GABAA/benzodiazepine receptor heterogeneity: neurophysiological implications

3-Phenyl-6-(2-pyridyl)methyloxy-1,2,4-triazolo[3,4-a]phthalazines and Analogues: High-Affinity γ-Aminobutyric Acid-A Benzodiazepine Receptor Ligands with α2, α3, and α5-Subtype Binding Selectivity over α1

Studies with our screening lead 5 and the literature compound 6 led to the identification of 6-benzyloxy-3-(4-methoxy)phenyl-1,2,4-triazolo[3,4-a]phthalazine 8 as a ligand with binding selectivity for the gamma-aminobutyric acid-A (GABA-A) alpha 3- and alpha 5-containing receptor subtypes over the GABA-A alpha 1 subtype (K(i): alpha 2 = 850 nM, alpha 3 = 170 nM, alpha 5 = 72 nM, alpha 1 = 1400 nM). Early optimization studies identified the close analogue 10 (K(i): alpha 2 = 16 nM, alpha 3 = 41 nM, alpha 5 = 38 nM, alpha 1 = 280 nM) as a suitable lead for further study. High-affinity ligands were identified by replacing the 6-benzyloxy group of compound 10 with 2-pyridylmethoxy (compound 29), but binding selectivity was not enhanced (K(i): alpha 2 = 1.7 nM, alpha 3 = 0.71 nM, alpha 5 = 0.33 nM, alpha 1 = 2.7 nM). Furthermore, on evaluation in xenopus oocytes,(22) 29 was discovered to be a weak to moderate inverse agonist at all four receptor subtypes (alpha 1, -7%; alpha 2, -5%; alpha 3, -16%; alpha 5, -5%). Replacement of the 3-phenyl group of 29 with alternatives led to reduced affinity, and smaller 3-substituents led to reduced efficacy. Methyl substitution of the benzo-fused ring of 29 at the 7-, 8-, and 10-positions resulted in increased efficacy although selectivity was abolished. Increased efficacy and retention of selectivity for alpha 3 over alpha 1 was achieved with the 7,8,9,10-tetrahydro-(7,10-ethano)-phthalazine 62. Compound 62 is currently one of the most binding selective GABA-A alpha 3-benzodiazepine-site partial agonists known, and although its selectivity is limited, its good pharmacokinetic profile in the rat (33% oral bioavailability after a 3 mg/kg dose, reaching a peak plasma concentration of 179 ng/mL; half-life of 1 h) made it a useful pharmacological tool to explore the effect of a GABA-A alpha 2/alpha 3 agonist in vivo.

Distribution of GABAA receptor mRNA in the motor cortex of ALS patients

The pathomechanism of amyotrophic lateral sclerosis (ALS) remains unclear. There is some evidence that excitotoxic cell death is involved in the degeneration of the motor nervous system, and that ligand-gated receptor channels play a role in the pathogenesis of the disease. Several electrophysiological and anatomical studies support the pathophysiological concept of an impaired inhibitory, namely GABAergic, control of the motoneurons in the cerebral cortex of ALS patients. The aim of our study was to investigate the expression of GABAA-receptor subunit mRNAs and the GABA synthesizing enzyme glutamic acid decarboxylase (GAD) in the motor cortex of ALS patients compared to tissue of control persons. We performed in situ hybridization histochemistry (ISH) on human postmortem motor cortex sections of ALS patients (n = 5) and age matched controls with no history of neurological disease (n = 5). The most intriguing finding was a significantly reduced mRNA expression of the alpha1-subunit in ALS patients while the level of beta1-subunit mRNA was elevated in the patients group. This may indicate specific alterations of the GABAA receptor subunit composition and result in distinct physiological and pharmacological properties of these receptors in ALS patients.

Phosphorylation influences neurosteroid modulation of synaptic GABAA receptors in rat CA1 and dentate gyrus neurones

The neurosteroid 5beta-pregnan-3alpha-ol-20-one (5beta3alpha) is a potent, endogenous, positive allosteric modulator of the GABA(A) receptor. Relatively low concentrations of 5beta3alpha (10-100 nM), thought to occur physiologically, caused a concentration-dependent slowing of the decay of GABA-mediated miniature inhibitory postsynaptic currents (mIPSCs) recorded from hippocampal CA1 pyramidal neurones. However, much greater concentrations of this neurosteroid (> or =300 nM) were required to similarly influence dentate granule cell mIPSCs. By contrast, the allosteric modulators pentobarbitone and flunitrazepam were equi-effective in prolonging mIPSCs in both neuronal types. Hence, the neurosteroid selectively differentiates between the synaptic GABA(A) receptors of these hippocampal neurones. Inhibition of either protein kinase A, or C, greatly reduced the sensitivity of CA1 synaptic GABA(A) receptors to 5beta3alpha, but not pentobarbitone, whereas stimulation of PKC had no effect on steroid sensitivity. However, in dentate gyrus granule cells, activation of PKC made mIPSCs sensitive to a previously ineffective concentration of 5beta3alpha. Collectively, these results suggest that the GABA-modulatory effects of physiological levels of the neurosteroid will not be uniformly experienced throughout the central nervous system, or even within the same brain region such as the hippocampus, but will be neurone-specific and will be dependent on the phosphorylation status of the GABA(A) receptor, or associated proteins.

Repeated neonatal handling with maternal separation permanently alters hippocampal GABAA receptors and behavioral stress responses

Increasing evidence suggests that postnatal events, such as handling or maternal separation, can produce long-term changes in brain function. These are often expressed as changes in the profile of endocrine or behavioral responses to stress. Changes in gamma-aminobutyric acid type A receptors (GABARs), which mediate the majority of fast synaptic inhibition in adult brain, have been proposed as one potential mediator of these behavioral effects. In the current article, we use a combination of single-cell electrophysiology and antisense mRNA amplification to demonstrate permanent molecular and functional differences in GABARs within hippocampal dentate granule neurons after as few as two episodes of neonatal handling with brief maternal separation. Adult animals that as pups experienced handling with maternal separation maintained a more immature GABAR phenotype and exhibited increased activity in response to swim stress. These findings demonstrate the exquisite sensitivity of the developing GABAergic system to even subtle environmental manipulations and provide an unique molecular mechanism by which postnatal handling with maternal separation may alter stress-related behavior.

NMDA-NR1 and -NR2B subunits mRNA expression in the hippocampus of rats tolerant to Diazepam

The development of tolerance to the hypolocomotor effects of Diazepam (DZ) is thought to be a contingent or learning phenomenon. In previous reports, we demonstrated a positive correlation between the development of tolerance to the sedative effects of DZ and hippocampal synaptic plasticity. Furthermore, previous exposure to the drug administration context blocks both the tolerance to sedative effects of DZ and the increased hippocampal plasticity. The results of the present investigation show that the development of tolerance to hypolocomotor action of DZ (5 mg/kg/day) for 4 days results in a significant increase in the hybridization signals for mRNA for N-methyl-D-aspartate (NMDA) glutamatergic receptor NR1 and NR2B subunits in the hippocampal dentate gyrus. Furthermore, we have observed more benzodiazepine binding sites in the hippocampus of non-tolerant animals. We conclude that the increased hippocampal synaptic efficacy in DZ tolerant rats, may be NMDA receptor dependent due to an increased recombinant NR1-NR2B complex observed in the hippocampal formation of tolerant rats.

Neurosteroid modulation of GABAA receptors

Certain metabolites of progesterone and deoxycorticosterone are established as potent and selective positive allosteric modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor. Upon administration these steroids exhibit clear behavioural effects that include anxiolysis, sedation and analgesia, they are anticonvulsant and at high doses induce a state of general anaesthesia, a profile consistent with an action to enhance neuronal inhibition. Physiologically, peripherally synthesised pregnane steroids derived from endocrine glands such as the adrenals and ovaries function as hormones by crossing the blood brain barrier to influence neuronal signalling. However, the demonstration that certain neurons and glial cells within the central nervous system (CNS) can synthesize these steroids either de novo, or from peripherally derived progesterone, has led to the proposal that these steroids (neurosteroids) can additionally function in a paracrine manner, to locally influence GABAergic transmission. Steroid levels are known to change dynamically, for example in stress and during pregnancy. Given that GABA(A) receptors are ubiquitously expressed throughout the central nervous system, such changes in steroid levels would be predicted to cause a global enhancement of inhibitory neurotransmission throughout the brain, a scenario that would seem incompatible with a physiological role as a selective neuromodulator. Here, we will review emerging evidence that the GABA-modulatory actions of the pregnane steroids are highly selective, with their actions being brain region and indeed neuron dependent. Furthermore, the sensitivity of GABA(A) receptors is not static but can dynamically change. The molecular mechanisms underpinning this neuronal specificity will be discussed with particular emphasis being given to the role of GABA(A) receptor isoforms, protein phosphorylation and local steroid metabolism and synthesis.

