Myeloid-related protein 8/14 and high-sensitivity cardiac troponin I to differentiate type 2 myocardial infarction

Myocardial infarction (MI) not only defines acute MI with obstructed coronary arteries (T1MI) but also myocardial necrosis caused by myocardial oxygen supply/demand mismatch as type 2 MI (T2MI); only T1MI patients benefit from an early invasive management. Myeloid-related protein(MRP)-8/14 is a biomarker described in various inflammatory diseases and in MI patients. Here we evaluate the potential of MRP-8/14 and high-sensitivity troponin I (hs-cTnI) to differentiate T2MI from T1MI. Patients with final diagnosis NSTEMI (n = 254; 33.1% female) enrolled in a prospective biomarker registry between 08/2011 and 10/2016 were analysed. Median baseline MRP-8/14 levels were higher in T2MI (n = 55; 3.37(1.88-6.48)μg/mL) than in T1MI (n = 199; 2.4 [1.4-3.79]μg/mL) (p = .013) patients, in contrast to hs-cTnI (T2MI:52[11.65-321.4]ng/L vs. T1MI:436.5 [61.25-1973.8]ng/L; p < .001). To detect the strength of this association odds ratios(OR) were calculated with MRP-8/14 yielding 2.13(1.16-3.92; p = .015) to predict T2MI and 0.47(0.26-0.87; p = .015) for T1MI. As expected, hs-cTnI yielded an OR of to predict T2MI 0.34(0.17-0.65; p = .001) and 2.98(1.53-5.81; p = .001) for T1MI. Both markers show comparable and independent results if adjust to hs-cTnI/MRP-8/14, TIMI risk score and CRP. T2MI is associated with higher MRP-8/14 and lower hs-cTnI concentrations than T1MI. Our data suggest that MRP-8/14 as a marker of inflammation might provide usable discriminatory information complementing hs-cTnI in a diagnostic procedure evaluating the type of MI directly upon hospital admission.

P1730Serial high-sensitivity troponin I measurements to discriminate type 2 from type 1 myocardial infarction

Background

Acute myocardial infarction (MI) is associated with high morbidity and mortality. A robust differentiation between type 1 and type 2 MI (T1/T2MI) has prognostic and therapeutic implications. We investigated whether serial high-sensitivity cardiac troponin I measurements could reliably discriminate T1MI from T2MI in patients presenting with a non-ST elevation myocardial infarction (NSTEMI).

Methods

We used data from a prospective acute coronary syndrome biomarker registry of patients with suspected MI that presented at or were transferred to one of two study centres. Here, we analysed an unselected group of 265 NSTEMI patients (67.2% males). Blood was drawn on admission and after 3 hours. High-sensitivity troponin I (hs-cTnI) was measured in frozen samples by a technician blinded to patient characteristics. T1MI or T2MI was defined as the gold-standard study diagnosis by two independent cardiologists based on all available data according to the Third Universal Definition of MI.

Results

A diagnosis of T2MI was made in 55 patients (20.8%) in the NSTEMI cohort. T2MI patients did not differ from T1MI patients regarding age, gender, traditional risk factors, or percentage of those with a history of coronary artery disease. Median baseline hs-cTnI levels were higher in T1MI (436.25; IQR 63.7–1918.8 ng/L) than in T2MI patients (48.4; IQR 11.7–305.9 ng/L; p<0.001). Absolute change in hs-cTnI concentration between 0 and 3 h was greater in T1MI than in T2MI patients with Dhs-cTnI 93.6 ng/L (IQR 13.5–815.3 ng/L) vs. 20.4 ng/L (IQR 2.5–106.5 ng/L) (p<0.001). hs-cTnI yielded an area under the receiver operator characteristics (AUROC) curve for identifying T2MI at baseline of 0.71 (IQR 0.64–0.79) and after 3 h of 0.7 (IQR 0.61–0.78).Dhs-cTnI was associated with an AUROC of 0.68 (IQR 0.6–0.76). Regarding a rule-out approach, Youden-optimized cut-offs for hs-cTnI at baseline as well as for the absolute change in hs-cTnI concentration were calculated (186.5 ng/L; 154.4 ng/L). Use of these two criteria yielded a sensitivity of 89% (78–96%) and a negative predictive value of 95% (89–98%) to exclude T2MI. 49 of 55 T2MI patients would have been ruled out using this algorithm.

Conclusion

Our data show that hs-cTnI concentrations differ between patients presenting with T1 and T2MI. The concentration of hs-cTnI and its change over time has the potential to rule out T2MI and therefore to identify patients who might benefit from an early invasive management. The differentiation between T1MI and T2MI by using hs-cTnI is nevertheless challenging, and further research on specific algorithms is needed.

Acknowledgement/Funding

3German Center for Cardiovascular Research (DZHK), Partnersite Rhein Main, Bad Nauheim, Germany

Reorientierung des Drehimpulses von Li2-Molekülen beim Stoß mit Edelgasatomen / Reorientation of the Angular Momentum of Li2-Molecules due to Collisions with Noble Gas Atoms

Laser-induced fluorescence has been used to investigate the reorientation of the molecular angular momentum due to inelastic collisions with atoms. We report on experiments for the Li2 (1IIu)-rare gas system, where the degree of polarisation of the emitted fluorescence has been measured. Within the framework of a classical theory mean angles for reorientation have been derived. Their dependence on the collision partner and the initial and final angular momentum is discussed.

