Studies on the neuropharmacological activity of bicuculline and related compounds

A Structural Rationale for N-Methylbicuculline Acting as a Promiscuous Competitive Antagonist of Inhibitory Pentameric Ligand-Gated Ion Channels

Bicuculline, a valued chemical tool in neurosciences research, is a competitive antagonist of specific GABA_A receptors and affects other pentameric ligand-gated ion channels including the glycine, nicotinic acetylcholine and 5-hydroxytryptamine type 3 receptors. We used a fluorescence-quenching assay and isothermal titration calorimetry to record low-micromolar dissociation constants for N-methylbicuculline interacting with acetylcholine-binding protein and an engineered version called glycine-binding protein (GBP), which provides a surrogate for the heteromeric interface of the extracellular domain of the glycine receptor (GlyR). The 2.4 Å resolution crystal structure of the GBP:N-methylbicuculline complex, sequence and structural alignments reveal similarities and differences between GlyR and the GABA_A receptor-bicuculline interactions. N-methylbicuculline displays a similar conformation in different structures, but adopts distinct orientations enforced by interactions and steric blocks with key residues and plasticity in the binding sites. These features explain the promiscuous activity of bicuculline against the principal inhibitory pentameric ligand-gated ion channels in the CNS.

Increasing Local Excitability of Brainstem Respiratory Nuclei Reveals a Distributed Network Underlying Respiratory Motor Pattern Formation

The core circuit of the respiratory central pattern generator (rCPG) is located in the ventrolateral medulla, especially in the pre-Bötzinger complex (pre-BötC) and the neighboring Bötzinger complex (BötC). To test the hypothesis that this core circuit is embedded within an anatomically distributed pattern-generating network, we investigated whether local disinhibition of the nucleus tractus solitarius (NTS), the Kölliker-Fuse nuclei (KFn), or the midbrain periaqueductal gray area (PAG) can similarly affect the respiratory pattern compared to disinhibition of the pre-BötC/BötC core. In arterially-perfused brainstem preparations of rats, we recorded the three-phase respiratory pattern (inspiration, post-inspiration and late-expiration) from phrenic and vagal nerves before and after bilateral microinjections of the GABA(A)R antagonist bicuculline (50 nl, 10 mM). Local disinhibition of either NTS, pre-BötC/BötC, or KFn, but not PAG, triggered qualitatively similar disruptions of the respiratory pattern resulting in a highly significant increase in the variability of the respiratory cycle length, including inspiratory and expiratory phase durations. To quantitatively analyze these motor pattern perturbations, we measured the strength of phase synchronization between phrenic and vagal motor outputs. This analysis showed that local disinhibition of all brainstem target nuclei, but not the midbrain PAG, significantly decreased the strength of phase synchronization. The convergent perturbations of the respiratory pattern suggest that the rCPG expands rostrally and dorsally from the designated core but does not include higher mid-brain structures. Our data also suggest that excitation-inhibition balance of respiratory network synaptic interactions critically determines the network dynamics that underlie vital respiratory rhythm and pattern formation.

A novel role of intestine epithelial GABAergic signaling in regulating intestinal fluid secretion

γ-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system, and it is produced via the enzymatic activity of glutamic acid decarboxylase (GAD). GABA generates fast biological signaling through type A receptors (GABA(A)R), an anionic channel. Intriguingly, GABA is found in the jejunum epithelium of rats. The present study intended to determine whether a functional GABA signaling system exists in the intestinal epithelium and if so whether the GABA signaling regulates intestinal epithelial functions. RT-PCR, Western blot, and immunohistochemical assays of small intestinal tissues of various species were performed to determine the expression of GABA-signaling proteins in intestinal epithelial cells. Perforated patch-clamp recording was used to measure GABA-induced transmembrane current in the small intestine epithelial cell line IEC-18. The fluid weight-to-intestine length ratio was measured in mice that were treated with GABA(A)R agonist and antagonist. The effect of GABA(A)R antagonist on allergic diarrhea was examined using a mouse model. GABA, GAD, and GABA(A)R subunits were identified in small intestine epithelial cells of mice, rats, pigs, and humans. GABA(A)R agonist induced an inward current and depolarized IEC-18. Both GABA and the GABA(A)R agonist muscimol increased intestinal fluid secretion of rats. The increased intestinal secretion was largely decreased by the GABA(A)R antagonist picrotoxin or gabazine, but not by tetrodotoxin. The expression levels of GABA-signaling proteins were increased in the intestinal epithelium of mice that were sensitized and challenged with ovalbumin (OVA). The OVA-treated mice exhibited diarrhea, which was alleviated by oral administration of gabazine or picrotoxin. An endogenous autocrine GABAergic signaling exists in the mammalian intestinal epithelium, which upregulates intestinal fluid secretion. The intestinal GABAergic signaling becomes intensified in allergic diarrhea, and inhibition of this GABA-signal system alleviates the allergic diarrhea.

Cholinergic suppression of visual responses in primate V1 is mediated by GABAergic inhibition

Acetylcholine (ACh) has been implicated in selective attention. To understand the local circuit action of ACh, we iontophoresed cholinergic agonists into the primate primary visual cortex (V1) while presenting optimal visual stimuli. Consistent with our previous anatomical studies showing that GABAergic neurons in V1 express ACh receptors to a greater extent than do excitatory neurons, we observed suppressed visual responses in 36% of recorded neurons outside V1's primary thalamorecipient layer (4c). This suppression is blocked by the GABA(A) receptor antagonist gabazine. Within layer 4c, ACh release produces a response gain enhancement (Disney AA, Aoki C, Hawken MJ. Neuron 56: 701-713, 2007); elsewhere, ACh suppresses response gain by strengthening inhibition. Our finding contrasts with the observation that the dominant mechanism of suppression in the neocortex of rats is reduced glutamate release. We propose that in primates, distinct cholinergic receptor subtypes are recruited on specific cell types and in specific lamina to yield opposing modulatory effects that together increase neurons' responsiveness to optimal stimuli without changing tuning width.

