Benzodiazepines specifically modulate GABA-mediated postsynaptic inhibition in cultured mammalian neurones

The central and basolateral amygdala differentially mediate the anxiolytic effects of benzodiazepines

Microinfusions of the benzodiazepine anxiolytic midazolam into the central or basolateral amygdaloid nuclei produced different anxiolytic effects in two tests of rat 'anxiety'. Infusions into the basolateral nucleus impaired open-arm avoidance in the elevated plus-maze test, but did not impair shock-probe avoidance in the shock-probe burying test. In contrast, infusions into the central nucleus impaired shock-probe avoidance, but did not impair open-arm avoidance. Both of these site-specific, midazolam-induced anxiolytic effects were blocked by a pre-infusion of the benzodiazepine receptor antagonist Ro 15-1788 (flumazenil). None of the treatments affected defensive burying. These results suggest that benzodiazepine receptors in the central and basolateral amygdaloid nuclei differentially mediate the anti-anxiety effects of benzodiazepine anxiolytics.

Trichloroethanol potentiation of gamma-aminobutyric acid-activated chloride current in mouse hippocampal neurones

1. The action of 2,2,2-trichloroethanol on gamma-aminobutyric acid (GABA)-activated Cl- current was studied in mouse hippocampal neurones in tissue culture by use of whole-cell patch-clamp recording. 2. Trichloroethanol increased the amplitude of currents activated by 1 microM GABA or 0.1 microM muscimol. Trichloroethanol, 1-25 mM, potentiated current activated by 1 microM GABA in a concentration-dependent manner with an EC50 of 3.0 +/- 1.4 mM and a maximal response (Emax) of 576 +/- 72% of control. 3. Trichloroethanol potentiated currents activated by GABA concentrations < 10 microM, but did not increase the amplitude of currents activated by concentrations of GABA > or = 10 microM. Despite marked potentiation of currents activated by low concentrations of GABA, trichloroethanol did not significantly alter the EC50, slope, or Emax of the GABA concentration-response curve. 4. Trichloroethanol, 5 mM, potentiated GABA-activated current in neurones in which ethanol, 10-500 mM, did not. The effect of trichloroethanol was not altered by the putative ethanol antagonist, Ro 15-4513. Trichloroethanol did not potentiate currents activated by pentobarbitone. 5. In the absence of exogenous GABA, trichloroethanol at concentrations > or = 2.5 mM activated a current that appeared to be carried by Cl- as its reversal potential changed with changes in the Cl- gradient and as it was inhibited by the GABAA antagonists, bicuculline methiodide and picrotoxin. 6. Since trichloroethanol is thought to be the active metabolite of chloral hydrate and other chloral derivative anaesthetics, potentiation of the GABA-activated current in central nervous system neurones by trichloroethanol may contribute to the sedative/hypnotic effects of these agents.

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)

Tolerance to the effects of diazepam, clonazepam and bretazenil on GABA-stimulated Cl- influx in flurazepam tolerant rats

The effect of chronic flurazepam treatment on the GABA (gamma-aminobutyric acid) receptor/chloride channel complex was studied using GABA-stimulated 36Cl- influx into brain microsacs, and its potentiation by diazepam, clonazepam and bretazenil. Rats were given flurazepam for 1 week, then microsacs were prepared from cerebral cortices of rats that were still receiving flurazepam, and from those that had stopped treatment 48 h earlier. Diazepam and clonazepam produced concentration-dependent increases in GABA-stimulated 36Cl- influx while bretazenil produced a much smaller effect, which did not reach statistical significance in the tissue from control rats. There was no significant change in the basal or 10 microM GABA-stimulated 36Cl- influx between control and treated groups. Tolerance was shown by a significantly reduced effect of diazepam and clonazepam to enhance GABA-stimulated 36Cl- influx in the tissue prepared from non-withdrawn rats. However, for both diazepam and clonazepam, there was no tolerance 48 h after chronic treatment. The results suggest that changes in the GABA receptor/Cl- channel complex on cerebral cortical neurons contribute to cross-tolerance from flurazepam to other benzodiazepines.

Penicillin-induced triphasic modulation of GABAA receptor-operated chloride current in frog sensory neuron

Effects of penicillin-G (PCN) on GABA-evoked Cl- current (IGABA) were investigated in freshly dissociated frog sensory neurons by the use of the concentration-clamp technique combined with the suction-pipette method. Under conditions where the internal and external solutions allowed only Cl- permeability, PCN elicited triphasic modulation on IGABA, consisting of two modes of blockade on IGABA and a following rebound (rebound-like transient IGABA). Simultaneously applied PCN and GABA depressed IGABA immediately (phasic blockade), with the depressed IGABA slightly recovering in amplitude to achieve a stable level of blockade (tonic blockade). When a solution containing a mixture or PCN and GABA was quickly replaced by one containing GABA alone, a rebound-like transient Cl- current (IR) was evoked. Each component of the PCN actions on IGABA was PCN- and GABA-concentration-dependent. The reversal potential for each component of the PCN actions on IGABA was close to the chloride equilibrium potential (ECl) calculated using the Nernst equation. The current-voltage (I-V) relations for both the phasic and tonic blockade revealed inward rectification, while I-V curves for the control IGABA and the IR were outwardly rectified. The degree of IGABA-desensitization and the amplitude of the IR correlated well. The data suggest that partial removal of the GABAA receptor-desensitization may result in generation of the IR.

Sensitivities of Achatina giant neurones to putative amino acid neurotransmitters

1. GABA receptors in Achatina identifiable giant neurones were classified into the muscimol I, muscimol II and baclofen types. Muscimol I and II type GABA receptors were sensitive to GABA and muscimol but insensitive to baclofen, whereas baclofen type receptors were sensitive to GABA and baclofen but insensitive to muscimol. Muscimol I and baclofen types were associated with the inhibition caused by GABA, while muscimol II type with the GABA excitation. 2. GABA, muscimol and TACA produced a transient outward current (Iout) with an increase in membrane conductance (g) of an Achatina neurone, TAN, having the muscimol I type GABA receptors. Their relative potency values (RPV) at GABA ED50 (approximately 10(-4) M) were: GABA:muscimol:TACA = 1:0.6:0.3. The GABA effects were potentiated by pentobarbitone, antagonized competitively by pitrazepin and non-competitively by picrotoxin and diazepam, and unaffected by bicuculline. The ionic mechanism of effects of GABA and its two analogues was the increase in membrane Cl- conductance (gCl). 3. GABA and (+/-)-baclofen produced a slow Iout with a g increase of another Achatina neurone, RPeNLN, having the baclofen type GABA receptors. The two compounds were almost equipotent (ED50: approximately 3 x 10(-4) M). The ionic mechanism of their effects was the increase in gk. The two compounds hardly affected the voltage-gated and slowly inactivating calcium current. Iout produced by GABA and (+/-)-baclofen were reduced by TEA, but unaffected by 4-AP, bicuculline, pitrazepin and picrotoxin. 4. Beta-hydroxy-L-glutamic acid (L-BHGA) showed the marked effects on the Achatina giant neurones; the two neurones were excited by the compound, whereas the three inhibited. D-BHGA, L-Glu, D-Glu and NMDA were less effective than L-BHGA or almost ineffective. Erythro-L-BHGA was more or less effective than threo-L-BHGA according to the neurones tested. 5. alpha-Kainic acid and domoic acid excited the two neurones, which were excited by L-BHGA. L-Quisqualic acid showed the similar effects to L-BHGA, which were mostly much stronger than L-BHGA. Erythro-L-tricholomic acid and DL-ibotenic acid showed the effects similar to L-BHGA selectively on some neurones. 6. It was pointed out that the pharmacological features of GABA on the Achatina neurones are simpler than those of L-BHGA, due to the simpler structure of the former compound having less binding sites than the latter.