Prediction of Daily Milk, Fat, and Protein Production by a Random Regression Test-Day Model

Test-day genetic evaluation models have many advantages compared with those based on 305-d lactations; however, the possible use of test-day model (TDM) results for herd management purposes has not been emphasized. The aim of this paper was to study the ability of a TDM to predict production for the next test day and for the entire lactation. Predictions of future production and detection of outliers are important factors for herd management (e.g., detection of health and management problems and compliance with quota). Because it is not possible to predict the herd-test-day (HTD) effect per se, the fixed HTD effect was split into 3 new effects: a fixed herd-test month-period effect, a fixed herd-year effect, and a random HTD effect. These new effects allow the prediction of future production for improvement of herd management. Predicted test-day yields were compared with observed yields, and the mean prediction error computed across herds was found to be close to zero. Predictions of performance records at the herd level were even more precise. Discarding herds enrolled in milk recording for <1 yr and animals with very few tests in the evaluation file improved correlations between predicted and observed yields at the next test day (correlation of 0.864 for milk in first-lactation cows as compared with a correlation of 0.821 with no records eliminated). Correlations with the observed 305-d production ranged from 0.575 to 1 for predictions based on 0 to 10 test-day records, respectively. Similar results were found for second and third lactation records for milk and milk components. These findings demonstrate the predictive ability of a TDM.

Within-Herd Effects of Age at Test Day and Lactation Stage on Test-Day Yields

Variance ratios were estimated for random within-herd effects of age at test day and lactation stage, on test-day yield and somatic cell score to determine whether including these effects would improve the accuracy of estimation. Test-day data starting with 1990 calvings for the entire US Jersey population and Holsteins from California, Pennsylvania, Wisconsin, and Texas were analyzed. Test-day yields were adjusted for across-herd effects using solutions from a regional analysis. Estimates of the relative variance (fraction of total variance) due to within-herd age effects were small, indicating that regional adjustments for age were adequate. The relative variances for within-herd lactation stage were large enough to indicate that accuracy of genetic evaluations could be improved by including herd stage effects in the model for milk, fat, and protein, but not for somatic cell score. Because the within-herd lactation stage effect is assumed to be random, the effect is regressed toward the regional effects for small herds, but in large herds, lactation curves become herd specific. Model comparisons demonstrated the greater explanatory power of the model with a within-herd-stage effect as prediction error standard deviations were greater for the model without this effect. The benefit of the within-herd-stage effects was confirmed in a random regression model by comparing variance components from models with and without random within-herd regressions and through log-likelihood ratio tests.

Cloning and functional characterization of two glycine receptor alpha-subunits from the perch retina

Glycine receptors are ligand-gated ion channel proteins mediating synaptic inhibition in the spinal cord, retina and brain of vertebrates. We have cloned and functionally characterized two glycine receptor alpha-subunits from the perch (Roccus americana) retina. Based on sequence homology with the mammalian counterparts, we termed these subunits alpha 1 and alpha 3. RT-PCR revealed the presence of both subunits in retina and brain, whereas alpha1 was predominant in spinal cord. A short splice variant of alpha1 was detected in the brain but not in the retina. Functional expression of the perch subunits in HEK-293 cells yielded robust glycine-gated currents sensitive to strychnine. The perch receptors displayed a high efficacy for taurine and GABA and thus differ from the mammalian counterparts. Because the retina is a rich source for taurine, this finding could be of physiological importance. The structural features of the ligand binding domain strongly support the notion of increased glycine/GABA discrimination in higher vertebrates.

GABA immunoreactivity in the human cerebellar cortex: a light and electron microscopical study

The distribution of gamma-aminobutyric acid (GABA) in surgical samples of human cerebellar cortex was studied by light and electron microscope immunocytochemistry using a polyclonal antibody generated in rabbit against GABA coupled to bovine serum albumin with glutaraldehyde. Observations by light microscopy revealed immunostained neuronal bodies and processes as well as axon terminals in all layers of the cerebellar cortex. Perikarya of stellate, basket and Golgi neurons showed evident GABA immunoreactivity. In contrast, perikarya of Purkinje neurons appeared to be negative or weakly positive. Immunoreactive tracts of longitudinally- or obliquely-sectioned neuronal processes and punctate elements, corresponding to axon terminals or cross-sectioned neuronal processes, showed a layer-specific pattern of distribution and were seen on the surface of neuronal bodies, in the neuropil and at microvessel walls. Electron microscope observations mainly focussed on the analysis of GABA-labelled axon terminals and of their relationships with neurons and microvessels. GABA-labelled terminals contained gold particles associated with pleomorphic vesicles and mitochondria and established symmetric synapses with neuronal bodies and dendrites in all cortex layers. GABA-labelled terminals associated with capillaries were seen to contact the perivascular glial processes, basal lamina and endothelial cells and to establish synapses with subendothelial unlabelled axons.

Measurement of fatigue in HIV-positive adults: reliability and validity of the Global Fatigue Index

Fatigue is among the most common and distressing symptoms in patients with HIV/AIDS. Little is known about the clinical assessment of fatigue, especially in patients using highly active antiretroviral regimens. The purpose of this study was to evaluate the psychometric properties of the Global Fatigue Index (GFI) in a community-based sample of 209 patients with HIV/AIDS. The GFI is a measure that quantifies five dimensions of fatigue from the Multidimensional Assessment of Fatigue instrument into one score. To assess construct validity, the study included measures of depression, perceived stress, activities of daily living (ADLs), health behaviors, and clinical markers. Cronbach's alpha was calculated for internal consistency reliability, and factor analysis and bivariate correlations were conducted. The GFI was found to be easily self-administered, reliable, and a valid measure of overall fatigue burden in an HIV population. This instrument may be used by clinicians and researchers for assessing fatigue.