GABA maintains the proliferation of progenitors in the developing chick ciliary marginal zone and non-pigmented ciliary epithelium

GABA is more than the main inhibitory neurotransmitter found in the adult CNS. Several studies have shown that GABA regulates the proliferation of progenitor and stem cells. This work examined the effects of the GABA(A) receptor system on the proliferation of retinal progenitors and non-pigmented ciliary epithelial (NPE) cells. qRT-PCR and whole-cell patch-clamp electrophysiology were used to characterize the GABA(A) receptor system. To quantify the effects on proliferation by GABA(A) receptor agonists and antagonists, incorporation of thymidine analogues was used. The results showed that the NPE cells express functional extrasynaptic GABA(A) receptors with tonic properties and that low concentration of GABA is required for a baseline level of proliferation. Antagonists of the GABA(A) receptors decreased the proliferation of dissociated E12 NPE cells. Bicuculline also had effects on progenitor cell proliferation in intact E8 and E12 developing retina. The NPE cells had low levels of the Cl-transporter KCC2 compared to the mature retina, suggesting a depolarising role for the GABA(A) receptors. Treatment with KCl, which is known to depolarise membranes, prevented some of the decreased proliferation caused by inhibition of the GABA(A) receptors. This supported the depolarising role for the GABA(A) receptors. Inhibition of L-type voltage-gated Ca(2+) channels (VGCCs) reduced the proliferation in the same way as inhibition of the GABA(A) receptors. Inhibition of the channels increased the expression of the cyclin-dependent kinase inhibitor p27(KIP1), along with the reduced proliferation. These results are consistent with that when the membrane potential indirectly regulates cell proliferation with hyperpolarisation of the membrane potential resulting in decreased cell division. The increased expression of p27(KIP1) after inhibition of either the GABA(A) receptors or the L-type VGCCs suggests a link between the GABA(A) receptors, membrane potential, and intracellular Ca(2+) in regulating the cell cycle.

Antagonistic properties of a natural product-Bicuculline with the gamma-aminobutyric acid receptor: studied through electrostatic potential mapping, electronic and vibrational spectra using ab initio and density functional theory

(+)-Bicuculline (hereinafter referred to as bicuculline), a phthalide isoquinoline alkaloid is of current interest as an antagonist of gamma-aminobutyric acid (GABA). Its inhibitor properties have been studied through molecular electrostatic potential (MEP) mapping of this molecule and GABA receptor. The hot site on the potential surface of bicuculline, which is also isosteric with GABA receptor, has been used to interpret the inhibitor property. A systematic quantum chemical study of the possible conformations, their relative stabilities, FT-Raman, FT-IR and UV-vis spectroscopic analysis of bicuculline has been reported. The optimized geometries, wavenumber and intensity of the vibrational bands of all the conformers of bicuculline have been calculated using ab initio Hartree-Fock (HF) and density functional theory (DFT) employing B3LYP functional and 6-311G(d,p) basis set. Mulliken atomic charges, HOMO-LUMO gap ΔE, ionization potential, dipole moments and total energy have also been obtained for the optimized geometries of both the molecules. TD-DFT method is used to calculate the electronic absorption parameters in gas phase as well as in solvent environment using integral equation formalism-polarizable continuum model (IEF-PCM) employing 6-31G basis set and the results thus obtained are compared with the UV absorption spectra. The combination of experimental and calculated results provides an insight into the structural and vibrational spectroscopic properties of bicuculline.

Synergistic roles of GABAA receptors and SK channels in regulating thalamocortical oscillations

Rhythmic oscillations throughout the cortex are observed during physiological and pathological states of the brain. The thalamus generates sleep spindle oscillations and spike-wave discharges characteristic of absence epilepsy. Much has been learned regarding the mechanisms underlying these oscillations from in vitro brain slice preparations. One widely used model to understand the epileptiform oscillations underlying absence epilepsy involves application of bicuculline methiodide (BMI) to brain slices containing the thalamus. BMI is a well-known GABAA receptor blocker that has previously been discovered to also block small-conductance, calcium-activated potassium (SK) channels. Here we report that the robust epileptiform oscillations observed during BMI application rely synergistically on both GABAA receptor and SK channel antagonism. Neither application of picrotoxin, a selective GABAA receptor antagonist, nor application of apamin, a selective SK channel antagonist, alone yielded the highly synchronized, long-lasting oscillations comparable to those observed during BMI application. However, partial blockade of SK channels by subnanomolar concentrations of apamin combined with picrotoxin sufficiently replicated BMI oscillations. We found that, at the cellular level, apamin enhanced the intrinsic excitability of reticular nucleus (RT) neurons but had no effect on relay neurons. This work suggests that regulation of RT excitability by SK channels can influence the excitability of thalamocortical networks and may illuminate possible pharmacological treatments for absence epilepsy. Finally, our results suggest that changes in the intrinsic properties of individual neurons and changes at the circuit level can robustly modulate these oscillations.

Blockade of GABA, type A, receptors in the rat pontine reticular formation induces rapid eye movement sleep that is dependent upon the cholinergic system

The brainstem reticular formation is an area important to the control of rapid eye movement (REM) sleep. The antagonist of GABA-type A (GABA(A)) receptors, bicuculline methiodide (BMI), injected into the rat nucleus pontis oralis (PnO) of the reticular formation resulted in a long-lasting increase in REM sleep. Thus, one factor controlling REM sleep appears to be the number of functional GABA(A) receptors in the PnO. The long-lasting effect produced by BMI may result from secondary influences on other neurotransmitter systems known to have long-lasting effects. To study this question, rats were surgically prepared for chronic sleep recording and additionally implanted with guide cannulas aimed at sites in the PnO. Multiple, 60 nl, unilateral injections were made either singly or in combination. GABA(A) receptor antagonists, BMI and gabazine (GBZ), produced dose-dependent increases in REM sleep with GBZ being approximately 35 times more potent than BMI. GBZ and the cholinergic agonist, carbachol, produced very similar results, both increasing REM sleep for about 8 h, mainly through increased period frequency, with little reduction in REM latency. Pre-injection of the muscarinic antagonist, atropine, completely blocked the REM sleep-increase by GBZ. GABAergic control of REM sleep in the PnO requires the cholinergic system and may be acting through presynaptic modulation of acetylcholine release.

