New 1,5-benzodiazepine compounds: activity at native GABA(A) receptors

Various new 1,5-benzodiazepine compounds were synthesized and tested for their biological activity in terms of effects on GABA(A) receptors of rat cerebellar granules in culture. Their effects were compared to those of a 1,4-benzodiazepine agonist, flunitrazepam and the already known 1,5-benzodiazepine antiepileptic clobazam. The effects were evaluated for the two different GABA(A) receptor populations present in these neurons, one mediating phasic inhibition and the other one mediating tonic inhibition. Many such compounds display a profile of inverse agonist to both GABA(A) receptor populations. One of them presents a profile of full agonist at the component mediating phasic inhibition. Interestingly, substitution of just one oxygen atom in that compound with sulphur in a specific position of a morpholine ring resulted in a remarkable change of activity from full agonist to a probable inverse agonist. This indicates such a position as a proton accepting one for the ligand within the benzodiazepine binding pocket of the relevant GABA(A) receptors. In addition, that position appears to be critical for the pharmacological activity.

GABA(A) receptors of cerebellar granule cells in culture: explanation of overall insensitivity to ethanol

GABA-activated chloride currents were studied in cerebellar granule cells put in culture from neonatal rats. As previously described, 10 microM GABA perfusion of these cells recorded by whole cell patch-clamp elicits chloride currents displaying a peak and a steady-state component. The two components were studied in the presence of 1 mM furosemide, 1 microM Zn(2+) and a combination of the two in order to evaluate the contribution of the different types of GABA(A) receptors. Furosemide inhibits alpha(6) containing receptors whereas low levels of Zn(2+) specifically block incomplete GABA(A) receptors made up of alpha and beta subunits only. The results show that the peak component involves the following receptors: alpha(x) beta(y), 25%; alpha(1) beta(y) gamma(2), 45%; alpha(6) beta(y) gamma(2) plus alpha(1) alpha(6) beta(y) gamma(2), 30%. The steady state component is made up by alpha(x) beta(y), 38%; alpha(1) beta(y) delta, 62%. Ethanol at relatively high concentration, 100 mM, slows further down the desensitization of alpha(1) beta(y) delta receptors. The results indicate that the relative insensitivity to ethanol of GABA(A) receptors of neonatal cerebellar granule cells in culture is due to the absence of mature alpha(6) beta(y) delta receptors, a major receptor brand involved in tonic inhibition.

Activation of cerebellar granule cells GABA(A) receptors by guanidinoacetate

The extracellular concentration of guanidinoacetate (GAA) in the brain increases in guanidino acetate methyl transferase (GAMT) deficiency, an inherited disorder. We tested whether the levels which this substance can reach in the brain in GAMT deficiency are able to activate GABA(A) receptors in key cerebellar neurons such as the cerebellar granules. GAA in fact activates these receptors in rat cerebellar granules in culture although at quite high concentrations, in the millimolar range. However, these millimolar GAA levels are not reached extracellularly in the brain in GAMT deficiency. In addition, GAA does not act as a partial agonist on granules' GABA(A) receptors. This appears to deny an effect by this molecule on cerebellar function in the disease via interference with granule cells' GABA(A) receptors. Study of partial blockage by furosemide of chloride currents activated by GABA and GAA in granule cells allowed us to distinguish two populations of GABA(A) receptors presumably involved in granule cells' tonic inhibition. One is devoid of alpha6 subunit and another one contains it. The latter when activated by GABA has a decay kinetics much slower than the former. GAA does not distinguish between these two populations. In any case, the very high extracellular GAA concentrations able to activate them are not likely to be reached in GAMT deficiency.

Only high concentrations of ethanol affect GABAA receptors of rat cerebellum granule cells in culture

In the experiments described in the present report, we evaluated the effects of ethanol on the activity of GABAA receptors of cerebellar granule cells in culture. Only very high ethanol concentrations (100-300 mM) showed a clear and significant stimulatory effect on the activity of such receptors. This result was unexpected. In fact, previous reports from other groups would have suggested high ethanol sensitivity of at least one population of GABAA receptors expressed by granule cells.

Calcium accumulation in neurites and cell bodies of rat cerebellar granule cells in culture: effects on GABA(A) receptor function

Accumulation of calcium in rat cerebellar granule cells in culture was studied by two photon laser scanning microscopy. Depolarizations by high extracellular potassium induced short-lived increases in calcium in both cell bodies and neurites. However, although the increase in neurites subsided completely after the initial peak, in cell bodies there was a persistent plateau until the high potassium stimulus was removed. On the contrary, the calcium signal due to NMDA receptors activation was persistent in both cell bodies and neurites and remained until the agonist was present. The nature of these calcium signals provides an interpretation key for the effects of NMDA receptors activation on GABA(A) receptors. In particular, the persistent calcium increase in neurites may explain the decrease in GABA activated chloride currents which are related to activation of dendritic/synaptic GABA(A) receptors.

Immunocytochemical study by two photon fluorescence microscopy of the distribution of GABAA receptor subunits in rat cerebellar granule cells in culture

An immunocytochemical investigation of the expression of alpha(1), alpha(6), beta(2/3), gamma(2) and delta subunits was performed on rat cerebellum granule cells in culture by the two photon microscopy technique. The first four subunits appear to be expressed abundantly in these cells, whereas the delta one seems to be expressed at a lower level. Another major difference in the distribution of these subunits is that whereas alpha(6), beta(2/3) and gamma(2) appear only on plasma membranes alpha(1) and delta are present mainly in the cell bodies cytoplasm. Still another difference was found in that the presence of gamma(2) on neurites is "polarized", preferentially labelling neurites with the appearance of dendrites. The subunits alpha(6) and beta(2/3) appear to label all types of neurites, with beta(2/3) being by far the most heavily expressed subunit type. A final distinct characteristic is that alpha(6) and, even more, gamma(2) appear to accumulate in the cytoplasmic domains immediately below the cone of emergence of neurites. This suggests a conspicuous transport of such subunits from the site of synthesis in the cell body to the site of final expression in the neurites (dendrites and axon terminals).