Brain regional pharmacology of GABA(A) receptors in alcohol-preferring AA and alcohol-avoiding ANA rats

Compounds interacting with the GABA(A) receptor system modulate voluntary alcohol consumption in alcohol-preferring AA (Alko, Alcohol) rats. Therefore, we compared the central GABA(A) receptor pharmacology of the AA rats to that of their counterpart, alcohol-avoiding ANA (Alko, Non-Alcohol) rats with receptor autoradiography. Total flumazenil-sensitive [(3)H]Ro 15-4513 binding to the benzodiazepine site of GABA(A) receptor was slightly lower in the hippocampus, striate cortex and lateral hypothalamus of the AA than ANA rats. The proportions of zolpidem- and diazepam-sensitive components were similar in both rat lines. Basal picrotoxin-sensitive [(35)S]TBPS binding to the convulsant site of GABA(A) receptor was similar in most regions between the rat lines, but the up-modulation of the binding by 10 microM diazepam in the hippocampal, amygdaloid and entorhinal cortical areas was greater in the AA than ANA rats. These results do not reveal any general genetic defect in the GABA(A) receptors of AA or ANA rats, but the regional profile of the ligand binding differences between the lines, especially in the coupling of the benzodiazepine and chloride channel sites, suggests receptor subtype-specific changes in brain regions implicated in behavioural reward and anxiolysis.

Sex differences in GABA(A)ergic system in rat substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR) is involved in the control of movement disorders including seizures through its GABAergic neurons. Microinfusions of muscimol (a GABA(A) receptor agonist) produce specific effects on seizures depending on sex, infusion site (SNR(anterior) or SNR(posterior)) and age. To assess whether these effects are due to sex differences in GABAergic indices within the SNR we analyzed the expression of alpha(1) subunit mRNA of the GABA(A) receptor and the levels of GABA immunoreactivity (IR) of male and female rats at postnatal day 15 (PN15) and PN30. In each age, within the same SNR region, expression of alpha(1) subunit mRNA and intensity of GABA IR per neuron was higher in females compared to males. At PN15, in both sexes, there were no regional differences in expression of alpha(1) subunit mRNA and intensity of GABA IR. However, at PN30 in both sexes, expression of alpha(1) subunit mRNA and intensity of GABA IR per cell was higher in SNR(anterior) than in SNR(posterior). These results demonstrate that expression of alpha(1) subunit mRNA for GABA(A) receptor and levels of GABA IR in the SNR are sex- and site-specific, which may contribute to sex-, regional- and age-related differences in the expression of movement disorders and seizures.

Distribution and binding parameters of GABAA receptors in the thalamic nuclei of Macaca mulatta and changes caused by lesioning in the globus pallidus and reticular thalamic nucleus

Ascending output from the basal ganglia to the primate motor thalamus is carried by GABAergic nigro- and pallido-thalamic pathways, which interact with intrinsic thalamic GABAergic systems represented in primates by local circuit neurons and axons of the reticular thalamic nucleus. Disease-triggered pathological processes in the basal ganglia can compromise any of these pathways either directly or indirectly, yet the effects of basal ganglia lesioning on its thalamic afferent-receiving territories has not been studied in primates. Two GABA(A) receptor ligands, [(3)H]muscimol and [(3)H]flunitrazepam, were used to study the distribution and binding properties of the receptor in intact monkeys, those with kainic acid lesions in the globus pallidus, and those with ibotenic acid lesions in the reticular nucleus using quantitative autoradiographic technique on cryostat sections of fresh frozen brain tissue. In control monkeys the binding affinities for [(3)H]muscimol averaged 50 nM in the thalamic nuclei and 86 nM in the basal ganglia while the binding densities varied (maximum density of binding sites [Bmax] range of 99.4-1000.1 fmol/mg of tissue). Binding affinities and Bmax values for [(3)H]flunitrazepam averaged 2.02 nM and 81-113 fmol/mg of tissue, respectively. Addition of 100-microM GABA increased average affinity to 1.35 nM whereas Bmax values increased anywhere from 1-50% in different nuclei. Zolpidem (100 nM) decreased binding by 68-80%. Bmax values for both ligands were decreased at the two survival times in both medial and lateral globus pallidus implying involvement of both nuclei in the lesion. Statistically significant, 40% decrease (P=0.055) of Bmax for [(3)H]muscimol was observed in the ventral anterior nucleus pars densicellularis (VAdc, the main pallidal projection territory in the thalamus) 1 week after globus pallidus lesioning and a 36% decrease (P=0.017) 4 months post-lesioning. In contrast, [(3)H]flunitrazepam Bmax values in the VAdc of the same animals were increased by 23% (P=0.021) at 1 week and 28% (P=0.005) 4 months postlesion, respectively. One week after the reticular nucleus lesioning, the binding densities of [(3)H]muscimol and [(3)H]flunitrazepam were decreased in the thalamic nuclei receiving projections from the lesioned reticular nucleus sector by approximately 50% (P<0.05) and 10-33% (P<0.05), respectively. The results suggest that different GABA(A) receptor subtypes are associated with different GABAergic systems in the thalamus which react differently to deafferentation.

Visualization of alpha5 subunit of GABAA/benzodiazepine receptor by 11C Ro15-4513 using positron emission tomography

Although [(11)C]Ro15-4513 and [(11)C]flumazenil both bind to the central benzodiazepine (BZ) receptors, the distributions of the two ligands are not identical in vivo. Moreover, the in vivo pharmacological properties of [(11)C]Ro15-4513 have not been thoroughly examined. In the present study, we examined the pharmacological profile of [(11)C]Ro15-4513 binding in the monkey brain using positron emission tomography (PET). [(11)C]Ro15-4513 showed relatively high accumulation in the anterior cingulate cortex, hippocampus, and insular cortex, with the lowest uptake being observed in the pons. Accumulation in the cerebral cortex was significantly diminished by the BZ antagonist flumazenil (0.1 mg/kg, i.v.), but not that in the pons. Using the pons as a reference region, the specific binding of [(11)C]Ro15-4513 in most of the cerebral cortex including the limbic regions clearly revealed two different affinity sites. On the other hand, specific binding in the occipital cortex and cerebellum showed only a low affinity site. Zolpidem with affinity for alpha1, alpha2, and alpha3 subunits of GABA(A)/BZ receptor fully inhibited [(11)C]Ro15-4513 binding in the occipital cortex and cerebellum, while only about 23% of the binding was blocked in the anterior cingulate cortex. Diazepam with affinity for alpha1, alpha2, alpha3, and alpha5 subunits inhibited the binding in all brain regions. Since Ro15-4513 has relatively high affinity for the alpha5 subunit in vitro, these in vivo bindings of [(11)C]Ro15-4513 can be interpreted as the relatively high accumulation in the fronto-temporal limbic regions representing binding to the GABA(A)/BZ receptor alpha5 subunit.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Human GABA A receptors on dopaminergic neurons in the pars compacta of the substantia nigra

The gamma-aminobutyric acid A (GABA(A)) receptor subunit expression of the dopaminergic cells of the substantia nigra (SN) was investigated in the present study. Especially the dopaminergic cells, located in the pars compacta of SN (SNc), are of great neurologic interest, because the functional deficit and depletion of these cells are the correlate of Parkinson's disease. We used a combination of in situ hybridization histochemistry (ISH) and immunohistochemistry (IHC) on sections of human postmortem mesencephalon to investigate the expression of GABA(A) receptor subunit messenger RNAs (mRNAs) and of the receptor protein in dopaminergic SN cells. Immunohistochemical detection of tyrosine hydroxylase (TH), the pivotal enzyme of dopamine synthesis, was used to define the boundaries of SN pars reticulata (SNr) and pars compacta subregions. In SNr, all neurons were labeled by subunit-specific oligonucleotide probes and the amount of GABA(A) receptor mRNA expression was quantified as alpha(1) = beta(2) > gamma(2) > alpha(3). In contrast, in SNc, only around 25% of neurons expressed mRNA transcripts of GABA(A) receptor subunits, quantified as alpha(1) = beta(2) > gamma(2) > alpha(3) > alpha(4) = beta(3). In approximately the same percentage of neurons, which were labeled by alpha(1)-subunit-specific probe, the alpha(1)-subunit also was detected at the protein level by a specific monoclonal antibody. We, therefore, could demonstrate that a subset of dopaminergic neurons in human SNc receive inhibitory synaptic input by means of GABA(A) receptors mainly of the alpha(1)beta(2)gamma(2) subtype. This might represent a negative feedback loop between the striatum and the SNc and be a target of pharmacologic interventions in neurodegenerative diseases such as Parkinson's disease.