Differential effects of iontophoretic in vivo application of the GABA(A)-antagonists bicuculline and gabazine in sensory cortex

We have compared the effects of microiontophoretic application of the GABA(A)-receptor antagonists bicuculline (BIC) and gabazine (SR95531) on responses to pure tones and to sinusoidally amplitude-modulated (AM) tones in cells recorded extracellularly from primary auditory cortex (AI) of Mongolian gerbils. Besides similar effects in increasing spontaneous and stimulus-evoked activity and their duration, both drugs elicited differential effects on spectral tuning and synchronized responses to AM tones. In contrast to gabazine, iontophoresis of the less potent GABA(A)-antagonist BIC often resulted in substantial broadening of frequency tuning for pure tones and an elimination of synchronized responses to AM tones, particularly with high ejecting currents. BIC-induced effects which could not be replicated by application of gabazine were presumably due to the well-documented, non-GABAergic side-effects of BIC on calcium-dependent potassium channels. Our results thus provide strong evidence that GABA(A)-mediated inhibition in AI does not sharpen frequency tuning for pure tones, but rather contributes to the processing of fast temporal modulations of sound envelopes. They also demonstrate that BIC can have effects on neuronal response selectivity which are not due to blockade of GABAergic inhibition. The results have profound implications for microiontophoretic studies of the role of intracortical inhibition in sensory cortex.

GABAergic mechanisms in regulating the activity state of substantia nigra pars reticulata neurons

Substantia nigra reticulata is the major output structure of the basal ganglia involved in somatosensory integration and organization of movement. While previous work in vitro and in anesthetized animal preparations suggests that these neurons are autoactive and points to GABA as a primary input regulating their activity, single-unit recording coupled with iontophoresis was used in awake, unrestrained rats to further clarify the role of tonic and phasic GABA input in maintenance and fluctuations of substantia nigra reticulata neuronal activity under physiologically relevant conditions. In contrast to glutamate, which was virtually ineffective at stimulating substantia nigra reticulata neurons in awake rats, all substantia nigra reticulata neurons tested were inhibited by iontophoretic GABA and strongly excited by bicuculline, a GABA-A receptor blocker. The GABA-induced inhibition had short onset and offset latencies, a fading response pattern (a rapid decrease in rate followed by its relative restoration), and was independent of basal discharge rate. The bicuculline-induced excitation was inversely related to discharge rate and current (dose)-dependent in individual units. However, the average discharge rate during bicuculline applications at different currents increased to a similar plateau ( approximately 60 impulses/s), which was about twice the mean basal rates. The excitatory effects of bicuculline were phasically inhibited or completely blocked by brief GABA applications and generally mimicked by gabazine, another selective GABA antagonist. These data as well as neuronal inhibitions induced by nipecotic acid, a selective GABA uptake inhibitor, suggest that substantia nigra reticulata neurons in awake, quietly resting conditions are under tonic, GABA-mediated inhibition. Therefore, because of inherent autoactivity and specifics of afferent inputs, substantia nigra reticulata neurons are very sensitive to phasic alterations in GABA input, which appears to be the primary factor determining fluctuations in their activity states under physiological conditions. While these cells are relatively insensitive to direct activation by glutamate, and resistant to a continuous increase in GABA input, they appear to be very sensitive to a diminished GABA input, which may release them from tonic inhibition and determine their functional hyperactivity.

BICUCULLINE METHIODIDE ATTENUATES HEPATIC INJURY AND DECREASES MORTALITY IN SEPTIC RATS: ROLE OF CYTOKINES

Bicuculline methiodide attenuates inflammation by inhibiting the production of proinflammatory cytokines, such as tumor necrosis factor-alpha, and by increasing the production of the anti-inflammatory cytokine interleukin-10, both of which play important roles in the pathogenesis of sepsis. The aim of this study was to examine the effects of bicuculline methiodide on sepsis in the cecal ligation and puncture septic-rat model. Cytokine production was measured by enzyme-linked immunosorbent assay. Oxidative stress was assessed by determining serum lipid peroxidation and nitrite levels. Hepatic injury was evaluated by determining the levels of serum aspartate aminotransferase, alkaline phosphatase, and total bilirubin. Mortality was recorded within 24 h. Bicuculline methiodide potently decreased the production of tumor necrosis factor-alpha and interleukin-1beta but increased interleukin-10 in serum. Bicuculline methiodide significantly decreased serum lipid peroxidation and nitrite levels. Further, bicuculline methiodide attenuated hepatic injury and reduced mortality after cecal ligation and puncture. Therefore, the alteration of cytokine production may be involved in the effects of bicuculline methiodide on hepatic injury and mortality in septic rats.

Effects of bicuculline methiodide on fast (>200 Hz) electrical oscillations in rat somatosensory cortex

Fast oscillatory activity (more than approximately 200 Hz) has been attracting increasing attention regarding its possible role in both normal brain function and epileptogenesis. Yet, its underlying cellular mechanism remains poorly understood. Our prior investigation of the phenomenon in rat somatosensory cortex indicated that fast oscillations result from repetitive synaptic activation of cortical pyramidal cells originating from GABAergic interneurons (). To test this hypothesis, the effects of topical application of the gamma-aminobutyric acid-A (GABA(A)) antagonist bicuculline methiodide (BMI) on fast oscillations were examined. At subconvulsive concentrations (approximately 10 microM), BMI application resulted in a pronounced enhancement of fast activity, in some trials doubling the number of oscillatory cycles evoked by whisker stimulation. The amplitude and frequency of fast activity were not affected by BMI in a statistically significant fashion. At higher concentrations, BMI application resulted in the emergence of recurring spontaneous slow-wave discharges resembling interictal spikes (IIS) and the eventual onset of seizure. High-pass filtering of the IIS revealed that a burst of fast oscillations accompanied the spontaneous discharge. This activity was present in both the pre- and the postictal regimes, in which its morphology and spatial distribution were largely indistinguishable. These data indicate that fast cortical oscillations do not reflect GABAergic postsynaptic currents. An alternate account consistent with results observed to date is that this activity may instead arise from population spiking in pyramidal cells, possibly mediated by electrotonic coupling in a manner analogous to that underlying 200-Hz ripple in the hippocampus. Additionally, fast oscillations occur within spontaneous epileptiform discharges. However, at least under the present experimental conditions, they do not appear to be a reliable predictor of seizure onset nor an indicator of the seizure focus.

