Bimodal action of furosemide on convulsant [3H]EBOB binding to cerebellar and cortical GABA(A) receptors

Synthesis and Pharmacological Evaluation of [11C]4-Methoxy-N-[2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-yl]benzamide as a Brain Penetrant PET Ligand Selective for the δ-Subunit-Containing γ-Aminobutyric Acid Type A Receptors

The α_4/6βδ-containing GABA_A receptors are involved in a number of brain diseases. Despite the potential of a δ-selective imaging agent, no PET radioligand is currently available for in vivo imaging. Here, we report the characterization of DS2OMe (1) as a candidate radiotracer, ¹¹C-labeling, and subsequent evaluation of [¹¹C]DS2OMe in a domestic pig as a PET radioligand for visualization of the δ-containing GABA_A receptors.

Flufenamic acid prevents behavioral manifestations of salicylate-induced tinnitus in the rat

INTRODUCTION

Tinnitus is defined as a phantom auditory sensation, the perception of sound in the absence of external acoustic stimulation. Given that flufenamic acid (FFA) blocks TRPM2 cation channels, resulting in reduced neuronal excitability, we aimed to investigate whether FFA suppresses the behavioral manifestation of sodium salicylate (SSA)-induced tinnitus in rats.

MATERIAL AND METHODS

Tinnitus was evaluated using a conditioned lick suppression model of behavioral testing. Thirty-one Wistar rats, randomly divided into four treatment groups, were trained and tested in the behavioral experiment: (1) control group: DMSO + saline (n = 6), (2) SSA group: DMSO + SSA (n = 6), (3) FFA group: FFA (66 mg/kg bw) + saline (n = 9), (4) FFA + SSA group: FFA (66 mg/kg bw) + SSA (400 mg/kg bw) (n = 10). Localization of TRPM2 to the plasma membrane of cochlear nucleus neurons was demonstrated by confocal microscopy.

RESULTS

Pavlovian training resulted in strong suppression of licking, having a mean value of 0.05 ±0.03 on extinction day 1, which is below the suppression training criterion level of 0.20 in control tinnitus animals. The suppression rate for rats having both FFA (66 mg/kg bw) and SSA (400 mg/kg bw) injections was significantly lower than that for the rats having SSA injections (p < 0.01).

CONCLUSIONS

We suggest that SSA-induced tinnitus could possibly be prevented by administration of a TRPM2 ion channel antagonist, FFA at 66 mg/kg bw.

Modifications of diflunisal and meclofenamate carboxyl groups affect their allosteric effects on GABAA receptor ligand binding

Gamma-aminobutyric acid type A receptors (GABAAR) are allosterically modulated by the nonsteroidal anti-inflammatory drugs diflunisal and fenamates. The carboxyl group of these compounds is charged at physiological pH and therefore penetration of the compounds into the brain is low. In the present study we have transformed the carboxyl group of diflunisal and meclofenamate into non-ionizable functional groups and analyzed the effects of the modifications on stimulation of [(3)H]muscimol binding and on potentiation of γ-aminobutyric acid-induced displacement of 4'-ethenyl-4-n-[2,3-(3)H]propylbicycloorthobenzoate. N-Butylamide derivative of diflunisal modulated radioligand binding with equal or higher potency than the parent compound, while diflunisalamide showed reduced allosteric effect as compared to diflunisal. Amide derivative of meclofenamate equally affected radioligand binding parameters, while both diflunisal and meclofenamate methyl esters were less active than the parent compounds. Our study clearly demonstrates that an intact carboxyl group in diflunisal and meclofenamate is not indispensable for their positive GABAAR modulation. Further derivatization of the compound might yield compounds with higher selectivity for GABAARs that could be utilized in drug development.