Prenatal exposure to diazepam and alprazolam, but not to zolpidem, affects behavioural stress reactivity in handling-naïve and handling-habituated adult male rat progeny

A gentle long-lasting handling produces persistent neurochemical and behavioural changes and attenuates the impairment in the behavioural reactivity to novelty induced by the prenatal exposure to diazepam (DZ) in adult male rat progeny. This study investigated the consequences of a late prenatal treatment with three GABA/BDZ R agonists (DZ) alprazolam (ALP) and zolpidem (ZOLP)), on different stress-related behavioural patterns, in non-handled (NH), short-lasting handled (SLH) and long-lasting handled (LLH) adult male rats exposed to forced swim test (FST), acoustic startle reflex (ASR) and Vogel test (VT). The effects on motor activity were evaluated in the open field and in the Skinner box. The seizure sensitivity to picrotoxin (PTX) was investigated as an index of the functional state of GABA/BDZ Rs. A single daily s.c. injection of DZ (1.25-2.50 mg/kg) and ALP (0.125-0.250 mg/kg) over gestational days 14-20 induced a decrease in immobility time in the FST in NH rats, no change in SLH rats and an increase in LLH rats; DZ induced an increase in the peak amplitude of the ASR in NH rats, no change in SLH rats and a reduction in LLH rats; ALP was ineffective in all groups. DZ and ALP reduced the number of punished licks in the VT in NH, SLH and LLH rats while the unpunished licks were not modified. DZ decreased locomotion and the lever pressing responses while ALP increased them. DZ and ALP increased the seizure sensitivity to PTX (2.5-4.0 mg/kg i.p.). These findings indicate a convergence on anxiety-related behaviours in the effects of prenatal exposure to DZ and ALP and a differentiation on motor activity. Long-lasting handling was able to overcompensate the increased behavioural stress reactivity induced by the prenatal exposure to DZ and ALP.

Functional characterization of GABA(A) receptors in neonatal hypothalamic brain slice

The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA(A) receptors are formed from a number of distinct subunits, designated alpha, beta, gamma, delta, epsilon, and theta, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA(A) receptors present on hypothalamic neurons. To gain insight into which GABA(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA(A) receptor-mediated currents were detected in all neurons tested and had an average EC(50) of 20 +/- 1.6 microM. Hypothalamic GABA(A) receptors were modulated by diazepam (EC(50) = 0.060 microM), zolpidem (EC(50) = 0.19 microM), loreclezole (EC(50) = 4.4 microM), methyl-6,7-dimethoxy-4-ethyl-beta-carboline (EC(50) = 7.7 microM), and 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). Conversely, these receptors were inhibited by Zn(2+) (IC(50) = 70.5 microM), dehydroepiandrosterone sulfate (IC(50) = 16.7 microM), and picrotoxin (IC(50) = 2.6 microM). The alpha4/6-selective antagonist furosemide (10-1,000 microM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA(A) receptor subtypes that incorporate alpha1 and/or alpha2 subunits, beta2 and/or beta3 subunits, and the gamma2 subunit. Our results suggest receptors expressing alpha3-alpha6, beta1, gamma1, and delta, if present, represent a minor component of functional hypothalamic GABA(A) receptors.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

GABA(A)-benzodiazepine receptor complex sensitivity in 5-HT(1A) receptor knockout mice on a 129/Sv background

Previous studies in 5-HT(1A) receptor knockout (1AKO) mice on a mixed Swiss Websterx129/Sv (SWx129/Sv) and a pure 129/Sv genetic background suggest a differential gamma-aminobutyric acid (GABA(A))-benzodiazepine receptor complex sensitivity in both strains, independent from the anxious phenotype. To further investigate these discrepancies, various GABA(A)-benzodiazepine receptor ligands were tested in different behavioral paradigms in 1AKO and wild type (WT) mice on a 129/Sv background. 1AKO and WT mice responded comparably to alprazolam, flumazenil, alcohol and pentylenetetrazol as measured in the stress-induced hyperthermia paradigm. In addition, sedative-anesthetic effects of pentobarbital measured via the righting reflex were similar and a selected dose of diazepam exerted similar anxiolytic effects in both genotypes in the elevated plus maze. In conclusion, 1AKO mice on a 129/Sv background have undisturbed GABA(A)-benzodiazepine receptor sensitivity in contrast to those described on a mixed Swiss Websterx129/Sv background. The anxious phenotype of 1AKO mice seems to occur independent of the GABA(A)-benzodiazepine receptor complex functioning.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Participation of GABAA receptors in the modulation of experimental anxiety by tachykinin agonists and antagonists in mice

Mice were acutely intraperitoneally treated with diazepam (DZP), pentylenetetrazol (PTZ) or NaCl 0.9% (control group), and 15 min later, the DZP-treated group received substance P (SP), neurokinin A (NKA; NK1 and NK2 natural preferential agonists), [Trp7 beta-Ala8] NKA(4-10) (Trp-7; NK3 antagonist) or vehicle intracerebroventricularly, whereas the PTZ-treated group was intracerebroventricularly administered with FK 888, SR 48968 (NK1 and NK2 antagonists, respectively) or senktide (SENK--[succinil-Asp6, MePhe8] substance P(6-11); NK3 agonist), or vehicle immediately before they were submitted to the elevated plus-maze (EPM) test. Another group of animals was repeatedly treated with increasing doses of DZP or NaCl 0.9% intraperitoneally for 28 days, and 3 days after the last injection (test day), animals received DZP, FK 888, SR 48968, SENK or vehicle intracerebroventricularly, or DZP (NaCl 0.9%) intraperitoneally before the EPM evaluation. The anxiolytic action of the acute treatment with DZP was inhibited by the central administration of NKA and Trp-7 but not by SP. NK1 and NK2 antagonists, but not NK3 agonist, blocked the anxiogenic action of PTZ, as evaluated in the plus-maze test. Flumazenil (FLM), a benzodiazepine antagonist, was not able to inhibit the anxiolytic profile of action induced by the NK2 antagonist. Central administration of FK 888 and SR 48968 promoted anxiolytic effects in both control and DZP-withdrawn animals, suggesting a clear relationship between the GABAergic and the tachykinergic systems, mostly involving NK1 and NK2 receptors, in the modulation of experimental anxiety in mice.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Modulation Of [35S]-tert-butylbicyclophosphorothionate binding by somatostatin in rat hypothalamus

1. The present study was designed to assess the effect of the tetradecapeptide somatostatin on the GABA(A) receptor complex in the rat hypothalamus. 2. GABA(A) receptors were labelled with [35S]-tert-butylbicyclophosphorothionate (TBPS), which binds in or near the chloride channel, and binding as assessed by in vitro quantitative autoradiography using a computer-assisted image analysis system. 3. Somatostatin inhibited the binding of [35S]-TBPS to the convulsant site of the hypothalamic GABA(A) receptor complex of rat slide-mounted hypothalamic structures in a concentration-dependent manner with an affinity in the micromolar range (10(-6) to 3 x 10(-6) mol/L). Somatostatin appeared to mimic the effects of the neurosteroid 5alpha-pregnane-3alpha ol-one (5alpha3alphaP), GABA and picrotoxin on [35S]-TBPS binding in the rat hypothalamus in all structures examined. Furthermore, GABA or muscimol (a GABA(A) receptor agonist), when added to the incubation medium, enhanced the capacity of somatostatin to inhibit [35S]-TBPS binding, with an IC50 of 10(-7) mol/L. However, incubation with bicuculline (a GABA(A) receptor antagonist) led to the abolition of the inhibitory effect of somatostatin on [35S]-TBPS specific binding in rat hypothalamus. 4. The present results demonstrate the presence of a modulatory effect of somatostatin on the GABA(A) receptor complex in rat hypothalamic structures. Furthermore, the data suggest that somatostatin allosterically modifies [35S]-TBPS binding through a mechanism similar to that of GABA. Taken together, these results provide evidence for the presence of somatostatin- GABA interactions in rat hypothalamus.