Inhibition of nicotinic acetylcholine receptors by bicuculline

A study was made on the effects of bicuculline, the classical gamma-aminobutyric acid-A receptor antagonist, on heteromeric mouse muscle alphabetagammadelta, heteromeric neuronal rat alpha2beta4 and alpha4beta2 and homomeric human alpha7 nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes. Bicuculline reduced the ACh-induced currents in a rapid and reversible way, with IC50 values of 34+/-1.5 microM for mouse muscle alphabetagammadelta and 12.4+/-0.7 and 18+/-1 microM for rat neuronal alpha2beta4 and alpha4beta2 nAChRs, respectively. Therefore, the three types of heteromeric receptors are inhibited by bicuculline but the neuronal alpha2beta4 and alpha4beta2 receptors were more sensitive than the muscle alphabetagammadelta receptor. The Hill coefficients for ACh-current inhibition were close to one for all types of receptors, suggesting a single site of action for bicuculline inhibition of nAChRs. Bicuculline shifted the ACh-dose-current response curve to the right and the maximal current was reduced, a reduction that for the heteromeric receptors was not overcome by high concentrations of ACh. The effect of bicuculline was examined at different membrane potentials, and the ACh-current-membrane potential relationships obtained indicate that the inhibition by bicuculline is voltage-dependent for muscle alphabetagammadelta and neuronal alpha2beta4 and alpha4beta2 nAChRs. All these results are consistent with the notion that bicuculline blocks the heteromeric muscle and neuronal nAChRs in a non-competitive way. Studies were also made on the wild type (wt alpha7) and mutant leu-to-threo (L248T) homomeric human neuronal alpha7-nAChRs. In sharp contrast to the heteromeric ACh receptors examined, bicuculline blocked in a competitive way the homomeric wt alpha7-nAChRs, as evidenced by a parallel shift of the bicuculline dose-ACh-current inhibition on raising the ACh concentration. Moreover, similar to the effects of serotonin on wt and mutant alpha7 ACh receptors, the mutation converted bicuculline from an antagonist into a competitive agonist. All this suggests that bicuculline may serve as a lead molecule to design new anticholinergic substances.

Evidence for a non-GABAergic action of quaternary salts of bicuculline on dopaminergic neurones

Intracellular recordings were made from neurones, presumed to be dopaminergic, in the rat midbrain slice preparation. Bicuculline methiodide (BMI) and methochloride (BMC) reversibly blocked the slow, apamin-sensitive component of the afterhyperpolarization in these cells. The IC50 for this effect was about 26 microM. In comparison, BMC antagonized the input resistance decrease evoked by muscimol (3 microM) with an IC50 of 7 microM. The base of bicuculline was less potent in blocking the slow afterhyperpolarization. SR95531 (2-[carboxy-3'-propyl]-3-amino-6-paramethoxy-phenyl-pyridaziniu m bromide), another competitive GABA(A) antagonist, and picrotoxin, a non-competitive GABA(A) antagonist, also blocked the action of muscimol (IC50s: 2 and 12 microM respectively), but had no effect on the afterhyperpolarization at a concentration of up to 100 microM. Moreover, 100 microM SR95531 did not affect the blockade of the afterhyperpolarization by BMC. This blockade persisted in the presence of tetrodotoxin and was apparently not due to a reduction of calcium entry, suggesting that it involved a direct action on the channels which mediate this afterhyperpolarization. These results strongly suggest that quaternary salts of bicuculline act on more than one target in the central nervous system. Thus, the involvement of GABA(A) receptors in a given effect cannot be proven solely on the basis of its blockade by these agents.

Bicuculline methiodide potentiates NMDA-dependent burst firing in rat dopamine neurons by blocking apamin-sensitive Ca2+-activated K+ currents

Apamin, a bee venom toxin which blocks a Ca2+-dependent K+ current, potentiates N-methyl-D-aspartate (NMDA)-induced burst firing in dopamine neurons. We now report that burst firing is also potentiated by an apamin-like effect of bicuculline methiodide (BMI) at the same concentration (30 microM) which blocks GABA(A) receptors in vitro. Using microelectrodes to record intracellularly from rat dopamine neurons in the midbrain slice, BMI reduced the apamin-sensitive afterhyperpolarization in all cells tested. BMI also mimicked apamin (100 nM) by potentiating burst firing produced by a concentration of NMDA (10 microM) which is too low to evoke burst firing when perfused alone. When recording under voltage-clamp, both BMI and apamin reduced a depolarization-activated outward current which was also sensitive to perfusate containing no-added Ca2+. Although picrotoxin (100 microM) and bicuculline free base (30 microM) blocked the inhibition of firing produced by the GABA(A) agonist isoguvacine (100 microM), neither had apamin-like effects. We conclude that BMI potentiates burst firing by blocking an apamin-sensitive Ca2+-activated K+ current.

The Drosophila easily shocked gene: a mutation in a phospholipid synthetic pathway causes seizure, neuronal failure, and paralysis

We have characterized easily shocked (eas), a Drosophila "band-sensitive" paralytic mutant. Electrophysiological recordings from flight muscles in the giant fiber pathway of adult eas flies reveal that induction of paralysis with electrical stimulation results in a brief seizure, followed by a failure of the muscles to respond to giant fiber stimulation. Molecular cloning, germline transformation, and biochemical experiments show that eas mutants are defective in the gene for ethanolamine kinase, which is required for a pathway of phosphatidylethanolamine synthesis. Assays of phospholipid composition reveal that total phosphatidylethanolamine is decreased in eas mutants. The data suggest that eas bang sensitivity is due to an excitability defect caused by altered membrane phospholipid composition.

