (GABA-T) immunohistochemistry was used to identify GABAergic neurons and a histochemical protocol was used as a marker of neuronal NO synthase-related NADPH diaphorase activity in enteric layers. GABA-T immunoreactive neurons were seen in the ganglionated nerve networks of the myenteric and submucosal layers. GABA-T immunoreactive fibres were also present in the longitudinal and circular muscle layers. A subpopulation of GABA-T immunoreactive neurons within both the myenteric and submucosal ganglia express NO synthase-related activity. This colocalization extends further to a subpopulation of fibers within the muscle layers. These findings strongly suggest that in addition to its role in inhibitory motor neurons, NO may also be a transmitter of enteric interneurons.

Synaptic vesicle-associated glutamate decarboxylase: identification and relationship to insulin-dependent diabetes mellitus

Glutamic acid decarboxylase (GAD) catalyzes the biosynthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). GAD has been suggested as an autoantigen in insulin-dependent diabetes mellitus and stiff-man syndrome. Recently, three forms of membrane-associated GAD (MGAD) have been characterized in porcine brain, but the subcellular localization and function of these proteins are unknown. We present evidence that GAD activity is associated with synaptic vesicles from porcine brain. These vesicles contain a 60 kDa protein recognized by serum from patients with insulin-dependent diabetes mellitus, probably MGADII, as shown by subcellular fractionation and immunoblotting. These results raise the possibility that the association of MGADII with synaptic vesicles may be crucial for its role as an autoantigen in insulin-dependent diabetes mellitus.

Insulin-like effects of vanadate and selenate on the expression of glucose-6-phosphate dehydrogenase and fatty acid synthase in diabetic rats

Insulin is capable of regulating cellular and metabolic processes as well as gene expression. In recent years, enthusiasm has surfaced for using insulin-mimetics to study the mechanism of action of insulin. Vanadate and selenate are two compounds that have been found to mimic the action of insulin on control of blood glucose levels in vivo. Vanadate has also been shown to regulate the expression of several enzymes both in vivo and in vitro, however, studies concerning selenate's ability to regulate expression have not been reported. In this study we show that administration of vanadate or selenate to streptozotocin-induced diabetic rats not only normalizes blood glucose levels similarly to insulin but also positively affects the expression of two key metabolic enzymes, glucose-6-phosphate dehydrogenase (G6PDH) and fatty acid synthase (FAS). Both G6PDH and FAS activity are significantly decreased in diabetic animals compared to non-diabetic control. Treatment of the diabetic animals with either insulin, vanadate or selenate restored both activities to about 80-90% of control. All treatment conditions exhibited activities significantly higher than those determined for the diabetic group but did not differ significantly from each other. Increases in G6PDH or FAS activity are due to increases in mRNA level. Increase in both G6PDH and FAS mRNA was comparable to the observed increase in activity suggesting that regulation of expression by the mimetics occurs pretranslationally.

Structure/function studies of QS-21 adjuvant: assessment of triterpene aldehyde and glucuronic acid roles in adjuvant function

QS-21, a purified Quillaja saponaria saponin immunologic adjuvant, contains two functional groups that we hypothesized to be involved in the adjuvant mechanism of action through charge or Schiff base interaction with a cellular target. Derivatives, prepared by modification of these sites, were prepared and tested for their ability to augment the immunogenicity of the antigen ovalbumin (OVA) in C57BL/6 mice. QS-21 derivatives that were modified at the carboxyl group on an anionic sugar, glucuronic acid, retained adjuvant activity for antibody stimulation, inducing relative increases in antibody titers similar to those induced by QS-21, although the minimum adjuvant dose required for this stimulation was increased several fold relative to the dose of unmodified QS-21. One of these derivatives also retained significant activity for induction of OVA-specific cytotoxic T-lymphocytes. In contrast, QS-21 derivatives modified at an aldehyde on the triterpene did not show adjuvant activity for antibody stimulation or for induction of cytotoxic T-lymphocytes, suggesting that this functional group may be involved in the adjuvant mechanism.

