Antagonism between bicuculline and GABA in the cat brain

Comparative investigation of analgesic tolerance to taurine, sodium salicylate and morphine: Involvement of peripheral muscarinic receptors

Nowadays various analgesic medications are used for the management of acute and chronic pain. Among these opioid and non-steroidal anti-inflammatory drugs stand in the first line of therapy, however, prolonged administration of these substance is generally challenged by development of analgesic tolerance in patients. Therefore, it is highly valuable to find new pharmacological strategies for prolonged therapeutic procedures. In this respect, Taurine, a free amino acid, has been shown to induce significant analgesia at both spinal and peripheral levels through cholinergic mechanisms. In the present study, we used hot-plate analgesic test to investigate how taurine either as a single medication or in combination with sodium salicylate and morphine may affect both acute response to pain and development of analgesic tolerance. The effect of taurine was also tested on morphine withdrawal syndrome. Hyoscine butyl bromide was used to assess the role of muscarinic receptors in taurine-mediated effects. Finally, biochemical assay was done to reveal how the activity of brain acetylcholinesterase may change in relation with muscarinic receptor activity. Results indicated that acute administration of taurine-sodium salicylate combination causes more potent analgesia compared to the use of tau (but not SS alone) and this seems to be mediated via activity of muscarinic receptors in peripheral nervous system. Furthermore, the effect of this combination undergoes less analgesic tolerance during time. Combination of taurine and morphine is an effective strategy to attenuate both morphine analgesic tolerance and dependence and this also seems to depend on activity of muscarinic receptors, however through differential cellular mechanisms.

Simultaneous Monitoring of pH and Chloride (Cl-) in Brain Slices of Transgenic Mice

Optosensorics is the direction of research possessing the possibility of non-invasive monitoring of the concentration of intracellular ions or activity of intracellular components using specific biosensors. In recent years, genetically encoded proteins have been used as effective optosensory means. These probes possess fluorophore groups capable of changing fluorescence when interacting with certain ions or molecules. For monitoring of intracellular concentrations of chloride ([Cl-]i) and hydrogen ([H+] i) the construct, called ClopHensor, which consists of a H+- and Cl--sensitive variant of the enhanced green fluorescent protein (E2GFP) fused with a monomeric red fluorescent protein (mDsRed) has been proposed. We recently developed a line of transgenic mice expressing ClopHensor in neurons and obtained the map of its expression in different areas of the brain. The purpose of this study was to examine the effectiveness of transgenic mice expressing ClopHensor for estimation of [H+]i and [Cl-]i concentrations in neurons of brain slices. We performed simultaneous monitoring of [H+]i and [Cl-]i under different experimental conditions including changing of external concentrations of ions (Ca2+, Cl-, K+, Na+) and synaptic stimulation of Shaffer's collaterals of hippocampal slices. The results obtained illuminate different pathways of regulation of Cl- and pH equilibrium in neurons and demonstrate that transgenic mice expressing ClopHensor represent a reliable tool for non-invasive simultaneous monitoring of intracellular Cl- and pH.

Potential Mechanisms Involved in the Anticonvulsant Effect of Methanol Extract of Pyrenancantha staudtii in Mice

OBJECTIVE

To determine the potential effect of Pyrenancantha staudtii extract on experimentally induced seizures in mice and to evaluate the role of benzodiazepines, naloxone, and serotonin within these pathways.

METHODS

Animal behaviours were evaluated using open field, hexobarbitone-induced sleep model, and anticonvulsant activity using picrotoxin-, or strychnine-, or isoniazid-induced convulsions. Attempt to understand the mode of action of the anticonvulsant activity of the plant, three notable antagonists (flumazenil, 3 mg/kg; naloxone 5 mg/kg, i.p., and cyproheptadine, 4 mg/kg, i.p) were used.

RESULTS

The results revealed a significant (p < 0.05) reduction in the frequency of rearing and grooming episodes compared with the control. The extract of P. staudtii potentiates the sleeping time of hexobarbitone-induced hypnosis in a dose-related manner. P. staudtii stem bark extracts significantly (p<0.05) prolonged the onset of a seizure and attenuated the duration of seizure in a dose-dependent manner in picrotoxin- and or isoniazid-induced seizures. While, P. staudtii stem bark extract at all doses (100, 200, and 400 mg kg-1) though significantly prolonged the onset of action, but did not confer any significant changes on the duration, as well as mortality in this strychnine-induced seizure model. However, the anticonvulsant activity of the methanolic extract of P. staudtii was significantly reversed following intraperitoneal pre-treatment with flumazenil (GABA receptor antagonist) and naloxone (opioid receptor antagonist) but not cyproheptadine (5-HT2 receptor antagonist) in picrotoxin-induced convulsion.

CONCLUSION

The data obtained suggest that methanol extract of P. staudtii possessed significant anticonvulsant effect, thereby confirming the traditional uses of P. staudtii in the treatment of epilepsy; mechanisms of which could involve the interaction with GABAergic and or opioidergic system.

Transitions between asynchronous and synchronous states: a theory of correlations in small neural circuits

The study of correlations in neural circuits of different size, from the small size of cortical microcolumns to the large-scale organization of distributed networks studied with functional imaging, is a topic of central importance to systems neuroscience. However, a theory that explains how the parameters of mesoscopic networks composed of a few tens of neurons affect the underlying correlation structure is still missing. Here we consider a theory that can be applied to networks of arbitrary size with multiple populations of homogeneous fully-connected neurons, and we focus its analysis to a case of two populations of small size. We combine the analysis of local bifurcations of the dynamics of these networks with the analytical calculation of their cross-correlations. We study the correlation structure in different regimes, showing that a variation of the external stimuli causes the network to switch from asynchronous states, characterized by weak correlation and low variability, to synchronous states characterized by strong correlations and wide temporal fluctuations. We show that asynchronous states are generated by strong stimuli, while synchronous states occur through critical slowing down when the stimulus moves the network close to a local bifurcation. In particular, strongly positive correlations occur at the saddle-node and Andronov-Hopf bifurcations of the network, while strongly negative correlations occur when the network undergoes a spontaneous symmetry-breaking at the branching-point bifurcations. These results show how the correlation structure of firing-rate network models is strongly modulated by the external stimuli, even keeping the anatomical connections fixed. These results also suggest an effective mechanism through which biological networks may dynamically modulate the encoding and integration of sensory information.

