Glutamatergic synapses on oligodendrocyte precursor cells in the hippocampus

Fast excitatory neurotransmission in the central nervous system occurs at specialized synaptic junctions between neurons, where a high concentration of glutamate directly activates receptor channels. Low-affinity AMPA (alpha-amino-3-hydroxy-5-methyl isoxazole propionic acid) and kainate glutamate receptors are also expressed by some glial cells, including oligodendrocyte precursor cells (OPCs). However, the conditions that result in activation of glutamate receptors on these non-neuronal cells are not known. Here we report that stimulation of excitatory axons in the hippocampus elicits inward currents in OPCs that are mediated by AMPA receptors. The quantal nature of these responses and their rapid kinetics indicate that they are produced by the exocytosis of vesicles filled with glutamate directly opposite these receptors. Some of these AMPA receptors are permeable to calcium ions, providing a link between axonal activity and internal calcium levels in OPCs. Electron microscopic analysis revealed that vesicle-filled axon terminals make synaptic junctions with the processes of OPCs in both the young and adult hippocampus. These results demonstrate the existence of a rapid signalling pathway from pyramidal neurons to OPCs in the mammalian hippocampus that is mediated by excitatory, glutamatergic synapses.

The metabotropic glutamate receptor (mGluR1 alpha) is concentrated at perisynaptic membrane of neuronal subpopulations as detected by immunogold reaction

An antiserum to mGluR1 alpha labeled a 160 kd protein in immunoblots of membranes derived from rat brain or cells transfected with mGluR1 alpha. Immunoreactivity for mGluR1 alpha was present in discrete subpopulations of neurons. The GABAergic neurons of the cerebellar cortex were strongly immunoreactive; only some Golgi cells were immunonegative. Somatostatin/GABA-immunopositive cells in the neocortex and hippocampus were enriched in mGluR1 alpha. The hippocampal cells had spiny dendrites that were precisely codistributed with the local axon collaterals of pyramidal and granule cells. Electron microscopic immunometal detection of mGluR1 alpha showed a preferential localization at the periphery of the extensive postsynaptic densities of type 1 synapses in both the cerebellum and the hippocampus. The receptor was also present at sites in the dendritic and somatic membrane where synapses were not located.

The accuracy of reverse transcriptase from HIV-1

A study was conducted to determine the fidelity of DNA synthesis catalyzed in vitro by the reverse transcriptase from a human immunodeficiency virus type 1 (HIV-1). Like other retroviral reverse transcriptases, the HIV-1 enzyme does not correct errors by exonucleolytic proofreading. Measurements with M13mp2-based fidelity assays indicated that the HIV-1 enzyme, isolated either from virus particles or from Escherichia coli cells infected with a plasmid expressing the cloned gene, was exceptionally inaccurate, having an average error rate per detectable nucleotide incorporated of 1/1700. It was, in fact, the least accurate reverse transcriptase described to date, one-tenth as accurate as the polymerases isolated from avian myeloblastosis or murine leukemia viruses, which have average error rates of approximately 1/17,000 and approximately 1/30,000, respectively. DNA sequence analyses of mutations generated by HIV-1 polymerase showed that base substitution, addition, and deletion errors were all produced. Certain template positions were mutational hotspots where the error rate could be as high as 1 per 70 polymerized nucleotides. The data are consistent with the notion that the exceptional diversity of the HIV-1 genome results from error-prone reverse transcription.

Perisynaptic location of metabotropic glutamate receptors mGluR1 and mGluR5 on dendrites and dendritic spines in the rat hippocampus