Prostaglandin B2-induced pulmonary hypertension is mediated by TxA2/PGH2 receptor stimulation

We investigated whether the physiological effects of prostaglandin B2 (PGB2) in the pulmonary circulation might be due to stimulation of thromboxane A2-prostaglandin H2 (TxA2/PGH2) receptors. In seven anesthetized rabbits, intravenous infusion of PGB2 (5.0 micrograms/kg) caused pulmonary hypertension as evidenced by increases in right ventricular systolic blood pressure. The magnitude of the pulmonary hypertension was comparable to that observed after infusion of the TxA2 mimetic U-46619 at a significantly lower dose (0.5 micrograms/kg), indicating that the effects of PGB2 in the intact animal are similar to TxA2 but less potent. Additionally, the TxA2/PGH2-receptor antagonist SQ-29548 blocked the pulmonary blood pressure responses elicited by PGB2. Receptor-binding studies using the TxA2 receptor ligand [3H]SQ-29548 indicated that PGB2 was a potent competitor for TxA2/PGH2 receptor binding. In agreement with the results from the intact animal, however, the efficacy of inhibition with PGB2 was significantly less than that measured for the TxA2 agonist U-46619. All of these results are consistent with the hypothesis that the physiological effects of PGB2 are mediated by stimulation of TxA2/PGH2 receptors.

Interleukin-12 enhances antigen-specific proliferation of peripheral blood mononuclear cells from HIV-positive and negative donors in response to Mycobacterium avium

OBJECTIVE

To determine if the addition of recombinant (r) human interleukin (IL)-12 enhances in vitro proliferative responses to Mycobacterium avium of peripheral blood mononuclear cells (PBMC) from HIV-positive donors with CD4 cell counts < 100 x 10(6)/l.

DESIGN AND METHODS

PBMC proliferative responses to virulent and avirulent serovars of M. avium in the presence and absence of exogenously added IL-12 were determined in 24 HIV-positive and 11 HIV-negative donors by 3H-thymidine uptake assay. Changes in CD4 and CD8 cell populations after IL-12 treatment and M. avium stimulation were analyzed by FACS.

RESULTS

IL-12 significantly enhanced proliferation of PBMC to both virulent and avirulent M. avium from all 24 HIV-positive donors (P = 0.0001) although the magnitude varied for each donor. In contrast, addition of IL-12 to PBMC from HIV-negative donors only increased the proliferative responses to the virulent M. avium serovar 4 (P = 0.0044). PBMC from HIV-positive donors in the presence of IL-12 responded better to the avirulent serovar of M. avium than the virulent serovar 4. Proliferative responses of HIV-positive donors to M. avium alone, however, were significantly less (P = 0.0013) than that of HIV-negative donors. Increased proliferative responses of HIV-positive donors were independent of CD4 counts. No significant changes in the ratio of CD4+ to CD8+ T cells occurred in either HIV-positive or negative donors under any culture conditions.

CONCLUSION

In vitro proliferative responses of PBMC from HIV-positive donors to M. avium were significantly enhanced by the addition of human rIL-12, which was not dependent on their CD4 cell counts. The use of IL-12 as an enhancer of cell-mediated immunity in AIDS patients against M. avium infections deserves further study.

Induction of antigen-specific killer T lymphocyte responses using subunit SIVmac251 gag and env vaccines containing QS-21 saponin adjuvant

Subunit vaccines based on recombinant proteins have proved useful for inducing antibody responses and they are safe for widespread use because they do not contain any live components. Unfortunately, they do not typically induce the types of cell-mediated immune responses required to control viral pathogens; specifically, they do not induce CD8+ cytotoxic T lymphocyte (CTL) responses. To increase the immunogenicity of recombinant proteins, we have used the QS-21 saponin adjuvant in subunit vaccine formulations. In the current study, experimental subunit vaccine formulations containing recombinant p55gag or gp120env proteins from the mac251 strain of the simian immunodeficiency virus (SIVmac251) and the QS-21 adjuvant were used to immunize rhesus macaques. These formulations induced SIV gag- or env-specific cellular immunity that was detectable in vitro and included killer cell activity. The induction of killer cells required prior vaccination and the responses were antigen specific for the immunogens contained in the vaccine formulations. Autologous target cells were required to detect these responses, suggesting genetic restriction, and effector cells appeared to be present in both the CD4+ and CD8+ T lymphocyte subpopulations. These data suggest that the vaccine-induced killer cell activity that was detected was mediated by both CD4+ and CD8+ lymphocytes. Despite the presence of these killer cells, all of the animals became infected with the SIVmac251 on experimental challenge. These findings demonstrated that antigen-specific killer cell responses could be induced by a subunit vaccine formulated with the QS-21 saponin adjuvant. The characteristics of the responses suggested that the effector cells were T lymphocytes, expressing either CD4 or CD8.(ABSTRACT TRUNCATED AT 250 WORDS)