Flufenamic acid suppresses epileptiform activity in hippocampus by reducing excitatory synaptic transmission and neuronal excitability

PURPOSE

In this study, we explore the antiepileptic effects of flufenamic acid (FFA) in order to identify the cellular mechanisms that underlie the potential anticonvulsant properties of this nonsteroidal antiinflammatory compound.

METHODS

The mechanisms of FFA action were analyzed using an in vitro model in which epileptiform activity was induced in hippocampal slices by perfusion with 100 microm 4-aminopyridine (4-AP) added to a modified Mg(2+)-free solution. The activity of CA1 pyramidal neurons as well as the synaptic connection between CA3 and CA1 was monitored using extracellular and patch-clamp recordings.

RESULTS

Epileptiform activity was suppressed in hippocampal neurons by FFA at concentrations between 50 and 200 microm. Glutamatergic excitatory synaptic transmission was diminished by FFA without modifying recurrent gamma-aminobutyric acid (GABA)ergic synaptic inhibition. Several lines of evidence indicated that FFA did not decrease neurotransmitter release probability, implicating a postsynaptic mechanism of action. FFA also potently reduced neuronal excitability, but did not alter the amplitude, duration, or undershoot of action potentials.

CONCLUSIONS

Our results suggest that FFA exerts an anticonvulsive effect on hippocampal pyramidal neurons by simultaneously decreasing glutamatergic excitatory synaptic activity and reducing neuronal excitability. Therefore, our study provides experimental evidence that FFA may represent an effective pharmacologic agent in the treatment of epilepsy in the mammalian central nervous system.

Properties of gap junction blockers and their behavioural, cognitive and electrophysiological effects: animal and human studies

Gap junctions play an important role in brain physiology. They synchronize neuronal activity and connect glial cells participating in the regulation of brain metabolism and homeostasis. Gap junction blockers (GJBs) include various chemicals that impair gap junction communication, disrupt oscillatory neuronal activity over a wide range of frequencies, and decrease epileptic discharges. The behavioural and clinical effects of GJBs suggest that gap junctions can be involved in the regulation of locomotor activity, arousal, memory, and breathing. Severe neuropsychiatric side effects suggest the involvement of gap junctions in mechanisms of consciousness. Unfortunately, the available GJBs are not selective and can bind to targets other than gap junctions. Other problems in behavioural studies include the possible adverse effects of GJBs, for example, retinal toxicity and hearing disturbances, changes in blood-brain transport, and the metabolism of other drugs. Therefore, it is necessary to design experiments properly to avoid false, misleading or uninterpretable results. We review the pharmacological properties and electrophysiological, behavioural and cognitive effects of the available gap junction blockers, such as carbenoxolone, glycyrrhetinic acid, quinine, quinidine, mefloquine, heptanol, octanol, anandamide, fenamates, 2-APB, several anaesthetics, retinoic acid, oleamide, spermine, aminosulfonates, and sodium propionate. It is concluded that despite a number of different problems, the currently used gap junction blockers could be useful tools in pharmacology and neuroscience.

Multiple functions of cation-chloride cotransporters in the fish retina

A GABA- or glycine-induced increase in Cl(-) permeability can produce either a depolarization or hyperpolarization, depending on the Cl(-) equilibrium potential. It has been shown that retinal neurons express the chloride cotransporters, Na-K-2Cl (NKCC) and K-Cl (KCC), the primary molecular mechanisms that control the intracellular Cl(-) concentration. We thus studied (1) the localization of these cotransporters in the fish retina, and (2) how suppression of cotransporter activity in the fish retina affects function. Specific antibodies against NKCC and KCC2 revealed that both cotransporters were expressed in the outer and inner plexiform layers, and colocalized in many putative amacrine cells and in cells of the ganglion cell layer. However, the somata of putative horizontal cells displayed only NKCC immunoreactivity and many bipolar cells were only immunopositive for KCC2. In the outer retina, application of bumetanide, a specific inhibitor of NKCC activity, (1) increased the steady-state extracellular concentration of K+ ([K+](o)) and enhanced the light-induced decrease in the [K+](o), (2) increased the sPIII photoreceptor-dependent component of the ERG, and (3) reduced the extracellular space volume. In contrast, in the outer retina, application of furosemide, a specific inhibitor of KCC activity, decreased sPIII and the light-induced reduction in [K+](o), but had little effect on steady-state [K+](o). In the inner retina, bumetanide increased the sustained component of the light-induced increase in [K+](o). These findings thus indicate that NKCC and KCC2 control the [K+](o) and extracellular space volume in the retina in addition to regulating GABA- and glycine-mediated synaptic transmission. In addition, the anatomical and electrophysiological results together suggest that all of the major neuronal types in the fish retina are influenced by chloride cotransporter activity.