Glutamic acid decarboxylase-positive neuronal cell bodies and terminals in the human cerebellar cortex

The distribution of gamma-aminobutyric acid (GABA) in the human cerebellar cortex was studied using immunohistochemistry for glutamic acid decarboxylase (GAD), the enzyme that catalyses GABA synthesis. Observations by light microscopy revealed, in all layers of the cerebellar cortex, strong, punctate positivity for GAD, related to putative GABAergic nerve terminals, as well as a diffuse cytoplasmic immunoreactivity within neuronal cell bodies. GAD-positive nerve terminals were found in close relationship with the walls of the cerebellar cortex microvessels. Observations by electron microscopy revealed positive nerve terminals in contact with the astrocyte perivascular sheath of capillaries. GAD immunoreactivity was also detected within astroglial perivascular endfeet and endothelial cells. The findings provide further insights into the GABAergic synapses of the circuitry of the human cerebellar cortex. The detection of 'vascular' GAD immunoreactivities suggests that GABAergic mechanisms may regulate cerebellar microvessel function.

The 'ABC' of GABA receptors

In the conventional view, GABA acts at either ionotropic GABAA or metabotropic GABAB receptors. Recently, novel ionotropic GABA receptors that are composed of rho-subunits have been identified in the vertebrate retina. These bicuculline- and baclofen-insensitive GABA receptors are frequently called GABAC, following an early suggestion by Graham Johnston and colleagues. An IUPHAR committee has recommended that the term GABAC be avoided and subclassifies the retinal receptors as GABAA0r. However, new evidence regarding the pharmacology, structure, function, genetics and cellular localization of ionotropic GABA receptors strengthens the case for the existence of two major classes of these receptors, GABAA and GABAC.

Dopamine receptor subtypes modulate olfactory bulb gamma-aminobutyric acid type A receptors

The gamma-aminobutyric acid type A (GABAA) receptor is the predominant Cl- channel protein mediating inhibition in the olfactory bulb and elsewhere in the mammalian brain. The olfactory bulb is rich in neurons containing both GABA and dopamine. Dopamine D1 and D2 receptors are also highly expressed in this brain region with a distinct and complementary distribution pattern. This distribution suggests that dopamine may control the GABAergic inhibitory processing of odor signals, possibly via different signal-transduction mechanisms. We have observed that GABAA receptors in the rat olfactory bulb are differentially modulated by dopamine in a cell-specific manner. Dopamine reduced the currents through GABA-gated Cl- channels in the interneurons, presumably granule cells. This action was mediated via D1 receptors and involved phosphorylation of GABAA receptors by protein kinase A. Enhancement of GABA responses via activation of D2 dopamine receptors and phosphorylation of GABAA receptors by protein kinase C was observed in mitral/tufted cells. Decreasing or increasing the binding affinity for GABA appears to underlie the modulatory effects of dopamine via distinct receptor subtypes. This dual action of dopamine on inhibitory GABAA receptor function in the rat olfactory bulb could be instrumental in odor detection and discrimination, olfactory learning, and ultimately odotopic memory formation.

Synaptic clustering of GABA(C) receptor rho-subunits in the rat retina

Polyclonal antibodies which recognize the rho-subunits of the GABA(C) receptor were applied to sections of the rat retina. Strong punctate immunoreactivity was found in the inner plexiform layer (IPL), which was shown by electron microscopy to represent a clustering of the GABA(C) receptors at synaptic sites. During postnatal development diffuse rho-immunoreactivity was first observed at postnatal day P3. Distinct labelling of bipolar cells appeared at P7 and punctate, synaptic labelling was observed at P10. In order to show that the rho-immunoreactive puncta coincide with the axons of bipolar cells, double immunostainings of retinal sections with an antiserum against syntaxin 3 and with the rho-antiserum were performed. The experiments showed that rho-immunoreactive puncta are preferentially located on the axon terminals of rod and cone bipolar cells. In order to determine whether GABA(C) receptor rho-subunits coassemble with GABA(A) receptor subunits, double-labelling experiments were performed with subunit specific antisera. Punctate, putative synaptic clustering was observed with all antisera applied, however, GABA(C) receptor expressing puncta did not coincide with GABA(A) receptor containing puncta. This suggests that there are no synaptic GABA receptors in the retina in which GABA(A) and GABA(C) receptor subunits are coassembled. Similar double-labelling experiments were also performed to find out whether GABA(C) receptors and glycine receptors are colocalized. They were clustered at different synapses. This suggests that synaptic GABA(C) receptors consist of rho-subunits and are not coassembled with GABA(A)- or glycine-receptor subunits.