Distributed aspects of the response to siphon touch in Aplysia: spread of stimulus information and cross-correlation analysis

We examined two aspects of the response to siphon stimulation in an attempt to test the hypothesis that the Aplysia CNS functions as a distributed system. First, we estimated the number of central neurons that respond to a light touch to the siphon skin. We made voltage-sensitive dye recordings from the abdominal, pleural, pedal, and cerebral ganglia. From these recordings we estimated that 220 abdominal neurons, 110 pleural neurons, and 650 pedal neurons were affected by the light touch. Thus, the information about this mild and localized stimulus is very widely distributed within the Aplysia CNS. This result allows the possibility that the Aplysia CNS functions as a distributed system. If only a small number of neurons had responded to the touch, it would have supported the conclusion that the gill-withdrawal reflex could be generated by a small, dedicated circuit. Second, we searched for correlations between the spike times of the individual abdominal ganglion neurons. Two time scales were examined: a millisecond time scale corresponding to the duration of a fast synaptic potential and a seconds time scale corresponding to the duration of the gill-withdrawal movement. Neuron pairs with highly correlated spike activity on a millisecond time scale must be connected by (or have a common input that uses) relatively powerful, fast, excitatory synapses. We expected that this kind of synaptic interaction would be relatively rare in nervous systems that functioned in a distributed manner. Indeed, only 0.3% of the neuron pairs had correlation coefficients of 0.15 or greater. These correlations accounted for approximately 2% of the action potentials generated in response to siphon stimulation. Thus, large, fast excitatory synaptic interactions appear to be relatively unimportant. This result is consistent with the hypothesis that the abdominal ganglion functions as a distributed system. When the longer time scale was used for the cross-correlograms, a large fraction of the cell pairs had correlated activity because many neurons are activated by the stimulus. It was not possible to interpret the slow correlations in terms of actual synaptic interactions between individual neurons. Our results are consistent with the possibility that the abdominal ganglion functions in a distributed manner. However, this evaluation is indirect and thus only tentative conclusions can be drawn. Evidence from several sources suggests that the neuronal interactions for generating the Aplysia gill-withdrawal reflex are complex.

Multiple mediastinal ganglioneuromas: report of a case

A 23-year-old male known to have an asymptomatic mediastinal mass for four years was admitted due to progressive pitting edema of his arms and legs lasting for one week. The edema subsided prior to surgical extirpation of the left paraspinal mass. Initially recognized radiologically as a well-defined soft tissue mass with mottled calcification, on operation the mass was seen to be two well-encapsulated ganglioneuromas. Another small, round, nodular lesion abutted the contralateral side of the vertebral body on a computed tomographic scan of the chest. The postoperative course was unremarkable except for subsequently developed but well-tolerated regional anhidrosis involving the left side of the chest wall and the left upper limb.

Effect of L-glutamic acid on acid secretion and immunohistochemical localization of glutamatergic neurons in the rat stomach

Glutamatergic neurons in the rat stomach were localized immunohistochemically using antibodies against L-glutamate (L-Glu) as well as glutamate synthesizing enzyme, glutaminase (GLNase). Myenteric ganglia and nerve bundles in the circular muscle and the longitudinal muscle were found to contain GLU- and GLNase-positive nerve fibers, while submucosa and mucosa were devoid of glutamatergic innervation. The distribution of glutamatergic neurons and their processes in both myenteric ganglia and circular muscle is heterogeneous within the stomach. The effect of L-Glu on gastric acid secretion was investigated on an everted preparation of isolated rat stomach. L-Glu at 10(-7) and 10(-8) M alone had no effect on acid secretion. It was found that the oxotremorine-, histamine-, or gastrin-stimulated acid secretion was markedly reduced by L-Glu at 10(-8) M, whereas L-Glu had little effect on the acid secretion stimulated by dimethyl-phenylpiperazinium (DMPP) at this concentration. However, at higher concentration, e.g., 10(-7) M, L-Glu also markedly reduced DMPP-induced acid secretion. Among L-Glu receptor agonists tested, quisqualic acid (QA) is most potent, followed by kainic acid (KA) and N-methyl-D-aspartic acid (NMDA) in inhibiting oxotremorine-stimulated acid secretion. Furthermore, this inhibitory effect of L-Glu on oxotremorine-stimulated acid secretion is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a specific non-NMDA receptor antagonist. All these results suggest that glutamatergic neurons are involved in the modulation of gastric acid secretion via ionotropic QA/KA receptors, probably through openings of Ca2+ channels.