GABA Receptors in the Medial Preoptic Area Modulate the Onset of Oestradiol-Induced Maternal Behaviour in Hysterectomised-Ovariectomised, Pregnant Rats

We studied the participation of GABA neurotransmission in the medial preoptic area (mPOA) with respect to the onset of the pup retrieval response and nest building. Pregnant female rats were implanted with bilateral cannulae in the mPOA on day 12 of pregnancy and, on day 16, the females were hysterectomised and ovariectomised and given 200 μg/kg of oestradiol benzoate. Two days later, the females received one of the following intracerebral drug treatments: GABA_B agonist baclofen (200 ng); GABA_B antagonist phaclofen (1 μg); GABA_A antagonist bicuculline (60 ng); or physiological saline. Five minutes after intracerebral infusion, three foster pups were introduced into the females' home cage. The subjects were observed for pup grouping (retrieval) during 15 min, after which the pups were left with the female. During the next 12 h, an observation was made every 1 h to determine whether the pups had been grouped (retrieved) or not. The GABA_B agonist baclofen reduced the proportion of females retrieving pups from 4 to 8 h following pup introduction. By contrast, both the GABA_A antagonist bicuculline and the GABA_B antagonist phaclofen enhanced the proportion of females retrieving pups during the first 3 h of observation. The latency to pup retrieval in subjects treated with the GABA_B agonist baclofen was significantly longer than that in subjects given any of the antagonists. All females built a nest but baclofen reduced nest quality. These data show that activation of GABA_B receptors in the mPOA has an inhibitory effect on basic maternal behaviours, whereas blockade of either the GABA_A or GABA_B receptor facilitates pup retrieval. It is possible that reduced GABAergic tone in the mPOA is a key element in the initiation of maternal behaviours in postparturient rats.

Chronic neonatal nicotine exposure increases excitation in the young adult rat hippocampus in a sex-dependent manner

Smoking during pregnancy exposes the fetus to nicotine, resulting in nicotine-stimulated neurotransmitter release. Recent evidence suggests that the hippocampus develops differently in males and females with delayed maturation in males. We show that chronic nicotine exposure during the first postnatal week has sex-specific long-term effects. Neonatal rat pups were chronically treated with nicotine (6mg/kg/day) (CNN) from postnatal day 1 to 7 or milk only (Controls), and hippocampal slices were prepared from Control- and CNN-treated young adults. Field excitatory postsynaptic potentials (fEPSPs) or population spikes (PSs) were recorded from the CA1 hippocampus following CA1 s. radiatum stimulation. Input/Output curves constructed from fEPSP data indicated that CNN-males, but not females, had significantly increased excitatory responses compared to Controls (p<0.05, n=10 Con, n=11 CNN). Long-term potentiation (LTP) was not significantly changed by CNN. In the presence of bicuculline, which blocks inhibitory GABA(A) receptors, an epileptiform burst consisting of a series of PSs was evoked. The amplitude of the first PS was significantly larger in CNN-males and females compared to Controls (males: p<0.01, n=8 Con, n=8 CNN; females: p<0.05, n=9 Con, n=7 CNN). Only CNN-males also had significantly larger second PSs (p<0.05, n=8 con, n=8 CNN). Epileptiform activity evoked by zero Mg(2+) incubation did not differ in amplitude or duration of bursts in CNN-males or females compared to Controls. These data indicate that neonatal nicotine exposure has long lasting effects and results in increased excitation within the CA1 hippocampus in adulthood, with males showing increased sensitivity to nicotine's effects.

GABA(B) receptor gene expression in monkey thalamus

Expression of gamma-amino butyric acid type B (GABA[B]) receptor gene transcripts was examined in the macaque monkey thalamus by in situ hybridization, using monkey-specific cRNA probes. GABA(B) transcript expression was widespread and of much higher density in the dorsal thalamus than in the reticular nucleus and other parts of the ventral thalamus and was highest in the epithalamus. In the dorsal thalamus, highest mRNA levels were found in the anteroventral nucleus and in the parafascicular nucleus. Sensory relay nuclei showed moderate GABA(B) mRNA levels. Neurons of all sizes were labeled, suggesting expression in relay cells and interneurons, and there was no labeling of neuroglial cells. Following 10-day periods of monocular deprivation, levels of GABA(B) mRNA were decreased in the deprived magno- and parvo-cellular laminae of the dorsal lateral geniculate nuclei, indicating activity-dependent regulation. High levels of GABA(B) receptors in the dorsal thalamus are likely to reflect the high density of synaptic inputs from the reticular nucleus while low expression in the reticular nucleus implies weak, GABA(B)-mediated intrareticular inhibition.

Electrophysiological evidence that noradrenergic neurons of the rat locus coeruleus are tonically inhibited by GABA during sleep

It is well known that noradrenergic locus coeruleus (LC) neurons decrease their activity during slow wave sleep (SWS) and are virtually quiescent during paradoxical sleep (PS). It has been proposed that a GABAergic input could be directly responsible for this sleep-dependent neuronal inactivation. To test this hypothesis, we used a new method combining polygraphic recordings, microiontophoresis and single-unit extracellular recordings in unanaesthetized head-restrained rats. We found that iontophoretic application of bicuculline, a specific GABA(A)-receptor antagonist, during PS and SWS restore a tonic firing in the LC noradrenergic neurons. We further observed that the application of bicuculline during wakefulness (W) induced an increase of the discharge rate. Of particular importance for the interpretation of these results, using the microdialysis technique, Nitz and Siegel (Neuroscience, 1997; 78: 795) recently found an increase of the GABA release in the cat LC during SWS and PS as compared with waking values. Based on these and our results, we therefore propose that during W, the LC cells are under a GABAergic inhibitory tone which progressively increases at the entrance and during SWS and PS and is responsible for the inactivation of these neurons during these states.

