Benzodiazepine receptor ligand actions on GABA responses. Beta-carbolines, purines

Effects of pentylenetetrazole and glutamate on metabolism of [U-(13)C]glucose in cultured cerebellar granule neurons

This study was performed to analyze the effects of glutamate and the epileptogenic agent pentylenetetrazole (PTZ) on neuronal glucose metabolism. Cerebellar granule neurons were incubated for 2 h in medium containing 3 mM [U-(13)C]glucose, with and without 0.25 mM glutamate and/or 10 mM PTZ. In the presence of PTZ, decreased glucose consumption with unchanged lactate release was observed, indicating decreased glucose oxidation. PTZ also slowed down tricarboxylic acid (TCA) cycle activity as evidenced by the decreased amounts of labeled aspartate and [1,2-(13)C]glutamate. When glutamate was present, glucose consumption was also decreased. However, the amount of glutamate, derived from [U-(13)C]glucose via the first turn of the TCA cycle, was increased. The decreased amount of [1,2-(13)C]glutamate, derived from the second turn in the TCA cycle, and increased amount of aspartate indicated the dilution of label due to the entrance of unlabeled glutamate into TCA cycle. In the presence of glutamate plus PTZ, the effect of PTZ was enhanced by glutamate. Labeled alanine was detected only in the presence of glutamate plus PTZ, which indicated that oxaloacetate was a better amino acid acceptor than pyruvate. Furthermore, there was also evidence for intracellular compartmentation of oxaloacetate metabolism. Glutamate and PTZ caused similar metabolic changes, however, via different mechanisms. Glutamate substituted for glucose as energy substrate in the TCA cycle, whereas, PTZ appeared to decrease mitochondrial activity.

Functional and biochemical evidence for diazepam as a cyclic nucleotide phosphodiesterase type 4 inhibitor

1. The responses of the electrically-driven right ventricle strip of the guinea-pig heart to diazepam were recorded in the absence and in the presence of different selective cyclic nucleotide phosphodiesterase (PDE) inhibitors. 2. Diazepam, at concentrations ranging from 1 microM to 100 microM, was devoid of effect on the contractile force in this preparation. 3. Conversely, diazepam (5 microM-100 microM) produced a consistent positive inotropic response in the presence of a concentration (1 microM), that was without effect in the absence of diazepam, of either of the selective PDE 3 inhibitors milrinone or SK&F 94120, but not in the presence of the selective PDE 4 inhibitor rolipram. 4. This effect of diazepam was not gamma-aminobutyric acid (GABA)-dependent, since it was neither mimicked nor potentiated by GABA, and was not affected by either a high concentration (5 microM) of the antagonists of the benzodiazepine/GABA/channel chloride receptor complex, picrotoxin, flumazenil and beta-carboline-3-carboxylic acid methyl ester (betaCCMe), or by the inverse agonists, beta-carboline-3-carboxylic acid N-methylamide (betaCCMa) and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM, 0.1 microM). Furthermore, a specific antagonist of the peripheral benzodiazepine receptors, PK 11195 (5 microM), did not influence the effect of diazepam. 5. Biochemical studies with isolated PDEs, confirmed that diazepam selectively inhibits type 4 PDE from guinea-pig right ventricle rather than the other PDEs present in that tissue. The compound inhibited this enzyme in a non-competitive manner. Diazepam was also able to inhibit PDE 5, the cyclic GMP specific PDE absent from cardiac muscle, with a potency close to that shown for PDE 4. 6. Diazepam displaced the selective type 4 PDE inhibitor, rolipram from its high affinity binding site in rat brain cortex membranes, and also potentiated the rise in cyclic AMP levels induced by isoprenaline in guinea-pig eosinophils, where only type 4 PDE is present. 7. The PDE inhibitory properties of diazepam were shared, although with lower potency, by other structurally-related benzodiazepines, that also displaced [3H]-rolipram from its high affinity binding site. The order of potency found for these compounds in these assays was not related to their potencies as modulators of the GABA receptor through its benzodiazepine binding site. 8. The pharmacological and biochemical data presented in this study indicate that diazepam behaves as a selective type 4 PDE inhibitor in cardiac tissue and this effect seems neither to be mediated by the benzodiazepine/GABA/channel chloride receptor complex nor by peripheral type benzodiazepine receptors.