Effects of the epileptogenic agent bicuculline methiodide on membrane currents induced by N-methyl-D-aspartate and kainate (oocyte; Xenopus laevis)

The actions of bicuculline methiodide (BIC) on N-methyl-D-aspartate (NMDA)- and kainate (KA)-activated ion channels were studied using Xenopus oocytes, previously microinjected with RNA from rat brains. BIC reduced NMDA- and, with a lower efficiency, KA-induced membrane currents in a dose-dependent manner. The BIC effect on both agonist responses showed a voltage dependency with a lower grade of reduction at more positive holding potentials. By increasing the concentrations of the agonists, the reduction of the agonist-induced membrane currents by BIC was increased.

Does bicuculline antagonize NMDA receptors? Further evidence in the rat striatum

In two areas of the rat striatum, the in vitro N-methyl-D-aspartate (NMDA, 50 microM)-evoked release of [3H]dopamine was studied in the presence of bicuculline (5 and 50 microM), an antagonist of GABAA receptors. The responses observed with the higher concentration (50 microM) is compatible with an antagonistic activity of bicuculline on NMDA receptor, as recently reported by Wright and Nowak.

gamma-Aminobutyric acidA receptor pharmacology in rat cerebral cortical synaptoneurosomes

Equilibrium binding interactions at the gamma-aminobutyric acid (GABA) and benzodiazepine recognition sites on the GABAA receptor-Cl- ionophore complex were studied using a vesicular synaptoneurosome (microsacs) preparation of rat brain in a physiological HEPES buffer similar to that applied successfully in recent GABAergic 36Cl- flux measurements. NO 328, a GABA reuptake inhibitor, was included in the binding assays to prevent the uptake of [3H]muscimol. Under these conditions, the equilibrium dissociation constant (KD) values for [3H]muscimol and [3H]diazepam bindings are 1.9 microM and 40 nM, respectively. Binding affinities for these and other GABA and benzodiazepine agonists and antagonists correlate well with the known physiological doses required to elicit functional activity. This new in vitro binding protocol coupled with 36Cl- flux studies should prove to be of value in reassessing the pharmacology of the GABAA receptor complex in a more physiological environment.

The role of GABA and glycine in the physostigmine-induced alterations of spinal cord reflexes

The purpose of this study was to determine which inhibitory pathway(s) mediate the alterations in the monosynaptic (MSR) and polysynaptic (PSR) reflexes after two different doses of physostigmine. It was found previously that 0.8 mg/kg physostigmine facilitated the MSR and 2.0 mg/kg initially depressed and then facilitated the MSR. Both doses facilitated the PSR. In this study, the animals were pretreated with either strychnine (0.1 mg/kg) or bicuculline (0.5 mg/kg), prior to the administration of either dose of physostigmine. It was found that both strychnine and bicuculline blocked the facilitation produced by the small dose of physostigmine, while bicuculline alone blocked the depression of the MSR produced by the large dose of physostigmine. Strychnine partially blocked the effects of both doses of physostigmine on the PSR, while bicuculline only partially blocked the effects of the small dose of physostigmine. These data suggest that the depression of the MSR was the result of a GABA-mediated pathway, while the facilitation of MSR involved both glycine and GABA.

Bicuculline blocks nicotinic acetylcholine response in isolated intermediate lobe cells of the pig

1. The effect of bicuculline on nicotinic acetylcholine (ACh) responses in isolated intermediate lobe (IL) cells of the pig was investigated by use of patch-clamp techniques. Bicuculline was found to reduce ACh-evoked whole-cell currents (IACh) in all cells tested (n = 40). 2. The blocking effect of bicuculline on IACh was dose-dependent, the concentration producing half-maximal blockade being 43.8 microM. 3. The blockade of IACh by bicuculline was not voltage-dependent at membrane potentials above -60 mV, but a slight voltage-dependence was observed at holding potentials (HP) of -80 and -100 mV. 4. The inhibitory effect of bicuculline on IACh was partially competitive at a HP of -60 mV. 5. Neither SR 95531, a pyridazinyl gamma-aminobutyric acid derivative, nor t-butylbicyclophosphorothionate (TBPS) blocked IACh in IL cells. 6. It is concluded that bicuculline interacts directly with the ACh receptor-ionophore complex on porcine IL cells.

Effects of GABA antagonists, SR 95531 and bicuculline, on GABAA receptor-regulated chloride flux in rat cortical synaptoneurosomes

Synaptoneurosomes isolated from cerebral cortices of male Sprague-Dawley rats were used for studying GABAA receptor-regulated chloride influx. The in vitro effects of GABA antagonists, SR 95531 (a pyridazinyl GABA derivative) and bicuculline, on pentobarbital-stimulated, muscimol-stimulated or flunitrazepam-enhanced, muscimol-stimulated chloride uptake were studied. The chloride uptake was determined at 30 degrees C, for 5 sec. Pentobarbital and muscimol produced a maximal stimulation of chloride uptake in cortical synaptoneurosomes at 500 microM and 50 microMs, respectively. SR 95531 as well as bicuculline had no effect on the basal uptake of chloride. Whereas, SR 95531 (0.3 - 30 microM) and bicuculline (0.1 - 100 microM), when added 5 min before muscimol (50 microM), produced a significant concentration-dependent inhibition of muscimol (50 microM)-stimulated chloride uptake (IC50S of 0.89 +/- 0.11 microM and 13.45 +/- 2.10 microM, respectively). In studies of the inhibitory effects of SR 95531 and bicuculline on pentobarbital (500 microM)-stimulated chloride uptake, the IC50S were 0.81 +/- 0.12 microM and 3.86 +/- 1.14 microM, respectively. SR 95531 exhibited a more potent inhibitory effect than bicuculline on flunitrazepam-enhanced, muscimol-stimulated chloride uptake. The results revealed that SR 95531 has a more potent antagonistic effect than bicuculline on GABAA-regulated chloride flux.