Neurosteroid modulation of recombinant and synaptic GABAA receptors

Certain pregnane steroids are now established as potent, positive allosteric modulators of the gamma-aminobutyric acid type A (GABAA) receptor. These compounds are known to be synthesized in the periphery by endocrine glands, such as the ovaries and the adrenal glands, and can rapidly cross the blood-brain barrier. Therefore, such steroids could act as endogeneous modulators of the major inhibitory receptor in the mammalian central nervous system. However, the demonstration that certain neurons and glia can synthesize the pregnane steroids (i.e., neurosteroids) additionally suggests that they may serve a paracrine role by influencing GABAA-receptor function through their local release in the brain itself. Here, we demonstrate that these neurosteroids are highly selective and extremely potent modulators of the GABAA receptor. The subunit composition of the GABAA receptor may influence the actions of the neurosteroids, particularly when considering concentrations of these agents thought to occur physiologically, which may underlie their reported differential effects at certain inhibitory synapses. However, recent work suggests that the phosphorylation status of either the synaptic GABAA receptor or its associated proteins may also influence neurosteroid sensitivity; these findings are discussed. Upon administration, the neurosteroids exhibit clear behavioral effects, including sedation, anticonvulsant actions, and behaviors predictive of anxiolysis; when given at high doses, they induce general anesthesia. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit these properties, and these data are reviewed in this chapter. However, targeting the brain enzymes that synthesize and metabolize the neurosteroids may offer a new approach to exploit this novel endocrine-paracrine neurotransmitter interaction.

Preclinical development of neurosteroids as neuroprotective agents for the treatment of neurodegenerative diseases

Recent literature has emphasized the unique role that the neurosteroid subclass of steroids, which includes dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS), play in the developing and adult central nervous system (CNS). Both DHEA and DHEAS are found in abundance in the CNS (Majewska, 1995), and both can be synthesized and metabolized in the brain of many species (Baulieu, 1981, 1998; Corpéchot et al., 1981, 1983; Zwain and Yen, 1999). DHEA and DHEAS have been implicated as potential signaling molecules for neocortical organization during neuronal development, suggesting that they have trophic factor-like activity (neurotrophic or neurotropic) or can interact with various neurotransmitter systems to promote neuronal remodeling (Compagnone and Mellon, 1998; Mao and Barger, 1998). Consistent with a neurotrophic role for these steroids, studies have shown that DHEAS protects certain neuronal populations against neurotoxic insults inflicted by the excitatory amino acid glutamate (Kimonides et al., 1998; Mao and Barger, 1998). This finding suggests that DHEAS may be useful in treating neurodegenerative diseases in which excitotoxicity is believed to be the underlying cause or a major contributor to cell death. Moreover, because DHEA and DHEAS are multifunctional and exhibit a variety of properties in the CNS, including memory consolidation, neuroprotection, and reduction of neurodegeneration (Majewska, 1992, 1995; Lapchak et al., 2000), their potential therapeutic benefits may be extended to include the treatment of other neurodegenerative diseases not directly linked to excitotoxicity.

Differential sensitivity to inverse agonists of GABA(A)/benzodiazepine receptors in rats with genetic absence-epilepsy

A strain of Wistar rats, genetic absence epilepsy rats from Strasbourg (GAERS), was selected and inbred over 40 generations for occurrence of spontaneous spike-wave discharges characteristic of absence seizures, simultaneously with a strain of non-epileptic rats (NER). GAERS demonstrate an excessive sensitivity to antagonists of the GABA(A) receptor. The sensitivity to convulsions induced by various inverse agonists of the GABA(A)/benzodiazepine receptor was compared in GAERS and NERs. The beta-carbolines FG 7142 and DMCM, and the imidazobenzodiazepines RO 19-4603 and the alpha 5-selective RY 024 were several times more convulsant in GAERS than in NERs. The largest differences were found with the non-selective RO 19-4603- and FG 7142. The proconvulsant imidazobenzodiazepine RO 15-4513, binding also to diazepam-insensitive receptors, had low efficacy. The high affinity binding of GABA(A)/BZD receptors with (3H) RO 15-1788 in the brain of naive rats and after administration of FG 7142 did not differ in GAERS and NERs. The data indicate that the hypersensitivity of GAERS to various inverse agonists of the GABA(A)/benzodiazepine receptor involves cortical GABA(A) receptors and is not related to differential activity of a subunit-selective receptor.

Modulation of native and recombinant GABA(A) receptors by endogenous and synthetic neuroactive steroids

Upon administration, certain pregnane steroids produce clear behavioural effects including, anxiolysis, sedation, analgesia, anaesthesia and are anti-convulsant. This behavioural profile is characteristic of compounds that act to enhance the actions of GABA acting at the GABA(A) receptor. In agreement, numerous studies have now demonstrated these steroids to be potent, positive allosteric modulators of the GABA(A) receptor. The pregnane steroids are synthesized in the periphery by endocrine glands such as the adrenals and the ovaries, but are also made by neurons and glial cells in the central nervous system itself. Hence, these compounds could play both an endocrine and a paracrine role to influence neuronal excitability by promoting inhibition. Here we review evidence that the pregnane steroids are highly selective and extremely potent GABA(A) receptor modulators and that their effects at 'physiological' concentrations (low nanomolar) may be influenced by the subunit composition of the GABA(A) receptor. This feature may underlie recent findings demonstrating the effects of the neurosteroids on inhibitory synaptic transmission to be brain region dependent, although recent reports suggest that phosphorylation mechanisms may additionally influence neurosteroid sensitivity of the GABA(A) receptor. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit the behavioural effects of the pregnane steroids and progress with this approach will be discussed. However, the demonstration that the steroids may be made within the central nervous system offers the alternative strategy of targeting the enzymes that synthesize/metabolise the neurosteroids to exploit this novel endocrine/paracrine interaction.

Complete genomic sequence of 195 Kb of human DNA containing the gene GABRG2

GABA (gamma-aminobutyric acid), as the main inhibitory neurotransmitter in the brain, plays an essential role for the overall balance between neuronal excitation and inhibition by acting on GABAA receptors, which are ligand-gated chloride channels. Impaired GABAergic function contributes to certain forms of epilepsy, schizophrenia, Alzheimer's Disease, and other neurological disorders. In order to identify possible genetic features and to further study biological regulation of GABAA receptor genes whose promoter elements and sequence anomalies may contribute to epileptic disorders, as an initial step, we shot-gun sequenced a BAC clone, dj082c10 (195,909-bp in size), encompassing human gamma(2) subunit of GABAA receptor (GABRG2). It is, we believe, the first genomic sequence of the GABA receptor gamma subunit family. Four contigs were assembled from 2950 reads prior to gap in an average redundancy of eight folds over the entire region. The precision of the consensus sequence was predicted to be 99.999% after closing gaps and finishing weak regions. The nine exons of GABRG2 spans an 85-kb region that had 81 SINEs comprising 22.32%, and nine L1 elements comprising 3.40%, respectively. However, the density of L1 in the regions flanking GABRG2 gene (29.45% by 45 elements) is significantly higher than that within the gene. The length of GABRG2 introns varies in the range of 1.5 kb to 38.1 kb.

Hippocampus and locus coeruleus activity on rats chronically treated with diazepam

The neural mechanisms underlying benzodiazepine (BZD) dependence remain equivocal. The present studies tested the hypothesis that similar neural circuitry might be involved in the effects of chronic 7-chloro-1-methyl-5-phenyl-3H-1,4-benzodiazepine-2(1H)-one, diazepam (DZ, Roche), administration and withdrawal. The results of our study showed an increased hippocampal synaptic plasticity in slices from rats chronically treated with DZ (5 mg/kg/18 days), assessed as a decrease of the threshold in the stimulation rate for long-term potentiation (LTP) elicitation. Rats with the same schedule of DZ administration but without signs of withdrawal behaved similarly to vehicle-treated ones (VEH), in the threshold to induce LTP. Furthermore, the activity of locus coeruleus (LC) norepinephrine (NE) neurons in rats tested 24 h after the last DZ injection showed a significant increase. On the other hand, rats that after chronic DZ administration did not develop signs of withdrawal and exhibited a similar pattern of discharge on LC-NE nucleus compared with their controls. We conclude that chronic DZ administration enhances both hippocampal synaptic plasticity and activity of LC-NE neurons. This neural system could be the biological substrate underlying the behavioral alterations accompanying chronic DZ administration and withdrawal.