Functional GABA(A) receptors on human glioma cells

Glioma cells in acute slices and in primary culture, and glioma-derived human cell lines were screened for the presence of functional GABA(A) receptors. Currents were measured in whole-cell voltage clamp in response to gamma-aminobutyric acid (GABA). While cells from the most malignant glioma, the glioblastoma multiforme, did not respond to GABA, an inward current (under our experimental conditions with high Cl- concentration in the pipette) was induced in gliomas of lower grades, namely in 71% of oligodendroglioma cells and in 62% of the astrocytoma cells. Glioma cell lines did not express functional GABA(A) receptors, irrespective of the malignancy of the tumour they originate from. The currents elicited by application of GABA were due to activation of GABA(A) receptors; the specific agonist muscimol mimicked the response, the antagonists bicuculline and picrotoxin blocked the GABA-activated current and the benzodiazepine receptor agonist flunitrazepam augmented the GABA-induced current and the benzodiazepine inverse agonist DMCM decreased the GABA current. Cells were heterogeneous with respect to the direction of the current flow as tested in gramicidin perforated patches: in some cells GABA hyperpolarized the membrane, while in the majority it triggered a depolarization. Moreover, GABA triggered an increase in [Ca2+]i in the majority of the tumour cells due to the activation of Ca2+ channels. Our results suggest a link between the expression of GABA receptors and the growth of glioma cells as the disappearance of functional GABA(A) receptors parallels unlimited growth typical for malignant tumours and immortal cell lines.

Benzodiazepine antagonists reduce epileptiform discharges in rat hippocampal slices

PURPOSE

The antiepileptic effects of benzodiazepine-receptor (BZR) agonists have been well documented. Surprisingly, an antiepileptic effect for the BZR antagonist, flumazenil, has also been described, the mechanism of which is unknown. We investigated the effects of nanomolar concentrations of flumazenil and a structurally dissimilar BZR antagonist, propyl-beta-carboline-3-carboxylate (beta-CCP), on normal synaptic responses and epileptiform discharges induced by a variety of methods in the CA1 region of rat hippocampal slices.

METHODS

Extracellular field potentials were recorded from stratum pyramidale of the CA1 region. Orthodromic stimulation was delivered by a bipolar electrode placed in the stratum radiatum at the border of the CA2/CA3 regions. Drugs were bath applied, and epileptiform discharges were quantified by using the Coastline Bursting Index, which calculates the total length of the discharge waveform of evoked multiple population spikes. For statistical comparisons, we calculated the Coastline Bursting Index for the average of five traces at the end of the control period (20 min), drug application (20 min), and washout (20-40 min).

RESULTS

Flumazenil was without effect on normal synaptic responses; however, flumazenil reduced epileptiform discharges evoked in the presence of high [K+]o, leu-enkephalin, the BZR inverse agonist, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), or after a cold-shock procedure. beta-CCP exhibited an action similar to that observed for flumazenil, suggesting that the antiepileptic effect is due to properties common to BZR antagonists.

CONCLUSIONS

We suggest that the antiepileptic effect we observed for flumazenil and beta-CCP is mediated at the BZR and might be due to competition with endogenous BZR inverse agonists released preferentially during epileptiform activity.