Modulation by Zn(2+) and Cd(2+) of GABA(A) receptors of rat cerebellum granule cells in culture

This study aims to characterize more closely the different populations of GABA(A) receptors present on the cerebellar granule cells of the rat. The effects of two divalent cations, Zn(2+) and Cd(2+), on GABA-activated chloride currents were studied using the whole-cell patch-clamp technique. Zinc cations inhibit differently the peak and the steady-state current elicited by 10 micro M GABA. In fact, Zn(2+) appears to be more potent in inhibiting the steady-state component, with a lower IC(50). The inhibition of the peak component is of the competitive type, whereas the inhibition of the steady-state one is mixed, being partly competitive and partly allosteric. In addition, Cd(2+) has an inhibitory effect on GABA-activated chloride currents. In terms of the peak component, its effect is limited in extent with a maximal inhibition of only 26%, but with a high affinity (IC(50) as low as 0.03 micro M). The steady-state component is inhibited by 20% independently from the Cd(2+) concentration, in the 10(-2)-10(2) micro M range. In this case, the inhibitory mechanism appears to be of the competitive type for the peak component and of the allosteric type for the steady-state one. We suggest these data are a further confirmation that the rapidly and slowly desensitizing components of the GABA-activated chloride currents, corresponding respectively to the peak and the steady-state components, are made up of two different receptor populations.

Regulation of the expression of low affinity GABAA receptors in rat cerebellar granule cells

GABAA receptors of cerebellar granule cells obtained from neonatal rats and kept in culture were studied by labelled muscimol binding. The data show that, according to the maturational state of those cells in vivo, one or two binding components appear. The low affinity component seems to be the one appearing later. The expression of this component seems to be regulated by protein tyrosine phosphorylation. In fact, its expression is down regulated by the protein tyrosine kinase (PTK) inhibitor, genistein. Viceversa, its expression is upregulated by insulin like growth factor I (IGF-I), most probably via PTK activation. A possible interpretation of the data is that in vivo IGF-I is one of the endogenous messages leading to the expression of this component during development. Another endogenous factor involved may be GABA itself. Low affinity GABAA receptors appear to be the ones involved in inhibitory synaptic transmission at glomeruli. Whereas the high affinity ones probably correspond to extrasynaptic GABAA receptors mediating the tonic form of inhibition in cerebellar granules.

Three-dimensional mapping of cholinergic molecules by confocal laser scanning microscopy in sea urchin larvae

Confocal laser scanning microscopy (CLSM) was used to examine molecules related to the cholinergic neurotransmission system and detected at all the larval stages of Paracentrotus lividus, by histochemical and immunohistochemical methods. CLSM, providing spatial resolution of the cells located both at the larval surface and at depth, allows a complete mapping in a three-dimensional volumetric frame. At early larval stages acetylcholinesterase- as well as choline acetyltransferase-like molecules were found mainly in the gut wall cells, and along the ciliary bands of the arms, together with muscarinic acetylcholine receptors. At perimetamorphic stages, cholinergic molecules were present in the ciliate strands along the arms, in the larval body and in the rudiment. At metamorphosis, positivity to cholinergic molecules translocated to the juvenile, where a high frequency of mAChR- and ChAT-like positive cells was found.

Fluid phase and receptor-mediated endocytosis in Paramecium primaurelia by fluorescence confocal laser scanning microscopy

In ciliated protozoa, most nutrients are internalized via phagocytosis by food vacuole formation at the posterior end of the buccal cavity. The uptake of small-sized molecules and external fluid through the plasma membrane is a localized process. That is because most of the cell surface is internally covered by an alveolar system and a fibrous epiplasm, so that only defined areas of the cell surface are potential substance uptake sites. The purpose of this study is to analyze, by fluorescence confocal laser scanning microscopy, the relationship between WGA (Triticum vulgaris agglutinin) and dextran internalization in Paramecium primaurelia cells blocked in the phagocytic process, so that markers could not be internalized via food vacuole formation. WGA, which binds to surface constituents of fixed and living cells, was used as a marker for membrane transport and dextran as a marker for fluid phase endocytosis. After 3 min incubation, WGA-FITC is found on plasma membrane and cilia, and successively within small cytoplasmic vesicles. After a 10-15 min chase in unlabeled medium, the marked vesicles decrease in number, increase in size and fuse with food vacuoles. This fusion was evidenced by labeling food vacuoles with BSA-Texas red. Dextran enters the cell via endocytic vesicles which first localize in the cortical region, under the plasma membrane, and then migrate in the cytoplasm and fuse with other endocytic vesicles and food vacuoles. When cells are fed with WGA-FITC and dextran-Texas red at the same time, two differently labeled vesicle populations are found. Cytosol acidification and incubation in sucrose medium or in chlorpromazine showed that WGA is internalized via clathrin vesicles, whereas fluid phase endocytosis is a clathrin-independent process.

Two-photon microscopy and spectroscopy based on a compact confocal scanning head

We have combined a confocal laser scanning head modified for TPE (two-photon excitation) microscopy with some spectroscopic modules to study single molecules and molecular aggregates. The behavior of the TPE microscope unit has been characterized by means of point spread function measurements and of the demonstration of its micropatterning abilities. One-photon and two-photon mode can be simply accomplished by switching from a mono-mode optical fiber (one-photon) coupled to conventional laser sources to an optical module that allows IR laser beam (two-photon/TPE) delivery to the confocal laser scanning head. We have then described the characterization of the two-photon microscope for spectroscopic applications: fluorescence correlation, lifetime and fluorescence polarization anisotropy measurements. We describe the measurement of the response of the two-photon microscope to the light polarization and discuss fluorescence polarization anisotropy measurements on Rhodamine 6G as a function of the viscosity and on a globular protein, the Beta-lactoglobulin B labeled with Alexa 532 at very high dilutions. The average rotational and translational diffusion coefficients measured with fluorescence polarization anisotropy and fluorescence correlation methods are in good agreement with the protein size, therefore validating the use of the microscope for two-photon spectroscopy on biomolecules.

Confocal microscopic study of GABA(A) receptors in Xenopus oocytes after rat brain mRNA injection: modulation by tyrosine kinase activity

The expression of GABA(A) receptors in Xenopus oocytes injected with rat brain mRNA was studied by immunocytochemistry and evaluation of the distribution of fluorescent probes at the confocal microscope. The beta(2/3) subunit distributed exclusively on the membrane at the animal pole of the oocytes. Treatment of oocytes for 20 min with the protein tyrosine kinase inhibitor genistein, 200 microM, resulted in a lower presence of GABA(A) receptors on the membrane. The inactive genistein analogue daidzein, 200 microM, had no effect even with a 30 min treatment. Alkaline phosphatase but not a protein tyrosine phosphatase, when injected into oocytes, reduced GABA(A) receptor membrane expression. The data indicate that protein tyrosine phosphorylation modulates the expression on the plasma membrane of presynthesized GABA(A) receptors.