Influence of anxiolytic drugs on the effects of specific serotonin reuptake inhibitors in the forced swimming test in mice

This study aimed at investigating the effect of several selective serotonin reuptake inhibitors (SSRIs), given alone or in combination with anxiolytic drugs, on the time spent immobile in the forced swimming test in mice. The time spent immobile was dose-dependently reduced by acute administration of fluoxetine (4-64 mg/ kg, i.p.), paroxetine (1-32 mg/kg, s.c.) or sertraline (4-32 mg/kg, s.c.), indalpine was active at only one dose (16 mg/kg, i.p.), fluvoxamine (up to 16 mg/kg, i.p.) and citalopram (up to 4 mg/kg, i.p.) were inactive. The anti-immobility effect of fluoxetine (32 mg/kg) was antagonized by an acute co-administration of all anxiolytics tested, the GABAA/BZD receptor agonists, diazepam (2 mg/kg, i.p.), chlordiazepoxide (8 mg/kg, i.p.), lorazepam (0.125 mg/kg, i.p.), triazolam (0.06 mg/kg, i.p.) and alpidem (8 mg/kg, i.p.) and the 5-HT1A receptor partial agonist, buspirone (0.5 mg/kg, s.c.). The sedative neuroleptic, thioridazine (4 mg/kg, i.p.), was also found to counteract the effect of fluoxetine. Lorazepam, triazolam and buspirone also reversed the anti-immobility effect of paroxetine and sertraline, while diazepam and chlordiazepoxide did not. Alpidem reduced the effect of sertraline but not paroxetine, whereas the reverse was found with thioridazine. These data indicate that the influence of anxiolytics on the action of SSRI antidepressants is variable, depending on both the SSRI and the anxiolytic considered. The co-administration of the GABAA/BZD receptor antagonist, flumazenil (16 mg/kg, i.p.), with behaviourally inactive doses of fluoxetine, fluvoxamine and citalopram, resulted in a reduction of immobility. The 5-HT1A receptor antagonist, (+)-WAY 100135 (8 mg/kg, s.c.), combined with a subactive dose of fluoxetine, but not with fluvoxamine, significantly reduced the time spent immobile. The 5-HT2A receptor antagonist, ketanserin (32 mg/kg, s.c.), which reduced immobility when given alone, did not interfere with fluoxetine given at a subactive dose. Although non-specific sedative and/or motor effects cannot be totally ruled out, these results suggest that pharmacodynamic interactions exist between various anxiolytics and SSRIs. These interactions probably involve both serotonergic and GABAergic processes.

Evaluation of native GABA(A) receptors containing an alpha 5 subunit

The type A receptor for gamma-aminobutyric acid (GABA), or GABA(A) receptor, is a pentamer of highly variable quaternary structure. It includes two alpha subunits, drawn from a pool of six genes, which largely determine benzodiazepine pharmacology of the receptor. In brain sections, both [(3)H]RY-80 (ethyl-8-acetylene-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a][1,4]benzodiazepine-3-carboxylate) and [(3)H]L-655,708 (ethyl (S)-11,12,13,13a-tetrahydro-7-methoxy-9-oxo-9H-imidazo[1,5-a]pyrrolo[2,1-c][1,4]benzodiazepine-1-carboxylate), which are selective for the benzodiazepine site of alpha 5 subunit-containing receptors, showed high-affinity, specific binding, but to fewer regions than did the nonselective benzodiazepine, [(3)H]flunitrazepam. The pattern mirrored alpha 5 mRNA distribution, and was similar to that previously reported for [(3)H]L-655,708 binding. Displacement of [(3)H]RY-80 bound to hippocampal homogenates, and of [(3)H]flunitrazepam bound to cerebellar and hippocampal homogenates showed comparable displacement by flumazenil (K(i)'s 5--7 nM). However, the K(i)'s for diazepam and for clobazam to displace [(3)H]RY-80 binding in hippocampus were about fourfold higher than for [(3)H]flunitrazepam, and the K(i) for clonazepam was sixfold larger, suggesting that these benzodiazepine receptor agonists bind with relatively lower affinity at hippocampal alpha 5-containing receptors.

Sex steroid regulation of glutamate decarboxylase mRNA expression in goldfish brain is sexually dimorphic

Testosterone and oestradiol can modulate GABA synthesis in sexually regressed goldfish. Here we investigated their effects on the mRNA expression of two isoforms of the GABA synthesizing enzyme glutamate decarboxylase (GAD(65) and GAD(67), EC 4.1.1.15). Full-length GAD clones were isolated from a goldfish cDNA library and sequenced. Goldfish GAD(65) encodes a polypeptide of 583 amino acid residues, which is 77% identical to human GAD(65). Goldfish GAD(67) encodes a polypeptide of 587 amino acid residues and is 82% identical to human GAD(67). Goldfish GAD(65) and GAD(67) are 63% identical. Sexually regressed male and female goldfish were implanted with solid silastic pellets containing testosterone, oestradiol or no steroid. Semiquantitative PCR analysis showed that oestradiol significantly increased GAD(65) mRNA expression in female hypothalamus and telencephalon, while testosterone resulted in a significant increase only in telencephalon. GAD(67) mRNA levels were not affected by steroids in females. In contrast, both steroids induced significant decreases of GAD(65) and GAD(67) mRNA levels in male hypothalamus, but had no effect on GAD mRNA expression in male telencephalon. Our results indicate that modulation of GAD mRNA expression is a possible mechanism for steroid action on GABA synthesis, which may have opposite effects in males and females.

Characterization of gamma-aminobutyrate type A receptors with atypical coupling between agonist and convulsant binding sites in discrete brain regions

Gamma-ainobutyric acid type A (GABA(A)) receptor ionophore ligand t-[35S]butylbicyclophosphorothionate ([35S]TBPS) was used in an autoradiographic assay on brain cryostat sections to visualize and characterize atypical GABA-insensitive [35S]TBPS binding previously described in certain recombinant GABA(A) receptors and the cerebellar granule cell layer. Picrotoxinin-sensitive but 1-mM GABA-insensitive [35S]TBPS binding was present in the rat cerebellar granule cell layer, many thalamic nuclei, subiculum and the internal rim of the cerebral cortex, amounting in these regions up to 6% of the basal binding determined in the absence of exogenous GABA. Similar binding properties were detected also in human and chicken brain sections. Like the GABA-sensitive [35S]TBPS binding, GABA-insensitive binding was profoundly decreased by pentobarbital, pregnanolone, loreclezole and Mg2+. The binding was reversible and apparently dependent on Cl- ions. Localization of the GABA-insensitive [35S]TBPS binding was not identical to that of high-affinity [3H]muscimol binding and diazepam-insensitive [3H]Ro 15-4513 binding, two previously established receptor subtype-dependent binding heterogeneities in the rat brain. The present study reveals a component of the GABA-ionophore enriched in the thalamus and cerebellar granule cells, possibly representing poorly desensitized or desensitizing receptors.

Decreased binding of [11C]flumazenil in Angelman syndrome patients with GABA(A) receptor beta3 subunit deletions

We used positron emission tomography (PET) to study brain [11C]flumazenil (FMZ) binding in four Angelman syndrome (AS) patients. Patients 1 to 3 had a maternal deletion of 15q11-q13 leading to the loss of beta3 subunit of gamma-aminobutyric acidA/benzodiazepine (GABA(A)/BZ) receptor, whereas Patient 4 had a mutation in the ubiquitin protein ligase (UBE3A) saving the beta3 subunit gene. [11C]FMZ binding potential in the frontal, parietal, hippocampal, and cerebellar regions was significantly lower in Patients 1 to 3 than in Patient 4. We propose that the 15q11-q13 deletion leads to a reduced number of GABA(A)/BZ receptors, which could partly explain the neurological deficits of the AS patients.