Midazolam inhibits long-term potentiation through modulation of GABAA receptors

Benzodiazepine drugs (BZ) are used for anxiety, insomnia, and seizures. They worsen memory, especially in large doses, but the mechanism of this action is uncertain. In micromolar concentrations, benzodiazepines have been shown to reduce long-term potentiation (LTP), which could be a cellular basis for their amnesic action. We have found that the LTP-inhibiting effects of BZ occur in the nanomolar concentrations attained in humans, and that this effect occurs through modulation of GABAA receptor function. We recorded extracellular synaptic input/output (I/O) curves for population spikes (PS) and EPSPs in rat hippocampal slices before and after induction of LTP. LTP increased maximal PS and EPSPs and shifted I/O curves for PS and EPSPs to the left, reflecting increased synaptic responsiveness after LTP. Curves relating EPSPs to PS were also shifted, so that after LTP larger PS were elicited for the same size EPSP (E-S potentiation). Midazolam (0.5 microM) markedly inhibited the left-shift in PS I/O curves due to E-S potentiation but did not significantly affect other parameters. 8-Phenyltheophylline (10 microM), an adenosine receptor antagonist, did not prevent midazolam inhibition of LTP. Bicuculline, a GABAA receptor antagonist, caused a dose-dependent antagonism of midazolam's LTP inhibition. Our results suggest that benzodiazepines reduce LTP primarily through reduction of E-S potentiation, and that this effect occurs through modulation of GABAA receptor function. This could in part account for the ability of benzodiazepines to disturb new memory formation.

The beta-carboline derivative DMCM decreases gamma-aminobutyric acid responses and Ca(2+)-mediated K(+)-conductance in rat neocortical neurons in vitro

Electrophysiological recordings from neurons of rat frontal neocortical slices have been used to investigate the action of the beta-carboline methyl-6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylate (DMCM), on responses to gamma-aminobutyric acid (GABA) and on the excitability of the neurons. Iontophoretic application of GABA close to the intracellularly recorded cells (resting membrane potential -74 +/- 0.9 mV) elicited a depolarization associated with a decrease of input resistance, mediated by GABAA receptors. Bath application of DMCM (0.1-1 microM) reduced these GABA responses decreasing the affinity of the receptors for GABA. This effect was blocked by the benzodiazepine receptor (BZR) antagonist ZK 93426 (1 microM). DMCM (0.1 microM) also decreased the hyperpolarization that followed a train of action potentials (AHP), mediated by Ca(2+)-dependent K+ conductance, and increased the duration of Ca(2+)-dependent action potentials recorded after blockade of Na+ and K+ conductances. Neither effect was blocked by BZR antagonists. These results indicate that DMCM increases the excitability of neurons not only by reducing the gain of the GABAA/BZR complex, but also by modulating intrinsic membrane mechanisms.

Benzodiazepine and beta-carboline regulation of single GABAA receptor channels of mouse spinal neurones in culture

1. The effects of the benzodiazepine receptor agonist, diazepam (DZ), and the inverse agonist, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), on gamma-aminobutyric acid (GABAA) receptor single channel currents were characterized. Outside-out patches were obtained from somata of cultured mouse spinal cord neurones and voltage clamped at -75 mV (ECl = 0 mV). 2. GABA (2 microM) alone or with DZ (20-1000 nM) or DMCM (20-100 nM) was applied to patches by pressure ejection from blunt micropipettes. DZ enhanced GABAA receptor currents with an inverted U-shaped concentration-response curve. Mean steady-state currents were increased by low concentrations of DZ (20-50 nM). At higher concentrations of DZ, the enhancement was diminished. Mean steady-state currents were decreased by DMCM at all concentrations. 3. GABAA receptor channels opened most frequently to a 27 pS main conductance level and less frequently to a 19 pS subconductance level. Neither DZ nor DMCM altered the proportion of time spent at either of the conductance levels. The kinetic properties of the main conductance level were studied. 4. Neither DZ nor DMCM altered the mean GABAA receptor channel open or burst durations. Sums of three exponential functions were required to fit best open and burst duration-frequency histograms for GABA alone or with DZ or DMCM. No significant changes in the three time constants or areas of the three exponential functions for open or burst duration histograms were produced by DZ or DMCM. 5. With increasing concentrations of DZ up to 50 nM, GABA evoked an increased frequency of channel openings and bursts. With higher DZ concentrations, the magnitudes of the increase in channel opening and burst frequencies were reduced. At all concentrations of DMCM, GABA evoked a decreased frequency of channel openings and bursts. 6. Closed duration-frequency histograms for GABA alone or with DZ or DMCM were best fitted by sums of at least six exponential functions. The three shortest closed duration time constants were unchanged by DZ or DMCM. The three longest closed duration time constants were altered by DZ and DMCM, consistent with alterations in opening frequency. 7. DZ increased and DMCM decreased steady-state GABAA receptor current by increasing or decreasing channel opening frequency without altering mean channel open duration. We propose that DZ and DMCM alter GABAA receptor current by acting reciprocally to increase or decrease only, respectively, the apparent agonist association rate at the first of two proposed GABA binding steps without altering channel gating.(ABSTRACT TRUNCATED AT 400 WORDS)