GABA-gated Cl- channels in the rat retina

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, and its physiological action is well established. GABA receptors have been localized immunocytochemically in the retina of different mammalian species, and all major retinal cell types have been found to express GABAA receptor subunits. Recently, a new type of GABA receptor with pharmacological and electrophysiological properties different from the known GABAA and GABAB receptors, has been described. These GABAC receptors are found predominantly in the vertebrate retina. This review concentrates on the electrophysiological characterization of GABA receptors expressed by amacrine and bipolar cells of the rat retina. We recorded GABA-induced currents from cultured neonatal amacrine and bipolar cells as well as from isolated bipolar cells of adult animals. While amacrine cells contain a homogeneous population of GABAA receptors, bipolar cells exhibit both GABAA and GABAC responses. Although both receptors gate chloride-selective ion channels, their biophysical and pharmacological properties differ markedly. These functional differences and the cellular distribution of GABAA and GABAC receptors suggest that they have different inhibitory functions in the rat retina.

Immunocytochemical localization of the GABA(C) receptor rho subunits in the cat, goldfish, and chicken retina

Polyclonal antibodies against the N-terminus of the rat rho1 subunit were used to study the distribution of gamma-aminobutyric acid C (GABA(C)) receptors in the cat, goldfish, and chicken retina. Strong punctate immunoreactivity was present in the inner plexiform layer (IPL) of all three species. The punctate labelling suggests a clustering of the GABA(C) receptors at synaptic sites. Weak label was also found in the outer plexiform layer (OPL) and over the cell bodies of bipolar cells. Double immunostaining of vertical sections with an antibody against protein kinase C (PKC) showed the punctate immunofluorescence to colocalize with bipolar cell axon terminals. In the goldfish retina, the axon terminals of Mb1 bipolar cells were enclosed by rho-immunoreactive puncta. In the chicken retina, several distinct strata within the IPL showed a high density of rho-immunoreactive puncta. The results suggest a high degree of sequence homology between the rho subunits of different vertebrate species, and they show that the retinal localization of GABA(C) receptors is similar across different species.

Immunocytochemical localization of the GABAc receptor rho subunits in the mammalian retina

Polyclonal antibodies against the N terminus of the rat rho 1 subunit were generated to study the distribution of GABAc receptors in the mammalian retina. The specificity of the antibodies was tested in Western blots and transfected HEK-293 cells. No cross-reactivity with the GABAA receptor subunits alpha 1-3, beta 1-3, gamma 2, delta or with the glycine receptor subunits alpha 1 and beta could be detected. In contrast, the rho 1, rho 2, and rho 3 subunits were all recognized by the antibodies. In vertical sections of rat, rabbit, cat, and macaque monkey retinae, strong punctate immunoreactivity was present in the inner plexiform layer. Weaker immunoreactivity was also present in the outer-plexiform layer, and cell bodies of bipolar cells were faintly labeled. Double immunostaining of vertical sections and immunostaining of dissociated rat retinae showed the punctate immunofluorescence to colocalize with bipolar cell axon terminals. The puncta possibly represent clustering of the rho subunits at postsynaptic sites.

Synaptic organization of an organotypic slice culture of the mammalian retina

Vertical slices of postnatal day 6 (P6) rat retina were cut and cultured using the roller-tube technique. The organotypic differentiation during a culture period of up to 30 days has been described in a previous study (Feigenspan et al., 1993a). Here we concentrated on the synaptic organization in the retinal slice culture. Electron microscopy revealed the presence of ribbon synapses in the outer plexiform layer and conventional and ribbon synapses in the inner plexiform layer. Immunofluorescence with antibodies that recognize specific subunits of GABAA or glycine receptors revealed a punctate distribution of the receptors. They were aggregated in "hot spots" that correspond to a concentration of receptors at postsynaptic sites. Different isoforms of GABAA and glycine receptors occurred in the slice cultures. The experiments show that there is a differentiation of synapses and a diversity of transmitter receptors in the slice cultures that is comparable to the in vivo retina.

GABAC receptors

gamma-Aminobutyric acid (GABA) is an important neurotransmitter that mediates inhibition in the vertebrate CNS. Until recently, two receptor subtypes were known: bicuculline-sensitive GABAA and baclofen-sensitive GABAB receptors. Several lines of evidence now indicate the existence of a third class of GABA receptor, which is distinct pharmacologically from GABAA and GABAB receptors and is found predominantly in the vertebrate retina. These novel GABAC receptors are Cl- pores. They are insensitive to drugs that modulate GABAA and GABAB receptors and are activated selectively by cis-4-aminoacrotonic acid.

A single point mutation decreases picrotoxinin sensitivity of the human GABA receptor rho 1 subunit

The GABA receptor rho subunits are thought to form bicuculline-insensitive and picrotoxinin-sensitive GABAC receptors. We have investigated the role of the amino acid at position 309 in transmembrane segment M2 of the human rho 1 subunit as a determinant for picrotoxinin sensitivity. The mutant rho 1P309S was constructed by exchanging proline 309 for serine, the corresponding amino acid of the human rho 2 subunit. Whole-cell recordings from HEK-293 cells transfected with rho 1P309S cDNA revealed that the sensitivity of the rho 1P309S channels for picrotoxinin was four-fold lower than that of the wild type rho 1 subunit. The affinity of the mutant receptor for GABA was only slightly changed. These results provide direct evidence that the amino acid at position 309 is an important determinant for the picrotoxinin sensitivity of GABA receptors formed by the rho subunits.