Role of Ca2+ in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated polyphosphoinositide turnover in primary neuronal cultures

Excitatory amino acid (EAA) receptors and EAA-mediated stimulation of polyphosphoinositide (poly-PI) turnover were studied in cultured neurons at different days in vitro (DIV). Six main observations have emerged from these studies: (a) Neurons increased their sensitivity to EAAs as a function of time in culture, indicated by increasing EAA-mediated poly-PI turnover. (b) Extracellular Ca2+ concentration played an important role in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-stimulated poly-PI turnover in cells at 4 DIV, whereas poly-PI turnover mediated by L-glutamate and trans-1-amino-cyclopentane-1,3-dicarboxylic acid was not Ca(2+)-dependent. (c) A marked stimulation of poly-PI turnover by AMPA was seen in the cultured neurons at 4 DIV, but not at 17 DIV, suggesting that a distinct EAA receptor sensitive to AMPA is transiently expressed. (d) The Ca2+ ionophore A23187 increased poly-PI turnover in cultured neurons, suggesting that Ca2+ entry is involved in stimulating poly-PI turnover. (e) Stimulation of poly-PI turnover by carbachol was greater in neurons at 17 DIV as compared with 4 DIV, and appeared to be Ca(2+)-dependent across DIV. (f) 6-Cyano-7-nitroquinoxaline-2,3-dione, an antagonist for non-N-methyl-D-aspartate ionotropic EAA receptors, inhibited 100% and 35% of AMPA- and quisqualate-induced poly-PI turnover, respectively, suggesting an involvement of ionotropic AMPA/quisqualate receptors in stimulating poly-PI turnover.

Use of fixed autologous stimulator cells to correctly present human immunodeficiency virus type 1 viral peptides to nonhuman primate lymphocytes in proliferation and cytotoxic T-lymphocyte assays

Autologous, virus-transformed lymphoblastoid cell lines were established by using peripheral blood lymphocytes from rhesus monkeys that were previously immunized with recombinant human immunodeficiency virus type 1 strain IIIB glycoprotein 160. These autologous cell lines were used to present human immunodeficiency virus type 1 viral antigens in a processed and cell-associated manner to T lymphocytes. This was accomplished by either infecting the cells with recombinant vaccinia viruses or pulsing them with synthetic peptides and then subjecting them to a mild fixation step with glutaraldehyde. Fixed antigen-presenting cells were then used as stimulator cells in vitro to measure cell-mediated immune responses. Both the vaccinia virus-infected and peptide-pulsed autologous cells stimulated antigen-specific cellular proliferative responses. The magnitude of the responses correlated with the immunization histories of the animals and other measures of immunity, such as antibody titers. Autologous vaccinia virus-infected cells were also capable of inducing the in vitro maturation of CD4+ and CD8+ precursor cytotoxic T lymphocytes into antigen-specific mature cytotoxic T lymphocytes. The use of stimulator cells to present viral peptides in a cell-associated manner appeared to be a very sensitive and versatile manner in which to measure cell-mediated immune responses with peripheral blood lymphocytes from nonhuman primates. It is likely that a similar approach will function with peripheral blood lymphocytes from humans.

An integral membrane protein form of brain L-glutamate decarboxylase: purification, characterization and its relationship to insulin-dependent diabetes mellitus

A new and novel form of L-glutamate decarboxylase (GAD; EC 4.1.1.15) was purified from whole porcine brain to apparent homogeneity by a combination of column chromatographies on DE-52, ultragel AcA 34, hydroxylapatite and Sephadex G-200, and native gel electrophoresis. The purified GAD was established as an integral membrane protein based on hydrophobic interaction chromatography and membrane extraction studies. This membrane GAD (MGAD) has a native molecular weight of 120 +/- 5 kDa and is a homodimer of 60 +/- 2 kDa. Immunoprecipitation and immunoblotting tests using the sera from insulin-dependent diabetes mellitus (IDDM) patients revealed the presence of antibodies against this newly identified MGAD in IDDM. The role of MGAD in the pathogenesis of IDDM and related autoimmune disorders is also discussed.