Thermodynamic analysis of binding to the cerebellar type I benzodiazepine receptor

The temperature dependence of binding to the type I benzodiazepine receptor in rat cerebellum was determined using [3H]Ro15-1788 and regression analysis techniques. The ligands chosen were from diverse chemical families and display different pharmacological properties. Included were the agonists flunitrazepam, CL 218,872, zolpidem and alpidem; the antagonists Ro15-1788 and propyl-beta-carboline-3-carboxylate (beta-CCP); the inverse agonists ethyl-beta-carboline-3-carboxylate (beta-CCE) and methyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM); and the selective muscle relaxant AHR-11797. Assays were performed at 0 degrees C, 20 degrees C and 37 degrees C and the K(i) at each temperature was used to construct a van't Hoff plot for each compound. The binding of all ligands, with the exception of DMCM, was enthalpy-driven. However, enthalpy alone does not determine the rank order of affinity. There was no relationship between the thermodynamic behavior of binding and the observation of agonism, antagonism or inverse agonism, indicating that activation and recognition are distinct steps in this receptor system.

Guanidino compounds that are increased in hyperargininemia inhibit GABA and glycine responses on mouse neurons in cell culture

The effects of arginine, homoarginine, alpha-keto-delta-guanidinovaleric acid and argininic acid (guanidino compounds that were found to be increased in hyperargininemia) were evaluated on responses to gamma-aminoburtyric acid (GABA) and glycine (Gly) on mouse neurons in primary dissociated cell culture. GABA and Gly were applied iontophoretically and intracellular microelectrode recording techniques were used. The guanidino compounds rapidly and reversibly inhibited both GABA and Gly responses. The guanidino compounds inhibited GABA responses in a concentration-dependent manner and inhibited Gly responses at a concentration of 10 mM. Argininic acid was the most potent in reducing inhibitory amino acid responses, followed in decreasing potency by alpha-keto-delta-guanidinovaleric acid, homoarginine and arginine. The guanidino compounds were equally potent in decreasing Gly and GABA responses. Co-application of CGS 9896, a benzodiazepine receptor antagonist, did not antagonize the guanidino compound-induced inhibition of GABA responses. These findings suggest that the guanidino compounds inhibited responses to the inhibitory neurotransmitters GABA and Gly by blocking the chloride channel. This effect might underlie the in vivo epileptogenicity of some of the guanidino compounds and might contribute to the pathogenesis of seizures in hyperargininemia.

Guanidino compounds that are increased in cerebrospinal fluid and brain of uremic patients inhibit GABA and glycine responses on mouse neurons in cell culture

Four guanidino compounds that have been found to be markedly increased in cerebrospinal fluid and brain tissue of uremic patients, namely, guanidine, methylguanidine, creatinine, and guanidinosuccinic acid, were applied to mouse spinal cord neurons in primary dissociated cell culture to evaluate their effects on postsynaptic responses to gamma-aminobutyric acid (GABA) and glycine. Intracellular microelectrode recording techniques were used. Guanidine, methylguanidine, creatine, and guanidinosuccinic acid reversibly and in a dose-dependent manner inhibited both GABA and glycine responses. Guanidinosuccinic acid was the most potent inhibitor of the amino acid responses, followed in decreasing potency by methylguanidine, guanidine, and creatinine. Guanidinosuccinic acid inhibited responses to GABA and glycine, at concentrations similar to those found in cerebrospinal fluid and brain tissue of patients with terminal renal insufficiency. The other guanidino compounds tested exerted their effects only at concentrations higher than those found in uremic biological fluids and tissues. The inhibitory effect of guanidine and methylguanidine on responses to GABA was additive. The effect of the guanidino compounds on GABA responses was not antagonized by coapplication of the benzodiazepine-receptor antagonist CGS 9896. The results suggest that guanidine, methylguanidine, creatinine, and guanidinosuccinic acid inhibited responses to the inhibitory neurotransmitters GABA and glycine by blocking the chloride channel. The observed action of the studied guanidino compounds might contribute to the pathogenesis of the complex neurological symptomatology encountered in uremia.