Penicillin-induced potentiation of glycine receptor-operated chloride current in rat ventro-medial hypothalamic neurones

1. Effects of penicillin G (PCN) on glycine (Gly)-evoked Cl- current (IGly) were investigated in acutely dissociated rat ventro-medial hypothalamic (VMH) neurones by the whole cell mode of patch clamp technique. 2. When PCN was applied simultaneously with Gly, PCN depressed IGly like a Cl- channel blocker. 3. The PCN-induced blocking action was clearly observed at a low PCN concentration (30 u), while the maximal blockade was achieved by 600 u (units per 10 ml) PCN. 4. When tested solution containing both PCN and Gly was quickly substituted with one containing Gly only, a new rebound-like transient current (I(T)) which also passed through Cl- channel, was elicited. 5. The peak amplitude of I(T) induced by PCN at concentrations higher than 100 u was greater than that induced by glycine alone. We termed this phenomenon PCN-induced potentiation of IGly. In all cells tested, PCN potentiated IGly. 6. At a lower PCN concentration below 30 u, I(T) generation was not clear in the presence of 10(-5) M gamma-aminobutyric acid. With PCN a higher concentration than 300 u, I(T) amplitude was greater than that of the original peak IGly. This was observed in 18 neurones out of 21. The maximal amplitude of the I(T) was achieved with 600 u PCN.

Modulation of GABAA and glycine receptors by chlormethiazole

The influence of chlormethiazole, on currents evoked by gamma-aminobutyric acid (GABA) and glycine, was investigated under voltage-clamp conditions, in bovine chromaffin cells and murine spinal neurones, respectively. Chlormethiazole (30 and 100 microM) dose dependently potentiated currents activated by either inhibitory neurotransmitter. The potentiation of the GABA-evoked response occurred without altering the reversal potential and was not influenced by the benzodiazepine receptor antagonist Ro 15-1788 (300 nM). GABA-gated channels, recorded from outside-out membrane patches, showed increased probability of being in the conducting state in the presence of chlormethiazole. High concentrations of chlormethiazole (3 mM) activated bicuculline (1 microM)-sensitive whole-cell currents with a reversal potential similar to the chloride equilibrium potential. Chlormethiazole potentiates GABA- and glycine-activated currents and at higher doses, directly activates the GABAA receptor.

Non-competitive inhibition of GABAA responses by a new class of quinolones and non-steroidal anti-inflammatories in dissociated frog sensory neurones

1. The interaction of a new class of quinolone antimicrobials (new quinolones) and non-steroidal anti-inflammatory agents (NSAIDs) with the GABAA receptor-Cl- channel complex was investigated in frog sensory neurones by use of the internal perfusion and 'concentration clamp' techniques. 2. The new quinolones and the NSAIDs (both 10(-6)-10(-5) M) had little effect on the GABA-induced chloride current (ICI) when applied separately. At a concentration of 10(-4) M the new quinolones, and to a lesser degree the NSAIDs, produced some suppression of the GABA response. 3. The co-administration of new quinolones and some NSAIDs (10(-6)-10(-14) M) resulted in a marked suppression of the GABA response. The size of this inhibition was dependent on the concentration of either the new quinolone or the NSAID tested. The inhibitory potency of new quinolones in combination with 4-biphenylacetic acid (BPAA) was in rank order norfloxacin (NFLX) much greater than enoxacin (ENX) greater than ciprofloxancin (CPFX) much greater than ofloxacin (OFLX), and that of NSAIDs in combination with ENX was BPAA much greater than indomethacin = ketoprofen greater than naproxen greater than ibuprofen greater than pranoprofen. Diclofenac, piroxicam and acetaminophen did not affect GABA responses in the presence of ENX. 4. In the presence of ENX or BPAA, there was a small shift to the right of the concentration-response curve for GABA without any effect on the maximum response. However, the co-administration of these drugs suppressed the maximum of the GABA concentration-response curve, indicating a non-competitive inhibition, for which no voltage-dependency was observed.5. Simultaneous administration of ENX and BPAA also suppressed pentobarbitone (PB)-gated Icl. On the other hand, both PB and phenobarbitone reversed the inhibition of GABA-induced Ic, by coadministration of ENX and BPAA.6. The effect on GABAA responses of co-administration of new quinolones and NSAIDs was not via an interaction with benzodiazepine receptors coupled to the GABAA receptor, since this effect was not reversed by Rol5-1788 or diazepam.7. It is concluded that the co-administration of new quinolones and some of the NSAIDs inhibit GABAergic transmission, and could result in convulsions.

Kinetic properties and regulation of GABAA receptor channels

Single-channel recordings of GABAA receptor single-channel currents have been obtained from mouse spinal cord neurons in cell culture. Detailed kinetic analysis of single-channel main-conductance level currents has allowed development of a preliminary kinetic scheme which describes the gating of the GABAA receptor channel. The essential features of this kinetic scheme are presented in scheme 1 (see above). In this scheme, the GABAA receptor channel is envisioned to exist in multiple open and closed states. Properties can be broken into three main categories. First, the receptor can exist in a closed and nondesensitized set of states. In the kinetic model it is envisioned that there is an unbound (C13), a singly bound (C12), and two doubly bound (C11 and C10) closed states. The singly bound and doubly bound closed states are thought to open to three open states (O1, O2, O3). However, each of the open states opens to two distal closed states whose kinetic properties are similar for all three open states (C4-C9). Only one desensitized state (D0) has been incorporated into the model. While this characterization of desensitization is certainly incomplete, it is an initial step toward including the desensitization process which is clearly evident in whole-cell and single-channel recordings. This kinetic scheme should be considered only an initial working model. A number of features appear to be correct. First, all analyses of open time frequency histograms for GABA- and GABA agonist-induced single-channel openings have demonstrated the presence of at least three distinct open time constants. Furthermore, the concentration-dependent change in the relative frequency of occurrence of the three open states suggests that the open states occur from singly and doubly bound forms of the receptor. Second, the presence of two brief closed states adjacent to the open states appears fairly secure. However, it should be noted that the kinetic analysis primarily suggests that each open state opens to two brief adjacent closed states in a concentration-independent manner. While we have indicated that these two closed states are distal to the three open states, the actual assignment of the location of these states is unclear. Another interpretation of these data is that there is only a single distal closed state and that the proximal, extraburst closed states have very brief durations that are similar to each other. What appears clear, however, is that the open states can close to either brief closed state.(ABSTRACT TRUNCATED AT 400 WORDS)

Bay K 8644 potentiates the anxiolytic effect of ethanol

The possible role of voltage-sensitive calcium channels (VSCCs) in the anxiolytic effect of ethanol was investigated using three different doses of ethanol (0.5, 1.0 and 2.0 g/kg) with calcium agonist Bay K 8644 (0.5 mg/kg) and calcium antagonist nifedipine (5 mg/kg) in rats. Ethanol produced an anxiolytic effect in a dose-dependent manner. The Bay K 8644-potentiated anxiolytic effect of ethanol, however, Bay K 8644 did not alter anxiety when used alone. Nifedipine itself showed an anxiolytic effect but did not change the ethanol-induced anxiolytic effect. This finding may lead to the consideration of the neurochemical mechanisms of the anxiolytic effects of ethanol and nifedipine as they vary from each other.