A rat brain cDNA encodes enzymatically active GABA transaminase and provides a molecular probe for GABA-catabolizing cells

cDNAs encoding gamma-aminobutyric acid aminotransferase (GABA-T) were isolated from a lambda ZAP rat hippocampal cDNA expression library by two independent cloning methods, immunological screening with an antimouse GABA-T antibody and plaque hybridization with a GABA-T cDNA probe derived by polymerase chain reaction. We have produced enzymatically active GABA-T from a rat brain cDNA containing the full-length GABA-T coding region. Our rat brain GABA-T cDNAs hybridize to mRNAs in brain and peripheral tissues, including liver, kidney, and testis. We have also detected GABA-T mRNA in GABAergic cells of rat cerebellar cortex by in situ hybridization. Our rat brain GABA-T probe hybridizes to Purkinje, basket, stellate, and Golgi II cells, the same GABAergic neurons previously shown to contain glutamate decarboxylase GAD65 and GAD67.

Accessory cell requirements for saponin adjuvant-induced class I MHC antigen-restricted cytotoxic T-lymphocytes

The in vivo and in vitro accessory cell requirements of class I major histocompatability complex (MHC) antigen-restricted cytotoxic T-lymphocyte (CTL) responses were determined using cell-depletion experiments coupled with active immunizations using ovalbumin (OVA) as the immunogen and saponin adjuvant (QS-21). To paralyze macrophage activity in vivo, C57BL/6 mice were treated with particulate silica or carrageenan. In vivo depletion of helper T-lymphocytes was accomplished by treatment with GK1.5 rat monoclonal antibody, which is specific for the murine CD4 antigen, and by genetic depletion of class II MHC antigens. Following treatments, the mice were immunized with formulations containing OVA alone or mixed with QS-21 saponin adjuvant, which induces MHC class I antigen-restricted CTL responses. In vivo treatment to paralyze macrophages abrogated these CTL responses but not antigen-specific antibody or lymphocyte proliferative responses. Depletion of CD4+ T-lymphocytes had no effect on CTL responses but significantly reduced proliferation and antibody responses. In vitro depletion and reconstitution experiments were done to compare the contributions of different antigen-presenting cells (APC), specifically dendritic cells (DC) and macrophages. Again, the requirement for macrophages was absolute but there was no indication that DC were involved. These data suggest that antigen processing and presentation functions are critical to the induction of CTL and that they are a function of macrophages but that CD4+ helper T-lymphocyte functions are not required.

Purification and characterization of a novel form of brain L-glutamate decarboxylase. A Ca(2+)-dependent peripheral membrane protein

L-Glutamic acid decarboxylase (GAD) catalyzes the one-step biosynthesis of gamma-aminobutyric acid (GABA), which is widely accepted as the major inhibitory neurotransmitter in the mammalian brain. In this paper, we report the identification and purification of a new and novel form of peripheral membrane GAD (MGAD) referred to as MGADIII, using a combination of chromatography on DE52, AcA 34, hydroxylapatite and Sephadex G-200, and native gel electrophoresis. The purified MGADIII migrated as a single protein band on a native 5-25% gradient polyacrylamide gel electrophoresis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent molecular mass of 118 +/- 6 and 60 +/- 4 kDa, respectively, suggesting that it is a homodimer of 60 kDa. MGADIII was established as a Ca(2+)-dependent peripheral membrane protein based on Triton X-114 phase partitioning assay, liposome binding experiment, and membrane extraction studies. Several lines of evidence are presented to show that the association of MGADIII with membranes occurs during depolarization of nerve terminal and that this is a reversible process. Based on these results and previous findings that GAD associates with synaptic vesicles and is regulated by phosphorylation, a model for regulation of GAD in the nerve terminal is proposed.