An electrophysiological investigation of the characteristics and function of GABAA receptors on bovine adrenomedullary chromaffin cells

The characteristics and function of gamma-aminobutyric acidA (GABAA) receptors expressed on bovine chromaffin cells in culture have been investigated using patch-clamp techniques. In voltage-clamped whole-cells, locally applied GABA (100 microM) evoked a transmembrane chloride current which demonstrated outward rectification. The amplitude of such currents was reversibly suppressed by the GABAA receptor antagonists bicuculline, picrotoxin and RU5135, and enhanced by the general anaesthetic propanidid. Glycine (100 microM) and baclofen (100 microM) were ineffective as agonists. In support of a physiological role for GABA in the adrenal medulla, the co-existence of GABAA and nicotinic acetylcholine (ACh) receptors was demonstrated on whole cells and outside-out membrane patches. Ionophoretically applied GABA reduced the amplitude of depolarization and action potential discharge occurring in response to locally applied ACh (100 microM), but had no effect upon the underlying ACh-induced current. In addition, an excitatory action of GABA was demonstrated by recording action potential waveforms in cell-attached patches. The results are discussed in the context of a GABA-ergic regulation of catecholamine secretion.

Differential effects of iodide and chloride on allosteric interactions of the GABAA receptor

t-[35S]Butylbicyclophosphorothionate [( 35S]TBPS) has been shown to bind to the GABAA receptor complex. The binding is modulated allosterically by drugs that interact at components of the receptor complex. The present studies were designed to evaluate the influence of ionic environment and state of equilibrium on the allosteric modification of [35S]TBPS binding. In both I- and Cl- under nonequilibrium conditions, diazepam, gamma-aminobutyric acid (GABA), and pentobarbital (PB) stimulate and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) inhibits [35S]TBPS binding. In addition, there is an inhibitory component to the effect of GABA and PB at higher drug concentrations. These effects are blocked by the appropriate antagonists for each drug. In Cl-, the stimulation of [35S]TBPS binding by drugs disappears at equilibrium, whereas the inhibition by GABA and PB persists. The inhibitory effect of DMCM in Cl- also disappears at equilibrium. When assayed in I- at equilibrium, however, DMCM stimulates [35S]TBPS binding. In addition, bicuculline, which is without effect under nonequilibrium conditions in either Cl- or I-, stimulates [35S]TBPS binding in I- at equilibrium. The persistent effects of DMCM, bicuculline, and GABA in I- are accompanied by alterations in the affinity of [35S]TBPS for its receptor. In addition, the stimulation of [35S]TBPS binding by GABA is associated with a decreased number of [35S]TBPS binding sites. These data demonstrate that receptor complex interactions with anions influence the responsiveness to drug binding.

Importance of a novel GABAA receptor subunit for benzodiazepine pharmacology

Neurotransmission effected by GABA (gamma-aminobutyric acid) is predominantly mediated by a gated chloride channel intrinsic to the GABAA receptor. This heterooligomeric receptor exists in most inhibitory synapses in the vertebrate central nervous system (CNS) and can be regulated by clinically important compounds such as benzodiazepines and barbiturates. The primary structures of GABAA receptor alpha- and beta-subunits have been deduced from cloned complementary DNAs. Co-expression of these subunits in heterologous systems generates receptors which display much of the pharmacology of their neural counterparts, including potentiation by barbiturates. Conspicuously, however, they lack binding sites for, and consistent electrophysiological responses to, benzodiazepines. We now report the isolation of a cloned cDNA encoding a new GABAA receptor subunit, termed gamma 2, which shares approximately 40% sequence identity with alpha- and beta-subunits and whose messenger RNA is prominently localized in neuronal subpopulations throughout the CNS. Importantly, coexpression of the gamma 2 subunit with alpha 1 and beta 1 subunits produces GABAA receptors displaying high-affinity binding for central benzodiazepine receptor ligands.