GABA receptor mediated suppression of defensive rage behavior elicited from the medial hypothalamus of the cat: role of the lateral hypothalamus

Recently, our laboratory has demonstrated that predatory attack behavior in the cat, elicited by electrical stimulation of the lateral hypothalamus, is suppressed following activation of the region of the medial hypothalamus from which defensive rage behavior is elicited [Han, Y., Shaikh, M.B., Siegel, A., Medical amygdaloid suppression of predatory attack behavior in the cat: II. Role of a GABAergic pathway from the medial to the lateral hypothalamus, Brain Res., 716 (1996) 72-83.]. The mechanism for this suppression is a direct GABAergic projection from the medial to lateral hypothalamus. The present study tested the hypothesis that the inhibitory relationship between these two regions of hypothalamus is reciprocal, namely, that a GABAergic neuron, which also projects from the lateral to medial hypothalamus, serves to suppress defensive rage elicited from the medial hypothalamus. Monopolar stimulating electrodes were implanted into lateral hypothalamic sites from which predatory attack behavior was elicited. In addition, cannula-electrodes were implanted into the medial hypothalamus for elicitation of defensive rage behavior and for microinjections of GABA compounds. Initially, in the absence of drug administration, the effects of dual stimulation of the lateral and medial hypothalamus upon response latencies were compared with those following single stimulation of the medial hypothalamus alone. Dual stimulation significantly (p<0.01) suppressed defensive rage behavior elicited from the medial hypothalamus. Then, administration of the GABAA receptor antagonist, bicuculline (10-60 pmol), into medial hypothalamic sites from which defensive rage was elicited blocked the suppressive effects of lateral hypothalamic stimulation. The GABAA receptor agonist, muscimol (0.3-30 pmol), microinjected into the medial hypothalamus, suppressed defensive rage elicited by single stimulation of the medial hypothalamus in a dose dependent manner. These suppressive effects of muscimol upon defensive rage were blocked following pretreatment with bicuculline (60 pmol). Administration of muscimol into adjoining regions of the lateral hypothalamus had no effect upon defensive rage, indicating its site specificity. Bicuculline (60 pmol) delivery into the medial hypothalamus had no effect upon defensive rage, suggesting the, presence of a phasic rather than tonic mechanism. A combination of immunocytochemical and retro grade tracing procedures were then employed to determine the origin of the putative GABAergic pathway projecting to the medial hypothalamus. In this experiment, the retrograde tracer, Fluoro-Gold (8%, 0.5 microl), was microinjected through a cannula-electrode in the medial hypothalamus from which defensive rage had been elicited. Following survival periods of 5-6 days, cats were perfused with 4% paraformaldehyde and brain tissue was processed for immunocytochemical staining of GABA neurons. Retrogradely labeled, immunopositively labeled, as well as Fluoro-Gold and GABA labeled cells, were identified in the lateral hypothalamus. Each type of neuron was distributed over wide regions of the lateral hypothalamus, extending from the area immediately caudal to the optic chiasm to the level of the posterior hypothalamus. Together, the behavioral pharmacological and anatomical data provide evidence of a direct inhibitory projection from the lateral to medial hypothalamus whose functions are mediated by GABAA receptors. When coupled with our previous findings, these results reveal the presence of reciprocal GABAergic inhibitory pathways between the medial and lateral hypothalamus. The findings suggest that functions associated with either the lateral or medial hypothalamus, but not both, can be activated at a given time.

Review : Inhibitory Processes in the Visual Cortex

Theoretical and experimental studies have predicted and confirmed, respectfully, the presence of inhibitory processes in the visual cortex. To date, however, the precise role of inhibition in shaping these processes remains unclear. Numerous studies provide evidence that inhibition acts at the single-neuron level, endowing selectivity in these neurons for various stimulus characteristics. Similarly, other studies seem to suggest that inhibition is employed by larger ensembles of neurons, endowing individual neuronal characteristics only through the behavior of the entire network. This article addresses previous views of inhibitory processes and the ways they may be used in developing characteristic properties of neurons in the visual cortex. NEUROSCIENTIST 4:45-52, 1998

Nucleus-specific expression of GABA(A) receptor subunit mRNAs in monkey thalamus

Expression of 10 GABAA receptor subunit genes was examined in monkey thalamus by in situ hybridization using cRNA probes specific for alpha 1, alpha 2, alpha 3, alpha 4, alpha 5, beta 1, beta 2, beta 3, gamma 1, and gamma 2 subunit mRNAs. These displayed unique hybridization on patterns with significant differences from rodents. Alpha 1, beta 2, and gamma 2 transcripts were expressed at high levels in all dorsal thalamic nuclei, but expression was significantly higher in sensory relay nuclei-especially the dorsal lateral geniculate nucleus. Other transcripts showed nucleus-specific differences in levels of expression and in the range expressed. Alpha 5 and alpha 4 subunit transcripts were expressed in all nuclei except the intralaminar nuclei. Levels of alpha 2, alpha 3, beta 1, beta 3, and gamma 1 expression were very low, except in intralaminar nuclei. In the reticular nucleus, most subunit transcripts were not expressed, and only gamma 2 transcripts were consistently detected at modest levels. Thalamic GABAA receptors may be assembled from nucleus-specific groupings of subunit polypeptides.

Medial amygdaloid suppression of predatory attack behavior in the cat: II. Role of a GABAergic pathway from the medial to the lateral hypothalamus