Mapping convulsants' binding to the GABA-A receptor chloride ionophore: a proposed model for channel binding sites

Gamma-aminobutyric acid (GABA) type A receptors play a key role in brain inhibitory neurotransmission, and are ligand-activated chloride channels blocked by numerous convulsant ligands. Here we summarize data on binding of picrotoxin, tetrazoles, beta-lactams, bicyclophosphates, butyrolactones and neurotoxic pesticides to GABA-A ionophore, and discuss functional and structural overlapping of their binding sites. The paper reviews data on convulsants' binding sensitivity to different point mutations in ionophore-lining second trans-membrane domains of GABA-A subunits, and maps possible location of convulsants' sites within the chloride ionophore. We also discuss data on inhibition of glycine, glutamate, serotonin (5-HT3) and N-acetylcholine receptors by GABA-A channel blockers, and examine the applicability of this model to other homologous ionotropic receptors. Positioning various convulsant-binding sites within ionophore of GABA-A receptors, this model enables a better understanding of complex architectonics of ionotropic receptors, and may be used for developing new channel-modulating drugs.

Dendritic compartmentalization of chloride cotransporters underlies directional responses of starburst amacrine cells in retina

The mechanisms in the retina that generate light responses selective for the direction of image motion remain unresolved. Recent evidence indicates that directionally selective light responses occur first in the retina in the dendrites of an interneuron, i.e., the starburst amacrine cell, and that these responses are highly sensitive to the activity of Na-K-2Cl (NKCC) and K-Cl (KCC), two types of chloride cotransporter that determine whether the neurotransmitter GABA depolarizes or hyperpolarizes neurons, respectively. We show here that selective blockade of the NKCC2 and KCC2 cotransporters located on starburst dendrites consistently hyperpolarized and depolarized the starburst cells, respectively, and greatly reduced or eliminated their directionally selective light responses. By mapping NKCC2 and KCC2 antibody staining on these dendrites, we further show that NKCC2 and KCC2 are preferentially located in the proximal and distal dendritic compartments, respectively. Finally, measurements of the GABA reversal potential in different starburst dendritic compartments indicate that the GABA reversal potential at the distal dendrite is more hyperpolarized than at the proximal dendrite due to KCC2 activity. These results thus demonstrate that the differential distribution of NKCC2 on the proximal dendrites and KCC2 on the distal dendrites of starburst cells results in a GABA-evoked depolarization and hyperpolarization at the NKCC2 and KCC2 compartments, respectively, and underlies the directionally selective light responses of the dendrites. The functional compartmentalization of interneuron dendrites may be an important means by which the nervous system encodes complex information at the subcellular level.