Ionotropic and metabotropic (mGluR1a) glutamate receptors were reported to be segregated from each other within the postsynaptic membrane at individual synapses. In order to establish whether this pattern of distribution applies to the hippocampal principal cells and to other postsynaptic metabotropic glutamate receptors, the mGluR1a/b/c and mGluR4 subtypes were localized by immunocytochemistry. Principal cells in all hippocampal fields were reactive for mGluR5, the strata oriens and radiatum of the CA1 area being most strongly immunolabelled. Labelling for mGluR1b/c was strongest on some pyramids in the CA3 area, weaker on granule cells and absent on CA1 pyramids. Subpopulations of non-principal cells showed strong mGluR1 or mGluR5 immunoreactivity. Electron microscopic pre-embedding immunoperoxidase and both pre- and postembedding immunogold methods consistently revealed the extrasynaptic location of both mGluRs in the somatic and dendritic membrane of pyramidal and granule cells. The density of immunolabelling was highest on dendritic spines. At synapses, immunoparticles for both mGluR1 and mGluR5 were found always outside the postsynaptic membrane specializations. Receptors were particularly concentrated in a perisynaptic annulus around type 1 synaptic junctions, including the invaginations at 'perforated' synapses. Measurements of immunolabelling on dendritic spines showed decreasing levels of receptor as a function of distance from the edge of the synaptic specialization. We propose that glutamergic synapses with an irregular edge develop in order to increase the circumference of synaptic junctions leading to an increase in the metabotropic to ionotropic glutamate receptor ratio at glutamate release sites. The perisynaptic position of postsynaptic metabotropic glutamate receptors appears to be a general feature of glutamatergic synaptic organization and may apply to other G-protein-coupled receptors.

Cell type and pathway dependence of synaptic AMPA receptor number and variability in the hippocampus

It has been suggested that some glutamatergic synapses lack functional AMPA receptors. We used quantitative immunogold localization to determine the number and variability of synaptic AMPA receptors in the rat hippocampus. Three classes of synapses show distinct patterns of AMPA receptor content. Mossy fiber synapses on CA3 pyramidal spines and synapses on GABAergic interneurons are all immunopositive, have less variability, and contain 4 times as many AMPA receptors as synapses made by Schaffer collaterals on CA1 pyramidal spines and by commissural/ associational (C/A) terminals on CA3 pyramidal spines. Up to 17% of synapses in the latter two connections are immunonegative. After calibrating the immunosignal (1 gold = 2.3 functional receptors) at mossy synapses of a 17-day-old rat, we estimate that the AMPA receptor content of C/A synapses on CA3 pyramidal spines ranges from <3 to 140. A similar range is found in adult Schaffer collateral and C/A synapses.

Inhaled nitric oxide in persistent pulmonary hypertension of the newborn

Nitric oxide (NO) has vasodilatory effects on the pulmonary vasculature in adults and animals. We examined the effects on systemic oxygenation and blood pressure of inhaling up to 80 parts per million by volume of NO at FiO2 0.9 for up to 30 minutes by 6 infants with persistent pulmonary hypertension of the newborn (PPHN). In all infants this treatment rapidly and significantly increased preductal oxygen saturation (SpO2); in 5 infants postductal SpO2 and oxygen tensions also increased. Inhalation of NO did not cause systemic hypotension or raise methaemoglobin. These data suggest that low levels of inhaled NO have an important role in the reversal of hypoxaemia due to PPHN.

Inhaled nitric oxide and persistent pulmonary hypertension of the newborn. The Inhaled Nitric Oxide Study Group

BACKGROUND

Persistent pulmonary hypertension of the newborn causes systemic arterial hypoxemia because of increased pulmonary vascular resistance and right-to-left shunting of deoxygenated blood. Inhaled nitric oxide decreases pulmonary vascular resistance in newborns. We studied whether inhaled nitric oxide decreases severe hypoxemia in infants with persistent pulmonary hypertension.

METHODS

In a prospective, multicenter study, 58 full-term infants with severe hypoxemia and persistent pulmonary hypertension were randomly assigned to breathe either a control gas (nitrogen) or nitric oxide (80 parts per million), mixed with oxygen from a ventilator. If oxygenation increased after 20 minutes and systemic blood pressure did not decrease, the treatment was considered successful and was continued at lower concentrations. Otherwise, it was discontinued and alternative therapies, including extracorporeal membrane oxygenation, were used.