The medial amygdala is known to powerfully suppress predatory attack behavior elicited by electrical stimulation of the lateral hypothalamus of the cat. In the preceding paper, it was shown that the initial limb of a pathway subserving suppression of predatory attack from the medial amygdala to the lateral hypothalamus projects to the ventromedial hypothalamus and its functions are mediated by substance P. The present study tested the hypothesis that the second limb of the pathway subserving medial amygdaloid suppression of predatory attack behavior projects from the medial to lateral hypothalamus and its functions are mediated by GABA. Cannula electrodes were implanted into the lateral hypothalamus for elicitation of predatory attack behavior as well as for the microinfusion of GABA compounds. Monopolar stimulating electrodes were implanted into sites within the medial amygdala from which subseizure levels of stimulation could suppress predatory attack behavior. Initially, the effects of dual stimulation of the medial amygdala and lateral hypothalamus upon response latencies for predatory attack were compared with single stimulation of the lateral hypothalamus alone. Dual stimulation was shown to significantly suppress predatory attack elicited from the lateral hypothalamus. Then, the GABAA receptor antagonist, bicuculline, was microinjected into sites within the lateral hypothalamus from which predatory attack was elicited in doses of 0.015, 0.075 and 0.15 nmol and paired trials of single and dual stimulation were again repeated in a manner identical to that applied prior to drug administration. Drug infusion produced a blockade of medial amygdaloid suppression of predatory attack in a time- and dose-dependent manner. Conversely, microinfusions of the GABAA receptor agonist, muscimol (10, 25 and 50 pmol), into the same lateral hypothalamic 'attack' site in the absence of medial amygdaloid stimulation suppressed predatory attack, thus simulating the effects of medial amygdaloid stimulation. Furthermore, pretreatment with bicuculline microinjected into the lateral hypothalamus blocked the suppressive effects of substance P, that was infused into the ventromedial hypothalamus, upon predatory attack. Receptor autoradiography demonstrated the presence of high affinity binding for GABAA receptors in the lateral hypothalamus. A combination of immunocytochemical and retrograde axonal tract tracing procedures, in which Fluoro-Gold was microinjected into the lateral hypothalamic attack sites, revealed the presence of populations of neurons labeled for both Fluoro-Gold and GABA in the ventromedial hypothalamus. These findings provide new evidence for the existence of a pathway from the medial to lateral hypothalamus whose functions are mediated by GABA. Thus, the overall findings provide support for the view that the pathway from the medial amygdala to the lateral hypothalamus underlying suppression of predatory attack behavior involves a two-neuronal arc: the first neuron projects from the medial amygdala to the medial hypothalamus and its functions are mediated by substance P: the second neuron involves a GABAergic pathway originating in the ventromedial hypothalamus and which projects to the lateral hypothalamus.

[Evoked somatosensory potentials in the baboon: intracortical localization and nature studies using pharmacology and analysis of sources of current]

SEPs were elicited by stimulation of the sciatic (proprioceptive) or sural (exteroceptive) nerves. SEPs were recorded through epidural chronic electrodes implanted in the related S1 cortical surface. They were studied after systemic or local cortical administration of subconvulsive doses of bicuculline (a specific GABAa antagonist). A powerful increase in the amplitude of the P16 component, along with an inhibition of the N30 component were observed. From a cortical Current Source Densities analysis, the P16 facilitation was shown to result from blockade of the GABAa inhibitory synapses on the somas of pyramidal cells that are responsible for the P16 wave. Reduction of the N30 wave was attributed to a bicuculline-induced reduction of an axo-dendritic depolarisation of the apical dendrites belonging to pyramidal cells. A neurophysiological model of the SEP primary waves elicited by the thalamocortical proprioceptive or cutaneous inputs is suggested.

Studies on the constituents of the leaves of Nerium oleander on behavior pattern in mice

Fresh, undried and uncrushed leaves of Nerium oleander were subjected to methanol extraction and bioassay directed fractionation. This led to the isolation of two purified fractions namely, B-1 and B-3. Fractions B-1 and B-3 were studied with respect to their actions on the central nervous system and behavior pattern in mice. Both fractions were found to produce reduction in locomotor activity, rota rod performance and potentiation of hexobarbital sleeping time. These fractions also showed analgesic activity. When tested against picrotoxin induced convulsions fraction B-1 showed 40% protection, while fraction B-3 exhibited 60% protection against bicuculline induced convulsions. These findings suggest that both fractions possess a CNS depressant action.

Bicuculline enhances the late GABAB receptor-mediated paired-pulse inhibition observed in rat hippocampal slices

The inhibition of CA1 pyramidal neurones in rat hippocampal slices was studied using extracellular recordings of population spike potential responses to paired orthodromic stimulation. Variation of the interpulse interval allowed the separation of an early phase of inhibition (interpulse interval 5-20 ms), blocked by the GABAA receptor antagonist bicuculline (1 microM; n = 11), and a late phase (interpulse interval 200-400 ms) blocked by the GABAB receptor antagonist phaclofen (1 mM; n = 5) but enhanced by bicuculline (n = 11). Similar enhancement was not observed when conditioning response amplitudes were increased by increasing the stimulus strength, rather than bicuculline. Orthodromic stimulation leads to synaptic excitation of both pyramidal neurones and inhibitory interneurones, and may also lead to activation of inhibitory inputs onto interneurones. Bicuculline could prevent inhibition of the interneurones, and hence enhance the late, GABAB receptor-mediated inhibition. Conversely, the therapeutic administration of benzodiazepines would be postulated to enhance the inhibition of inhibitory interneurones, leading to an iatrogenic decrease in GABAB receptor-mediated inhibition.

Barbiturate-reversible reduction of water diffusion coefficient in flurothyl-induced status epilepticus in rats

Rat brains (n = 17) with flurothyl-induced status epilepticus (SE) have been imaged with a gradient-echo diffusion-weighted imaging sequence at 2.0 T. The apparent water diffusion coefficient (ADC) decreased during seizure discharges. The magnitude of the ADC reduction correlated well with the duration of flurothyl exposure. A 17% reduction in the water ADC compared with preseizure condition was observed in rats with the longest flurothyl exposure time. In 13 rats, pentobarbital was used to arrest the electrographic seizure activity. ADC values began to return to normal a few minutes after the injection. In four rats with no pentobarbital administration, ADC values remained depressed up to 1 h after seizure onset. The results suggest that diffusion-weighted MR imaging may be useful for mapping recent intense seizure activity in human patients with medically intractable epilepsy.

Comparison of the actions of glycine and related amino acids on isolated third order neurons from the tiger salamander retina

Whole cell voltage and current clamp recordings were obtained from third order neurons isolated from the salamander retina. Using cross desensitization, the structure-function relationship of short chain amino acids on the glycine receptor were examined. L-Serine, L-alanine, beta-alanine and taurine all cross desensitized with glycine, but did not show significant cross desensitization with GABA. This indicates that these amino acids act at the glycine receptor. The order of potency was glycine >> beta-alanine > taurine >> L-alanine > L-serine. TAG, a reputed selective taurine antagonist, was equally effective in blocking taurine and glycine currents. There is no evidence for distinct receptors for taurine. Amino acids with larger moieties at the alpha carbon, such as threonine and valine, produced inactive ligands. Placing a methyl group on the amine of glycine or esterification of the carboxyl group also greatly reduced activity. Based on these modifications of the glycine molecule, it appears that selectivity at the glycine receptor results in part from steric restrictions at all three sites in the glycine chain. The steric interference is most critical at the carboxyl and amino ends, and less limiting at the alpha carbon. Doses of L-serine that had only slight effects in voltage clamp experiments, nevertheless produced large effects in current clamp experiments. This indicates that several endogenous amino acids can have significant effects on membrane voltage, even when their shunting activity may be small. High concentrations of agonists produced desensitization in the voltage clamp records, but there was little evidence of desensitization in the current clamp experiments. These results indicate that several endogenous amino acids can activate the glycine receptor, but there is no evidence for a discrete receptor for taurine, beta-alanine, L-alanine or L-serine. Since all these endogenous amino acids have similar amino and acid terminals, reduction in potency results from steric interference around the alpha carbon. This graded potency may have functional significance in mediating inhibition.