The action of GABA receptor agonists and antagonists on muscle membrane conductance in Schistocerca gregaria

1. The properties of postsynaptic gamma-aminobutyric acid (GABA) receptors in the extensor tibiae muscle of Schistocerca gregaria were studied by conventional electrophysiological recording techniques. 2. GABA and other active GABA receptor agonists produced rapid, dose-dependent, reversible increases in membrane conductance. 3. In two microelectrode experiments the ED50 for GABA was approximately 1 mM. In three microelectrode experiments (assuming short cable theory conditions) the ED50 for GABA was 2.3 mM. The Hill coefficient for GABA estimated from the latter experiments was 1.4. 4. The relative potency of muscimol/GABA at the ED50 for GABA was 1.36. 3-Aminopropane sulphonic acid (3-APS) and isonipecotic acid were weakly active, baclofen and piperidine-4-sulphonic acid (P4S) were inactive. Isoguvacine produced depolarizations and increases in conductance in preparations which hyperpolarized in response to GABA. These depolarizations were enhanced by both picrotoxin and pitrazepin although the increases in input conductance were depressed. 5. Picrotoxin (20 microM), (+)-bicuculline (20-100 microM) and pitrazepin (1-10 microM) all reversibly antagonized GABA-induced responses. Such antagonism was not competitive in the case of picrotoxin and (+)-bicuculline but was competitive for pitrazepin. Schild plot analysis gave an average pA2 value of 5.5 for pitrazepin. 6. The significance of these results is briefly discussed.

Vertebrate cyclodiene insecticide resistance: role of gamma-aminobutyric acid and diazepam binding sites

Certain populations of the mosquitofish (Gambusia affinis) are highly resistant to cyclodiene and cyclodiene-type insecticides that competitively interact with the picrotoxinin binding site of the gamma-aminobutyric acid (GABA) receptor-ionophore complex in the central nervous system. Resistance involves a reduction in affinity of the picrotoxinin binding site. The present study reports that GABA receptor binding is increased in resistant brain membranes compared to membranes from susceptible fish at concentrations of free radioligand above 0.2 microM. The increase appears to be due to a greater number of binding sites (Bmax) in the resistant population. Diazepam binding affinity (Kd) and Bmax were not different in membranes from resistant fish compared to those from susceptible fish. Up-regulation of GABA binding sites in the resistant fish population may compensate for a possible reduction of GABAergic transmission caused by chronic environmental exposure to cyclodiene insecticides. However, a lack of cross-resistance to bicuculline (a competitive GABA antagonist) indicates that an increase in GABA sites is not a mechanism of cyclodiene resistance.

Bicuculline blocks the response to acetylcholine and nicotine but not to muscarine or GABA in isolated insect neuronal somata

The isolated somata of neurons from the thoracic ganglia of the locust, Locusta migratoria, respond to pressure microapplication of gamma-aminobutyric acid (GABA) and acetylcholine. The acetylcholine receptors fall into two groups. ACh1 (activated by nicotine) and ACh2 (activated by muscarine). The GABA receptor and the ACh1 receptor differ in pharmacology from the known vertebrate receptors. The GABA receptor is insensitive to bicuculline and its salts up to a concentration of 10(-4) M. In contrast, bicuculline is a moderately potent, at least partially competitive antagonist of the ACh1 receptor-mediated response in the thoracic neuronal somata. These observations suggest that classical diagnostic compounds such as bicuculline may show greater cross-reactivity than hitherto suspected among the members of the superfamily of ligand-activated channels.

GABA-mediated synaptic inhibition of projection neurons in the antennal lobes of the sphinx moth, Manduca sexta

Responses of neurons in the antennal lobe (AL) of the moth Manduca sexta to stimulation of the ipsilateral antenna by odors consist of excitatory and inhibitory synaptic potentials. Stimulation of primary afferent fibers by electrical shock of the antennal nerve causes a characteristic IPSP-EPSP synaptic response in AL projection neurons. The IPSP in projection neurons reverses below the resting potential, is sensitive to changes in external and internal chloride concentration, and thus is apparently mediated by an increase in chloride conductance. The IPSP is reversibly blocked by 100 microM picrotoxin or bicuculline. Many AL neurons respond to application of GABA with a strong hyperpolarization and an inhibition of spontaneous spiking activity. GABA responses are associated with an increase in neuronal input conductance and a reversal potential below the resting potential. Application of GABA blocks inhibitory synaptic inputs and reduces or blocks excitatory inputs. EPSPs can be protected from depression by application of GABA. Muscimol, a GABA analog that mimics GABA responses at GABAA receptors but not at GABAB receptors in the vertebrate CNS, inhibits many AL neurons in the moth.

Nociceptive responses to altered GABAergic activity at the spinal cord

GABA agonists and antagonists were injected intrathecally at the spinal cord, to determine their effect on nociceptive thresholds. Tactile stimulation, applied against the flank by a medium diameter von Frey fiber (5.5 g force), elicited distress vocalizations after, but not before injection of the GABA antagonists, bicuculline MI or picrotoxin (0.25 and 1 microgram dosages). Vocalization threshold to tail shock was significantly reduced by bicuculline MI or picrotoxin. Tail flick withdrawal latency from radiant heat was not altered by GABA antagonists. The GABA agonist, muscimol, significantly elevated vocalization threshold to tail shock at a 5 micrograms dose. At a lower dose level (1 microgram), muscimol significantly reduced vocalization threshold to tail shock. Tail flick latency was significantly prolonged by the 5 micrograms dose of muscimol; however, flaccid paralysis of the hind limbs was also evident. Nociceptive thresholds were not altered by GABA or saline injection. These findings indicate that GABAergic activity contributes to the tonic modulation of nociception at the spinal cord.