Effects of antiepileptic drugs on GABA responses and on reduction of GABA responses by PTZ and DMCM on mouse neurons in cell culture

The mechanisms of action of antiepileptic drugs effective against generalized absence seizures (antiabsence AEDs) remain uncertain. Antiabsence AEDs are generally effective against seizures induced in experimental animals by pentylenetetrazol (PTZ) and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), drugs which reduce GABAergic inhibition. Thus, antiabsence AEDs have been suggested to enhance GABAergic inhibition. We studied the effects of several AEDs on GABA responses recorded from mouse spinal cord neurons grown in primary dissociated cell culture. Four antiabsence AEDs were included: ethosuximide (ESM), dimethadione (DMO), sodium valproate (VPA), and diazepam (DZP). Two experimental AEDs, CGS 9896 and ZK 91296, with anticonvulsant action against PTZ- or DMCM-induced seizures were also included. Possible effects of the antiabsence and experimental AEDS on PTZ- and DMCM-induced inhibition of GABA responses were also evaluated. PTZ and DMCM reversibly reduced GABA responses in a concentration-dependent manner. PTZ completely inhibited GABA responses at 10 mM (IC50 of 1.1 mM), whereas DMCM-induced inhibition of GABA responses reached a plateau level of 39% of control values at 1 microM (IC50 of 33 nM). ESM (1,200 microM), DMO (6 mM), VPA (200 microM), CGS 9896 (2 microM), and ZK 9896 (2 microM) did not alter GABA responses. DZP enhanced GABA responses in a concentration-dependent manner. The inhibition of GABA responses produced by PTZ 1 mM was unaltered by ESM (600 microM), DMO (6 mM), CGS 9896 (1 microM), or ZK 9896 (1 microM). Coapplication of VPA (200 microM) and PTZ (1 mM) slightly enhanced the PTZ effect. DZP (greater than 10 nM), however, reversed the PTZ-induced reduction of GABA responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of alpha-keto-delta-guanidinovaleric acid on inhibitory amino acid responses on mouse neurons in cell culture

The experimentally proven convulsant alpha-keto-delta-guanidinovaleric acid (alpha-K-delta-GVA) was applied to mouse spinal cord neurons in primary dissociated cell culture to assess its effects on postsynaptic gamma-aminobutyric acid (GABA)- and glycine (GLY)-responses. Intracellular microelectrode recording techniques were used. alpha-K-delta-GVA reversibly inhibited both GABA- and GLY-responses in a concentration-dependent manner. The effect of alpha-K-delta-GVA on GABA-responses was not antagonized by co-application of the benzodiazepine receptor antagonist CGS 9896. The results suggest that alpha-K-delta-GVA inhibited responses to the inhibitory neurotransmitters GABA and GLY by blocking the chloride channel. This action might underlie the convulsant effect of this compound in rabbit. The possible pathophysiological importance of alpha-K-delta-GVA in hyperargininemic patients is discussed.

Characteristics of flunitrazepam binding to intact primary cultured spinal cord neurons and its modulation by GABAergic drugs

The interaction of [3H]flunitrazepam and its modulation by various drugs was studied in intact primary cultured spinal cord neurons. In the intact cells, the [3H]-flunitrazepam binding was rapid and saturable. The benzodiazepine binding sites exhibited high affinity and saturability, with an apparent KD of 6.1 +/- 1.6 nM and Bmax of 822 +/- 194 fmol/mg protein. The association and dissociation of [3H]flunitrazepam binding exhibited monoexponential kinetics. Specifically bound [3H]flunitrazepam was displaced in a concentration-dependent manner by benzodiazepines like flunitrazepam, clonazepam, diazepam, Ro 15-1788, and beta-carbolines like methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3'-carboxylate. Specific [3H]flunitrazepam binding to intact cells was enhanced in a concentration-dependent manner by gamma-aminobutyric acid (GABA) agonists and drugs which facilitate GABAergic transmission like etazolate, (+)-etomidate, and pentobarbital. The enhancing effect of GABA agonists was antagonized by bicuculline and picrotoxinin. These results suggest that the intact cultured spinal cord neurons exhibit the properties of benzodiazepine GABA receptor-ionophore complex. Since these cells can also be studied in parallel for characterizing GABA-induced 36Cl-influx, they provide an ideal in vitro assay preparation to study GABA synaptic pharmacology.