Expression of GABA receptor rho 1 and rho 2 subunits in the retina and brain of the rat

We have investigated the distribution of GABA receptor rho 1 and rho 2 subunits in the rat central nervous system. Cloning of rat rho 1 and rho 2 cDNA fragments revealed similarities to the corresponding human sequences of 99% (rho 1) and 88% (rho 2) at the protein level. Whereas the human rho 2 subunit has no consensus sequence for phosphorylation by protein kinase C, the cytoplasmic loop of the rat sequence contains two such sites. Use of the polymerase chain reaction with reverse-transcribed total RNA (RT-PCR) from different brain tissues revealed that transcript for the rho 1 subunit was present in the retina only. The rho 2 mRNA was detected in all brain regions, with the highest level of expression in the retina. In situ hybridization of retinal sections revealed that rho 1 and rho 2 transcripts are present in the inner nuclear layer. RT-PCR and in situ hybridization of isolated retinal cells showed that both rho subunits are present in rod bipolar cells. Since these cells express bicuculline-insensitive GABA receptors, our results further support the idea that rho subunits are part of the GABAc receptor.

Expression of glycine receptor subunits and gephyrin in single bipolar cells of the rat retina

We studied the expression of glycine receptor (GlyR) subunits and gephyrin in the adult rat retina. Reverse transcribed RNA was amplified by polymerase chain reaction (RT-PCR) with primers designed to recognize GlyR alpha 1, alpha 2, alpha 3, beta subunits, and gephyrin. Using RNA isolated from the whole retina, signals for all four GlyR subunits and gephyrin could be observed. In rod bipolar cells, in contrast, we detected a subset of GlyR subunits, alpha 1 and beta, and no gephyrin. Patch-clamp recording employing two subtype-specific blockers of the GlyR, picrotoxinin and cyanotriphenylborate (CTB), indicated that the GlyR in rod bipolar cells is a heteromeric protein composed of the alpha 1 and beta subunit. Moreover, the absence of detectable amounts of gephyrin mRNA suggests that the anchor protein is not required for the function of GlyRs in rod bipolar cells.

Modulation of hippocampal glycine receptor channels by protein kinase C

The inhibitory glycine receptor (GlyR) is composed of polypeptide subunits that contain intracellular consensus sequences for phosphorylation by protein kinase C (PKC). During whole-cell recording from rat hippocampal neurones, we observed a time-dependent increase of the glycine-induced membrane current. After 22 min the amplitude was 260 + 13% of the initial control response. PKC was involved in the modulation of hippocampal glycine receptors, since the observed effect was more prominent when the phorbol ester PMA, an activator of PKC, was included in the patch pipette. The action of PMA was mimicked by applying the '5-HT2 receptor agonist, alpha-methyl-serotonin, to the cells. The time-dependent increase in glycine responses was reduced by either tamoxifen, an inhibitor of PKC, or by alkaline phosphatase. Protein kinase A and tyrosine kinase were not involved as modulatory drugs of these kinases had no effect. These results provide direct evidence for the regulation of GlyR function by PKC in rat hippocampal neurones.

Differential pharmacology of GABAA and GABAC receptors on rat retinal bipolar cells

GABAA and GABAC receptors were studied on cultured or freshly isolated rat retinal bipolar cells. The cells displayed GABA-induced whole-cell currents, which were only partially blocked by high concentrations (100 microM) of the GABAA receptor antagonist bicuculline. The bicuculline-resistant (GABAC) component was insensitive to the GABAA receptor modulators flunitrazepam (1 microM) and pentobarbital (50 microM). The bicuculline-sensitive portion of the current was strongly augmented by both drugs, indicating that it was mediated by conventional GABAA receptors. The GABAC and GABAA receptor subtypes displayed a 7-fold difference in their binding affinity for GABA, the EC50 values being 4.2 microM and 27.1 microM, respectively. The Hill coefficient was approximately 2 for both receptors. The bicuculline-insensitive GABAC receptors were markedly blocked by 100 microM picrotoxinin, 2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide (SR-95531) and gamma-hexachlorocyclohexane, drugs known to be antagonists of GABAA receptors. Examination of single-channel currents indicated main-state conductances of 7.9 pS and 29.6 pS for GABAC and GABAA receptors, respectively. The pore diameter of open GABAC receptor channels was 5.1 A, i.e. close to the value of 5.6 A reported for the GABAA receptor. These results demonstrate that rod bipolar cells possess two populations of pharmacologically distinct GABA receptors, GABAA and novel-type GABAC receptors, which might subserve different physiological functions in controlling visual transduction in the retina.

Modulation of GABAC receptors in rat retinal bipolar cells by protein kinase C

1. The intracellular phosphorylation of bicuculline- and baclofen-insensitive GABAC receptors was investigated in rat retinal bipolar cells. The cells were recorded in organotypic slice cultures by using the whole-cell configuration of the patch-clamp technique. 2. Peak GABA responses recorded in the presence of bicuculline decreased with repetitive GABA applications. Intracellular application of the phorbol ester, phorbol 12-myristate, 13-acetate (PMA) increased this run-down, whilst it was prevented by both tamoxifen and phosphatase. 3. Perfusing the cells extracellularly with L-AP4, trans-(+/-)-1-amino-1,3-cyclopentane dicarboxylate (ACPD) or alpha-methyl serotonin accelerated the run-down of GABAC responses. 4. Modulation of GABAC responses could be induced by intracellular application of GTP gamma S, indicating involvement of G-proteins in the transduction cascade. 5. These results suggest that retinal GABAC receptors in bipolar cells are modulated by protein kinase C. Receptors which stimulate phospholipase C, presumably via Gi or Go, such as some of the metabotropic glutamate receptors or the 5-HT2 receptor, appear to be linked to this regulatory pathway.