RESULTS

Inhaled nitric oxide successfully doubled systemic oxygenation in 16 of 30 infants (53 percent), whereas conventional therapy without inhaled nitric oxide increased oxygenation in only 2 of 28 infants (7 percent). Long-term therapy with inhaled nitric oxide sustained systemic oxygenation in 75 percent of the infants who had initial improvement. Extracorporeal membrane oxygenation was required in 71 percent of the control group and 40 percent of the nitric oxide group (P=0.02). The number of deaths was similar in the two groups. Inhaled nitric oxide did not cause systemic hypotension or increase methemoglobin levels.

CONCLUSIONS

Inhaled nitric oxide improves systemic oxygenation in infants with persistent pulmonary hypertension and may reduce the need for more invasive treatments.

Differential plasma membrane distribution of metabotropic glutamate receptors mGluR1 alpha, mGluR2 and mGluR5, relative to neurotransmitter release sites

Two group I metabotropic glutamate receptor subtypes, mGluR1 and mGluR5, have been reported to occur in highest concentration in an annulus surrounding the edge of the postsynaptic membrane specialisation. In order to determine whether such a distribution is uniform amongst postsynaptic mGluRs, their distribution was compared quantitatively by a pre-embedding silver-intensified immunogold technique at electron microscopic level in hippocampal pyramidal cells (mGluR5), cerebellar Purkinje cells (mGluR1 alpha) and Golgi cells (mGluR2). The results show that mGluR1 alpha, mGluR5 and mGluR2 each have a distinct distribution in relation to the glutamatergic synaptic junctions. On dendritic spines, mGluR1 alpha and mGluR5 showed the highest receptor density in a perisynaptic annulus (defined as within 60 nm of the edge of the synapse) followed by a decreasing extrasynaptic (60-900 nm) receptor level, but the gradient of decrease and the proportion of the perisynaptic pool (mGluR1 alpha, approximately 50%; vs mGluR5, approximately 25%) were different for the two receptors. The distributions of mGluR1 alpha and mGluR5 also differed significantly from simulated random distributions. In contrast, mGluR2 was not closely associated with glutamatergic synapses in the dendritic plasma membrane of cerebellar Golgi cells and its distribution relative to synapses is not different from simulated random distribution in the membrane. The somatic membrane, the axon and the synaptic boutons of the GABAergic Golgi cells also contained immunoreactive mGluR2 that is not associated with synaptic specialisations. In the hippocampal CA1 area the distribution of immunoparticles for mGluR5 on individual spines was established using serial sections. The results indicate that dendritic spines of pyramidal cells are heterogeneous with respect to the ratio of perisynaptic to extrasynaptic mGluR5 pools and about half of the immunopositive spines lack the perisynaptic pool. The quantitative comparison of receptor distributions demonstrates that mGluR1 alpha and mGluR5, but not mGluR2, are highly compartmentalised in different plasma membrane domains. The unique distribution of each mGluR subtype may reflect requirements for different transduction and effector mechanisms between cell types and different domains of the same cell, and suggests that the precise placement of receptors is a crucial factor contributing to neuronal communication.

Target-cell-specific concentration of a metabotropic glutamate receptor in the presynaptic active zone

The probability of synaptic neurotransmitter release from nerve terminals is regulated by presynaptic receptors responding to transmitters released from the same nerve terminal or from terminals of other neurons. The release of glutamate, the major excitatory neurotransmitter, is suppressed by presynaptic autoreceptors. Here we show that a metabotropic glutamate receptor (mGluR7) in the rat hippocampus is restricted to the presynaptic grid, the site of synaptic vesicle fusion. Pyramidal cell terminals presynaptic to mGluR1alpha-expressing interneurons have at least a ten-fold higher level of presynaptic mGluR7 than terminals making synapses with pyramidal cells and other types of interneuron. Distinct levels of mGluR7 are found at different synapses made by individual pyramidal axons or even single boutons. These results raise the possibility that presynaptic neurons could regulate the probability of transmitter release at individual synapses according to the postsynaptic target.