GABA(A) receptor modulation in rat cerebellum granule cells

The inhibitory GABA(A) receptor is a key element in determining the pattern of nerve cell electrical activity. Thus, modulation of its function is of paramount impact in shaping neuronal functional activity under physiological and pathological conditions. This applies to cerebellar granule neurons as to all the other neurons in the brain. The culture of cerebellar granules from newborn rats is a convenient means by which to approach these cells for electrophysiological studies provided that they maintain, as far as GABA(A) receptors are concerned, the same characteristics as in situ. Thus, the regulation of GABA(A) receptor activity in these neurons has been studied by the patch-clamp technique, both in the whole-cell and outside-out configuration. An obvious first level of control of such receptors' activity is their desensitization under continued agonist application, with biphasic kinetics. The data do not allow one to conclude whether one is dealing with two different populations of receptors or with a single population with two desensitization phases; although the presence of two GABA(A) receptor populations is suggested by a host of observations. The granule cell GABA(A) receptors are modulated by changes in extracellular pH with lower pH resulting in an enhanced receptor activity. They display, under the conditions of whole-cell recording, a run-down phenomenon which is most probably due to a tyrosine phosphatase activity which is in turn under control by a protein serine kinase. Thus, in situ tyrosine phosphorylation is a key element in determining the efficiency of GABA mediated inhibition. Activation of protein kinase A or protein kinase G (PKG) down-regulates GABA(A) receptors' activity. This last event is involved in the depression of those receptors' activity by L-arginine via the production of nitric oxide. In addition, the activity of calmodulin-activated adenylate cyclase I is controlled by GABA(B) receptors. Dendritic GABA(A) receptor activity is partially blocked by previous activation of N-methyl-D-aspartate (NMDA) receptors via calcineurin mediated dephosphorylation/activation of protein tyrosine phosphatase and concomitant production of nitric oxide and PKG activation. The site phosphorylated by PKG is evidently not available for calcineurin-mediated serine dephosphorylation, due to calcineurin-specific membrane localization in respect of the GABA(A) receptor. Overall, a complex network of biochemical signals appear to keep granule cells GABA(A) receptors under a fine balance between up- and down-regulatory mechanisms. The overall data appear also to indicate the presence of two GABA(A) receptor populations: a dendritic one which can be modulated by Ca++ entering via NMDA receptors and a cell body one. The two populations are probably different in terms of desensitization kinetics and benzodiazepine sensitivity.

Modulation by lanthanum ions of gamma-aminobutyric acid(A) receptors of rat cerebellum granule cells in culture: clues on their subunit composition

Gamma-aminobutyric acid (GABA) activated chloride currents were studied in rat cerebellum granule cells in culture by the whole cell patch-clamp technique. Both the peak and steady state currents were inhibited by 100 microM lanthanum. In the first case, inhibition is due to an increase of the EC50 for GABA. The inhibitory effect of lanthanum on the peak current at 3 microM GABA increased with the cation concentration. A tendency towards the same behavior was found also for the inhibition of the steady state current, at 3 microM GABA, as a function of lanthanum concentration, although inhibition in this case was lower. The comparison of the results with published data about the effects of lanthanum on recombinant GABA(A) receptors likely to occur in granule cells allows suggestions about the receptor types giving, respectively, the peak and the steady state component.

Single-pinhole confocal imaging of sub-resolution sparse objects using experimental point spread function and image restoration

Sparse fluorescent pointlike subresolution objects have been imaged using a diffraction limited single-pinhole confocal fluorescence microscope. A Maximum likelihood image restoration algorithm has been used in conjunction with a measure of the experimental point spread function for improving the three-dimensional imaging of subresolution sparse objects. The experimental point-spread-function profiles have been improved by a factor of 1.95 in lateral direction and 3.75 in axial direction resulting in full-width half maximum (FWHM) values of 91 +/- 11 nm and 160 +/- 26 nm. This amounts to 1. 43 and 2.15 in optical units, respectively. The lateral and axial FWHM of the sparse pointlike subresolution objects is about 5 and 3 times smaller than the wavelength. This result points to the attractive possibility of utilising a compact confocal architecture for localising punctuate fluorescent objects having subresolution dimensions. The key resides in the utilisation of the measured point spread function coupled to an appropriate image restoration approach, and, of course, in the stability of the confocal system being used.

Apoptotic cell death and impairment of L-type voltage-sensitive calcium channel activity in rat cerebellar granule cells treated with the prion protein fragment 106-126

Prion diseases are neurodegenerative pathologies characterized by the accumulation, in the brain, of altered forms of the prion protein (PrP), named PrP(Sc). A synthetic peptide homologous to residues 106-126 of PrP (PrP106-126) was reported to maintain the neurodegenerative characteristics of PrP(Sc). We investigated the intracellular mechanisms involved in PrP106-126-dependent degeneration of primary cultures of cerebellar granule neurons. Prolonged exposure of such neurons to PrP106-126 induced apoptotic cell death. The L-type voltage-sensitive calcium channel blocker nicardipine reproduced this effect, suggesting that blockade of Ca(2+) entry through this class of calcium channels may be responsible for the granule cell degeneration. Microfluorometric analysis showed that PrP106-126 caused a reduction in cytosolic calcium levels, elicited by depolarizing K(+) concentrations in these neurons. Electrophysiological studies demonstrated that PrP106-126 and nicardipine selectively reduce the L-type calcium channel current. These data demonstrate that PrP106-126 alters the activity of L-type voltage-sensitive calcium channels in rat cerebellar granule cells and suggest that this phenomenon is related to the cell death induced by the peptide.