Continuous release of diazepam: electrophysiological, biochemical and behavioral consequences

Neuronal GABAergic sensitivity was assessed using electrophysiological, biochemical and behavioral techniques following the continuous release and maintenance of relatively constant brain levels of diazepam for greater than or equal to 21 days. Our studies indicate that long-term exposure to diazepam results in: (1) a decrease in iontophoretic sensitivity to GABA in the dorsal raphe nucleus, (2) an increase in the affinity of the GABA recognition site in brain tissue and (3) an increase in susceptibility to bicuculline-induced seizures in the intact animal. Since the decrease in GABAergic responsiveness was observed in the presence of measurable levels of diazepam, it was concluded that this subsensitivity phenomenon is associated with tolerance and not with withdrawal effects of the benzodiazepines.

Gabaergic modulation of mouse-killing in the rat

When GABA-potentiating compounds were administered IP to rats with prior experience of mouse-killing behaviour, a reduction of killing was observed with gamma-vinyl GABA (200 and 400 mg/kg) and nipecotic acid amide (400 mg/kg), while no significant effect was noted following injection of dipropylacetate or THIP. The inhibitory effects of gamma-vinyl GABA and nipecotic acid amide were not reversed by subsequent injection of picrotoxin and were associated with sedation as observed in open field and actograph tests. When GABA-potentiating compounds were administered to food-deprived rats exposed for the first time to a mouse (initial elicitation), administration of gamma-vinyl GABA, dipropylacetate, nipecotic acid amide or THIP increased the incidence of mouse-killing behaviour. Conversely, the incidence of mouse-killing under the same conditions was reduced following injections of picrotoxin. These results do not support the hypothesis that the general activation of GABAergic mechanisms inhibits mouse-killing behaviour in rats. On the contrary, data obtained in naive animals suggest that potentiation of these mechanisms actually facilitates the initial elicitation of this behaviour.

[3H]bicuculline methochloride binding to low-affinity gamma-aminobutyric acid receptor sites

The binding of [3H]bicuculline methochloride (BMC) to mammalian brain membranes was characterized and compared with that of [3H] gamma-aminobutyric acid ([3H]GABA). The radiolabeled GABA receptor antagonist showed significant displaceable binding in Tris-citrate buffer that was improved by high concentrations of chloride, iodide, or thiocyanate, reaching greater than 50% displacement in the presence of 0.1 M SCN-. An apparent single class of binding sites for [3H]BMC (KD = 30 nM) was observed in 0.1 M SCN- for fresh or frozen rat cortex or several regions of frozen and thawed bovine brain. The Bmax was about 2 pmol bound/mg of crude mitochondrial plus microsomal membranes from unfrozen washed and osmotically shocked rat cortex, similar to that for [3H]GABA. Frozen membranes, however, showed decreased levels of [3H]BMC binding with no decrease or an actual increase in [3H]GABA binding sites. [3H]BMC binding was inhibited by GABA receptor specific ligands, but showed a higher affinity for antagonists and lower affinity for agonists than did [3H]GABA binding. Kinetics experiments with [3H]GABA binding revealed that low- and high-affinity sites showed a similar pharmacological specificity for a series of GABA receptor ligands, but that whereas all agonists had a higher affinity for slowly dissociating high-affinity [3H]GABA sites, bicuculline had a higher affinity for rapidly dissociating low-affinity [3H]GABA sites. This reverse potency between agonists and antagonists during assay of radioactive antagonists or agonists supports the existence of agonist- and antagonist-preferring conformational states or subpopulations of GABA receptors. The differential affinities, as well as opposite effects on agonist and antagonist binding by anions, membrane freezing, and other treatments, suggest that [3H]BMC may relatively selectively label low-affinity GABA receptor agonist sites. This study, using a new commercially available preparation of [3H]bicuculline methochloride, confirms the report of bicuculline methiodide binding by Möhler and Okada (1978), and suggests that this radioactive GABA antagonist will be a valuable probe in analyzing various aspects of GABA receptors.

Substantia nigra: site of anticonvulsant activity mediated by gamma-aminobutyric acid

Localization of the anatomic substrate for anticonvulsant activity mediated by gamma-aminobutyric acid (GABA) was examined using intracerebral injections of GABA agonists. Blockade of tonic hindlimb extension in the maximal electroshock test and blockade of tonic and clonic seizures produced by pentylenetetrazole and bicuculline were obtained by elevating GABA in the ventral midbrain tegmentum. Elevation of GABA in forebrain and hindbrain areas had no effect on convulsant activity. Blockade of tonic and clonic seizures was also obtained after microinjections of the direct GABA receptor agonist, muscimol, into the midbrain. The substantia nigra was identified as the critical midbrain site for GABA-mediated anticonvulsant activity. Local injection of GABA agonists into the midbrain provided seizure protection without a widespread augmentation of GABA-mediated activity throughout the brain and without impairing either alertness or motor function. Synapses in the substantia nigra appear to represent an important control mechanism for inhibiting the propagation of generalized convulsions.

Membrane depolarization and prolongation of calcium-dependent action potentials of mouse neurons in cell culture by two convulsants: bicuculline and penicillin

The convulsant compounds bicuculline (BICUC) and penicillin (PCN) are antagonists of GABA-mediated synaptic inhibition. In addition, we have shown that BICUC and PCN produced membrane depolarization of mouse spinal cord neurons in primary dissociated cell culture by blocking a potassium conductance, a non-synaptic direct effect. Both compounds also prolonged calcium-dependent action potentials of mouse dorsal root ganglion and spinal cord neurons in cell culture. Thus, BICUC and PCN had both synaptic and non-synaptic actions. The possibility that both synaptic and non-synaptic actions of BICUC and PCN are involved in their convulsant mechanism of action is discussed.