Allosteric modulation of [3H]EBOB binding to GABAA receptors by diflunisal analogues

Allosteric modulatory effects of 12 biphenyl derivatives of diflunisal and two fenamates were studied on A-type receptors of GABA (GABAAR) via [3H]4'-ethynylbicycloorthobenzoate (EBOB) binding to synaptic membrane preparations of rat forebrain. A simplified ternary allosteric model was used to determine binding affinities of the compounds and the extents of cooperativity with GABA. Structure activity analysis revealed that 4-hydroxy substituents of the biphenyls contribute to their micromolar binding affinities more than 3-carboxyl groups. Electron-withdrawing fluorinated substituents, especially in ortho position, were also advantageous. These factors also strongly enhanced the cooperativity with GABA binding. The correlation between displacing potency of the allosteric agents and cooperativity with GABA suggests that these processes are associated with common mechanisms. The pharmacological relevance of these interactions is discussed. These data help to differentiate the structural requirements of these agents to act on GABAergic neurotransmission versus nonsteroidal anti-inflammatory effects.

Novel 17β-Substituted Conformationally Constrained Neurosteroids that Modulate GABAA Receptors

The goal of this study was to develop a series of allopregnanolone analogues substituted by conformationally constrained 17beta side chains to obtain additional information about the structure-activity relationship of 5alpha-reduced steroids to modulate GABA(A) receptors. Specifically, we introduced alkynyl-substituted 17beta side chains in which the triple bond is either directly attached to the 17beta-position or to the 21-position of the steroid skeleton. Furthermore, we investigated the effects of C22 and C20 modification. The in vitro binding affinity for the GABA(A) receptor of the new analogues was measured by allosteric displacement of the specific binding of [(3)H]4'-ethynyl-4-n-propyl-bicycloorthobenzoate (EBOB) to GABA(A) receptors on synaptosomal membranes of rat cerebellum. An allosteric binding model that has been successfully applied to ionotropic glycine receptors was employed. The most active derivative is (20R)-17beta-(1-hydroxy-2,3-butadienyl)-5alpha-androstane-3-ol (20), which possesses low nanomolar potency to modulate cerebellar GABA(A) receptors and is 71 times more active than the control compound allopregnanolone. Theoretical conformational analysis was employed in an attempt to correlate the in vitro results with the active conformations of the most potent of the new analogues.

Nanomolar allopregnanolone potentiates rat cerebellar GABAA receptors

The ionophore function of gamma-aminobutyric acid A (GABA(A)) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebellar cortex. Chloride currents elicited by 1 microM GABA were potentiated by allopregnanolone with a plateau of high affinity (EC(50) = 14 nM) and a peak of potentiation around 1 microM allopregnanolone. Furosemide (0.1 mM) eliminated the high affinity phase and increased the EC(50) to 685 nM. GABA(A) receptors of rat cerebellar synaptosomal membranes were labelled with [(3)H]ethynylbicycloorthobenzoate (EBOB). Allopregnanolone displaced [(3)H]EBOB binding with IC(50) = 320 nM. The displacing potency of allopregnanolone was strongly enhanced (IC(50) = 39 nM) in the presence of 400 nM GABA and 60 nM SR 95531. Nanomolar potentiation by allopregnanolone can be associated with cerebellar GABA(A) receptors containing alpha(6), beta(2-3) and delta subunits. This might be suitable for physiological modulation of tonic inhibitory neurotransmission via extrasynaptic GABA(A) receptors in cerebellar granule cells by neurosteroids.