Distribution of neurotransmitters, neuropeptides, and receptors in the vestibular nuclei complex of the rat: an immunocytochemical, in situ hybridization and quantitative receptor autoradiographic study

This article investigates the distribution of neurotransmitters, neuropeptides, and related receptors in the vestibular nuclei complex (VNC) of the adult rat by means of immunohistochemistry, in situ hybridization, and quantitative receptor autoradiography. The entire complex proves to be rich in muscarinic receptors and it shows a high density of imipramine and benzodiazepine binding sites. Peptidergic neurons and a few positive fibers are described in the caudal part of the VNC. In particular, the medial vestibular nucleus contains a number of neurons expressing both the enkephalin mRNA and peptide. This nucleus and the lateral vestibular nucleus are also rich in opiate receptors. Substance P, thyrotropin releasing hormone, and neurotensin receptors are also found in the medial and in the spinal vestibular nuclei. In spite of the presence of alpha 2 catecholaminergic receptors, no thyrosine-hydroxylase-immuno-reactive elements are seen in the caudal VNC. The possible functional meaning of these data is discussed.

Identification and function of glycine receptors in cultured cerebellar granule cells

Poly(A)+ mRNA was isolated from cultured mouse cerebellar granule cells and injected into Xenopus oocytes. This led to the expression of receptors that evoked large membrane currents in response to glycine. Current-responses were also obtained after application of beta-alanine and taurine, but these were very low relative to that of glycine (maximal beta-alanine and taurine responses were 8 and 3% of that of glycine, respectively). The role of glycine receptors on K(+)-evoked transmitter release in cultured cerebellar granule cells was also assayed. Release of preloaded D-[3H]aspartate evoked by 40 mM K+ was dose dependently inhibited by glycine, and the concentration producing half-maximal inhibition was 50 microM. Taurine, beta-alanine, and the specific GABAA receptor agonist isoguvacine also inhibited K(+)-evoked release, and the maximal inhibition was similar for all agonists (approximately 40%). The EC50 value was 200 microM for taurine, 70 microM for beta-alanine, and 4 microM for isoguvacine. Bicuculline (150 microM) antagonized the inhibitory effect of isoguvacine (150 microM) but not that of glycine (1 mM). In contrast, strychnine (20 microM) antagonized the inhibitory effect of glycine (1 mM) but not that of isoguvacine (150 microM). The pharmacology of the responses to beta-alanine and taurine showed that these agonists activate both glycine and GABAA receptors. The results indicate that cultured cerebellar granule cells translate the gene for the glycine receptor and that activation of glycine receptors produces neuronal inhibition.

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.

GABA inhibition of lateral hypothalamic neurons: role of reticular thalamic afferents

GABA and reticular thalamic (RT) stimulation induced inhibition of lateral hypothalamic (LH) neuronal activity was studied to determine if RT inhibitory input to the LH is mediated by stimulation of GABA receptors. Seven barrel electrodes were utilized to record simultaneously from the LH during microiontophoretic application of GABA, glycine, bicuculline, picrotoxin, and electrical stimulation of the RT. GABA produced an ejection current-related decrease in LH neuronal activity that was antagonized, in a dose-related manner, by the simultaneous administration of picrotoxin or bicuculline. LH neurons were much less sensitive to glycine inhibition which also was relatively insensitive to the GABA receptor antagonists. RT electrical stimulation provided a short latency inhibitory input to LH neurons that was mimicked by the microiontophoretic administration of GABA. In addition, the microiontophoretic application of picrotoxin or bicuculline blocked the RT stimulation induced inhibition of LH neuronal activity. These data indicate that the inhibitory effects of both GABA and RT synaptic inputs onto LH neurons are mediated by stimulation of GABAA receptors and substantiates the involvement of GABA as the inhibitory transmitter of thalamic to hypothalamic projections.

Subpopulations of GABA neurons containing somatostatin, neuropeptide Y, and parvalbumin in the dorsomedial cortex of the lizard Psammodromus algirus

Different subpopulations of GABA neurons containing the neuropeptides somatostatin and neuropeptide Y, and the calcium binding protein parvalbumin were studied by immunocytochemistry using light and electron microscopy in the dorsomedial cortex of the lizard Psammodromus algirus to investigate the connectivity of different subsets of GABA neurons in the lizard dorsomedial cortical circuitry and to compare cortical regions of reptiles and mammals. GABA neurons were classified into different subsets by using the peroxidase anti-peroxidase immunohistochemical method on adjacent Araldite-embedded semithin sections. GABA neurons in the dorsomedial cortex fall into three major subsets: 1) neurons with somatostatin (and neuropeptide Y), which accounted for about 44% of the GABA population; 2) neurons with parvalbumin, which accounted for about 13% of the GABA neurons; and 3) neurons without parvalbumin or neuropeptides, which represented 40% of all GABA cells. This division of GABA neurons in non-overlapping subpopulations of neuropeptide- and parvalbumin-containing cells is similar to that found in the mammalian hippocampal formation. On the basis of the nerve terminal fields, somatostatin- and parvalbumin-immunoreactive neuronal populations appear to be functionally different, acting on different portions of the projection neurons. Parvalbumin-immunoreactive neurons inhibit the pyramidal neurons at the cell body level, whereas somatostatin-immunoreactive neurons inhibit them on distal dendrites. The results of the present study add more similarities between the lizard dorsomedial cortex and parts of the mammalian hippocampus.