The affinities, potencies and efficacies of some benzodiazepine-receptor agonists, antagonists and inverse-agonists at rat hippocampal GABAA-receptors

The abilities of some benzodiazepine-receptor agonists, antagonists and inverse agonists to modulate the inhibitory potency of the gamma-aminobutyric acid (GABA)A-receptor agonist, isoguvacine, on the CA1 population spike recorded from slices of rat hippocampus, were determined. Concentration-response curves were constructed of the extent to which the benzodiazepine-receptor ligands shifted the isoguvacine concentration-response curve to the left or right. These were compared to their displacement curves of [3H]-Ro15-1788 binding to rat hippocampal membranes under near physiological assay conditions. The above comparisons suggest that the effect on the potency of isoguvacine produced by the benzodiazepine-receptor agonists, diazepam and flunitrazepam, and the partial agonists, Ro16-6028 and Ro17-1812, closely parallels their degree of benzodiazepine-receptor occupancy. Thus, the partial agonists, Ro16-6028 and Ro17-1812, were unable to produce as large a maximum response as the full agonists, diazepam and flunitrazepam. The maximum effects produced by diazepam, flunitrazepam, Ro16-6028, Ro17-1812, the antagonist, propyl-beta-carboline-3-carboxylate, and the inverse agonist, methyl-6, 7-dimethyl-4-ethyl-beta-carboline-3-carboxylate (DMCM), on the potency of isoguvacine in the hippocampal slice corresponded to the change in their affinities produced by the addition of GABA in the radioligand binding studies (GABA-shift). This suggests that the changes in affinity of benzodiazepine-receptor ligands produced by GABAA-receptor activation reflects their ability to modify GABAA-receptor function. The benzodiazepine-receptor antagonists, Ro15-1788 and CGS 8216, had apparent agonist and inverse agonist effects, respectively, on the potency of isoguvacine. These effects occurred at concentrations above those required for saturation of the benzodiazepine-receptor, as labelled by [3H]-Ro15-1788, and were not in agreement with the absence of any effect of GABAA-receptor stimulation in the GABA-shift experiments. This indicates that these events are not mediated by an action at the classical benzodiazepine-receptor site. 6 It is concluded that hippocampal GABAA-receptor function can be allosterically modulated in a manner consistent with the agonist/inverse-agonist model of benzodiazepine-receptor activation, and that compounds exist with varying efficacies throughout this range.

Benzodiazepine and beta-carboline modulation of GABA-stimulated 36Cl-influx in cultured spinal cord neurons

GABAA agonists stimulate 36Cl-influx in spinal cord cultured neurons in a concentration-dependent manner. This effect of GABAA receptor stimulation is enhanced by benzodiazepines like clonazepam, diazepam and flurazepam and attenuated by (+)bicuculline and picrotoxinin. The beta-carbolines, methyl-6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) and propyl-beta-carboline-3-carboxylate (beta-CCPr) exhibited opposite effects, with DMCM attenuating, while beta-CCPr potentiating GABA's effect. These results are consistent with the behavioral and electrophysiological effect of benzodiazepines and beta-carbolines with GABA receptor complex.

Blockade of a putative GABA-mediated neurotransmission in the cerebellum by benzodiazepine receptor inverse agonists

An exponential relationship was observed between the firing rate of cerebellar Purkinje cells in urethane-anaesthetized rats and the duration of inhibition evoked in these cells by electrical stimulation of the nearby cortical surface. Benzodiazepines, administered i.v., decreased cell firing and increased the duration of the inhibitory response but did not alter the relationship between the two parameters. These effects of one benzodiazepine, RU 32007, were reversed by the benzodiazepine antagonist Ro15-1788 which had little effect alone. The benzodiazepine inverse agonists methyl- or ethyl-beta-carboline-3-carboxylate increased cell firing with the expected reductions in duration of inhibitory response in some cases. However, in 50% of recordings the inhibitory response disappeared, independent of the firing rate. All the effects of the beta-carboline esters were reversed by Ro15-1788 or the benzodiazepine, RU 32007. This action of the benzodiazepine receptor inverse agonists represents an in vivo blockade of an endogenous synaptic inhibition which is thought to be mediated by release of GABA.