Modulation by extracellular pH of GABAA receptors expressed in Xenopus oocytes injected with rat brain mRNA

Rat brain poly(A)(+) mRNA was injected into Xenopus oocytes. After 72-96 hr, GABA(A) receptors expressed in this heterologous system were studied by perfusion of GABA and recording of GABA evoked chloride current under voltage-clamp conditions. The GABA activated currents were blocked by bicuculline and enhanced by flunitrazepam. Acidic (6.4) extracellular pH (pH(e) ) augmented, whereas basic pH (8.4) decreased the current evoked by 100 microM GABA in the respect of the current evoked at pH 7.4. Concentration-response curves for GABA evoked chloride currents were built at the three pHs. These data showed that acidic pH does not change the EC50 for GABA but it increases significantly I(max) in comparison to pH 7.4. At pH 8.4 there was a significant decrease of EC50 for GABA. However, there was also a very strong decrease of I(max), so that the overall effect at 100 microM GABA was a decrease of GABA activated chloride current in the respect of the one activated at neutral pH. These data may indicate that on average brain GABA(A) receptors are positively modulated by extracellular acidosis. The opposite may occur in extracellular alcalosis.

Two-photon excitation of fluorescence for three-dimensional optical imaging of biological structures

Techniques based on two-photon excitation (TPE) allow three-dimensional (3D) imaging in highly localized volumes, of the order of magnitude of a fraction of a femtolitre up to single-molecule detection. In TPE microscopy a fundamental advantage over conventional widefield or confocal 3D fluorescence microscopy is given by the use of infrared (IR) instead of ultraviolet (UV) radiation to excite those fluorophores requiring UV excitation, hence causing little damage to the specimen or to fluorescent molecules outside the volume of the TPE event and allowing a deeper penetration within the sample compared with conventional one-photon excitation of fluorescence. In our laboratory, within the framework of a national INFM project, we have realized a TPE fluorescence microscope, part of a multipurpose architecture also including lifetime imaging and fluorescence correlation spectroscopy modules. The core of the architecture is a mode-locked Ti:sapphire infrared pulsed laser pumped by a high-power (5 W, 532 nm) solid-state laser and coupled to an ultracompact scanning head. For the source we have measured a pulse width from 65 to 95 fs as a function of wavelength (690-830 nm). The scanning head allows conventional and two-photon confocal imaging. Point spread function measurements are reported with examples of applications to the study of biological systems.

Changes in the endoplasmic reticulum structure of Paramecium primaurelia in relation to different cellular physiological states

The fluorochrome 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)], a vital dye utilized to stain the endoplasmic reticulum (ER) of animal and plant cells, has been used to visualize the ER-type structures of Paramecium primaurelia under confocal laser scanning microscopy (CLSM). The morphology of the ER has been studied in paramecia in different physiological conditions. Cells are analysed in early and late logarithmic growth phases, in stationary and in death phases, during shift-up by refeeding after starvation and shift-down by using a starvation medium. In log-phase growing paramecia, the ER constitutes an anastomosing membrane system consisting of short tubules and flattened sacs forming a peripheral network, which is abundant in the cortical region around the trichocysts and the ciliary basal bodies. The tubular network and cytoplasmic membranes are reduced in stationary-phase cells; the original conditions are restored in starved cells after refeeding. The analysis of serial optical sections collected by CLSM at 0.5 microm intervals and three-dimensional reconstruction from these sections allow us to visualize differences between differently growing cells.

Evidence of two populations of GABA(A) receptors in cerebellar granule cells in culture: different desensitization kinetics, pharmacology, serine/threonine kinase sensitivity, and localization

GABA(A) receptors of rat cerebellar granule cells in culture have been studied by the whole cell patch clamp technique. The biphasic desensitization kinetic observed could be due either to different desensitization mechanisms of a single receptor population or to different receptor populations. The overall data indicate that the latter hypothesis is most probably the correct one. In fact, the fast desensitizing component was selectively potentiated by a benzodiazepine agonist and preferentially down-regulated by activation of the protein serine/threonine kinases A and G, as a consequence of the latter characteristic that receptor population was preferentially down-regulated by previous activation of N-methyl-d-aspartate glutamate receptors, via production of nitric oxide and PKG activation, most probably in dendrites. The other population is benzodiazepine insensitive and not influenced by activation of PKA or PKG. This slowly desensitizing population may correspond to the extrasynaptic delta subunit containing GABA(A) receptors described by other authors. Instead, the rapidly desensitizing population appears to represent dendritic synaptic GABA(A) receptors.

Adapting a compact confocal microscope system to a two-photon excitation fluorescence imaging architecture

Within the framework of a national National Institute of Physics of Matter (INFM) project, we have realised a two-photon excitation (TPE) fluorescence microscope based on a new generation commercial confocal scanning head. The core of the architecture is a mode-locked Ti:Sapphire laser (Tsunami 3960, Spectra Physics Inc., Mountain View, CA) pumped by a high-power (5 W, 532 nm) laser (Millennia V, Spectra Physics Inc.) and an ultracompact confocal scanning head, Nikon PCM2000 (Nikon Instruments, Florence, Italy) using a single-pinhole design. Three-dimensional point-spread function has been measured to define spatial resolution performances. The TPE microscope has been used with a wide range of excitable fluorescent molecules (DAPI, Fura-2, Indo-1, DiOC(6)(3), fluoresceine, Texas red) covering a single photon spectral range from UV to green. An example is reported on 3D imaging of the helical structure of the sperm head of the Octopus Eledone cirrhosa labelled with an UV excitable dye, i.e., DAPI. The system can be easily switched for operating both in conventional and two-photon mode.

GABA(B) receptor activation protects GABA(A) receptor from cyclic AMP-dependent down-regulation in rat cerebellar granule cells

Interaction between GABAA and GABA(B) receptors was studied in rat cerebellar granule cells in culture, by the whole-cell patch-clamp approach. Our data show that the GABA(B) agonist (-)baclofen is not able, per se, to significantly change the muscimol-activated chloride current. However, (-)baclofen dose-dependently prevents the reduction of GABA(A) receptor function by forskolin, an activator of adenylate cyclase. The effect of baclofen is mediated by a pertussis toxin-sensitive G protein. In fact, in cells treated with pertussis toxin, baclofen and forskolin, the toxin is able to block baclofen action, allowing forskolin to act fully. The protective effect by GABA(B) receptor activation under these circumstances is most probably related to the prevention of cyclic AMP increases after forskolin treatment. In fact, in these neurons cyclic AMP and protein kinase A activation result in a down-regulation of GABA(A) receptor function. On the whole, the data indicate the presence of complex modulation of GABA(A) receptors by GABA(B) receptor types in cerebellum granule cells.