Dual activation of GABAA and glycine receptors by beta-alanine: inverse modulation by progesterone and 5 alpha-pregnan-3 alpha-ol-20-one

The differential sensitivity of the glycine and GABAA receptors to modulation by progesterone and 5 alpha-pregnan-3 alpha-ol-20-one (5 alpha 3 alpha) was used to determine whether beta-alanine acts through its own receptor, or through the glycine and/or GABAA receptor(s). The response to beta-alanine resembles the glycine response as it is inhibited by strychnine (a competitive glycine antagonist) or progesterone (a negative modulator of the glycine response). Significantly, the response to beta-alanine also resembles the GABA response in that it is inhibited by 2-(carboxy-3'-propyl)-3-amino-6-paramethoxy-phenylpyridazinium+ ++ bromide (SR-95531; a competitive GABA antagonist) and potentiated by 5 alpha 3 alpha (a positive modulator of the GABA response). The efficacy of beta-alanine at the GABAA receptor is comparable to that of GABA. Similarly, the efficacy of beta-alanine at the glycine receptor is comparable to that of glycine. The greater potency of beta-alanine at the glycine receptor indicates that, if beta-alanine is a neurotransmitter, its effects are more likely to be mediated by glycine receptors than by GABAA receptors. However, activation of the GABAA receptor by beta-alanine may become important in the presence of steroid modulators such as progesterone or 5 alpha 3 alpha.

Modulation of f2-f1: evidence for a GABA-ergic efferent system in apical cochlea of the guinea pig

f2-f1, but not 2f1-f2, was reduced in amplitude during continuous stimulation of the test ear with the primary tones, and with single tones near the primary frequencies. Stimulation of the contralateral ear, either with broad band noise or with single tones near the primary frequencies, also reduced f2-f1. Ipsilateral and contralateral effects were additive and were restricted to the frequency range between about 2 kHz and 7 kHz. Contralateral, but not ipsilateral suppression, was blocked after systemic administration of strychnine. Ipsilateral suppression was eliminated by perfusion of the cochlea with tetrodotoxin. Both contralateral and ipsilateral suppression were abolished after perfusion of the cochlea with bicuculline. The results are evidence for a role for a GABA-ergic efferent system in the modulation of outer hair cell mechanics in the apical cochlea.

GABA-induced responses in Purkinje cell dendrites of the rabbit cerebellar slice

Pressure applications of GABA localized to Purkinje cell somas in a rabbit cerebellar slice produced uniphasic hyperpolarizing responses, whereas applications of GABA that were directed at the Purkinje cell dendrites produced complex, triphasic responses with hyperpolarizing and depolarizing components. Both somatic and dendritic application of GABA elicited fast hyperpolarization (GABAhf), but dendritic application also elicited a slower depolarization (GABAd) and a later, long-lasting hyperpolarization (GABAhl). All three types of responses were accompanied by increased conductance. Use of either GABA antagonist, bicuculline or picrotoxin, eliminated the GABAhf and GABAd responses but left the GABAhl response intact. Pressure delivery of the GABA agonist, baclofen, to the dendrites but not the soma elicited a GABAhl response. Application of baclofen paired with membrane depolarization sufficient to elicit local, calcium-dependent dendritic spiking produced a persistent reduction in the GABAhl response, whereas alternating presentations of baclofen and membrane depolarization or presentations of baclofen alone could not. The fact that GABA and baclofen inhibited Purkinje cell activity in the rabbit cerebellar slice and that picrotoxin and bicuculline eliminated some, but not all of the components of the GABA response suggests the presence of both GABAA and GABAB receptors. The ability of baclofen to inhibit Purkinje cells if it was applied to the dendrites but not if applied to the soma suggests that GABAB receptors are located predominantly on Purkinje cell dendrites. The pairing-specific change in the baclofen response suggests the existence of GABAB-mediated modifiability of Purkinje cell dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurine induces bicuculline/strychnine-insensitive dose-dependent inhibition of cortical visual evoked responses

Modulatory influences of taurine on cortical visual evoked responses and their susceptibility to GABAergic and glycinergic antagonists were studied in adult rats. Taurine topically administered to the visual cortex produced dose-dependent inhibition of the positive-negative fast component of the cortical visual evoked responses. Neither bicuculline nor strychnine antagonized the taurine effect, as revealed by absence of a shift to right, a change in slope or in the taurine IC50 value in the dose-response curve. Results suggest that taurine-induced depression of cortical responses evoked in the visual cortex are mediated by receptors other than the GABAA and glycine receptors.

The contribution of AMPA and NMDA receptors to graded bursting activity in the hippocampal CA1 region in an acute in vitro model of epilepsy

The AMPA/KA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the NMDA receptor antagonist 2-amino-5-phosphonovalerate (D-APV) were used to investigate the contribution of excitatory amino acid (EAA) receptors to graded bursting activity recorded in the CA1 region of the rat hippocampal slice following bath application of the convulsant drug bicuculline methiodide (BIC, 2-3 microM). CNQX (5-9 microM) significantly antagonised the burst in a reversible, concentration-dependent manner (n = 5). The effect involved a reduction in the amplitude but not the number of population spikes of the burst and also a depression of the underlying burst excitatory post-synaptic potential (EPSP). D-APV (5-25 microM), in contrast, reduced the amplitude and number of spikes in the burst but had no effect on the burst EPSP (n = 5). Following a single concentration of CNQX (5 microM), applied in the presence of bicuculline, it was observed that the components of epileptiform response which remained could be completely abolished with D-APV (10 microM; n = 10). It was also shown that, following elimination of synaptic transmission with CNQX (5 microM), application of bicuculline (2-3 microM) induced a small burst that could be reversibly antagonised with D-APV (10 microM). These results show that evoked epileptiform activity witnessed in the presence of bicuculline involves the activation of both AMPA and NMDA receptors, the AMPA receptor activation making the major contribution. The burst mediated by NMDA receptors is not dependent on prior activation of AMPA receptors.