Transient increase in [3H]Ro 15-4513 specific binding in the superficial gray layer of the rat superior colliculus induced by visual deafferentation

The imidazodiazepine compound [3H]Ro 15-4513, a partial inverse agonist of benzodiazepine receptors of the central type, binds with high affinity (order of 10(-8) M) to a single population of benzodiazepine binding sites in the mammalian central nervous system. A quantitative autoradiographic study was carried out to determine the effects of one eye removal on [3H]Ro 15-4513 specific binding to rat brain sections in the superficial gray layer or stratum griseum superficiale (SGS) of the superior colliculus. Retinal afferent degeneration due to right eye removal, performed 3 and 7 days before sacrifice, led to a significant and symmetrical increase in the [3H]Ro 15-4513 specific binding in both right and left SGS by enhancing the binding affinity of the radioligand. This transient phenomenon disappeared when a longer survival period of 45 days was allowed to elapse. Conversely, unilateral lesion of the primary visual areas had no apparent effects on the specific binding of the radioligand. The absence of any loss of binding sites after either type of lesion suggests that the benzodiazepine receptors are probably not situated on the optic nerve axon terminals, nor on the cortical axon terminals originating from primary visual areas. In the SGS, as in other rat brain structures, benzodiazepine receptors of the central type are functionally coupled with GABAA receptors and form 'GABAA receptors/benzodiazepine receptors/chloride channel' complexes. The involvement of the local GABAergic system in the postlesion plasticity of benzodiazepine receptors was studied by testing the effects of exogenously applied GABA on [3H]Ro 15-4513 specific binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular models for recognition and activation at the benzodiazepine receptor: a review

In this minireview an overview description of the different pharmacophores that have been proposed for the benzodiazepine receptor is presented. The methodology as well as the experimental information used in the development of each model will be described. Finally, the new experimental discoveries that are likely to affect the models presented are briefly indicated.

Modulation of GABA-mediated inhibition in rat cerebellar slices by benzodiazepine receptor ligands

1. Extracellular recordings were made from the Purkinje cell layer of rat cerebellar slices. Compounds were perfused over the slice and bipolar stimulating electrodes placed in the external layer of the slice close to the recording electrode. 2. Stimulus-evoked inhibition of Purkinje layer cell activity was sensitive to bicuculline methiodide and picrotoxin, suggesting it was gamma-aminobutyric acid (GABA) mediated. The benzodiazepine ligands RU 32007 and Ro 19-0528 reversibly increased the period of inhibition, as did pentobarbital. This benzodiazepine effect was antagonised by Ro 15-1788. 3. Five inverse agonists all reduced the period of stimulated inhibition and this effect was reversed by Ro 15-1788, suggesting the involvement of benzodiazepine receptors. 4. It is concluded that this system provides a convenient physiological and possibly quantitative model for studying the action of benzodiazepine receptor ligands.

Pentobarbital selectively blocks supraspinal morphine analgesia. Evidence for GABAA receptor involvement

The possibility that the GABAA receptor is involved in supraspinal morphine analgesia was investigated in rats using pentobarbital and other GABAA receptor-chloride channel ligands. Inhibition of tail-flick was used as an index of analgesia. Pentobarbital almost completely reversed intracerebroventricular (i.c.v.) morphine analgesia, but did not affect intrathecal (i.t.) morphine analgesia. Phenobarbital had a similar effect but was much weaker. Picrotoxin blocked the reversal effect of pentobarbital. Diazepam, when given together with pentobarbital, enhanced the effect of pentobarbital. Furthermore, muscimol reversed i.c.v. morphine analgesia without affecting i.t. morphine analgesia. Our results strongly suggest that the GABAA receptor is involved in supraspinal morphine analgesia.

Pharmacological characteristics of two different types of inhibitory GABA receptors on Achatina fulica neurones

GABA (gamma-aminobutyric acid) receptors of Achatina fulica neurones have been classified into two types associated with neuronal inhibition and one type with excitation. The pharmacological features of muscimol I and baclofen types associated with inhibition were investigated in this study. Activation of muscimol I type receptors on TAN (tonically autoactive neurone) by GABA, muscimol and trans-4-aminocrotonic acid (TACA) produced a transient outward current (Iout) with an increase in membrane conductance (g). Their relative potencies at GABA ED50 (approximately 10(-4) M) were: GABA: muscimol: TACA = 1:0.6:0.3. The relation between Iout and g increase (delta g) induced by various concentrations of these compounds was linear. The Hill coefficients for GABA were close to 1.0. The GABA effects were potentiated by pentobarbitone, antagonized competitively by pitrazepin and non-competitively by picrotoxin and diazepam, and unaffected by bicuculline. The reversal potentials of the effects of GABA, muscimol and TACA on TAN changed under various [Cl-]0 according to the Nernst equation for Ec1, but not under various [K+]0 and [Na+]0. Activation of baclofen type GABA receptors on RPeNLN (right pedal nerve large neurone) by GABA and (+/-)-baclofen produced a slow Iout with an increase in g. The two compounds were almost equipotent (ED50: approximately 3 x 10(-4) M). The relation between Iout and delta g produced by various concentrations was linear. The Hill coefficients for GABA were also close to 1.0. The reversal potentials of GABA and (+/-)-baclofen on RPeNLN changed under various [K+]0 according to the Nernst equation for EK, but not under various [Cl-]0 and [Na+]0. The two compounds hardly affected the voltage-gated and slowly inactivating calcium current. The Iout produced by GABA and (+/-)-baclofen was reduced by tetraethylammonium chloride, but was unaffected by 4-aminopyridine, bicuculline, pitrazepin and picrotoxin. In conclusion, the pharmacological features of muscimol I type GABA receptors are partly comparable to those of mammalian GABAA receptors, except for the influences of bicuculline and diazepam: the features of the baclofen type GABA receptor, which did not occur with muscimol I type receptors in the same neurone, were similar to those of GABAB.