Cerebellar granule cell GABA(A) receptors studied at the single-channel level: modulation by protein kinase G

Rat cerebellar granule cells GABA(A) receptors were studied at the single-channel level in outside-out patches. Three conductance levels were detected as activated by 0.1 microM GABA: 11, 20 and 30 pS. Single-channel I-V relationships were linear. The probability of opening did not vary over time within single patches. Kinetic analysis brought to a mean open time constant of 3.2, 2.9 and 2.8 ms respectively for each conductance level and a closed time histogram fitted by the sum of two exponential functions (tau c1 = 2.1 ms, 43%; tau c2 = 18.2 ms, 57%). Protein kinase G (PKG) activation did not affect single-channel conductances, but resulted in a reduction over time of single-channel open probability for all the conductance levels. Kinetically, protein kinase G modified the mean open time constants and the relative areas of the two components of the closed state distribution whereas the mean closed time constants remained unaffected. These results confirm and add details about cerebellar granule GABA(A) receptors down regulation by PKG.

Prion protein fragment 106-126 induces apoptotic cell death and impairment of L-type voltage-sensitive calcium channel activity in the GH3 cell line

The prion diseases are transmissible neurodegenerative pathologies characterized by the accumulation of altered forms of the prion protein (PrP), termed PrP(Sc), in the brain. Previous studies have shown that a synthetic peptide homologous to residues 106-126 of PrP (PrP 106-126) maintains many characteristics of PrP(Sc), i.e., the ability to form amyloid fibrils and to induce apoptosis in neurons. We have investigated the intracellular mechanisms involved in the cellular degeneration induced by PrP 106-126, using the GH3 cells as a model of excitable cells. When assayed in serum-deprived conditions (48 hr), PrP 106-126 (50 microM) induced cell death time-dependently, and this process showed the characteristics of the apoptosis. This effect was specific because a peptide with a scrambled sequence of PrP 106-126 was not effective. Then we performed microfluorimetric analysis of single cells to monitor intracellular calcium concentrations and showed that PrP 106-126 caused a complete blockade of the increase in the cytosolic calcium levels induced by K+ (40 mM) depolarization. Conversely, the scrambled peptide was ineffective. The L-type voltage-sensitive calcium channel blocker nicardipine (1 microM) also induced apoptosis in GH3 cells, suggesting that the blockade of Ca2+ entry through this class of calcium channels may cause GH3 apoptotic cell death. We thus analyzed, by means of electrophysiological studies, whether Prp 106-126 modulate L-type calcium channels activity and demonstrated that the apoptotic effect of PrP 106-126 was due to a dose-dependent inactivation of the L-type calcium channels. These data demonstrate that the prion protein fragment 106-126 induces a GH3 apoptotic cell death inducing a selective inhibition of the activity of the L-type voltage-sensitive calcium channels.

Involvement of phosphatase activities in the run-down of GABA(A) receptor function in rat cerebellar granule cells in culture

Run-down of GABA activated Cl- currents was found when rat cerebellar granule cells in culture were studied by the whole-cell patch-clamp technique in the absence of ATP in the pipette medium. This event could be prevented, even in the absence of ATP, by using the perforated-patch technique or by adding to the pipette medium either a blocker of protein tyrosine phosphatase, sodium vanadate, or deltamethrin, a blocker of the protein serine/threonine phosphatase calcineurin. Conversely, run-down could be partially induced, even in the presence of ATP, by blockers of tyrosine kinases. A reduction of GABA(A) receptor activity was also found in outside-out membrane patches when ATP was not on the membrane inside. The run-down phenomenon involved all three conductance levels found in these patches: 11, 20 and 30 pS. In all three cases it was due to a reduction of channels' open probability. The single-channel experiments showed that also in this case run-down was prevented by either sodium vanadate or deltamethrin on the membrane cytoplasmic side. Overall, through relatively unphysiological conditions (cells in culture and patch-clamp techniques), the study of the run-down phenomenon shows that the tyrosine phosphorylation state of GABA(A) receptors is of importance in maintaining it in a proper functional state. The data also show that tyrosine phosphorylation state is controlled by a protein tyrosine phosphatase, whose activity in turn is blocked via serine/threonine phosphorylation.

Effect of nitric oxide donors on GABA uptake by rat brain synaptosomes

The effect of nitric oxide donors and L-arginine on the uptake of GABA was studied in synaptosomes purified from rat brain. The neurotransmitter uptake was significantly reduced by S-nitrosoacetylpenicillamine and by sodium nitroprusside, although in this case to a lesser extent. A slight inhibitory effect was found preincubating rat brain synaptosomes with 1 mM L-arginine as well. The S-nitrosoacetylpenicillamine effect gradually disappeared with decomposition of the substance by exposure to light. The nitric oxide effect appears to be mainly due to a decrease in the V for synaptosomal GABA uptake and seems to be related to a partial collapse of nerve endings ionic gradients. Functionally, it could result over time in a reduced availability of GABA at the synapses involved.

A dual mechanism for impairment of GABAA receptor activity by NMDA receptor activation in rat cerebellum granule cells

The function of the GABAA receptor has been studied using the whole cell voltage clamp recording technique in rat cerebellum granule cells in culture. Activation of NMDA-type glutamate receptors causes a reduction in the effect of GABA. Full GABAA receptor activity was recovered after washing out NMDA and NMDA action was prevented in a Mg+2 containing medium. The NMDA effect was also absent when extracellular Ca+2 was replaced by Ba+2 and when 10 mM Bapta was present in the intracellular solution. Charge accumulations via voltage activated Ca+2 channels greater than the ones via NMDA receptors do not cause any reduction in GABAA receptor function, suggesting that Ca+2 influx through NMDA receptor channels is critical for the effect. The NMDA effect was reduced by including adenosine-5'-O-3-thiophosphate (ATP-gamma-S) in the internal solution and there was a reduction in the NMDA effect caused by deltamethrin, a calcineurin inhibitor. Part of the NMDA induced GABAA receptor impairment was prevented by prior treatment with L-arginine. Analogously, part of the NMDA effect was prevented by blockage of NO-synthase activity by N omega-nitro-L-arginine. A combination of NO-synthase and calcineurin inhibitors completely eliminated the NMDA action. An analogous result was obtained by combining the NO-synthase inhibitor with the addition of ATP-gamma-S to the pipette medium. The additivity of the prevention of the NMDA impairment of GABAA receptor by blocking the L-arginine/NO pathway and inhibiting calcineurin activity suggests an independent involvement of these two pathways in the interaction between NMDA and the GABAA receptor. On the one hand Ca+2 influx across NMDA channels activates calcineurin and dephosphorylates the GABAA receptor complex directly or dephosphorylates proteins critical for the function of the receptor. On the other hand, Ca+2 influx activates NO-synthase and induces nitric oxide production, which regulates such receptors via protein kinase G activity.