Facilitation of GABAergic signaling in the retina by receptors stimulating adenylate cyclase

The gamma-aminobutyric acid type A (GABAA) receptor is the predominant Cl(-)-channel protein mediating inhibition in the retina and elsewhere in the mammalian brain. We have observed a time-dependent increase of GABA-induced whole-cell currents when dopamine was applied externally to rat retinal amacrine cells. After 20 min, the peak current was increased to 208% +/- 10% of its initial value. A comparable effect was observed with the dopamine D1 receptor agonist (+)-1-phenyl-2,3,4,5-tetrahydro(1H)-3-benzazepine-7,8-diol hydrochloride (SKF-38393) but not with the D2 agonist bromocryptine. The action of dopamine involved phosphorylation of GABAA receptors by protein kinase A, as evident from intracellular application of protein kinase A, cAMP, and forskolin. Both guanosine 5'-[gamma-thio]triphosphate and cholera toxin augmented the GABA response, indicating a role for the guanosine 5'-triphosphate-binding protein Gs in the transduction cascade. Phosphorylation of GABAA receptors shifted the half-maximally effective GABA concentration from 71 microM to 47 microM without affecting the maximal response amplitude. The elevated binding affinity for GABA was caused by an increase of the open probability of the channels from 0.09 to 0.33 (2 microM GABA); conductance and mean open time did not change. Several other receptor agonists such as adenosine, histamine, somatostatin, enkephalin, and vasoactive intestinal peptide were found to couple to the same intracellular phosphorylation pathway. Since some of these cotransmitters colocalize with GABA in amacrine cells, they may fine-tune GABAergic inhibition in the retina.

Decreased agonist affinity and chloride conductance of mutant glycine receptors associated with human hereditary hyperekplexia

Hereditary hyperekplexia is a dominant neurological disorder associated with point mutations at the channel-forming segment M2 of the glycine receptor alpha 1 subunit. Voltage-clamp recordings from the heterologously expressed mutants (alpha 1R271L or alpha 1R271Q) revealed 146- to 183-fold decreased potencies of glycine to activate the chloride channel, and significantly reduced maximal whole-cell currents as compared with wild-type receptors. In contrast, the ability of the competitive antagonist strychnine to block glycine-induced currents was similar in all cases. Radioligand binding assays showed a 90- to 1365-fold reduction in the ability of glycine to displace [3H]strychnine from its binding site on the mutant receptors. Paralleling the reductions in whole-cell current, the elementary main-state conductances of the mutants (alpha 1R271L, 64 pS; alpha 1R271Q, 14 pS) were lower than that of the wild-type receptor (86 pS). The decreased agonist affinities and chloride conductances of the mutants are likely to cause neural hyperexcitability of affected patients by impairing glycinergic inhibition. In addition, our data reveal that structural modifications of the ion-channel region can affect agonist binding to the glycine receptor.

Cyanotriphenylborate: subtype-specific blocker of glycine receptor chloride channels

The inhibitory glycine receptor is a ligand-gated ion-channel protein existing in different homo- and heterooligomeric isoforms. Here we show that the chloride channel of the recombinant alpha 1-subunit homooligomeric glycine receptor is efficiently blocked by cyanotriphenylborate (CTB) with a concentration effecting 50% inhibition (IC50) of 1.3 microM in the presence of 50 microM glycine. The antagonistic effect of CTB is noncompetitive, use dependent, and more pronounced at positive membrane potentials, suggesting open-channel block. In contrast to alpha 1-subunit receptors, alpha 2-subunit homooligomers are resistant to CTB (IC50 >> 20 microM). By exchanging the channel-lining transmembrane segment M2 of the alpha 1 polypeptide by that of the alpha 2 polypeptide, we could transfer this resistance to alpha 1 channels, indicating that a single glycine residue at position 254 of the alpha 1 subunit is critical for CTB sensitivity. The blocker did not affect the cation-selective channel of the nicotinic acetylcholine receptor. Thus, CTB may prove useful as a tool to probe the subunit structure of native glycine receptors in mammalian neurons.

Structure, diversity and synaptic localization of inhibitory glycine receptors

The inhibitory glycine receptor (GlyR) mediates postsynaptic inhibition in spinal cord, brain stem and other regions of the vertebrate central nervous system. Biochemical and molecular approaches have identified different developmentally and regionally regulated GlyR isoforms that result from the differential expression of at least four genes coding for different variants of the ligand-binding alpha subunit. Molecular studies have allowed identification of GlyR subunit domains implicated in ligand binding, channel formation and receptor assembly. At the postsynaptic membrane, the GlyR colocalizes with a 93-kDa tubulin-binding peripheral membrane protein, gephyrin. Antisense inhibition of gephyrin expression prevents GlyR accumulation at postsynaptic membrane specialization. Thus, gephyrin is essential for postsynaptic receptor topology.