DBI (diazepam binding inhibitor): the precursor of a family of endogenous modulators of GABAA receptor function. History, perspectives, and clinical implications

Biochemical, electrophysiological, and lately, molecular biological techniques have shown that GABAA receptors are heterogeneous supramolecular complexes and can be divided into at least three major subgroups: GABAA1, GABAA2, and GABAA3. They differ mainly in the structural and functional properties of the allosteric modulatory center associated with each one of them. This paper will review the present state of research based on the evidence that DBI (diazepam binding inhibitor) and its natural processing products can selectively modulate GABAergic transmission at different GABAA receptor subtypes. Furthermore, the possibility that the DBI family of peptides represents a novel and meaningful neurochemical correlate for neuropsychiatric pathology, sustained by an alteration of GABAergic transmission, will be discussed.

Effect of diazepam on electroacupuncture-induced changes in regional gamma-aminobutyric acid of the rat central nervous system

Electroacupuncture-induced analgesia (EAA), as assessed in terms of tail flick latency in adult male albino rats, was reduced or completely withdrawn by co-treatment of diazepam with electroacupuncture (EA) (10 Hz, 1 volt), although diazepam (5-20 mg/kg, i.p.) alone had no analgesic effect. Further, it was found that only the gamma-aminobutyric acid (GABA) system of thalamus and pons-medulla regions were involved in EAA. The EA-induced inhibition of GABAergic activity in the thalamus and pons-medulla was disinhibited when diazepam was pre-administered to rats treated with a single EA and reduced the EAA.

Augmentation of GABA-induced chloride current in frog sensory neurons by diazepam

The effect of diazepam (DZP) on the GABA-induced macroscopic and microscopic Cl- current was investigated in isolated frog sensory neurons using both 'concentration-clamp' and patch-clamp techniques. At concentration range between 10(-9) and 10(-4) M, DZP itself evoked no response but potentiated time- and dose-dependently the subthreshold GABA responses, though at high DZP concentrations beyond 10(-5) M the potentiation ratio decreased. The potentiation effect was long-lasting and desensitized slowly over the course of several 10 minutes after washing-out of DZP. DZP potentiated GABA response without shifting the GABA reversal potential. The entire GABA dose-response curve was shifted in a parallel manner to the left by adding DZP without changing cooperatively: the Hill slope was 2.0. The potentiation of GABA response by DZP did not depend on either inward or outward direction of the Cl- current but slightly on the membrane potential. The time constants of activation of desensitization of GABA-gated Cl- current consisted of fast and slow components, respectively. The slow components were concentration-dependent, and significantly changed in the presence of DZP, while DZP had little effects on fast components. In the 'inside-out' configuration, the addition of DZP activated GABA-receptor ionophore complexes under subthreshold without changing the single Cl- channel conductance. It is concluded that DZP may act at a site to modulate GABA binding, in which DZP increases GABA binding affinity and also affects the kinetics of GABA-gated Cl- channels, indicating that DZP has dual action on the GABA-induced responses.

Antiepileptic drug actions

Antiepileptic drugs (AEDs) vary in their efficacy against generalized tonic-clonic, myoclonic, and absence seizures, suggesting different mechanisms of action. Phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA) reduced the ability of mouse central neurons to sustain high-frequency repetitive firing of action potentials (SRF) at therapeutic free serum concentrations. Phenobarbital (PB) and the benzodiazepines (BZDs), diazepam (DZP), clonazepam (CZP), and lorazepam (LZP), also reduced SRF, but only at supratherapeutic free serum concentrations achieved in treatment of generalized tonic-clonic status epilepticus. These AEDs interact with sodium channels to slow the rate of recovery of the channels from inactivation. The BZDs and PB enhanced gamma-aminobutyric acid (GABA) responses evoked on mouse central neurons by binding to two different sites on the GABAA receptor channel. BZDs increased the frequency of GABA receptor channel openings. In contrast, barbiturates increased the open duration of these channels. VPA enhanced brain GABA concentration and may enhance release of GABA from nerve terminals. Ethosuximide (ESM) reduced a small transient calcium current which has been shown to be involved in slow rhythmic firing of certain neurons. Reduction of SRF, enhancement of GABA-ergic inhibition, and reduction of calcium current may be, in part, the bases for AED action against generalized tonic-clonic, myoclonic, and absence seizures, respectively.

Benzodiazepines in the treatment of alcoholism

This chapter comprises three sections that cover the main aspects of benzodiazepines and alcohol: (1) the basic pharmacology of benzodiazepines; (2) use of benzodiazepines in the treatment of withdrawal; and (3) the use of benzodiazepines in treating alcoholics. The basic studies suggest that a major site of action of alcohol may be the GABA/benzodiazepine receptor complex and that compensatory alterations in this complex may underly withdrawal. In the section on alcohol withdrawal, interactions between the GABA/benzodiazepine receptor complex, sympathetic nervous system, and hypothalamic-pituitary-adrenal axis are discussed. Use of benzodiazepines in the treatment of the alcohol withdrawal syndrome are reviewed, including the possibility that the benzodiazepines may prevent withdrawal-induced "kindling." Lastly, we review indications for, and efficacy of, benzodiazepines in long-term treatment of patients with alcoholism. Benzodiazepines are not indicated for the treatment of alcoholism. Furthermore, they have very few indications in alcoholics and their dependency-producing potency has to be appreciated when they are used in patients with alcoholism.

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)

Distribution of benzodiazepine receptors in the rat superior colliculus: a light and electron microscope quantitative autoradiographic study

The distribution of benzodiazepine (Bdz) receptors of the central type was analysed in the superficial grey layer of the rat superior colliculus from light and electron microscope autoradiographs, using the highly specific partial reverse agonist [3H]Ro 15-4513, a radioligand which can be crosslinked to its binding sites by ultraviolet rays. Biochemical characteristics of the binding were first defined by liquid scintillation count on unfixed cryostat mesencephalic brain slices. Saturation curves (1.6-20 nM) and Scatchard plot indicated that the radioligand bound with a high affinity (Kd = 11 nM) to a single population of sites (Bmax = 650 fmol/mg dry tissue). A slight primary chemical fixation of the brain did not significantly modify the binding characteristics. The consolidation of the specific binding by ultraviolet light on prefixed brain slices was found to be optimal after a 45-min illumination period. The distribution of Bdz sites on light and electron microscope autoradiographs was then analysed by applying these binding conditions. Prefixed brain slices (50 micron thick, Vibratome) were incubated in the 15 nM radioligand in the absence (total binding) or in the presence (non-specific binding) of the non-radioactive antagonist Ro 15-1788 (10(-5) M). Quantitative light microscopic study of Epon-embedded semithin sections showed that 95% of the silver grains of the specific label were located on the neuropil to the detriment of the neuronal and glial cell compartments. In the electron microscopic study, the distribution of the specific binding sites was statistically analysed over a total of more than 10 identified single or junctional tissue compartments, using the 50% probability circle method (Williams, 1969). Apart from a slight labeling of varicose profiles, the specific labeling was found to be concentrated on two particular tissue compartments: the percentage of grains associated with contacts between varicosities and dendrites was 32%, and that associated with axodendritic synapses was 16% of the total specific labeling measured over all compartments combined. A low proportion (33%) of the labeled axodendritic interfaces was characterized by a synaptic differentiation. These results suggest that both synaptic and non-synaptic Bdz receptors are present in the rat superior colliculus, and may each modulate neuronal cell activity in a different way.