Consistency in nervous systems: trial-to-trial and animal-to-animal variations in the responses to repeated applications of a sensory stimulus in Aplysia

What is the internal noise in a nervous system? We studied this question by determining the trial-to-trial consistency of the neuronal response in the abdominal ganglion of Aplysia californica. Because our voltage-sensitive dye recordings detected the spike activity from a large fraction of the neurons in the ganglion, these results provide a reasonably complete characterization of the consistency of the response to a sensory stimulus. The consistency of each neuron was evaluated by the number and timing of spikes in the response. The variability in the spike count was described using the coefficient of variation. The spike count variations follow a Poisson distribution, indicating that most of these variations were the result of a random process. For each neuron the reliability of the response to touch was measured in two ways; both measures indicated a broad distribution of reliabilities within the neuron population. The time of the maximum response also varied substantially in some animals. These timing variations were in part due to random processes and in part due to systematic effects (changes in activity of many neurons that were highly correlated). The time course of the activity of individual neurons was compared with the time course of the gill withdrawal. In some animals the activity of individual neurons was only poorly correlated with the behavior; in contrast, the summed activity of groups of neurons matched the behavior quite well. This implies that the behavioral output of the system may be a distributed combination of the activity of many neurons. The differences between animals were substantially larger than the trial-to-trial differences in one animal. The responses made by different preparations differed along many dimensions.

Hormonal regulation of the gene for the type C ecotropic retrovirus receptor in rat liver cells

The infectibility of the regenerating rat liver by ecotropic retroviruses was studied relative to the expression of the gene coding for the ecotropic retrovirus receptor (Ecor) that functions as a cationic amino acid transporter. It is known that the gene for the receptor is expressed in primary hepatocytes and hepatoma cells but is absent in adult liver cells. Isolation of a 2.85-kb cDNA for the rat Ecor suggested that the rat viral receptor is 97% homologous to the mouse viral receptor and that it contains the envelope-binding domain that determines the host range of ecotropic murine retroviruses. This explains the efficient infection of rat cells by ecotropic retroviruses. Since cell division is required for liver cells to be infected, we determined the susceptibility of the regenerating rat liver to infection at different time points after partial hepatectomy (0 to 24 h) in relation to the presence of receptor mRNA. Infection of the liver occurred only when the liver was exposed to virus 4 h after partial hepatectomy. This time course of infection paralleled expression of the gene for the Ecor, which was rapidly induced between 2 and 6 h during liver regeneration. However, expression of the dormant receptor gene in quiescent liver cells can be induced by insulin, dexamethasone, and arginine, indicating that cell division is not required for expression of the receptor gene in liver cells. A diet high in carbohydrate (low in protein) significantly increased the concentration of receptor mRNA in liver cells, indicating that hormones play a role in the regulation of expression of this gene in vivo. We conclude that the gene for the viral receptor is expressed in the regenerating and quiescent liver when the urea cycle enzymes are down regulated. The infection of the regenerating rat liver by ecotropic retroviruses at the time point of expression of the receptor gene supports the requirement of expression of this transporter for infection.

Neuronal activity during different behaviors in Aplysia: a distributed organization?

The active neuronal populations in the Aplysia abdominal ganglion during spontaneous and evoked behaviors were compared with the use of multineuronal optical measurements. In some preparations, more than 90 percent of the neurons activated during the reflex withdrawal of the gill also were activated during respiratory pumping and during small spontaneous gill contractions. Although the same neurons made action potentials in all three behaviors, the activity patterns were different. There was a substantial interaction between the neural substrates underlying evoked and spontaneous behaviors when they were made to occur together. If a gill withdrawal reflex was elicited a few seconds after a respiratory pumping episode, the evoked neuronal activity in most neurons was clearly altered. These results suggest that a distributed organization involving a large number of neurons may be responsible for generating the two behaviors. Different behaviors appear to be generated by altered activities of a single, large distributed network rather than by small dedicated circuits.

A membrane form of brain L-glutamate decarboxylase: identification, isolation, and its relation to insulin-dependent mellitus

A membrane form of L-glutamate decarboxylase (GAD) was identified and purified to apparent homogeneity from hog brain. The purified GAD was established as an integral membrane protein by phase-partitioning assay, charge-shift electrophoresis, and chromatography on a hydrophobic interaction column. This membrane GAD has a native molecular mass of 96 +/- 5 kDa and is a homodimer of 48 +/- 3-kDa subunits. Immunoprecipitation and immunoblotting tests revealed the presence of antibodies against this membrane GAD in sera from patients with insulin-dependent diabetes mellitus. Since this form of GAD appears to be an integral membrane protein and is presumed to have extracellular domains exposed, it seems reasonable to suggest that membrane GAD is more likely than soluble GAD to be involved in the pathogenesis of insulin-dependent diabetes and related autoimmune disorders such as stiff-man syndrome.