Cell surface domain specific postsynaptic currents evoked by identified GABAergic neurones in rat hippocampus in vitro

1. Inhibitory postsynaptic currents (IPSCs) evoked in CA1 pyramidal cells (n = 46) by identified interneurones (n = 43) located in str. oriens were recorded in order to compare their functional properties and to determine the effect of synapse location on the apparent IPSC kinetics as recorded using somatic voltage clamp at -70 mV and nearly symmetrical [Cl-]. 2. Five types of visualised presynaptic interneurone, oriens-lacunosum moleculare (O-LMC), basket (BC), axo-axonic (AAC), bistratified (BiC) and oriens-bistratified (O-BiC) cells, were distinguished by immunocytochemistry and/or synapse location using light and electron microscopy. 3. Somatostatin immunoreactive O-LMCs, innervating the most distal dendritic shafts and spines, evoked the smallest amplitude (26 +/- 10 pA, s.e.m., n = 8) and slowest IPSCs (10-90 % rise time, 6.2 +/- 0.6 ms; decay, 20.8 +/- 1.7 ms, n = 8), with no paired-pulse modulation of the second IPSC (93 +/- 4 %) at 100 ms interspike interval. In contrast, parvalbumin-positive AACs evoked larger amplitude (308 +/- 103 pA, n = 7) and kinetically faster (rise time, 0.8 +/- 0.1 ms; decay 11.2 +/- 0.9 ms, n = 7) IPSCs showing paired-pulse depression (to 68 +/- 5 %, n = 6). Parvalbumin- or CCK-positive BCs (n = 9) terminating on soma/dendrites, BiCs (n = 4) and O-BiCs (n = 7) innervating dendrites evoked IPSCs with intermediate kinetic parameters. The properties of IPSCs and sensitivity to bicuculline indicated that they were mediated by GABAA receptors. 4. In three cases, kinetically complex, multiphasic IPSCs, evoked by an action potential in the recorded basket cells, suggested that coupled interneurones, possibly through electrotonic junctions, converged on the same postsynaptic neurone. 5. The population of O-BiCs (4 of 4 somatostatin positive) characterised in this study had horizontal dendrites restricted to str. oriens/alveus and innervated stratum radiatum and oriens. Other BiCs had radial dendrites as described earlier. The parameters of IPSCs evoked by BiCs and O-BiCs showed the largest cell to cell variation, and a single interneurone could evoke both small and slow as well as large and relatively fast IPSCs. 6. The kinetic properties of the somatically recorded postsynaptic current are correlated with the innervated cell surface domain. A significant correlation of rise and decay times for the overall population of unitary IPSCs suggests that electrotonic filtering of distal responses is a major factor for the location and cell type specific differences of unitary IPSCs, but molecular heterogeneity of postsynaptic GABAA receptors may also contribute to the observed kinetic differences. Furthermore, domain specific differences in the short-term plasticity of the postsynaptic response indicate a differentiation of interneurones in activity-dependent responses.

Inhaled nitric oxide in congenital heart disease

BACKGROUND

Congenital heart lesions may be complicated by pulmonary arterial smooth muscle hyperplasia, hypertrophy, and hypertension. We assessed whether inhaling low levels of nitric oxide (NO), an endothelium-derived relaxing factor, would produce selective pulmonary vasodilation in pediatric patients with congenital heart disease and pulmonary hypertension. We also compared the pulmonary vasodilator potencies of inhaled NO and oxygen in these patients.

METHODS AND RESULTS

In 10 sequentially presenting, spontaneously breathing patients, we determined whether inhaling 20-80 ppm by volume of NO at inspired oxygen concentrations (FIO2) of 0.21-0.3 and 0.9 would reduce the pulmonary vascular resistance index (Rp). We then compared breathing oxygen with inhaling NO. Inhaling 80 ppm NO at FIO2 0.21-0.3 reduced mean pulmonary artery pressure from 48 +/- 19 to 40 +/- 14 mm Hg and Rp from 658 +/- 421 to 491 +/- 417 dyne.sec.cm-5.m-2 (mean +/- SD, both p < 0.05). Increasing the FIO2 to 0.9 without adding NO did not reduce mean pulmonary artery pressure but reduced Rp and increased the ratio of pulmonary to systemic blood flow (Qp/Qs), primarily by increasing Qp (p < 0.05). Breathing 80 ppm NO at FIO2 0.9 reduced mean pulmonary artery pressure and Rp to the lowest levels and increased Qp and Qp/Qs (all p < 0.05). While breathing at FIO2 0.9, inhalation of 40 ppm NO reduced Rp (p < 0.05); the maximum reduction of Rp occurred while breathing 80 ppm NO. Inhaling 80 ppm NO at FIO2 0.21-0.9 did not alter mean aortic pressure or systemic vascular resistance. Methemoglobin levels were unchanged by breathing up to 80 ppm NO for 30 minutes.