Kinetic and pharmacological properties of the GABA-induced chloride current in Aplysia neurones: a 'concentration clamp' study

1. gamma-Aminobutyric acid (GABA) was applied by the 'concentration clamp' technique to isolated neurones of Aplysia. GABA induced a chloride current (ICl) due to activation of a single class of chloride-channel. 2. The concentration-response curve for the peak ICl gave an apparent dissociation constant of 6.4 X 10(-5) M and a Hill coefficient of 0.88. The current-voltage relationship was linear in the voltage range examined (-40 to +10 mV). 3. The activation phase of the ICl could be fitted to a single exponential function and desensitization followed the sum of two exponential functions. The time constants of activation and desensitization decreased with increasing concentrations of GABA but were voltage-independent. The recovery process from desensitization also followed the sum of two exponential functions. 4. As for the rate-limiting step of the channel activation, the hyperbolic relationship between the activation rate and GABA concentration showed that the rapid binding assumption holds, suggesting that the isomerization step is rate-limiting. The apparent channel closing rate constant was estimated to be 10 s-1 from the ordinate intercept of the linear part of the above relationship at lower concentrations. 5. Muscimol and beta-alanine induced a ICl, which cross-desensitized with that evoked by GABA. The GABA-ICl was not enhanced by diazepam (10(-6) M) or alpha-chloralose (10(-3) M), in fact depressant effects were evident. 6. Pentobarbitone decreased the GABA-ICl non-competitively without altering activation or desensitization kinetics. The concentration-inhibition curve gave a KD value of 8.9 x 10(-5) M and a Hill coefficient of 1.0. 7. These results suggest that GABA activates a single class of Cl channel in Aplysia neurones, which have one binding site for the agonist. The GABA receptor-Cl channel complex in Aplysia is pharmacologically and perhaps structurally different from that in vertebrates.

Intracellular picrotoxin blocks pentobarbital-gated Cl- conductance

Using the 'inside-out' configuration of frog sensory neurons, we studied the effect of intracellular picrotoxin on the pentobarbital-gated single channel response of Cl- -current (iCl). The pentobarbital-induced iCl showed no voltage-dependency and the single channel conductance (gamma Cl) was 16 +/- 3.1 pS (n = 6). Picrotoxin caused the pentobarbital-gated Cl- channels to react in a flickering pattern and then finally caused them to cease their opening altogether. This inhibitory action of picrotoxin was reversible.

Anticonvulsant drug actions on neurons in cell culture

Two actions of clinically used antiepileptic drugs have been studied using mouse neurons in primary dissociated cell culture. The antiepileptic drugs phenytoin, carbamazepine and valproic acid were demonstrated to limit sustained high frequency repetitive firing of action potentials at free serum concentrations that are achieved in patients being treated for epilepsy. Furthermore, an active metabolite of carbamazepine also limited sustained high frequency repetitive firing while inactive metabolites of phenytoin and carbamazepine did not limit sustained high frequency repetitive firing. Phenobarbital and clinically used benzodiazepines limited sustained high frequency repetitive firing of action potentials, but only at concentrations achieved during the treatment of generalized tonic-clonic status epilepticus. Ethosuximide did not limit sustained high frequency repetitive firing even at concentrations four times those achieved in the serum of patients treated for generalized absence seizures. Phenobarbital and clinically used benzodiazepines enhanced postsynaptic GABA responses at concentrations achieved free in the serum during treatment of generalized tonic-clonic or generalized absence seizures. However, phenytoin, carbamazepine, valproic acid and ethosuximide did not modify postsynaptic GABA responses at therapeutic free serum concentrations. These results suggest that the ability of antiepileptic drugs to block generalized tonic-clonic seizures and generalized tonic-clonic status epilepticus may be related to their ability to block high frequency repetitive firing of neurons. The mechanism underlying blockade of myoclonic seizures may be related to the ability of antiepileptic drugs to enhance GABAergic synaptic transmission. The mechanism underlying management of generalized absence seizures remains unclear.

Effects of non-sedative anxiolytic drugs on responses to GABA and on diazepam-induced enhancement of these responses on mouse neurones in cell culture

1. Intracellular microelectrode recording techniques were performed on mouse spinal cord and cerebral hemisphere neurones grown in primary dissociated cell culture. The effects of several anxiolytics applied by local pressure ejection on responses to gamma-aminobutyric acid (GABA) evoked by iontophoresis were investigated. Responses to GABA were depolarizing since intracellular chloride ion concentration was increased by injection from potassium chloride (3M)-filled recording micropipettes and neurones were held at large negative membrane potentials (-70 to -90 mV). The agents studied were six 'non-sedative anxiolytics', CL 218,872 (3-methyl-6-(3-trifluoromethyl-phenyl)1,2,4-triazolo(4,3-b) pyridazine), PK 8165 (2-phenyl-4-(2-(4-piperidinyl)ethyl)-quinoline), PK 9084 (2-phenyl-4-(2-(3-piperidinyl)ethyl)-quinoline), CGS 9896 (2-(4-chlorophenyl)-2,5-dihydropyrazolo(4,3-c)quinoline-3(3H)-one) , ZK 91296 (ethyl 5-benzyloxy-4-methoxymethyl-beta-carboline-3-beta-carboxylate), buspirone (8-4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl-8-azaspiro[4.5]decane- 7,9- dione), and two sedative anxiolytics, diazepam and zopiclone [( 6-(5-chloro-2-pyridyl)-6,7-dihydro-7-oxo-5H-pyrrolo[3,4-b]pyrazin- 5- yl]4-methyl-1-piperazinecarboxylate). 2. Direct effects on responses to GABA were studied for all drugs applied in varying concentrations. For the drugs which significantly altered responses to GABA, the effects of the benzodiazepine receptor antagonists Ro 15-1788 (ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo(1,5a)-(1,4)benzodi azepine - 3-carboxylate) and CGS 8216 (2-phenylpyrazolo(4,3-c)-quinolin-3(5H)-one) were evaluated. For the drugs devoid of significant direct effect on responses to GABA, the influence on diazepam-induced enhancement of responses to GABA was evaluated. 3. Diazepam, zopiclone and CL 218,872 concentration-dependently and reversibly enhanced responses to GABA. Maximal enhancement was 82% for diazepam (500 nM), 64% for zopiclone (10 microM) and 20% for CL 218,872 (10 microM). PK 8165 effects varied with concentration, enhancing responses to GABA (up to 18%) at nM concentrations and reducing responses to GABA (up to 90%) at microM concentrations. CGS 9896, ZK 9126, PK 9084 and buspirone, in concentrations ranging from 1 nM to 10 microM, lacked significant direct effects on responses to GABA. 4. The enhancing effects of diazepam, zopiclone, CL 218,872 and PK 8165 were antagonized by Ro 15-1788. However, the reducing effect on responses to GABA of PK 8165 at microM concentrations was not antagonized by CGS 8216. CGS 9896 and ZK 91296 concentration-dependently blocked the diazepam-induced enhancement of responses to GABA. However, PK 9084 and buspirone did not antagonize the diazepam-induced enhancement of responses to GABA. 5. These results indicate that diazepam and zopiclone may be full agonists, CL 218,872 and PK 8165 are partial agonists, and CGS 9896 and ZK 91296 are pure antagonists at benzodiazepine receptors. On the other hand, PK 9084 and buspirone do not interact with benzodiazepine receptors.