A single amino acid residue on the alpha(5) subunit (Ile215) is essential for ligand selectivity at alpha(5)beta(3)gamma(2) gamma-aminobutyric acid(A) receptors

Imidazobenzodiazepines such as RY-80 have been reported to exhibit both high affinity and selectivity for GABA(A) receptors containing an alpha(5) subunit. A single amino acid residue (alpha(5)Ile215) has been identified that plays a critical role in the high-affinity, subtype-selective effects of RY-80 and structurally related ligands. Thus, substitution of alpha(5)Ile215 with the cognate amino acid contained in the alpha(1) subunit (Val211) reduced the selectivity of RY-80 for alpha(5)beta(3)gamma(2) receptors from approximately 135- to approximately 8-fold compared with alpha(1)beta(3)gamma(2) receptors. This mutation produced a comparable reduction in the selectivity of RY-24 (a structural analog of RY-80) for alpha(5)beta(3)gamma(2) receptors but did not markedly alter the affinities of ligands (e.g., flunitrazepam) that are not subtype-selective. Conversely, substitution of the alpha(1) subunit with the cognate amino acid contained in the alpha(5) subunit (i.e., alpha(1)V211I) increased the affinities of alpha(5)-selective ligands by a approximately 20-fold and reduced by 3-fold the affinity of an alpha(1)-selective agonist (zolpidem). Increasing the lipophilicity (e.g., by substitution of Phe) of alpha(5)215 did not significantly affect the affinities (and selectivities) of RY-80 and RY-24 for alpha(5)-containing GABA(A) receptors. However, the effect of introducing hydrophilic and or charged residues (e.g., Lys, Asp, Thr) at this position was no greater than that produced by the alpha(5)I215V mutation. These data indicate that residue alpha(5)215 may not participate in formation of the lipophilic L(2) pocket that has been proposed to contribute to the unique pharmacological properties of alpha(5)-containing GABA(A) receptors. RY-80 and RY-24 acted as inverse agonists in both wild-type alpha(5)beta(3)gamma(2) and mutant alpha(5)I215Kbeta(3)gamma(2) receptors expressed in Xenopus laevis oocytes. However, both RY-24 and RY-80 acted as antagonists at mutant alpha(5)I215Vbeta(3)gamma(2) and alpha(5)I215Tbeta(3)gamma(2) receptors, whereas the efficacy of flunitrazepam was similar at all three receptor isoforms. The data demonstrate that amino acid residue alpha(5)215 is a determinant of both ligand affinity and efficacy at GABA(A) receptors containing an alpha(5) subunit.

Altered receptor subunit expression in rat neocortical malformations

PURPOSE

Identification of changes in neurotransmitter function in animal models of epilepsy provides a basis for rational drug development and an understanding of the mechanisms underlying epileptogenesis. We investigated changes in the efficacy of the benzodiazepine type I agonist zolpidem and the polyamine site N-methyl-D-aspartate receptor antagonist ifenprodil in a rat model of microgyria.

METHODS

Neonatal freeze lesions were used to produce a microsulcus in the normally lissencephalic rat neocortex with anatomical similarities to human polymicrogyria. Whole-cell voltage-clamp recordings were made from visually identified layer 2/3 pyramidal cells in acutely prepared brain slices from nonlesioned and lesioned rats.

RESULTS

The effect of 20 nmol/L zolpidem on the decay time constant of inhibitory postsynaptic currents was significantly less in neurons from brain slices containing the freeze lesion. A higher concentration (100 nmol/L) of zolpidem was equally efficacious in lesioned and nonlesioned cortex. In lesioned cortex, the threshold for evoking epileptiform discharges was significantly increased in the presence of 10 micromol/L ifenprodil. This effect was significant in both intrinsic hyperexcitability and partial disinhibition with 2 micromol/L bicuculline in lesioned cortex. Ifenprodil had significantly less effect on the threshold of discharges evoked in control cortex in the partial disinhibition model.

CONCLUSIONS

The decreased sensitivity of gamma-aminobutyric acid A receptors to 20 nmol/L zolpidem in the freeze-lesion model is consistent with a delayed or arrested maturation in this animal model. These data support a delay in the developmental switch from alpha2 to alpha1 subunits in gamma-aminobutyric acid A receptors of neocortical pyramidal cells in lesioned cortex. The increased ifenprodil sensitivity of the threshold for evoking epileptiform discharges in both control and disinhibited slices containing the microsulcus is explained by a delay in the expression of the 2A (NR2A) N-methyl-D-aspartate receptor subunit. Delayed development may be a hallmark of this type of cortical dysplasia.

Transduction of the discriminative stimulus effects of zolpidem by GABA(A)/alpha1 receptors

Zolpidem is an imidazopyridine with high affinity at gamma-aminobutyric acid(A) (GABA(A)) receptors expressing alpha1 subunits. In squirrel monkeys trained to discriminate a high dose of zolpidem (> or =3.0 mg/kg) from saline, zolpidem and another GABA(A)/alpha1 receptor-preferring agonist, zaleplon, substituted dose-dependently for zolpidem, whereas the non-selective agonists diazepam and triazolam were did not substitute at any dose tested. These findings offer the first evidence for a selective role of GABA(A)/alpha1 receptors in the interoceptive effects of high doses of zolpidem.

Does dihydrohonokiol, a potent anxiolytic compound, result in the development of benzodiazepine-like side effects?

The aims of this study were to assess whether dihydrohonokiol, 3'-(2-propenyl)-5-propyl-(1,1'-biphenyl)-2,4'-diol (DHH-B), a potent anxiolytic compound, developed benzodiazepine-like side effects. A 1 mg kg(-1) dose of diazepam, almost equivalent to the minimum dose for the anxiolytic effect, disrupted the traction performance, potentiated hexobarbital-induced sleeping and impaired learning and memory performance. DHH-B, even at a dose of 1 mg kg(-1) (i.e. five times higher than the minimum dose for significant anxiolytic effect) neither developed diazepam-like side effects nor enhanced the side effects of diazepam. Rather, the potentiation by diazepam of hexobarbital-induced sleeping was reduced by 1 mg kg(-1) DHH-B. Furthermore, mice treated with 10 daily administrations of 1 and 5 mg kg(-1) diazepam, but not 0.2-5 mg kg(-1) DHH-B, showed precipitated withdrawal symptoms characterized by hyper-reactivity, tremor and tail-flick reaction when they were challenged with flumazenil (10 mg kg(-1) i.p.). These results suggest that, unlike the benzodiazepine anxiolytic diazepam, DHH-B is less likely to induce motor dysfunction, central depression, amnesia or physical dependence at the effective dose required for the anxiolytic effect.

Benzodiazepine-sensitive GABA(A) receptors limit the activity of the NMDA/NO/cyclic GMP pathway: a microdialysis study in the cerebellum of freely moving rats

In the cerebellum, infusion of NMDA (200 microM) for 20 min evoked a marked (200%) increase of extracellular cyclic GMP (cGMP) levels. The selective GABA(A) receptor agonist muscimol (0.01-100 microM) was able to counteract the NMDA effect with an EC(50) of 0.65 microM; the inhibitory effect of muscimol (10 microM) was prevented by bicuculline (50 microM). Diazepam (10 microM) significantly potentiated the muscimol (1 microM) inhibition; furthermore, when coinfused with 0.1 microM muscimol (a concentration not affecting, on its own, the cGMP response to NMDA), diazepam (10 microM) reduced the NMDA effect. Similar results were obtained with zolpidem (0.1-1 microM). Finally, local infusion of the benzodiazepine site antagonist flumazenil (10 microM), together with muscimol and diazepam, almost completely restored the effect of NMDA on extracellular cGMP levels. It is concluded that GABA(A) receptors potently control the NMDA/nitric oxide/cGMP pathway in the cerebellum in vivo. In terms of the alpha subunit composition, we can deduce that the cerebellar GABA(A) receptor does not contain alpha(6) or beta(4) subunits because it is diazepam-sensitive. Moreover, the observation that zolpidem is active at a rather low concentration, in combination with localization studies present in the literature, tend to exclude the presence of alpha(5) subunits in the receptor composition and suggest the involvement of an alpha(1) subunit.