Pharmacology of GABA-mediated inhibition of spinal cord neurons in vivo and in primary dissociated cell culture

In this paper it is shown that the postsynaptic GABA-receptor chloride ion channel complex is composed of several functional subunits. There are probably at least two stereospecific locations on the receptor for GABA-binding and both must be occupied to obtain an increase in chloride conductance. The interaction between these sites is uncertain but there could be either positive cooperativity between the sites or only a requirement that both sites are occupied without occupation of either site affecting the affinity for GABA of the other site. There is a chloride conductance channel coupled to the GABA receptor which opens for an average of 20 msec and has an average conductance of 18 pS. The GABA-coupled chloride channel may or may not have the same composition as the glycine coupled chloride channel. In addition to the GABA-recognition site and the chloride ion channel, GABA-receptors must have additional binding sites or modulator sites where drugs can bind to modify GABA activation of the GABA receptor. The convulsant PICRO binds to a site which is independent of the GABA site and PICRO reduces GABA responses. Barbiturates and benzodiazepines augment GABA-responses without reducing GABA-binding and thus they must bind to a modulator site independent of the GABA recognition site. Whether or not this is the same site as the PICRO binding site is uncertain. Thus, the GABA-receptor-chloride ion channel complex is composed of at least: 1) two GABA-binding sites; 2) a chloride ion channel; 3) a convulsant binding site (PICRO-binding site) and 4) an anticonvulsant binding site. This organization serves several obvious purposes. First, since two GABA-molecules are required to activate GABA-coupled chloride ion channels, the dose-response relationship for GABA is sigmoidal and steep. Thus minor shifts in GABA affinity will produce large alterations in GABA-responses and the GABA receptor can be easily modulated. Second, since the receptors has binding sites for convulsant and anticonvulsant compounds which decrease and increase GABA-responses, GABAergic inhibition can easily be modulated.

The effects of muscimol and picrotoxin injections into the cat substantia nigra

The behaviour of cats after unilateral injections of muscimol, picrotoxin and bicuculline into the posterior and lateral parts of the substantia nigra was observed. The antagonism between muscimol and picrotoxin was limited to some effects of the drugs: (1) muscimol-induced sniffing, licking and enhanced locomotor activity was attenuated by picrotoxin in contrast to the muscimol-induced contralateral turning which was even enhanced by picrotoxin; (2) picrotoxin-induced balance disorders as well as characteristic motor disturbances in the hind legs remained unaffected following an additional treatment with muscimol. Furthermore, subcutaneously given apomorphine did not affect the picrotoxin-induced balance and hind leg disorders, although the apomorphine-induced stereotyped behaviour itself was enhanced and suppressed by intranigral administration of muscimol and picrotoxin respectively. The conclusion is reached that there are at least two distinct populations of GABA receptors within the substantia nigra: one group controlling or being controlled by dopaminergic neurons and one group operating fully independently of the former neurons. The similarities to experiments on rats are stressed. The mechanism underlying the muscimol-resistant picrotoxin-induced effects (balance disorders and motor disturbance of the hind legs) is discussed.

A re-examination of the GABA-inhibitory action of bicuculline on lobster muscle

The GABA-inhibitory action of bicuculline on lobster muscle was critically re-examined. Bicuculline (20-100 micrometers) depressed the GABA-evoked conductance increase in a reversible manner, the double reciprocal transformation of the GABA dose/conductance curves remaining linear. If bicuculline was assumed to be a pure non-competitive antagonist, then the observed depression of the GABA curves at high GABA concentrations was underestimated. Also, the action of bicuculline was not in accordance with a dualistic antagonism (competitive and pure non-competitive type). Regarding bicuculline as a "mixed" non-competitive antagonist however, gave a better overall fit to the experimental data. Combinations of bicuculline and picrotoxinin also depressed the GABA response in a manner expected from the combination of two "mixed" non-competitive antagonists. It was concluded that bicuculline (like picrotoxinin and picrotoxin) behaves as a "mixed" non-competitive rather than a pure non-competitive antagonist of GABA on lobster muscle. However bicuculline methochloride proved inactive on the lobster. Moreover, bicuculline itself became less effective in a more acidic solution (pH 5.6, where more of the protonated form would exist). An intracellular or intramembrane site of action of this antagonist was therefore postulated.

Differential solubilization of gamma-aminobutyric acid receptive sites from membranes of mammalian brain

Sodium-dependent (+Na) and sodium-independent (-Na) receptive sites for gamma-aminobutyric acid (GABA) residing in or on frozen synaptic plasma membranes (SPM) of bovine cerebral cortex were characterized as to binding constants, pharmacologic specificities, and sodium dependence. The SPM fraction was then treated with various concentrations of Triton X-100 resulting in the loss of pharmacologic specificity, binding characteristics, and sodium dependence associated with +Na GABA receptive sites in SPM. The resulting junctional complex preparation (JC), i.e., a fraction enriched in junctional complexes, possessed only the pharmacologic specificity and binding constants associated with -Na receptive sites whether assayed in the presence or absence of 100 mM-NaCl. This is probably due to the detergent dispersal or solubilization of the +Na GABA receptive site. The binding constants, KD and Bmax, for -Na GABA binding in SPM were 170 nM and 4.4 pmol/mg protein, while in JC they were 186 nM and 3.7 pmol/mg protein. Under repeated washing the KD was reduced to 60 +/- 6.9 nM and the Bmax was reduced to 2.5 +/- 0.5 pmol/mg protein in JC, probably owing to the removal of endogenous ligand or inhibitor, and not to inhibition by residual Triton X-100. Multiple extraction with 0.1% or 0.5% Triton X-100 did not alter the KD or Bmax values for the binding of [3H]GABA to JC. Sodium-independent GABA binding was lost from JC membranes with the use of sodium deoxycholate, probably through solubilization.

Stereospecific interaction of bicuculline with specific [3H]strychnine binding to rat spinal cord synaptosomal membranes

The gamma-aminobutyric acid (GABA) antagonist (+/-)-bicuculline inhibits specific [3H]strychnine binding to postsynaptic glycine receptor sites in rat spinal cord synaptosomal membranes with an inhibition constant of about 5 microM, which is fairly similar to its inhibition constant reported for the GABA receptor. This effect is highly stereospecific, since the affinity of (-)-bicuculline for the specific [3H]strychnine binding sites is more than ten times less than that of the pharmacologically active (+)-bicuculline. Besides an unspecific effect at the glycine receptor, the results could suggest that the glycine and the GABA receptors are located close together in spinal cord membranes, so that the antagonist states of both receptors may be able to interfere with each other in some mechanistic way.