Interaction of ethanol with the GABAA receptor in the rat brain: possible involvement of endogenous steroids

The effect of ethanol on the binding of gamma-aminobutyric acid (GABA) agonist, [3H]muscimol, to crude synaptosomal membranes prepared from various rat brain regions was investigated, in vitro, at 37 degrees C. Ethanol altered specific muscimol binding in a biphasic manner--reducing it at concentrations less than 10 mM and subsequently increasing specific binding at concentrations greater than 10 mM. The former effect was due to a decrease of the receptor affinity for an agonist, and the latter, due to an increase of the receptor density. Ethanol interfered also with the effects of "GABAergic" modulatory steroids on muscimol binding. This suggests that steroid-ethanol interactions, occurring at the level of the plasma membrane, may be involved in the molecular mechanism of action of ethanol on the GABAA receptor.

A comparison of the effects of midazolam and pentobarbital on the dose-response of GABA-gated Cl- influx in rat brain microsacs

gamma-Aminobutyric acid (GABA)-gated Cl- influx was studied in a rat brain 'microsac' preparation. Midazolam (a benzodiazepine), at 0.1-10 microM, increased GABA potency (maximum 2-fold) without affecting its efficacy or exerting GABA-mimetic effect. Pentobarbital (10-500 microM) increased GABA potency (maximum 12-fold) and efficacy (maximum 40%). Pentobarbital exhibited GABA-mimetic effect at concentrations above 200 microM. The differential effects of midazolam and pentobarbital in modulating GABA-mediated responses may account for some of the differences between these two classes of drugs.

Changes in central GABAergic function following acute and repeated stress

1. The function of gamma-aminobutyric acid (GABA)ergic systems in response to acute and repeated stressful manipulations was evaluated in both the corpus striatum and frontal cerebral cortex of the rat. 2. In the corpus striatum the activity of the synthetic enzyme for GABA (glutamic acid decarboxylase, GAD) and the levels of GABA were reduced by acute immobilization stress (1 h). GABA turnover was reduced only by acute cold stress (3 h, 4 degrees C). 3. In the frontal cerebral cortex no changes were observed after acute stressful manipulations, but repeated stress (0.5 h immobilization per day for 14 days) enhanced both GAD activity and GABA turnover, and reduced GABA levels. 4. In conclusion, it would appear that the GABAergic system in the corpus striatum of the rat is most sensitive to acute stress and that the system in the frontal cerebral cortex area is preferentially responsive to chronic stress. It is speculated that the cortical GABAergic system is responsible for adaptive responses to the adverse conditions prevailing during chronic stress.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Benzodiazepine enhancement of gamma-aminobutyric acid-mediated chloride ion flux in rat brain synaptoneurosomes

Benzodiazepine agonists such as Ro 11-6896 [B10(+)], diazepam, clonazepam, and flurazepam were found to enhance muscimol-stimulated 36Cl- uptake into rat cerebral cortical synaptoneurosomes. The rank order of potentiation was B10(+) greater than diazepam greater than clonazepam greater than flurazepam. These benzodiazepines had no effect on 36Cl-uptake in the absence of muscimol. Further, the inactive enantiomer, Ro 11-6893 [B10(-)], and the peripheral benzodiazepine receptor ligand Ro 5-4864 did not potentiate muscimol-stimulated 36Cl- uptake at concentrations up to 10 microM. In contrast, the benzodiazepine receptor inverse agonists ethyl-beta-carboline-3-carboxylate and 6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylic acid methyl ester inhibited muscimol stimulated 36Cl- uptake. Benzodiazepines and beta-carbolines altered the apparent K0.5 of muscimol-stimulated 36Cl- uptake, without affecting the Vmax. The effects of both benzodiazepine receptor agonists and inverse agonists were reversed by the benzodiazepine antagonists Ro 15-1788 and CGS-8216. These data further confirm that central benzodiazepine receptors modulate the capacity of gamma-aminobutyric acid receptor agonists to enhance chloride transport and provide a biochemical technique for studying benzodiazepine receptor function in vitro.

Bidirectional nature of benzodiazepine receptor ligands extends to effects on vigilance

The classification of benzodiazepine receptor ligands into agonists, antagonists and inverse agonists is based on biochemical, electrophysiological and behavioural evidence. Agonists potentiate the effects of gamma-aminobutyric acid (GABA) and exhibit anxiolytic, anticonvulsant, hypnotic, amnesic and muscle-relaxant properties; inverse agonists show mirror-image effects in that they may be convulsant and anxiogenic and may increase muscle tone. Antagonists antagonise the effects of both agonists and inverse agonists. Some of the most interesting ligands, however, are those substances with actions intermediate between either those of the agonists and the antagonists, or between those of the antagonists and the inverse agonists. These partial agonists and partial inverse agonists possess only some of the properties of the agonists and inverse agonists, respectively. The present experiments show that the agonist and inverse agonist properties of benzodiazepine receptor ligands can also be revealed in an animal continuous attention task in which rats were required to detect a brief signal during which operation of a lever was rewarded by food. Benzodiazepines and a beta-carboline benzodiazepine receptor agonist, ZK 93423, disrupted performance of this task, as did the antimuscarinic substance, scopolamine. Another beta-carboline, ZK 91296, which has anxiolytic and anticonvulsant properties like benzodiazepines, did not affect performance of the continuous attention task, demonstrating a separation of anxiolytic and sedative properties of such substances. A partial inverse agonist beta-carboline, FG 7142, was able to antagonise the disruptive effects of scopolamine on this task, as was, to a smaller extent, the antagonist ZK 93426. These results are discussed in terms of vigilance-enhancing properties of the inverse agonist beta-carbolines, and the possibility that such vigilance-enhancing effects might contribute to improvement of performance in learning tasks.