Flufenamic acid is a pH-dependent antagonist of TRPM2 channels

Like a number of other TRP channels, TRPM2 is a Ca(2+)-permeable non-selective cation channel, the activity of which is regulated by intracellular and extracellular Ca(2+). A unique feature of TRPM2 is its activation by ADP-ribose and chemical species that arise during oxidative stress, for example, NAD(+) and H(2)O(2). These properties have lead to proposals that this channel may play a role in the cell death produced by pathological redox states. The lack of known antagonists of this channel have made these hypotheses difficult to test. Here, we demonstrate, using patch clamp electrophysiology, that the non-steroidal anti-inflammatory compound flufenamic acid (FFA) inhibits recombinant human TRPM2 (hTRPM2) as well as currents activated by intracellular ADP-ribose in the CRI-G1 rat insulinoma cell line. All concentrations tested in a range from 50 to 1000 microM produced complete inhibition of the TRPM2-mediated current. Following FFA removal, a small (typically 10-15%) component of current was rapidly recovered (time constant approximately 3 s), considerably longer periods in the absence of FFA produced no further current recovery. Reapplication of FFA re-antagonised the recovered current and subsequent FFA washout produced recovery of only a small percentage of the reblocked current. Decreasing extracellular pH accelerated FFA inhibition of TRPM2. Additional experiments indicated hTRPM2 activation was required for FFA antagonism to occur and that the generation of irreversible antagonism was preceded by a reversible component of block. FFA inhibition could not be induced by intracellular application of FFA. ADP-ribose activated currents in the rat insulinoma cell line CRI-G1 were also antagonised by FFA with concentration- and pH-dependent kinetics. In contrast to the observations made with hTRPM2, antagonism of ADP-ribose activated currents in CRI-G1 cells could be fully reversed following FFA removal. These experiments suggest that FFA may be a useful tool antagonist for studies of TRPM2 function.

Cation--chloride cotransporters mediate neural computation in the retina

The ability of directionally selective (DS) retinal ganglion cells to respond selectively to stimulus motion in one direction is a classic unresolved example of computation in a local neural circuit. Recent evidence indicates that DS responses occur first in the retina in the dendrites of starburst amacrine cells (interneurons presynaptic to the ganglion cells). We report that the directional responses of starburst-cell dendrites and DS ganglion cells are highly sensitive to the polarity of the transmembrane chloride gradient. Reducing the transmembrane chloride gradient by ion substitution or by blocking the K-Cl cotransporter resulted in the starburst cells responding equally to light moving in opposite directions. Conversely, increasing the chloride gradient by blocking the Na-K-Cl cotransporter eliminated responses to light moving in either direction. Moreover, in each case, blocking the chloride cotransporters or reducing the transmembrane chloride gradient eliminated the directional responses of DS ganglion cells in a manner opposite that of the starburst cells. These results indicate that chloride cotransporters play a key role in the generation of direction selectivity and that the directional responses of starburst cells and DS ganglion cells are exquisitely sensitive to the chloride equilibrium potential. The findings further suggest that the directional responses of DS ganglion cells are mediated in part by the directional release of gamma-aminobutyric acid from starburst dendrites and that the asymmetric distribution of the two cotransporters along starburst-cell dendrites mediates direction selectivity. A model of direction selectivity in the retina that incorporates these and other findings is discussed.

Receptor subtype-dependent positive and negative modulation of GABA(A) receptor function by niflumic acid, a nonsteroidal anti-inflammatory drug

In addition to blocking cyclooxygenases, members of the fenamate group of nonsteroidal anti-inflammatory drugs have been proposed to affect brain GABAA receptors. Using quantitative autoradiography with GABAA receptor-associated ionophore ligand [35S]t-butylbicyclophosphorothionate (TBPS) on rat brain sections, one of the fenamates, niflumate, at micromolar concentration was found to potentiate GABA actions in most brain areas, whereas being in the cerebellar granule cell layer an efficient antagonist similar to furosemide. With recombinant GABAA receptors expressed in Xenopus laevis oocytes, we found that niflumate potentiated 3 microM GABA responses up to 160% and shifted the GABA concentration-response curve to the left in alpha1beta2gamma2 receptors, the predominant GABAA receptor subtype in the brain. This effect needed the gamma2 subunit, because on alpha1beta2 receptors, niflumate exhibited solely an antagonistic effect at high concentrations. The potentiation was not abolished by the specific benzodiazepine site antagonist flumazenil. Niflumate acted as a potent antagonist of alpha6beta2 receptors (with or without gamma2 subunit) and of alphaXbeta2gamma2 receptors containing a chimeric alpha1 to alpha6 subunit, which suggests that niflumate antagonism is dependent on the same transmembrane domain 1- and 2-including fragment of the alpha6 subunit as furosemide antagonism. This antagonism was noncompetitive because the maximal GABA response, but not the potency, was reduced by niflumate. These data show receptor subtype-dependent positive and negative modulatory actions of niflumate on GABAA receptors at clinically relevant concentrations, and they suggest the existence of a novel positive modulatory site on alpha1beta2gamma2 receptors that is dependent on the gamma2 subunit but not associated with the benzodiazepine binding site.