Effects of two convulsant beta-carboline derivatives, DMCM and beta-CCM, on regional neurotransmitter amino acid levels and on in vitro D-[3H]aspartate release in rodents

Clonic seizures were induced in Swiss or DBA/2 mice by methyl-6-7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), 0.048 mmol/kg i.p., or by methyl-beta-carboline-3-carboxylate (beta-CCM), 0.044 mmol/kg i.p. Measurement of regional brain (cortex, hippocampus, striatum, and cerebellum) amino acid levels after 15 min of seizure activity showed increases in gamma-aminobutyric acid (GABA) (in all regions after beta-CCM, and in cortex and hippocampus after DMCM), and an increase in glycine in the striatum after beta-CCM. Aspartate levels fell (in cortex and hippocampus) after DMCM, but were unchanged in all regions after beta-CCM. Glutamate levels fell in cortex after beta-CCM and in striatum after DMCM. Pretreatment with the excitatory amino acid receptor antagonist, 2-amino-7-phosphonoheptanoic acid, 0.5 mmol/kg i.p., 45 min prior to the beta-carboline, significantly increased the ED50 for DMCM-induced clonic seizures (4.68 mumol/kg vs. 9.39 mumol/kg). Similar pretreatment did not significantly alter the ED50 for beta-CCM (4.22 mumol/kg vs. 6.6 mumol/kg). Pretreatment with 2-amino-7-phosphonoheptanoic acid, 1.0 mmol/kg, blocked the increase in GABA content produced by DMCM but not the fall in cortical aspartate content. Potassium-induced release of preloaded D-[3H]aspartate from rat cortical or hippocampal minislices was enhanced in the presence of DMCM (100 microM). In contrast, stimulated release of D-[3H]aspartate (from cortex or hippocampus) was not altered in the presence of beta-CCM (100 microM). Although DMCM and beta-CCM are both considered to induce convulsion by acting at the GABA--benzodiazepine receptor complex, the convulsions differ in several pharmacological and biochemical respects. It is suggested that enhanced release of excitatory amino acid neurotransmitters plays a more important role in seizures induced by DMCM.

Benzodiazepine receptor ligand actions on GABA responses. Benzodiazepines, CL 218872, zopiclone

The effects on GABA (4-aminobutyric acid) responses of several benzodiazepine and nonbenzodiazepine benzodiazepine receptor ligands were examined using mouse spinal cord neurons in dissociated cell culture. Diazepam, clonazepam and nitrazepam enhanced GABA responses potently at low nanomolar concentrations. Diazepam and clonazepam were most potent with significant enhancement at 1 nM and peak enhancement of 80.7 and 50.2% at 10 nM respectively. Nitrazepam was least potent with no significant enhancement at 1 nM and enhancement of only 20.7% at 10 nM. The benzodiazepine antagonist, Ro 15-1788, blocked enhancement by diazepam but also weakly enhanced GABA responses at low micromolar concentrations, suggesting partial agonist activity. The convulsant benzodiazepine, Ro 5-4864, did not enhance GABA responses at any concentration tested but antagonized GABA responses at 1 microM and above. Diazepam shifted GABA dose-response curves to the left by decreasing the apparent KD but without altering the apparent Vmax (Lineweaver-Burk analysis). Two nonbenzodiazepine anxiolytic/anticonvulsants, CL 218872 and zopiclone, were weak enhancers of GABA responses at high nanomolar concentrations. These results with benzodiazepines, CL 218872 and zopiclone are consistent with their anxiolytic and anticonvulsant profile in vivo and with studies of their effects upon low affinity GABA binding in vitro.