Neurotransmitters in the Vestibular Commissural System of the Cat

The present study focused on the transmitters that control the vestibular neural activity and, in particular, that regulate commissural inhibition. Extracellular spikes of a single vestibular neuron were recorded in decerebrate cats. The seven barrels of the electrode, with the exception of the center barrel, were filled with transmitter candidates and their specific antagonists, while the center barrel was filled with 2 M NaCl for extracellular recording. After isolation of a type 1 neuron, chemicals were iontophoretically applied to examine their effects on its activity. The results were as follows: (1) GABA and glycine markedly decreased spontaneous firing of the neurons, while serotonin did not affect their activity. (2) Bicuculline abolished the inhibitory effects of GABA on the neurons. (3) Strychine abolished the effects of glycine. (4) Commissural inhibition induced by electrical stimulation of the contralateral labyrinth was not abolished by strychine but was abolished by bicuculline. We conclude that (1) vestibular type 1 neurons are controlled by GABAergic and glycinergic but not serotoninergic neurons, and (2) commissural inhibition is activated by the GABAA receptor, but not by the GABAB receptor.

Differential blockade of bicuculline and strychnine on GABA- and glycine-induced responses in dissociated rat hippocampal pyramidal cells

The inhibitory effects of bicuculline (BIC) and strychnine (STR) on GABA- and glycine-induced responses were studied in the rat dissociated hippocampal CA1 pyramidal neurons in whole-cell mode by using the conventional patch-clamp technique. Both GABA and glycine elicited inward Cl- currents in a dose-dependent manner and had almost the same maximal responses. The half-maximum dose (Ka) and Hill coefficient were 6.4 microM and 1.1 for the GABA response, and 74 microM and 1.5 for the glycine response. BIC and STR antagonized both GABA and glycine responses in a competitive manner. The blocking potency of BIC and STR on the GABA response was comparable. The half inhibition dose (IC50) was 2.7 microM for BIC and 6.7 microM for STR. STR blocked the glycine response about 3,000 x more effectively than BIC. The IC50 was 28 nM for STR and 100 microM for BIC. The BIC and STR did not have voltage-dependent blocking effects on either GABA or glycine responses. Neither GABA nor glycine showed outward rectification in their current-voltage relationships. The functional role of glycine in the rat hippocampal CA1 region is discussed.

Bursting properties of units in cat globus pallidus and entopeduncular nucleus: the effect of excitotoxic striatal lesions

The bursting properties of units recorded in globus pallidus and entopeduncular nucleus were studied in awake cats sitting quietly before and after ipsilateral excitotoxic striatal lesions. A computerized statistical procedure was used to identify and evaluate bursts in the recorded spike trains. Bursts were assigned a quantitative statistical measure of burst 'strength' (or improbability) - the surprise value. Before the lesion, 34% of units in the globus pallidus and 60% of units in the entopeduncular nucleus exhibited bursts. Burst units had a significantly slower discharge rate and a significantly greater variability of discharge than non-burst units. The mean length of the interspike intervals immediately preceding the bursts was significantly longer than the overall median intervals in burst units. After the lesion, 21% of units in the globus pallidus and 11% of the units in the entopeduncular nucleus exhibited bursts. Burst units had significantly higher discharge rates and lower discharge variability after the lesion. In contrast, the lesion had no significant effect on the rate or variability of non-burst units. The differences between bursting and non-bursting units in discharge rate and variability disappeared after the lesion. In globus pallidus, the lesion resulted in a significant reduction in the mean number of bursts per unit, surprise value per burst, mean length of bursts, and number of spikes per burst, and a significant increase in the mean discharge rate of burst units. In entopeduncular nucleus, the small number of bursts recorded after the lesion precluded a useful statistical comparison of the effect of striatal lesions on the properties of the bursts. This study demonstrates that removing striatal projections to globus pallidus and entopeduncular nucleus decreases bursting in these nuclei, indicating that intact striatal projections are necessary for the normal production of bursts in these regions.

Effects of enoxacin and its combination with 4-biphenylacetate, an active metabolite of fenbufen, on population spikes in rat hippocampal slices

The effects of enoxacin, a new quinolone antibacterial agent, and its combination with 4-biphenylacetate (BPA), an active metabolite of the non-steroidal antiinflammatory agent fenbufen, were examined on population spikes induced by electrical stimulation of the stratum radiatum in the CA1 pyramidal cell layer in rat hippocampal slices. Enoxacin (10(-4) M) and bicuculline (10(-6) M) increased the amplitude of the population spikes and anew elicited the second spikes (latency: 10 msec.), while BPA (10(-5) M) decreased the amplitude of the population spikes. However, the combination of enoxacin (10(-6), 10(-5) M) with BPA (10(-5) M) elicited the second spikes or epileptiform bursts with an increase of the population spike amplitude. The dose-response relationships showed that the effect of enoxacin was 100 times potentiated in the presence of BPA (10(-5) M). The second spikes induced by enoxacin (10(-4) M) were suppressed by muscimol (10(-6) M) and baclofen (10(-6) M), but not by clorazepate (5 x 10(-5) M) and pentobarbital (5 x 10(-5) M). The second spikes induced by bicuculline (10(-6) M) were suppressed by these four drugs. The second spikes by the combination of enoxacin (10(-6) M) with BPA (10(-5) M) were suppressed by muscimol (5 x 10(-6) M), but not by clorazepate (5 x 10(-5) M). These results suggest that the combination of enoxacin with BPA exerts a drug interaction to elicit the second spikes or epileptiform bursts with its mode of action different from that of bicuculline.

Strychnine-induced potassium current in isolated dorsal root ganglion cells of the rat

1. Effects of strychnine (Str) on the dissociated dorsal root ganglion (DRG) cells of the rat have been investigated in whole-cells configuration by a conventional patch-clamp technique. 2. gamma-Aminobutyric acid (GABA)-induced Cl- current (ICl) increased sigmoidally with increasing concentration. The half-maximal response (Ka) was 3 x 10(-5) M and the Hill coefficient was 1.5. Both Str and bicuculline inhibited the GABA-induced ICl in a concentration-dependent manner. 3. Str itself could elicit the current at concentrations over 10(-5) M, at which concentrations the GABA response was completely suppressed. The concentration-response curve for the Str-induced current was bell-shaped, and a nearly maximum response occurred at 3 x 10(-4) M. A transient 'hump' current appeared immediately after the wash-out of external solution containing high concentrations of Str over 3 x 10(-4) M. 4. The Str-induced outward current and a transient 'hump' current were augmented by the removal of extracellular K+ and were suppressed by the substitution of intracellular K+ for Cs+. But the current was not sensitive to extracellular Na+, Ca2+ and Cl-. 5. The reversal potential of Str-induced current (EStr) was -75 mV, which was close to the K+ equilibrium potential (EK = -76.3 mV). The change of EStr for a ten fold change in extracellular K+ concentration was 58 mV, indicating that the membrane behaves like a K+ electrode in the presence of Str. The reversal potential of the 'hump' current was also close to EK. 6. The Str-induced outward current was antagonized by K+ channel blockers such as Ba2+, tetraethylammonium (TEA)-chloride, and 4-aminopyridine (4-AP) in a concentration-dependent manner. 7. The Str-induced K+ current was not affected by internal perfusion of bis(gamma-aminophenoxy)ethane-N, N,N',N-tetraacetic acid (BAPTA), indicating that the Str response does not result in the activation of K+ conductance by the intracellular Ca2+.