Kinetic properties of the pentobarbitone-gated chloride current in frog sensory neurones

1. The kinetic properties of the activation and inactivation (desensitization) phases of pentobarbitone (PB)-induced inward Cl- current (ICl) were studied in isolated frog sensory neurones, following suppression of Na+, K+ and Ca2+ currents, using the concentration jump technique which combines the internal perfusion and the rapid exchange of the external solutions surrounding a neurone with time constants of 2-3 ms. The results were compared with those of the gamma-aminobutyric acid (GABA)-gated ICl. 2. The PB dose-response curve was bell-shaped and the maximum peak value was less than the current induced by 1.7 X 1.5(-5) M-GABA, the concentration at which GABA evoked a half-maximum response. 3. The activation and inactivation phases of PB-induced ICl consisted of double-exponential, fast and slow components, respectively. The time constant of the fast component (tau af) of the activation was relatively stable in a concentration range between 3 X 10(-4) and 6 X 10(-3) M. The time constant of the slow component (tau as) of the activation decreased with increasing PB concentrations. Both the fast and slow components (tau if and tau is) of the inactivation decreased with increasing PB concentrations. 4. Over a wide range of concentrations the tau af and tau as values of the PB-induced ICl were 10-30 times greater than the respective values of GABA-induced ICl. 5. At concentrations below 10(-3) M the PB-induced ICl was voltage dependent at more negative potentials than -20 mV. 6. The PB-induced ICl was blocked by bicuculline and by picrotoxin, but in a different manner. Bicuculline increased the time constants of the activation and inactivation. Picrotoxin had little effect on the activation phase but markedly facilitated the inactivation phase. 7. High concentrations of PB (over 10(-3) M) led to a decline in both the peak and plateau currents of the PB-induced ICl. A transient 'hump' current appeared with wash-out of the external solutions containing high concentrations of PB. This hump current was blocked by bicuculline in a dose-dependent manner. 8. The results suggest the possibilities that the PB receptor-ionophore complexes consist of at least two different components having different affinities and kinetics and that the PB and GABA binding sites are closely located.

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.

Responses to GABA by isolated insect neuronal somata: pharmacology and modulation by a benzodiazepine and a barbiturate

Mechanically dissociated neuronal somata from the thoracic ganglia of Locusta migratoria and Schistocerca gregaria were viable in vitro for hours and were current- and voltage-clamped to record the responses evoked by brief pressure applications of gamma-aminobutyric acid (GABA) in the presence of various modulators. The application of GABA and muscimol, but not baclofen, produced a hyperpolarization and concurrent increase in the membrane conductance. The current underlying this response reversed at -65 mV, was evoked in all cells tested and showed outward rectification. In 6 of 74 Locusta neurones but not in the neurones of Schistocerca, GABA and muscimol evoked a biphasic response. The initial, fast phase was indistinguishable from the GABA-evoked current seen in all neurones. The remaining predominant, slow and long-duration component of the response was an inward current over the membrane potential range 0 to -80 mV, increasing with hyperpolarization. The GABAA antagonists bicuculline and pitrazepin were without effect on the fast GABA response while picrotoxin was a potent blocker of both the fast and the slow GABA responses. Flunitrazepam enhanced the amplitude of the fast response by up to 70% without increasing its duration. Sodium pentobarbital enhanced both the amplitude and the duration of the fast GABA response. We conclude that the locust thoracic neuronal GABA receptor/channel complex resembles the vertebrate GABAA receptor in having associated modulatory receptor sites for benzodiazepines and barbiturates, but differs from it in terms of the pharmacology of the GABA receptor itself.

The GABA/benzodiazepine receptor-chloride ionophore complex: nature and modulation

1. A high affinity, saturable, stereospecific binding site for Benzodiazepines has been found to be functionally and possibly structurally related to a GABA receptor-chloride ionophore complex. 2. There are both central (CNS) as well as "peripheral" binding sites, involving multiple organs. 3. Evidence strongly suggests that mutually exclusive Benzodiazepine agonists and antagonists bind to the same receptor, possibly in an agonist-antagonist-inverse agonist continuum. 4. The search for an endogenous ligand has been inconclusive and the question of such a substance remains open. 5. Although the relationship between this receptor and the Limbic System remains unclear, it seems certain that the Benzodiazepine receptor plays an important role in the modulation of Limbic System excitability.

A diazepam binding inhibitor (DBI)-like neuropeptide is detected in human brain

Diazepam binding inhibitor (DBI), a 11,000 MW neuropeptide, which coexists with GABA and elicits proconflict responses in the rat, has been purified and partially sequenced from rat brain. We now report purification and characterization of a DBI-like neuropeptide from human brain. Its molecular weight and pharmacological profile is identical to that of rat DBI but differs in the amino acid composition and immunologically. The tryptic fragments of human DBI differ from rat DBI in the HPLC elution profile and in the amino acid sequence. Using high affinity specific human DBI antibodies, the distribution of DBI-like immunoreactivity in bioptic samples of human brain appeared to be similar to that of DBI found in rat brain. DBI-like immunoreactivity was also found in spinal fluid of human volunteers. The cerebrospinal fluid content of this peptide might be used as a probe to study whether spinal fluid DBI content changes in neuropsychiatric disorders.

Benzodiazepine pharmacology of cultured mammalian CNS neurons

Many neurons cultured from the embryonic mammalian central nervous system (CNS) express benzodiazepine receptors while some neurons differentiate specific transmitter phenotypes like glutamic acid decarboxylase (GAD), the synthetic enzyme for gamma-aminobutyric acid (GABA). The benzodiazepine receptors in these cultured neurons are often, if not always coupled to a practically ubiquitous GABA-mediated function, activation of Cl- ion conductance. The transmitter signal serves to inhibit neuronal excitability and is facilitated by clinically important benzodiazepines. Here we review some details regarding the pharmacological actions of benzodiazepines on membrane excitability.

Diazepam action on gamma-aminobutyric acid-activated chloride currents in internally perfused frog sensory neurons

The Cl- current (ICl) in gamma-aminobutyric acid (GABA)-sensitive frog sensory neuron was separated from other Na+, Ca2+, and K+ currents using a suction pipette technique which allows internal perfusion under a single-electrode voltage clamp. Diazepam (DZP) itself evoked no response but facilitated the dose- and time-dependently GABA-induced ICl without changing the GABA equilibrium potential (EGABA) at concentrations ranging widely, from 3 X 10(-9) to 10(-4) M. In the presence of DZP, the GABA dose-response curve shifted to the left without changing the maximum current, indicating that DZP modifies the interaction between GABA and its receptor rather than affecting directly the channel activation step. The enhancement of the GABA-induced ICl by DZP depended neither on the membrane voltage nor on the inward or outward direction of the ICl. DZP also potentiated the ICl elicited by GABA agonists such as beta-alanine, taurine, homotaurine, 5-aminovaleric acid, l-GABOB, d-GABOB, glycine, and muscimol. The GABA response enhanced by pentobarbital (PB) was further enhanced by adding DZP, indicating that DZP and PB do not act in the same way. Ro5-3663, a diazepam analogue, enhanced the GABA-induced ICl only in a narrow range of the concentrations but inhibited the current at concentrations higher than 2 X 10(-6) M.