Nitric oxide and GABAA receptor function in the rat cerebral cortex and cerebellar granule cells

The aim of the present work was to investigate the mechanism by which the diffusible factor nitric oxide regulates GABAA receptor function in the brain. The effect of nitric oxide on GABAA receptor function has been studied in two different neuronal preparations: rat cerebral cortex microsacs and rat cerebellum granule cells in culture. In the first case, GABA-stimulated 36Cl-accumulation was studied as an index of GABAA receptor function. The maximal rate of GABA-stimulated 36Cl- accumulation (Vmax) was reduced by treatment of microsacs with nitric oxide chemical donors such as sodium nitroprusside (-26%) and S-nitroso-acetyl-penicillamine (-11%). The greater effect of the former agent is due to an additional interference by its breakdown products. The biochemical precursor L-arginine (1 mM) produced the same Vmax decrease as S-nitroso-acetyl-penicillamine. This effect was reversed by a nitric oxide synthase blocker and appears truly nitric oxide mediated. The action of nitric oxide in this system does not seem to imply cyclic GMP formation. GABAA receptor function was studied by whole-cell patch-clamp in rat cerebellum granule cells in culture. In this case, L-arginine (100 microM) profoundly reduced the Cl- current elicited by 10 microM GABA and its effect subsided following washing out. The effect of L-arginine was observed almost exclusively on the rapidly desensitizing component of the GABA-activated current. The action of L-arginine was blocked by a protein kinase G inhibitor and mimicked by its activators. Thus, it appears that this effect in these cells involves nitric oxide formation, cyclic GMP accumulation and protein kinase G-catalysed phosphorylation of GABAA receptor.

Valinomycin acts as a channel in ultrathin lipid membranes

When the thickness of monolayer membranes formed by bolaform archaeal lipids is reduced to the approximate length of two valinomycin molecules, the zero-current conductance does not show any more a linear dependence on valinomycin concentration; instead, a quadratic behaviour is observed. This suggests that a dimer permeation pore is formed and therefore the conduction mechanism changes from carrier to channel.

Modulation by nitric oxide of rat brain GABAA receptors

The effect of nitric oxide (NO) on the function of GABAA receptors was studied in two different rat brain neuron populations. Cerebral cortex neuronal GABAA receptors were studied by preparing microsacs and evaluating 36Cl- accumulation. Whether nitric oxide was provided by sodium nitroprusside (SNP) or by the metabolic precursor precursor arginine there was a 15-25% reduction in the Vmax for GABA-stimulated 36Cl- accumulation. The arginine effect could be reversed by the NO synthase (NOS) inhibitor N omega-nitro-L-arginine. GABAA receptor mediated Cl- currents were studied in rat cerebellar granule cells by whole-cell patch clamp. S-Nitroso-N-acetylpenicillamine (SNAP), sodium nitroprusside and L-arginine reduced the Cl- current elicited by 10 microM GABA. The L-arginine effect was reversible upon its washing out. This circumstance indicates that NO produced by endogenous NOS can inhibit GABAA receptor function in cerebellar granule cells.

Modulation by extracellular pH of the activity of GABAA receptors on rat cerebellum granule cells

The Cl- currents activated by GABA via GABAA receptors in rat cerebellum granule cells in culture were studied by whole-cell patch-clamp. These currents were measured at various extracellular pH. The currents activated by 100 microM GABA, both the peak and the steady-state component, increase at acidic pH's and decrease at basic pH's. The transition point being at around 7.7. Interestingly, passing from pH 7.4 to 6.4 the GABA dose-response curve indicates that the increases in the peak current are related to an augmented maximal current. The increases in the steady-state component are mainly due to a higher affinity of the receptors for the neurotransmitter and disappear at saturating [GABA]. The study of the I-V curves for the GABA activated peak Cl- currents at pH 6.4, 7.4 and 8.4 reveals linearity in the latter instance. However, an outward rectification is present at the two more acidic pH's. This fact suggests that the protonation of basic amino acids at the acid pH does involve rectification of Cl- channel conductance. Overall, the data indicate that slight changes in in situ extracellular pH may have profound influences on GABAA receptor function.

Inactivation of voltage-dependent calcium current in an insulinoma cell line

We have studied the mechanism of Ca current inactivation in the beta-cell line HIT-T15 by conventional and perforated patch recording techniques, using two pulse voltage protocols and a combination of current and tail current measurements. In 5 mM Ca, from a holding potential of -80 mV, the maximum current showed a complex time course of inactivation: a relatively fast, double exponential inactivation (tau h1 approximately 12 ms and tau h2 approximately 60 ms) and a very slowly inactivating component (tau > 1 s). The faster component (tau h1) was due to the voltage-dependent inactivation of a low-threshold-activated (LVA), T-type current, which deactivates more slowly (tau approximately 3-5 ms) than the other components (tau approximately 0.2-0.3 ms). The intermediate component (tau h2) was due to the Ca-dependent inactivation of a portion of the high-threshold-activated (HVA) current. A saturating dose of the dihydropyridine (DHP) nifedipine (10 microM) did not affect the LVA current, but inhibited by 68 +/- 5% the transient, Ca-sensitive portion of the HVA current and by 33 +/- 12% the long lasting component. We suggest that three components of the calcium current can be resolved in HIT cells and the main target of DHPs is a HVA current, which inactivates faster than the DHP-resistant HVA component and does so primarily through calcium influx.

Asymmetric black membranes formed by one monolayer of bipolar lipids at the air/water interface

In this work a new technique is presented for the formation of black lipid membranes from a single monolayer of bipolar lipids at the air/water interface. The lipid, extracted from the thermophilic archaeobacterium Sulfolobus solfataricus, is characterized by two different polar heads. The membrane is formed with a technique similar to that introduced by Montal and Mueller; however, the lipid is spread only on one side of the teflon partition. Conductance in the presence of valinomycin, voltage-dependent capacitance, current-voltage measurements and electroporation indicate that, as expected, the membrane is asymmetric.