Modulation of [3H] TBOB binding to the rodent GABAA receptor by simple disaccharides

We have recently reported evidence that a simple beta-linked alkylated mannose reversibly increased the magnitude of GABA(A) receptor currents evoked in cultured rat pyramidal neurons whilst concomitantly reducing the incidence of spontaneous synaptic activity. In this present study, the effects of the simple beta-linked disaccharide, lactose was investigated using a [3H] TBOB (t-[3H] butylbicycloorthobenzoate) binding assay in adult rat forebrain and cerebellum membranes. Lactose elicited a significant potentiation of [3H] TBOB binding to well-washed forebrain and cerebellar membranes (mean E(max) values=367 and 287%; mean EC(50) values=1.5 and 30 microM, respectively, N=4). The alpha-linked disaccharides, maltose and sucrose also potentiated [3H] TBOB binding, but with 100-600-fold higher EC(50) values than lactose. The lactose-mediated potentiation of [3H] TBOB in the forebrain and cerebellum was completely abolished in the presence of 0.3 microM GABA. Over the concentration range in which significant potentiation of [3H] TBOB binding was detected, lactose elicited no significant effect upon [3H] flunitrazepam binding. This study demonstrated that lactose can modulate the GABA(A) receptor channel, allosterically coupled to the agonist site, but independent of the benzodiazepine site. Furthermore, lactose displayed differential effects upon forebrain and cerebellar GABA(A) receptors indicating that it may be a novel subtype selective agent.

Cation modulation of GABA(A) receptors in brain sections of AT and ANT rats

Changes in magnesium ion (Mg(2+)) concentration may be implicated in alcohol-related behaviors through modulation of neuronal excitability by actions on ligand-gated ion channels. To study whether putative Mg(2+)-binding sites differ between two rat lines, alcohol-insensitive (AT) and alcohol-sensitive (ANT) rats, selectively outbred for differential sensitivity to the motor-impairing effect of ethanol, we compared the effect of Mg(2+) on [35S]tert-butylbicyclophosphorothionate ([35S]TBPS) binding to GABA(A) receptors with the use of ligand autoradiographic analyses of brain sections from these rats. There were some slight differences between the rat lines in modulation of the binding in the forebrain. A low concentration of Mg(2+) (0.1 mM) inhibited basal [35S]TBPS binding more efficiently in the central gray matter and hippocampus in the ANT rats than in the AT rats. In the presence of gamma-aminobutyric acid, the effect of a low concentration of Mg(2+) was higher in the caudate-putamen and inner layer of the cerebral cortex in the AT rats than in the ANT rats. No difference between the rat lines was found at a higher (3 mM) Mg(2+) concentration. Furosemide, a GABA(A) antagonist selective for cerebellar granule cell-specific alpha6beta2/3 subunit-containing receptors, was less efficient in antagonizing the Mg(2+)-induced inhibition of [35S]TBPS binding in the ANT rats than in the AT rats. Another divalent cation, zinc ion, was less efficient in displacing [35S]TBPS binding from the cerebellar granule cell layer in the ANT rats than in the AT rats, whereas a trivalent cation, lanthanum ion, produced identical modulation of the binding in the two rat lines. The results indicate that the alcohol-sensitive ANT rats have altered cerebellar granule cell--specific alpha6 subunit--containing GABA(A) receptors and seem to indicate that these receptors might be implicated in the sensitivity difference of the rat lines to ethanol and sedative drugs.