Activation of Cl- channels by avermectin in rat cultured hippocampal neurons

The actions of the insecticide avermectin (AVM) were studied in rat cultured hippocampal neurons with patch-clamp techniques. Application of micromolar concentrations of AVM to voltage-clamped cells gave rise to whole-cell currents, which showed a slow time-course of activation in the order of 10 s, and wash-out periods of typically 20 min. Dose-response curves revealed a half-maximally activating AVM concentration (EC50) of 2.0 +/- 0.6 microM and a Hill coefficient of 1.5 +/- 0.9. The current activated by AVM was carried predominantly by Cl- ions, as demonstrated by ion-substitution experiments. The Cl- channel blocker picrotoxinin (100 microM) substantially but transiently reduced the AVM response. Outside-out patch recording showed that AVM opened Cl- channels with a conductance of 40 +/- 12 pS. The open-time distribution was characterized by two time constants of 11 ms and 259 ms. It is suggested that AVM directly activates Cl- channels in mammalian central neurons, which resemble the channels activated by the physiological transmitters GABA and glycine.

Residues within transmembrane segment M2 determine chloride conductance of glycine receptor homo- and hetero-oligomers

We have expressed glycine receptor (GlyR) alpha and beta subunit cDNAs in HEK-293 cells to study the functional properties of homo- versus hetero-oligomeric GlyR channels. Dose-response curves of whole-cell currents in cells expressing alpha 1 subunits revealed an average Hill coefficient of h = 4.2. Co-expression with the beta subunit markedly increased glycine-gated whole-cell currents, which now exhibited a mean Hill coefficient of only h = 2.5. For alpha 1, alpha 2 and alpha 3 homo-oligomers, the main-state single-channel conductances were 86, 111 and 105 pS, respectively, recorded at symmetrical Cl- concentrations of 145 mM. The mutant alpha 1 G221A gave rise to a main-state of 107 pS. This indicates that the main-state of alpha homo-oligomers depends on residue 221 which is located within transmembrane segment M2. Importantly, the main-state conductances of alpha 1/beta, alpha 2/beta and alpha 3/beta hetero-oligomers were only 44, 54 and 48 pS, respectively. The latter values are similar to those found in spinal neurons, suggesting that native GlyRs are predominantly alpha/beta hetero-oligomers. Co-expression of alpha 1 with mutant beta subunits revealed that residues within and close to segment M2 of the beta subunit determine the conductance differences between homo- and hetero-oligomers.

Functional heterogeneity of hippocampal GABAA receptors

gamma-Aminobutyric acid type A (GABAA) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose-response curves of rapidly and slowly desensitizing cells revealed EC50 values of 8.5 and 37.3 microM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABAA receptor channels was 28-31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn2+. These data suggest the presence of physiologically and pharmacologically distinct GABAA receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.

Organotypic slice culture of the mammalian retina

Vertical slices of 6-day postnatal (P6) rat retina were cut at a thickness of 100 microns and cultured using the roller-tube technique. After 14-21 days in vitro there was significant distortion of normal retinal architecture, but localized areas of the slices showed the typical pattern of layering of mature retina. The following immunocytochemical markers were used to characterize the different retinal cell types: antibodies against protein kinase C (PKC), calcium binding protein (CabP 28kD), neurofilaments (NF), glia-specific antibodies (GFAP, vimentin), and transmitter-specific antibodies (GABA, TH). The expression of these markers was compared in P6 retina, adult retina, and slice culture. To further characterize the cultured cells, patch-clamp recordings were performed in combination with intracellular injection of Lucifer Yellow (LY). Transmitter- and voltage-gated membrane currents were recorded from morphologically identified neurons. The experiments show that a mammalian slice culture can be used to study differentiation and function of retinal cell types.

Pharmacology of GABA receptor Cl- channels in rat retinal bipolar cells

gamma-Aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian nervous system, is known to operate bicuculline-sensitive Cl- channels through GABAA receptors and bicuculline-insensitive cation channels through GABAB receptors. Recent observations indicate that the retina may contain GABA receptors with unusual pharmacological properties. Here we report that GABA gates bicuculline-insensitive Cl- channels in rod bipolar cells of the rat retina, which were not modulated by flunitrazepam, pentobarbital and alphaxalone and were only slightly blocked by picrotoxinin. Moreover, the GABAB receptor agonist baclofen, and the antagonist 2-hydroxysaclofen had no effect. The underlying single-channel conductance was 7 pS and the open time 150 ms. These values are clearly different from those obtained for GABAA receptor channels recorded in other neurons of the same preparation, and in other parts of the brain. The bicuculline- and baclofen-insensitive GABA receptors were activated selectively by the GABA analogue cis-4-aminocrotonic acid (CACA). Hence they may be similar to those receptors termed GABAC receptors.

The atypical M2 segment of the beta subunit confers picrotoxinin resistance to inhibitory glycine receptor channels

Purified preparations of the inhibitory glycine receptor (GlyR) contain alpha and beta subunits, which share homologous primary structures and a common transmembrane topology with other members of the ligand-gated ion channel superfamily. Here, a beta subunit-specific antiserum was shown to precipitate the [3H]strychnine binding sites localized on alpha subunits from membrane extracts of both rat spinal cord and mammalian cells co-transfected with alpha and beta cDNAs. Further, inhibition of alpha homo-oligomeric GlyRs by picrotoxinin, a non-competitive blocker of ion flow, was reduced 50- to 200-fold for alpha/beta hetero-oligomeric receptors generated by cotransfection. Site-directed mutagenesis identified residues within the second predicted transmembrane segment (M2) of the beta subunit as major determinants of picrotoxinin resistance. These data implicate the M2 segment in blocker binding to and lining of the GlyR chloride channel.

gp120 of HIV-1 induces apoptosis in rat cortical cell cultures: prevention by memantine