Electric shock convulsions in the rabbit and brain cortex GABAA receptor function

The effect of electric shock convulsions (ESC) on the function of brain cortex GABAA receptors has been studied in the rabbit. Three single electroconvulsive shocks (ECS) were given at intervals of 48 hours and the brain cortex was sampled 36 hours after the last shock. The dose-response curve was determined for GABA-stimulated 36Cl-accumulation into brain cortex microsacs. The parameters of the curve (maximal accumulation rate, Ka and Hill coefficient, n) were constant when determined in two different series of experiences. Animals handled in the same way as the animals from the electric shock group but which did not receive the ECSs (sham ECS group) showed similar maximal accumulation rate and Ka. However, the average n coefficient was significantly higher in the electric shock group. Naive animals, taken from their cages just before the sacrifice, showed dose-response curves which varied from one experimental series to another. This last result (confirming previous observations) shows modifications and inconsistencies in the evaluation of GABAA receptor function in stressed handling-naive animals.

Electroporation in symmetric and asymmetric membranes

We present results of electrical measurements performed both on symmetric and asymmetric membranes in current-clamp conditions. The current-voltage characteristic curve of the membranes shows a reversible conductance transition to a higher level above a critical potential Vc. The experimental results are interpreted in the light of the electroporation theory, which allows estimates of the line tension to be made. These estimates are compared to previous experimental findings or theoretical calculations. The behaviour of symmetric membranes of different chain lengths or consisting of mixtures of short and long chains indicates a strong dependence of Vc on the chain composition and on the presence of charges on the polar head. The electroporation process is also analyzed in asymmetric bilayers consisting of a charged and an uncharged monolayer, a condition which mimics that of natural membranes. Therefore it is possible to analyze the electrical forces acting on the uncharged monolayer due to the presence of charges on the other one, under several ionic-strength conditions. It is shown that the instability arises in the uncharged monolayer, while the coupling between the two monolayers triggers the electroporation process.

Effect of protein kinase C activators on the uptake of GABA by rat brain synaptosomes

Synaptosomes were prepared from rat brain by a discontinuous Ficoll gradient method and used for studying the uptake of labelled GABA. Two GABA uptake components were evidenced, a high (Km = 3.13 microM) and a low (Km = 92.4 microM) affinity one. Preincubation of synaptosomes with two different activators of protein kinase C, phorbol 12, 13-diacetate (PDAc) and oleyl-acetyl glycerol (OAG), resulted in a change of GABA uptake. In particular, the low affinity component increased its Vmax by 58-74%, with no change in the Km. No statistically significant modification was detected for the high affinity component.

Regulation of GABAA receptor in cerebellar granule cells in culture: differential involvement of kinase activities

GABAA receptor function was studied in rat cerebellar granule cells in culture, by the whole-cell patch-clamp approach. The data show that GABA activates Cl- currents in these neurons which reverse at the appropriate membrane potential and are blocked by picrotoxin. The GABA-activated currents desensitize with time of application of the neurotransmitter at concentrations > or = 10(-6) M. The dose-response curve for the peak Cl- current gives a Ka value of 2.3 microM with a Hill coefficient of 1.2. The peak Cl- current elicited by GABA decreases with time of cell registration, with a time-constant of 7.3 min. Residual responsiveness though is maintained thereafter. This "run-down" phenomenon can be completely prevented by adding adenosine-5'-triphosphate + Mg2+ in the pipette solution. Treatments which directly (8-bromoadenosine-3',5'-cyclic-monophosphate; adenosine-3', 5'-cyclic-monophosphate) or indirectly (forskolin, isobutylmethylxanthine) increase the adenosine-3',5'-cyclic-monophosphate intracellular content reduce the GABA-induced Cl- current. Conversely, treatment with the protein kinase A and C inhibitor 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine potentiates the effect of GABA. On the whole, the data indicate that different protein kinase activities modulate the functional state of the GABAA receptors on granule cells from the rat cerebellum.

A transient outward current dependent on external calcium in rat cerebellar granule cells

The outward potassium current of rat cerebellar granule cells in culture was studied with the whole-cell patch-clamp method. Two voltage-dependent components were identified: a slow current, resembling the classical delayed rectifier current, and a fast component, similar to an IA-type current. The slow current was insensitive to 4-aminopyridine and independent of external Ca2+, but significantly inhibited by 3 mM tetraethylammonium. The fast current was depressed by external 4-aminopyridine, with an ED50 = 0.7 mM, and it was abolished by removal of divalent cations from the external medium. The sensitivity of the transient outward current to different divalent cations was investigated by equimolar substitution of Ca2+, Mn2+ and Mg2+. In 2.8 mM Mn2+, the transient potassium conductance was comparable to that in 2.8 mM Ca2+, while in 2.8 mM Mg2+ the transient component was drastically reduced, as in the absence of any divalent cations. However, when Ca2+ was present, Mg2+ up to 5 mM had no effect. The transient current increased with increasing concentrations of external Ca2+, [Ca2+]o, and the maximum conductance vs. [Ca2+]o curve could be approximated by a one-site model. In addition, the current recorded with 5.5 mM BAPTA in the intracellular solution was not different from that recorded in the absence of any Ca2+ buffer. These results suggest that divalent cations modulate the potassium channel interacting with a site on the external side of the cell membrane.

Voltage-dependent calcium currents in dissociated granule cells from rat cerebellum

Voltage-dependent calcium currents were investigated by the patch-clamp technique in whole-cell recording configuration in cultures from 8-day-old rat cerebella, which contained greater than or equal to 90% granule cells. In solutions designed to minimize the sodium and potassium conductances and in 20 mM barium, an inward current activated around -25 mV, reached a peak amplitude at +20 mV and reversed around +80 mV. In 20 mM calcium, this current was approximately 50% of that in barium. From one to three days in vitro only 16% of the cells tested (n = 20) had a current exceeding 50 pA in maximum amplitude, while after four days in vitro the current reached 100 pA in all neurons tested (n greater than 70). Verapamil (50-100 microM) reversibly depressed this current. The dihydropyridine agonist Bay K 8644 (1 microM) enhanced the maximum conductance by 25 +/- 8% (n = 4), caused a negative shift in the activation of 21 +/- 5 mV and a prolongation of the deactivation time course as the voltage was stepped back from +20 to -80 mV. The GABAB agonist baclofen (50 microM) reversibly depressed the current by 27 +/- 8% in 80% of the cells. The effect was similar to that of GABA (10 microM), when the GABAA response (chloride current) was partially blocked by bicucculine. This current can be classified as a dihydropyridine-sensitive high-voltage-activated calcium current. The modulation by GABAB agonists is likely to be significant for presynaptic inhibition.