Differential downregulation of GABAA receptor subunits in widespread brain regions in the freeze-lesion model of focal cortical malformations

Focal cortical malformations comprise a heterogeneous group of disturbances of brain development, commonly associated with drug-resistant epilepsy and/or neuropsychological deficits. Electrophysiological studies on rodent models of cortical malformations demonstrated intrinsic hyperexcitability in the lesion and the structurally intact surround, indicating widespread imbalances of excitation and inhibition. Here, alterations in regional expression of GABA(A) receptor subunits were investigated immunohistochemically in adult rats with focal cortical malformations attributable to neonatal freeze-lesions. These lesions are morphologically characterized by a three- to four-layered cortex with microsulcus formation. Widespread regionally differential reduction of GABA(A) receptor subunits alpha1, alpha2, alpha3, alpha5, and gamma2 was observed. Within the cortical malformation, this downregulation was most prominent for subunits alpha5 and gamma2, whereas medial to the lesion, a significant and even stronger decrease of all subunits was detected. Lateral to the dysplastic cortex, the decrease was most prominent for subunit gamma2 and moderate for subunits alpha1, alpha2, and alpha5, whereas subunit alpha3 was not consistently altered. Interestingly, the downregulation of GABA(A) receptor subunits also involved the ipsilateral hippocampal formation, as well as restricted contralateral neocortical areas, indicating widespread disturbances in the neocortical and hippocampal network. The described pattern of downregulation of GABA(A) receptor subunits allows the conclusion that there is a considerable modulation of subunit composition. Because alterations in subunit composition critically influence the electrophysiological and pharmacological properties of GABA(A) receptors, these alterations might contribute to the widespread hyperexcitability and help to explain pharmacotherapeutic characteristics in epileptic patients.

Characterization of neuronal migration disorders in neocortical structures: loss or preservation of inhibitory interneurons?

PURPOSE

Neuronal migration disorders (NMD) are often associated with therapy-resistant epilepsy. In human cerebral cortex, this hyperexcitability has been correlated with a loss of inhibitory interneurons. We used a rat model of focal cortical NMD (microgyria) to determine whether the expression of epileptiform activity in this model coincides with a decrease in inhibitory interneurons.

METHODS

In 2-to 4-month-old rats, the density of interneurons immunoreactive for gamma-aminobutyric acid (GABA), calbindin, and parvalbumin was determined in fronto-parietal cortex in nine 200-microm-wide sectors located up to 2.5 mm lateral and 2.0 mm medial from the lesion center in primary parietal cortex (Par1). Quantitative measurements in homotopic areas of age-matched sham-operated rats served as controls.

RESULTS

The freeze lesion performed in newborn rat cortex resulted in adult rats with a microgyrus extending in a rostro-caudal direction from frontal to occipital cortex. The density of GABA-and parvalbumin-positive neurons in fronto-parietal cortex was not significantly different between lesioned and control animals. Only the density of calbindin-immunoreactive neurons located 1.0 mm lateral and 0.5 mm medial from the lesion was significantly (Student t test, p < 0.05) larger in freeze-lesioned rats (5,817 +/- 562 and 6,400 +/- 795 cells per mm3, respectively; n = 12) compared with measurements in homotopic regions in Par1 cortex of controls (4,507 +/- 281 and 4, 061 +/- 319 cells per mm3, respectively; n = 5).

CONCLUSIONS

The previously reported widespread functional changes in this model of cortical NMD are not related to a general loss of inhibitory interneurons. Other factors, such as a decrease in GABA receptor density, modifications in GABAA receptor subunit composition, or alterations in the excitatory network, e.g., an increase in the density of calbindin-immunoreactive pyramidal cells, more likely contribute to the global disinhibition and widespread expression of pathophysiological activity in this model of cortical NMD.

Identification of a naturally occurring Pro385-Ser385 substitution in the GABA(A) receptor alpha6 subunit gene in alcoholics and healthy volunteers

In the rat, variation in alcohol and benzodiazepine sensitivity has been correlated with an inherited variant of the GABA(A)alpha6 receptor. Our goal was to identify polymorphisms in the human GABA(A)alpha6 receptor gene and determine whether a variant of the receptor is associated with alcoholism. The GABA(A)alpha6 receptor gene coding region was screened in 80 unrelated patients with alcoholism using single strand conformational polymorphism analysis. For rapid genotyping, a Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) assay was developed. A relatively abundant amino acid substitution and three synonymous DNA substitutions were detected. The synonymous variants, 35A>G, 665A>G, and 1031G>C had rare-allele frequencies of 0.25, 0.02, and 0.47, respectively. The Pro385Ser substitution is located in the second intracellular domain of the receptor adjacent to a putative phosphorylation site. Pro385Ser has rarer allele frequencies of 3.3% and 4.8% in 196 Finnish alcoholic patients and 189 controls, respectively (P = NS). A naturally occurring non-conservative Pro385Ser was detected in the GABA(A)alpha6 receptor. The variant is not associated with alcoholism.

The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish

Both glutamate and γ-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotransmitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarboxylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts more than 400 million years ago. There is good evidence for the involvement of GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA action to stimulate LH release appears to be a combination of effects on GnRH release, potentiation of gonadotropin hormone-releasing hormone (GnRH) action, and in some cases directly at the LH cell. These actions appear to be dependent on such factors as sex or sex steroid levels, and there may also be species differences. Nevertheless, the stimulatory effects of GABA on LH are present in at least 4 fish species. In contrast, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 fish species. The sites and mechanisms of action of amino acid neurotransmitters on LH release have yet to be fully characterized. Both N-methyl-D-aspartic acid (NMDA) and S-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABAA type which mediates the effects of GABA on LH release in fish, at least partially acting on the GnRH neuron, but likely directly acting at the gonadotroph as well. GABA may also be involved in regulating the release of other pituitary hormones in fish, namely follicle stimulating hormone (FSH = GTH-I), prolactin, and growth hormone. Based on the findings described in this review, a working model for the involvement of glutamate and GABA in the regulation of LH release in teleost fish is proposed.

Key words: glutamate, GABA, luteinizing hormone, muscimol, patch clamp electrophysiology, reproduction, fish.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Beta-CCT, a selective BZ-omega1 receptor antagonist, blocks the anti-anxiety but not the amnesic action of chlordiazepoxide in mice

The aim of this study was to test further the hypothesis that different benzodiazepine (BZ-omega) receptor subtypes may mediate anxiolytic and amnesic effects of BZ agonists, using the selective BZ-omega1 receptor antagonist beta-CCT (beta-carboline-3-carboxylate t-butyl-ester). Experiments were performed in Swiss mice using the elevated plus-maze anxiety test and two learning tasks - passive avoidance and the radial arm maze. In the elevated plus-maze test, beta-CCT (30 mg/kg, i.p.) completely abolished the increase in open-arm entries induced by the BZ chlordiazepoxide (5mg/kg, i.p.). Chlordiazepoxide decreased retention latency in the passive avoidance step-through procedure, and increased the number of errors in the radial arm maze. These effects were not modified by beta-CCT. Except for a slight, albeit significant, amnesic effect in the passive avoidance test, beta-CCT was devoid of intrinsic activity when administered alone. These results are in agreement with previous studies using selective BZ-omega1 agonists, and thus provide further evidence that BZ-omega1 receptors may be involved in the anxiolytic but not in the amnesic effects of BZ agonists.

Synchronous postnatal increase in alpha1 and gamma2L GABA(A) receptor mRNAs and high affinity zolpidem binding across three regions of rat brain

The objective of this study was to correlate postnatal changes in levels of mRNAs encoding predominant GABA(A) receptor subunits with a functional index of receptor development. This study is the first to quantify the temporal relationship between postnatal changes in predominant GABA(A) receptor mRNAs and zolpidem-sensitive GABA(A) receptor subtypes. In Experiment 1, we measured zolpidem displacement of 3H-flunitrazepam from rat cerebral cortex, hippocampus, and cerebellum at 0, 6, 14, 21, 29, and 90 postnatal days. Three independent 3H-flunitrazepam sites with high (K(i)=2. 7+/-0.6 nM), low (K(i)=67+/-4.8 nM), and very low (K(i)=4.1+/-0.9 mM) affinities for zolpidem varied in regional and developmental expression. In Experiment 2, we used RNAse protection assays to quantify levels of alpha1, alpha2, beta1, beta2, gamma2S and gamma2L mRNAs in the above regions at the same postnatal ages. Although there was a high degree of regional variation in the developmental expression of zolpidem-sensitive GABA(A) receptors and subunit mRNAs, a dramatic increase in high affinity zolpidem binding sites and alpha1 mRNA levels occurred within all three regions during the second postnatal week. Furthermore, a temporal overlap was observed between the rise in alpha1 mRNA and high affinity zolpidem binding and a more prolonged increase in gamma2L in each region. These results point to the inclusion of the alpha1 and gamma2L subunits in a GABA(A) receptor subtype with a high zolpidem affinity and suggest that a global signal may influence the emergence of this subtype in early postnatal life.