CONCLUSIONS

Inhaled NO is a potent and selective pulmonary vasodilator in pediatric patients with congenital heart disease complicated by pulmonary artery hypertension. Inhaling low levels of NO may provide an important and safe means for evaluating the pulmonary vasodilatory capacity of patients with congenital heart disease without producing systemic vasodilation.

Prolonged inhalation of low concentrations of nitric oxide in patients with severe adult respiratory distress syndrome. Effects on pulmonary hemodynamics and oxygenation

BACKGROUND

Nitric oxide (NO) inhalation selectively decreases pulmonary artery hypertension and improves arterial oxygenation in patients with the adult respiratory distress syndrome (ARDS). In this study of patients with severe ARDS, we sought to determine the effect of inhaled NO dose and time on pulmonary artery pressure and oxygen exchange and to determine which patients with ARDS are most likely to show this response.

METHODS

Thirteen patients with severe ARDS (hospital mortality 67%) inhaled 0-40 parts per million (ppm) NO. Seven of these patients continued to breathe 2-20 ppm NO for 2-27 days.

RESULTS

Inhaling 5-40 ppm NO decreased mean pulmonary artery pressure in a dose-related fashion (from 34 +/- 7 to 30 +/- 7 mmHg at 20 ppm NO). Systemic arterial pressure did not change. The ratio of arterial oxygen tension to inspired oxygen fraction increased (from 126 +/- 36 to 149 +/- 38 mmHg) and the venous admixture decreased (from 31.2 +/- 5.5 to 28.2 +/- 5.2%) without a clear dose-response effect. During prolonged NO inhalation, 2-20 ppm NO effectively reduced mean pulmonary artery pressure (38 +/- 7 vs. 31 +/- 6 mmHg) and increased arterial oxygen tension (79 +/- 10 vs. 114 +/- 27 mmHg) without evidence of tachyphylaxis. The decrease of pulmonary vascular resistance during NO inhalation correlated with the level of pulmonary vascular resistance without NO (r = -0.72). The reduction of venous admixture correlated with the level of venous admixture without NO (r = -0.78).

CONCLUSIONS

Long-term NO inhalation at low concentrations selectively decreases mean pulmonary artery pressure and improves arterial oxygen tension in patients with ARDS. The selective pulmonary vasodilation effect is most pronounced in ARDS patients with the greatest degree of pulmonary vasoconstriction.

Specialized nucleoprotein structures at the origin of replication of bacteriophage lambda: localized unwinding of duplex DNA by a six-protein reaction

The O protein of bacteriophage lambda localizes the initiation of DNA replication to a unique site on the lambda genome, ori lambda. By means of electron microscopy, we infer that the binding of O to ori lambda initiates a series of protein addition and transfer reactions that culminate in localized unwinding of the origin DNA, generating a prepriming structure for the initiation of DNA replication. We can define three stages of this prepriming reaction, the first two of which we have characterized previously. First, dimeric O protein binds to multiple DNA binding sites and self-associates to form a nucleoprotein structure, the O-some. Second, lambda P and host DnaB proteins interact with the O-some to generate a larger complex that includes additional DNA from an A + T-rich region adjacent to the O binding sites. Third, the addition of the DnaJ, DnaK, and Ssb proteins and ATP results in an origin-specific unwinding reaction, probably catalyzed by the helicase activity of DnaB. The unwinding reaction is unidirectional, proceeding "rightward" from the origin. The minimal DNA sequence competent for unwinding consists of two O binding sites and the adjacent A + T-rich region to the right of the binding sites. We conclude that the lambda O protein localizes and initiates a six-protein sequential reaction responsible for but preceding the precise initiation of DNA replication. Specialized nucleoprotein structures similar to the O-some may be a general feature of DNA transactions requiring extraordinary precision in localization and control.