GABA inhibits the rise in cytosolic free calcium concentration in depolarized immature cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA) reduced the peak rise or slowed the rate of rise in cytosolic free calcium concentration ([Ca]in) induced by quisqualate, kainate, or high KCl in immature cerebellar Purkinje cell bodies. The sustained rise or repeated transient of [Ca]in induced by tetraethylammonium, veratridine, or Bay-K-8644 was lowered to the basal level by adding GABA, although the inhibition by GABA of Bay-K-8644-induced rise in [Ca]in was only slight and transient in some cells. These findings indicate that GABA inhibits the rise in [Ca]in by hyperpolarizing the membrane potentials at Purkinje cell bodies.

Evidence for GABA-mediated control of putative nociceptive modulating neurons in the rostral ventromedial medulla: iontophoresis of bicuculline eliminates the off-cell pause

This study was undertaken to test the hypothesis that gamma-aminobutyric acid (GABA) is an endogeneous neurotransmitter regulating the activity of a class of putative nociceptive modulatory neurons (termed "off-cells") in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. Off-cells, which are believed to correspond to the RVM output neuron that inhibits nociceptive processing at the level of the spinal cord, exhibit an abrupt pause in firing that begins immediately prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by application of noxious heat to the tail. Single-unit recording and iontophoretic techniques were used to examine the ability of the GABAA receptor antagonist bicuculline methiodide (BIC) to antagonize selectively the characteristic off-cell pause. Iontophoretic application of BIC (5-30 nA) blocked the TF-related pause in each of the off-cells tested. This effect of BIC was generally slow in onset, and outlasted the period of application by several minutes. BIC iontophoresis also eliminated the cyclic alternation between active and silent periods that is often displayed by off-cells in lightly anesthetized rats. BIC application did not have a consistent effect on the firing of two other classes of RVM neurons ("on-cells" and "neutral cells"). Iontophoretically applied BIC antagonized the inhibitory effect of iontophoretically applied GABA, but not that produced by glycine. The glycine receptor antagonist strychnine did not mimic the action of BIC on off-cell activity. These data demonstrate antagonism of a synaptically evoked response using iontophoretic application of BIC, and provide strong evidence that the inhibitory neurotransmitter GABA mediates the TF-related off-cell pause. Taken together with behavioral experiments demonstrating that a GABA-mediated inhibitory process within RVM is crucial in permitting execution of the TF response, the present observations point to the significant functional relevance of GABA transmission within RVM in modulation of nociception.

Decreased sensitivity of cerebellar nuclei neurons to GABA and taurine: effects of long-term inferior olive destruction in the rat

The effects of iontophoretically applying the presumed Purkinje cell inhibitory neurotransmitters, GABA and taurine, were tested on neurons of the cerebellar nuclei in normal and in climbing-fiber-deafferented cerebella. Rats treated with 3-acetylpyridine to totally destroy the inferior olive were used for acute experiments 105-185 days after treatment. In controls, nearly all neuronal firing was dose-dependently depressed by both inhibitory amino acids. The depression in firing for both were antagonized by bicuculline and picrotoxin but not by strychnine while TAG specifically antagonized only responses to taurine. At sufficient doses, bicuculline and TAG induced disinhibitory responses (significant release of neuron discharge) in the absence of applied antagonist. In deafferented animals, the inhibitory efficacy of GABA and taurine were drastically reduced; most of the neurons failed to respond to these amino acids at the same iontophoretic parameters as for the control rats. Moreover, high doses of bicuculline and TAG did not induce any disinhibitory response (no significant increase in discharge rate) in most of the neurons tested. These results clearly demonstrate that climbing fiber deafferentation reduces postsynaptic sensitivity of the cerebellar nuclei neurons for the presumed Purkinje cell inhibitory neurotransmitters.

Neurotransmitters regulating vestibular commissural inhibition in the cat

The present study focuses on the transmitters which control the vestibular neural activity and, in particular, those which regulate commissural inhibition. Extracellular spike recordings were taken from single vestibular neurons of decerebrate cats. Seven-barreled electrodes were used. The barrels, except for the central one, were filled with several transmitter candidates and their specific inhibitors. After isolation of a type I neuron, the chemicals were iontophoretically applied to examine their effects on the activity of the neuron. The results were as follows: i) GABA and glycine markedly decreased spontaneous firing of the neurons, while serotonin did not change the activity. ii) Bicuculline abolished the inhibitory effects of GABA on the neurons. iii) Strychnine also abolished the effects of glycine, iv) Commissural inhibition caused by electrical stimulation of the contralateral labyrinth was not abolished by the application of strychnine but was abolished by bicuculline. We concluded that 1) vestibular type 1 neurons are controlled by GABAergic and glycinergic neurons, and that 2) the commissural inhibition is activated by the GABAA receptor, but not by the GABAB receptor.

Regulation of the cytosolic free calcium concentration by Na+ spikes in immature cerebellar neurons with N-methyl-D-aspartate receptors

N-Methyl-D-aspartate and glycine increased the cytosolic free calcium concentration ([Ca]in) in medium-sized cerebellar neurons. Spontaneous changes in [Ca]in were occasionally observed in NMDA-responsive cells, but large increases in [Ca]in were triggered only through depolarizations by adding veratridine or K+ channel blockers in every cell examined. The [Ca]in increase was suppressed by voltage-dependent Na+ and Ca2+ channel blockers and by an inhibitory transmitter (GABA), suggesting that the generation of Na+ spikes is involved in the increase in [Ca]in.

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.