After incubation of rat cortical cell cultures with the human immunodeficiency virus type 1 (HIV-1) coat protein gp120 for 12 h, cells showed fragmentation of DNA at internucleosomal linkers, the characteristic feature of apoptosis. In a quantitative approach, it was determined that the percentage of DNA fragmentation increased from 7%, in the absence of gp120, to 62% following incubation with 24 ng/ml of gp120. Simultaneously, the percentage of viable cells decreased from 94% to 33%. Memantine (1-amino-3,5-dimethyladamantane), a drug currently used in the therapy of spasticity and Parkinson's disease as well as the NMDA antagonist MK-801 both prevented the effects of gp120 at micromolar concentrations. In human cultured astrocytes, gp120 was ineffective with respect to DNA fragmentation and cell toxicity. From these data, we conclude that the gp120-induced apoptosis may contribute to the neurological complications frequently associated with the immunodeficiency syndrome. The cytoprotective effect of memantine in cortical cell cultures may qualify the drug for the treatment of AIDS-related dementia.

Electrophysiological characterization of diazepam binding inhibitor (DBI) on GABAA receptors

The gamma-aminobutyric acid (GABAA) receptor complex is a hetero-oligomeric protein which contains an integral chloride channel and several modulatory domains. The ligands of benzodiazepine recognition sites can up- or down-regulate the activity of the GABAA receptor. The effects of DBI (diazepam binding inhibitor) on GABAA receptors have been studied in cultured mammalian central neurons. Experiments performed with patch-clamp techniques, as well as with conventional intracellular microelectrodes, have revealed a reversible reduction of GABA-induced responses by micromolar concentrations of DBI. This effect was prevented by Ro 15-1788 (flumazenil), a selective benzodiazepine receptor antagonist. From these data, DBI is capable of reducing the activity of the GABAA receptor complex by specifically interacting with the benzodiazepine recognition site. The idea of DBI being a negative allosteric modulator of GABAA receptor channels is in agreement with biochemical, as well as behavioral, pharmacology data.

Effects of the 1-amino-adamantanes at the MK-801-binding site of the NMDA-receptor-gated ion channel: a human postmortem brain study

Recent studies from our laboratory have provided evidence that the 1-amino-adamantane derivative memantine (1-amino-3,5-dimethyl-adamantane) binds to the MK-801-binding site of the N-methyl-D-aspartate (NMDA)-receptor-gated ion channel. This action has been suggested to account for the antiparkinsonian and antispastic activity of the drug. In the present investigation we have extended our work by testing a series of 1-amino-adamantanes, including amantadine (1-amino-adamantane) and memantine, for their ability to compete with [3H]MK-801 binding in membrane homogenates of postmortem human frontal cortex. The most potent substance (1-amino-3,5-diethyl-adamantane) had a Ki-value of 0.19 +/- 0.06 microM while the weakest substance (1-N-methyl-amino-adamantane) had a Ki-value of 21.72 +/- 1.63 microM. The Ki-value of amantadine was 10.50 +/- 6.10 microM. In agreement with our earlier investigation, the Ki-value of memantine was 0.54 +/- 0.23 microM. The results indicate that 1-amino-adamantanes, in general, may produce their pharmacological effects through an interaction with the NMDA-receptor-gated ion channel. The displacement of [3H]MK-801 binding thus may provide the basis to predict the antiparkinsonian and antispastic activity of novel substituted 1-amino-adamantanes and possibly of other drugs.

Effect of atrial natriuretic factor (ANF) on nicotinic acetylcholine receptor channels in bovine chromaffin cells

Bovine chromaffin cells have binding sites for rat atrial natriuretic factor (ANF), as demonstrated autoradiographically by using the 125I-labelled peptide. Patch-clamp recording revealed that ANF reduces acetylcholine (ACh)-induced membrane currents in chromaffin cells at physiological membrane potentials. The effect was dose-dependent with the IC50 value being 5.2 microM ANF and the Hill coefficient close to 1. The channel block was absent at positive membrane potentials, indicating a non-competitive interaction of ANF with the open ACh receptor channel. Fragments of ANF had a much less pronounced action, which is possible due to their structure and molecular charge being different to ANF. The block of nicotinic ACh receptor channels may enable ANF to control the secretion of catecholamines from adrenal chromaffin cells.

Patch-clamp study of gamma-aminobutyric acid receptor Cl- channels in cultured astrocytes

The membrane channels operated by gamma-aminobutyric acid (GABA) were studied in cultured astrocytes from rat cerebral hemispheres by using patch-clamp techniques. The channel properties appeared to be very similar, in many respects, to those present in neuronal cell membranes. The Cl- -selective channels were activated after the sequential binding of two GABA molecules to the receptor, as deduced from the slope of the dose-response curve. Single-channel currents displayed multiple conductance states of 12 pS, 21 pS, 29 pS, and 43 pS, with the main-state conductance being 29 pS. The gating properties could be described by a sequential reaction scheme for agonist-activated channels. GABA-induced whole-cell currents were potentiated by the benzodiazepine receptor agonist diazepam and also, to a lesser extent, by methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate an inverse agonist. In neurons and chromaffin cells, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate reduces the sensitivity of the GABA receptor, indicating that neuronal and glial GABA/benzodiazepine receptor--Cl- channel complexes are different. Glial GABA receptor channels could be of functional importance in buffering extracellular Cl- in the cleft of the GABAergic synapse.