Regional and cellular localisation of GABA(A) receptor subunits in the human basal ganglia: An autoradiographic and immunohistochemical study

The regional and cellular localisation of gamma-aminobutyric acid(A) (GABA(A)) receptors was investigated in the human basal ganglia using receptor autoradiography and immunohistochemical staining for five GABA(A) receptor subunits (alpha(1), alpha(2), alpha(3), beta(2, 3), and gamma(2)) and other neurochemical markers. The results demonstrated that GABA(A) receptors in the striatum showed considerable subunit heterogeneity in their regional distribution and cellular localisation. High densities of GABA(A) receptors in the striosome compartment contained the alpha(2), alpha(3), beta(2, 3), and gamma(2) subunits, and lower densities of receptors in the matrix compartment contained the alpha(1), alpha(2), alpha(3), beta(2,3), and gamma(2) subunits. Also, six different types of neurons were identified in the striatum on the basis of GABA(A) receptor subunit configuration, cellular and dendritic morphology, and chemical neuroanatomy. Three types of alpha(1) subunit immunoreactive neurons were identified: type 1, the most numerous (60%), were medium-sized aspiny neurons that were immunoreactive for parvalbumin and alpha(1), beta(2,3), and gamma(2) subunits; type 2 (38%) were medium-sized to large aspiny neurons immunoreactive for calretinin and alpha(1), alpha(3), beta(2,3), and gamma(2) subunits; and type 3 (2%) were large sparsely spiny neurons immunoreactive for alpha(1), alpha(3), beta(2,3), and gamma(2) subunits. Type 4 neurons were calbindin-positive and immunoreactive for alpha(2), alpha(3), beta(2,3), and gamma(2) subunits. The remaining neurons were immunoreactive for choline acetyltransferase (ChAT) and alpha(3) subunit (type 5) or were neuropeptide Y-positive with no GABA(A) receptor subunit immunoreactivity (type 6). The globus pallidus contained three types of neurons: types 1 and 2 were large neurons and were immunoreactive for alpha(1), alpha(3), beta(2,3), and gamma(2) subunits and for parvalbumin alone (type 1) or for both parvalbumin and calretinin (type 2); type 3 neurons were medium-sized and immunoreactive for calretinin and alpha(1), beta(2, 3), and gamma(2) subunits. These results show that the subunit composition of GABA(A) receptors displays considerable regional and cellular variation in the human striatum but are more homogeneous in the globus pallidus.

Changes of [3H]muscimol binding and GABA(A) receptor beta2-subunit mRNA level by tolerance to and withdrawal from pentobarbital in rats

Effects of continuous pentobarbital administration on binding characteristics of [3H]muscimol were examined by autoradiography, and levels of GABA(A) receptor beta2-subunit mRNA were investigated by in situ hybridization histochemistry in the rat brain. In order to eliminate the induction of hepatic metabolism by systemic administration of pentobarbital, an i.c.v. infusion model of tolerance to and withdrawal from pentobarbital was used. An experimental model of barbiturate tolerance and withdrawal was developed using i.c.v. infusion of pentobarbital (300 microg/10 microl/hr for 7 days) by osmotic minipumps and abrupt withdrawal from pentobarbital. The levels of [3H]muscimol binding were elevated in cingulate of frontal cortex (46%) and granule layer of cerebellum (32%) of rats 24-hr after withdrawal from pentobarbital, while it was only elevated in cingulate (58%) of tolerant rats. The GABA(A) receptor beta2-subunit mRNA was increased in the withdrawal rats only: in the cortex (9-14%), hippocampus (15-21%), inferior colliculus (21%), and granule layer of cerebellum (24%). These results show the involvement of GABA(A) receptor and its beta2-subunit up-regulations in pentobarbital withdrawal rats, and suggest that the levels of [3H]muscimol binding and GABA(A) receptor beta2-subunit mRNA are altered in a region-specific manner during pentobarbital withdrawal.

Sex differences in long-term consequences of prenatal diazepam exposure: possible underlying mechanisms

Prenatal exposure to diazepam, a benzodiazepine (BZD) compound, leads to pronounced effects on responses to stressors in exposed animals when they reach adulthood. Many of the responses are sex specific. The mechanisms mediating the effects of the exposure on the organism have not been elucidated; however, the time course for the appearance of altered function following in utero drug exposure indicates that the exposure interfered with neural organization of mechanisms mediating responses to stressors. The article discusses possible mechanisms that relate to sites of action of the drug in the developing brain: the GABA(A) receptor, and the mitochondrial BZD receptor. The mechanisms mediating the sex-specific impact of diazepam on the developing brain appear to be complex and interactive.

Biphasic modulation of GABA(A) receptor binding by steroids suggests functional correlates

Neuroactive steroids and other positive modulators of GABA(A) receptors showed regional variation in both the efficacy and potency for modulation of [35S]TBPS binding to rat brain membrane homogenates, with biphasic concentration-dependence. GABA present in the binding assays prevented the enhancement phase of the steroid concentration-dependence plot while the antagonists bicuculline and RU5135 prevented the inhibition phase. Using recombinant GABA(A) receptors, expressed in insect cell line Sf9 using baculovirus, enhancement by steroids of [35S]TBPS binding was sensitive to the presence of the gamma2 subunit and the nature of the alpha subunit (alpha1 beta2 gamma2S > alpha1 beta2, alpha6 beta2, alpha6 beta2 gamma2S, and alpha6 beta2 delta). As in cerebellum, addition of RU5135 reduced the inhibitory phase and revealed a small enhancement of TBPS binding by neuroactive steroids. The subunit-dependent interactions of steroid and GABA site ligands are consistent with a three-state model in which the receptor mono-liganded by GABA or steroid has a different affinity for TBPS than the resting state, and the receptor biliganded by GABA, steroid, or both has little affinity for TBPS.

Modulation of bistratified cell IPSPs and basket cell IPSPs by pentobarbitone sodium, diazepam and Zn2+: dual recordings in slices of adult rat hippocampus

Simultaneous intracellular recordings from presynaptic Stratum pyramidale interneurons and postsynaptic pyramidal cells in adult rat hippocampal slices were performed to investigate the strength of the modulation of single-axon inhibitory postsynaptic potentials (IPSPs) by the GABAA receptor modulators pentobarbitone, diazepam and zinc. The processing of biocytin-filled interneurons for light microscopy revealed that these single-axon IPSPs were generated by basket cells (n = 33), bistratified cells (n = 18) and axo-axonic cells (n = 2). The IPSPs generated by these three groups of interneurons had amplitudes and widths at half amplitude with similar ranges, but when bistratified cell IPSPs were compared with basket cell IPSPs with similar half widths their rise times were slower. Pentobarbitone sodium (250 microM) powerfully enhanced 13 tested IPSPs generated by all three cell types. Amplitudes were enhanced by 82 +/- 56%, 10-90% rise times by 150 +/- 101% and the widths at half amplitude by 71 +/- 29%. Diazepam (1-2 microM) also increased all IPSPs tested, although the changes were more moderate in basket cell IPSPs (amplitudes increased by 19 +/- 11%, n = 8) than in bistratified cell IPSPs (amplitudes increased by 66 +/- 48%, n = 5). Basket cell IPSP 10-90% rise times and widths at half amplitude were not significantly increased. Bistratified cell IPSP 10-90% rise times were increased by 44 +/- 24% and the widths at half amplitude by 32 +/- 35%. The one tested IPSP generated by an axo-axonic cell was also diazepam-sensitive. Zinc, 250 microM, decreased four out of 10 IPSPs generated by basket cells and four out of five IPSPs generated by bistratified cells. The one tested axo-axonic cell IPSP was zinc-insensitive. These data suggest that IPSPs generated in CA1 pyramidal cells by basket and bistratified cells display different pharmacologies and may be mediated by different receptors or receptor combinations.