The gamma 2 subunit of the GABAA receptor is concentrated in synaptic junctions containing the alpha 1 and beta 2/3 subunits in hippocampus, cerebellum and globus pallidus

The gamma 2 subunit is necessary for the expression of the full benzodiazepine pharmacology of GABAA receptors and is one of the major subunits in the brain. In order to determine the location of channels containing the gamma 2 subunit in relation to GABA-releasing terminals on the surface of neurons, a new polyclonal antipeptide antiserum was developed to the gamma 2 subunit and used in high resolution, postembedding, immunoelectron-microscopic procedures. Dual immunogold labelling of the same section for two subunits, and up to three sections of the same synapse reacted for different subunits, were used to characterize the subunit composition of synaptic receptors. The gamma 2 subunit was present in type 2, "symmetrical" synapses in each of the brain areas studied, with the exception of the granule cell layer of the cerebellum. The gamma 2 subunit was frequently co-localized in the same synaptic junction with the alpha 1 and beta 2/3 subunits. The immunolabelling of synapses was coincident with the junctional membrane specialization of the active zone. Immunolabelling for the receptor often occurred in multiple clusters in the synapses. In the hippocampus, the gamma 2 subunit was present in basket cell synapses on the somata and proximal dendrites and in axo-axonic cell synapses on the axon initial segment of pyramidal and granule cells. Some synapses on the dendrites of GABAergic interneurones were densely labelled for the gamma 2, alpha 1 and beta 2/3 subunits. In the cerebellum, the gamma 2 subunit was present in both distal and proximal Purkinje cell dendritic synapses established by stellate and basket cell, respectively. On the soma of Purkinje cells, basket cell synapses were only weakly labelled. Synapses on interneuron dendrites were more densely labelled for the gamma 2, alpha 1 and beta 2/3 subunits than synapses on Purkinje or granule cells. Although immunoperoxidase and immunofluorescence methods show an abundance of the gamma 2 subunit in granule cells, the labelling of Golgi synapses was much weaker with the immunogold method than that of the other cell types. In the globus pallidus, many type 2 synapses were labelled for the gamma 2 subunit together with alpha 1 and beta 2/3 subunits. The results show that gamma 2 and beta 2/3 subunits receptor channels are highly concentrated in GABAergic synapses that also contain the alpha 1 and beta 2/3 subunits. Channels containing the gamma 2 subunit are expressed in synapses on functionally distinct domains of the same neuron receiving GABA from different presynaptic sources. There are quantitative differences in the density of GABAA receptors at synapses on different cell types in the same brain area.

Adenovirus-mediated gene transfer of cGMP-dependent protein kinase increases the sensitivity of cultured vascular smooth muscle cells to the antiproliferative and pro-apoptotic effects of nitric oxide/cGMP

Studies in vitro have underestimated the importance of cGMP-dependent protein kinase (PKG) in the modulation of vascular smooth muscle cell (SMC) proliferation and apoptosis in vivo. This is attributable, in part, to a rapid decline in PKG levels as vascular SMC are passaged in culture. We used a recombinant adenovirus encoding PKG (Ad.PKG) to augment kinase activity in cultured rat pulmonary artery SMC (RPaSMC). Incubation of Ad. PKG-infected RPaSMC (multiplicity of infection = 200) with 8-Br-cGMP decreased serum-stimulated DNA synthesis by 85% and cell proliferation at day 5 by 74%. The effect of 8-Br-cGMP on DNA synthesis in Ad.PKG-infected RPaSMC was blocked by KT5823 (PKG inhibitor), but not by KT5720 (cAMP-dependent protein kinase inhibitor). A nitric oxide (NO) donor compound, S-nitrosoglutathione, at concentrations as low as 100 nM, inhibited DNA synthesis in Ad. PKG-infected RPaSMC, but not in uninfected cells or in cells infected with a control adenovirus. In addition, 8-Br-cGMP and S-nitrosoglutathione induced apoptosis in serum-deprived RPaSMC infected with Ad.PKG, but not in uninfected cells or in cells infected with a control adenovirus. These results demonstrate that modulation of PKG levels in vascular SMC can alter the sensitivity of these cells to NO and cGMP. Moreover, these observations suggest an important role for PKG in the regulation of vascular SMC proliferation and apoptosis by NO and cGMP.