Adsorption of gamma-aminobutyric acid to phosphatidylserine membranes

The interaction of the negatively-charged phosphatidylserine (PS) and gamma-Aminobutyric acid (GABA) is examined in black lipid membranes (BLM) and inverse micelles. GABA does not permeate through PS membranes and, in concentrations of 10(-5)-10(-4) M, it reduces the negative potential at the membrane-aqueous solution interface. The effect is owing to the adsorption of the GABA cationic species and the consequent decrease of the negative surface charge density of the membrane. When the intrinsic pH of the membrane-solution interface is considered, the Gouy-Chapman-Stern theory describes the GABA screening effect and makes it possible to calculate the GABA-PS binding constant. This value is compared with that obtained measuring the partition of 14C-GABA between an organic phase containing PS and the aqueous solution. The results presented strongly suggest that the electrostatic force plays a major role in GABA-PS interaction.

Guanosine-5'-O-(3-thiotriphosphate) modifies kinetics of voltage-dependent calcium current in chick sensory neurons

Internal perfusion with the G-protein activator guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma S) mimics the effect of noradrenaline and dopamine on the voltage-dependent calcium current in chick dorsal root ganglion (DRG) cells. With 100 microM GTP-gamma S in the pipette, the current at +10 mV was depressed by approximately 50%, with a 10-fold increase of its time to peak. The activation time course of the control calcium current could be approximated with a single exponential curve, whereas with GTP-gamma S the activation time course was double exponential, with time constants tau 1 and tau 2. 2 mM Mg-ATP in the pipette prevented the GTP-gamma S-induced current decrease in 70% of the cells, but the time course of the current was always double exponential. From -50 mV, the current at +10 mV was best fitted with tau 1 = 1.7 +/- 0.5 and tau 2 = 25.6 +/- 5.5 in seven cells. Both time constants decreased with increasing depolarizations. In the first 2 min of recording, the current changed with time. However, both tau 1 and tau 2 were constant, whereas the relative contribution of the slow component increased from 10 to 70%. In addition, the effect was independent of the holding potential in the range from -100 to -30 mV. These results suggest that the activation of a G-protein causes a fraction of the high-threshold calcium channels to switch to a new closed state, with slower opening kinetics.

A transient voltage-dependent outward current in cultured cerebellar granules

Granule cells were dissociated from rat cerebella with a procedure that yields a 98% pure cell population. Potassium currents in these cells were studied using the patch-clamp technique. Depolarizing pulses of 10 mV step and 100 ms duration from a holding potential of -80 mV elicited two different potassium outward currents: a transient, low-voltage activated component and a long lasting, high-voltage activated component. At +30 mV, the total current reached an amplitude of 2 nA (mean value of 15 experiments). The reversal potential of the transient current, estimated by measuring tail currents, was -77 mV, close to that predicted by the Nernst equation. The transient current was half inactivated with a holding potential of -78 mV and completely inactivated with -50 mV or more positive holding potentials. Finally, the current decay could be fitted by the sum of two exponentials with time constants of about 20 and 250 ms.

Conductance transition induced by an electric field in lipid bilayers

A cooperative phenomenon showing a structural change in the organization of bilayer lipid membranes at a critical value of the applied electric field is presented. The transition is characterized by a sharp increase in conductance. The phenomenon can be observed under current-clamp conditions (rather than the usual voltage-clamp conditions) to avoid rupturing the membrane. At a critical potential value the conductance increases and therefore the potential decreases to keep the current constant. Results refer to membranes made of egg phosphatidylcholine (PC), diphytanoylphosphatidylcholine and cholesterol/egg PC. It is found that the critical potential at which the transition occurs depends dramatically on pH and ionic concentration, indicating that the electrical properties of the external surface determine the major characteristics of such a transition.

Dependence of the conductance of the alpha-latrotoxin channel on applied potential and potassium concentration

alpha-Latrotoxin, with a molecular weight 130,000, is the main component of black widow spider venom, and acts at the presynaptical level, inducing a notable release of neurotransmitters in the synapses of all vertebrates. In artificial lipid membranes, this neurotoxin induces the formation of cation-selective ionic channels, whose conductance depends on the intensity and direction of the applied potential. In fact, also in the presence of symmetrical solutions of potassium chloride, the voltage-current characteristics of the single channel strongly rectify. Such rectification, which depends on the concentration of the ions in solution, can be described by a one-site, one-ion model for a channel. The data fit provides the values of the three parameters describing the model. Moreover, a statistical analysis of the amplitude of the single channel, as a function of the concentration of potassium chloride, has made it possible to verify the consistency of the model used.

Permeation of divalent cations through alpha-latrotoxin channels in lipid bilayers: steady-state current-voltage relationships

alpha-Latrotoxin, a polypeptide neurotoxin known to cause massive release of transmitter from vertebrate nerve terminals, is thought to act by forming cation-selective channels in plasma membranes. This paper describes the steady-state current carried by Ca2+, Sr2+ and Ba2+ through pores of alpha-LaTx molecules incorporated in artificial bilayer membranes made of neutral lipids. Even when the solutions separated by the membrane are identical, the I-V relations rectify strongly, the current being higher when the side to which the toxin is added is positive. The polarity of the rectification is consistent with the hypothesis that the mechanism of action of the toxin is, at least in part, that of promoting inwardly directed flow of cations, and thus, accumulation of Ca2+ and other ions in the intracellular spaces. The dependence of the I-V characteristics on voltage and Ca2+ concentration is well described by a one-site, one-ion model for a channel. Three parameters of the model are deduced: the binding constant of the site for Ca2+, K = 1.5 M-1 (or K = 7 M-1 when activities are used instead of concentrations); the "electrical" distance of the site from the toxin-containing solution, alpha = 0.3; the free energy difference between the two barrier peaks, delta F = 0.26 kT. The values of the parameters deduced by studying the channel in the presence of Ca2+ give theoretical curves that also fit the data with Sr2+ and Ba2+, indicating a low level of discrimination among these three cations.