Electrophysiological studies in cultured mouse CNS neurones of the actions of an agonist and an inverse agonist at the benzodiazepine receptor

The action of agents which bind with the benzodiazepine (BZ) receptor has been investigated by use of intracellular recordings from dissociated mouse neurones grown in tissue culture. The agents tested were midazolam (an agonist at the BZ receptor) and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM-an inverse agonist at the BZ receptor). These were applied to the neurone under study by one of the following methods: iontophoresis; pressure application of known concentrations from blunt pipettes; directly in the perfusing medium. On only very few occasions did midazolam or DMCM have a direct effect on the membrane potential (EM) or conductance (GM) of the impaled neurone. For the neurones where direct effects were present, there was no consistent pattern of response. Neither substance affected the threshold for action potential generation. The effect of midazolam and DMCM on responses evoked by iontophoretic application of gamma-aminobutyric acid (GABA) was also investigated. Three parameters were used to quantify GABA responses: the depolarization (VGABA); the increase in GM (gGABA) measured with constant current pulses; using voltage clamp, the GABA current (IGABA). The GABA response should be quantified by a parameter which is linearly related to the number of GABA-operated channels which are conducting at any instant. VGABA does not fulfil this criterion. gGABA is an appropriate parameter, but is difficult to determine for large responses where the membrane is nearly short circuited. IGABA measured during voltage clamp fulfils this criterion. Midazolam (greater than 10(-6) M) reliably potentiated GABA responses with a parallel shift to the left of the dose-response curve. This is in agreement with biochemical studies where BZs increase the affinity of the GABA receptor for its ligand. The effect of DMCM on GABA responses was more variable. In the majority of cases GABA responses were reduced by DMCM. The threshold dose for this depression was usually around 10(-6) M, but was sometimes as low as 10(-8) M. Dose-response curves of IGABA or gGABA showed the inhibition to be of a non-competitive nature. The maximum inhibition achieved was around 70%. For a given neurone, and at doses which did not necessarily cause a reduction of the response to GABA, DMCM could antagonize the potentiating action of midazolam on GABA responses. A possible interpretation is that more than one BZ site per receptor complex must be occupied by a BZ agonist (or inverse agonist) before the functional changes for the complex as a whole can occur. Desensitization to GABA was increased by midazolam.

Excitation of mouse motoneurones by GABA-mediated primary afferent depolarization

Observations on the reflex properties of a mouse spinal cord preparation in vitro are reported. The findings show that the synaptically evoked, GABA-mediated, discharge of action potentials in primary afferent fibres, monitored as the dorsal root reflex, may lead to the excitation of motoneurones. Subthreshold, bicuculline-sensitive increases in motoneuronal excitability, followed by prolonged inhibition, may be seen in preparations in which the delayed reflex is not seen. Thus, primary afferent depolarization may both increase motoneuronal excitability and also cause presynaptic inhibition of afferent input.

Benzodiazepine- and barbiturate-interactions with GABAA receptor responses on lactotrophs

Modulation of the biphasic effect of muscimol on prolactin secretion by benzodiazepines and secobarbital was investigated, using an in vitro superfusion system. The stimulatory effect of low concentrations of muscimol was potentiated by both classes of drugs, and the effect of benzodiazepines appeared to be mediated by central-type benzodiazepine receptors. Neither benzodiazepines nor secobarbital affected the inhibitory response to muscimol. Clonazepam reduced the potency of bicuculline methiodide as an antagonist of the stimulatory effect, but did not alter the potency of picrotoxinin. These results demonstrate a selective potentiation of one component of the GABAA receptor effect on lactotrophs by benzodiazepines and barbiturates and provide evidence for a functional effect of these drugs at a site without the CNS.

Characterization of peripheral benzodiazepine binding sites in human term placenta

Peripheral benzodiazepine binding sites were characterized in human term placental membranes using [3H]PK 11195, which is a ligand specific for peripheral benzodiazepine binding sites. Binding of [3H]PK 11195 to human term placental membranes was found to be saturable. Scatchard analysis revealed a single population of binding sites (r = 0.98). Equilibrium dissociation constant (KD) was 2.1 +/- 0.3 nM, and density of binding sites (Bmax) was 920 +/- 105 fmol/mg protein. The KD value calculated from kinetic experiments was 3.6 +/- 0.2 nM. The ability of various drugs to displace [3H]PK 11195 from human term placental binding sites was tested: the inhibition constants (KI) for PK 11195, Ro 5-4864, and diazepam were 2.9, 11.8, and 177 nM, respectively, whereas clonazepam, methyl-beta-carboline-3-carboxylate, Ro 15-1788, chlordiazepoxide, atropine, and estradiol were inefficient in displacing [3H]PK 11195 (KI greater than 10(-5) M).

Bidirectional control of palatable food consumption through a common benzodiazepine receptor: theory and evidence

A classical approach to the control of food consumption has been to assume separate mechanisms for the arousal to eat, on the one hand, and the satiation of feeding responses, on the other. The present paper is concerned with a single, and a comparatively simple, neuronal mechanism which is endowed with properties to allow the complete determination of the level of feeding, from hyperphagia to anorexia. The model for the control of feeding, which is presented here, draws attention to the benzodiazepine receptor found distributed through the brain, and present in certain hypothalamic nuclei. Recent evidence which characterizes the receptor is reviewed, and the various categories of benzodiazepine receptor ligands are described. Pharmacological data, collected in a palatable food consumption model using non-food-deprived rats, demonstrate that benzodiazepine receptor agonists produce hyperphagia, benzodiazepine receptor inverse agonists produce anorexia, and benzodiazepine receptor antagonists block both effects. Hence, bidirectional control of food intake can be achieved through differential ligand action at a common set of receptors. Speculatively, these data can be extended, if it is assumed that two endogenous ligands exist in the brain which act like benzodiazepine agonist and inverse agonist, respectively. Evidence for the presence in hypothalamic nuclei of endogenous ligands of the latter kind is discussed. Benzodiazepine withdrawal-induced anorexia is also described, and is taken as evidence for the part played by feeding mechanisms in the development of benzodiazepine physical dependence.

Existence of sites for anions and divalent cations in the solubilized gamma-aminobutyric acid/benzodiazepine receptor complex

This study evaluated the ability of gamma-aminobutyric acid (GABA), baclofen, monovalent anions, divalent cations, and various combinations thereof to protect solubilized benzodiazepine (BZ) receptors of types 1 and 2, when contained together on the complex, against heat inactivation. Neither anions, cations, nor GABA alone provided significant protection of solubilized BZ receptors against heat, but inclusion of monovalent anions or divalent cations together with 500 microM GABA did afford protection. Monovalent anions combined with GABA (500 microM) provided 50% to full protection. Divalent cations, such as CaCl2 (2.5 mM) or MgCl2 (2.5 mM) in the presence of GABA (500 microM) yielded 45% and 24% protection, respectively. Other divalent cations tested (Zn2+, Hg2+, Co2+, and Ni2+) were poor protectors, even when combined with GABA. Monovalent anions (200 mM NaCl) and divalent cations (5 mM CaCl2) when tested together provided no protection. Similarly, baclofen (the GABA-B agonist) provided no protection, either alone or together with anions or divalent cations. These results indicate that the independent but interacting recognition sites of GABA, BZ, anions, and divalent cations, previously detected in the membrane-bound state, are retained in the solubilized state.