Immunocytochemical localization of the alpha 1 and beta 2/3 subunits of the GABAA receptor in relation to specific GABAergic synapses in the dentate gyrus

Dentate granule cells receive spatially segregated GABAergic innervation from at least five types of local circuit neurons, and express mRNA for at least 11 subunits of the GABAA receptor. At most two to four different subunits are required to make a functional pentamer, raising the possibility that cells have on their surface several types of GABAA receptor channel, which may not be uniformly distributed. In order to establish the subcellular location of GABAA receptors on different parts of dentate neurons, the distribution of immunoreactivity for the alpha 1 and beta 2/3 subunits of the receptor was studied using high-resolution immunocytochemistry. Light microscopic immunoperoxidase reactions revealed strong GABAA receptor immunoreactivity in the molecular layer of the dentate gyrus. Pre-embedding immunogold localization of the alpha 1 and beta 2/3 subunits consistently showed extrasynaptic location of the GABAA receptor on the somatic, dendritic and axon initial segment membrane of granule cells, but failed to show receptors in synaptic junctions. Using a postembedding immunogold technique on freeze-substituted, Lowicryl-embedded tissue, synaptic enrichment of immunoreactivity for these subunits was found on both granule and non-principal cells. Only the postembedding immunogold method is suitable for revealing relative differences in receptor density at the subcellular level, giving approximately 20 nm resolution. The immunolabelling for GABAA receptor occupied the whole width of synaptic junctions, with a sharp decrease in labelling at the edge of the synaptic membrane specialization. Both subunits have been localized in the synaptic junctions between basket cell terminals and somata, and between axo-axonic cell terminals and axon initial segments of granule cells, with no qualitative difference in labelling. Receptor-immunopositive synapses were found at all depths of the molecular layer. Some of the boutons forming these dendritic synapses have been shown to contain GABA, providing evidence that some of the GABAergic cells that terminate only on the dendrites of granule cells also act through GABAA receptors. Double immunolabelling experiments demonstrated that a population of GABA-immunopositive neurons expresses a higher density of immunoreactive GABAA receptor on their surface than principal cells. Interneurons were found to receive GABAA receptor-positive synapses on their dendrites in the hilus, molecular and granule cell layers. Receptor-immunopositive synapses were also present throughout the hilus on presumed mossy cells. The results demonstrate that both granule cells and interneurons exhibit a compartmentalized distribution of the GABAA receptor on their surface, the postjunctional membrane to GABAergic terminals having the highest concentration of receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhaled nitric oxide reverses pulmonary vasoconstriction in the hypoxic and acidotic newborn lamb

We determined whether inhaling low levels of nitric oxide (NO) gas could selectively reverse hypoxic pulmonary vasoconstriction in the near-term newborn lamb and whether vasodilation would be attenuated by respiratory acidosis. To examine the mechanism of air and NO-induced pulmonary vasodilation soon after birth, we measured plasma and lung cGMP levels in the newly ventilated fetal lamb. Breathing at FIO2 0.10 nearly doubled the pulmonary vascular resistance index in newborn lambs and decreased pulmonary blood flow primarily by reducing left-to-right blood flow through the ductus arteriosus. Inhaling 20 ppm NO at FIO2 0.10 completely reversed hypoxic pulmonary vasoconstriction within minutes. Maximum pulmonary vasodilation occurred during inhalation of > or = 80 ppm NO. Breathing 8% CO2 at FIO2 0.10 elevated the pulmonary vascular resistance index to a level similar to breathing at FIO2 0.10 without added CO2. Respiratory acidosis did not attenuate pulmonary vasodilation by inhaled NO. In none of our studies did inhaling NO produce systemic hypotension or elevate methemoglobin levels. Four minutes after initiating ventilation with air in the fetal lamb lung, cGMP concentration nearly doubled without changing preductal plasma cGMP concentration. Ventilation with 80 ppm NO at FIO2 0.21 increased both lung and preductal plasma cGMP concentration threefold. Our data suggest that inhaled NO gas is a rapid and potent selective vasodilator of the newborn pulmonary circulation with an elevated vascular tone due to hypoxia and respiratory acidosis that acts by increasing lung cGMP concentration.

Fidelity of mammalian DNA replication and replicative DNA polymerases

Current models suggest that two or more DNA polymerases may be required for high-fidelity semiconservative DNA replication in eukaryotic cells. In the present study, we directly compare the fidelity of SV40 origin-dependent DNA replication in human cell extracts to the fidelity of mammalian DNA polymerases alpha, delta, and epsilon using lacZ alpha of M13mp2 as a reporter gene. Their fidelity, in decreasing order, is replication greater than or equal to pol epsilon greater than pol delta greater than pol alpha. DNA sequence analysis of mutants derived from extract reactions suggests that replication is accurate when considering single-base substitutions, single-base frameshifts, and larger deletions. The exonuclease-containing calf thymus DNA polymerase epsilon is also highly accurate. When high concentrations of deoxynucleoside triphosphates and deoxyguanosine monophosphate are included in the pol epsilon reaction, both base substitution and frameshift error rates increase. This response suggests that exonucleolytic proofreading contributes to the high base substitution and frameshift fidelity. Exonuclease-containing calf thymus DNA polymerase delta, which requires proliferating cell nuclear antigen for efficient synthesis, is significantly less accurate than pol epsilon. In contrast to pol epsilon, pol delta generates errors during synthesis at a relatively modest concentration of deoxynucleoside triphosphates (100 microM), and the error rate did not increase upon addition of adenosine monophosphate. Thus, we are as yet unable to demonstrate that exonucleolytic proofreading contributes to accuracy during synthesis by DNA polymerase delta. The four-subunit DNA polymerase alpha-primase complex from both HeLa cells and calf thymus is the least accurate replicative polymerase. Fidelity is similar whether the enzyme is assayed immediately after purification or after being stored frozen.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcellular localization of a putative kainate receptor in Bergmann glial cells using a monoclonal antibody in the chick and fish cerebellar cortex

A monoclonal antibody, IX-50, that was raised against a kainate binding protein (Mr = 49,000) from chicken cerebellum, was used in light and electron microscopic immunocytochemical studies to localize putative kainate receptors. Pre- and postembedding immunoperoxidase and immunogold methods were used in the cerebellar cortices of one to 26-day old chickens and adult rainbow trout. Immunoreactivity was detected only in association with Golgi epithelial/Bergmann glial cells. Intracellular immunoreactivity was present in the granular and agranular endoplasmic reticulum, Golgi apparatus and in lysosomes, representing the sites of synthesis, glycosylation and degradation of the protein. In the fish the granular endoplasmic reticulum was not immunoreactive. Extracellular immunoreactivity was associated with the plasma membrane. In the fish it was established that the epitope is on the outer surface of the membrane. The protein seems to be uniformly distributed along the membrane including the somata, the radial stem processes and the leafy lamellae surrounding Purkinje cell dendrites. Areas of the glial membrane in contact with other glial cells were also immunopositive. High-resolution light microscopy demonstrated all the Bergmann glial plasma membrane in the cortex, providing a "negative" image of Purkinje cell dendrites. It is apparent that Bergmann glial processes selectively outline the dendrites of the Purkinje cells by surrounding the parallel fibre terminal/Purkinje cell spine synaptic complexes. The parallel fiber terminals were highly immunoreactive for glutamate, as shown by an immunogold procedure. The association of Bergmann glial processes, carrying the Mr = 49,000 kainate binding protein, with the Purkinje cell dendrites and spine synapses could provide a basis for neuronal signalling to the Bergmann glia, possibly by glutamate.