GABA receptor mechanisms in the central nervous system

In vitro modulation of gamma amino butyric acid (GABA) receptor expression by bone marrow stromal cells

Bone marrow stromal cells (BMSC) have the capability to undergo a change of morphology, reminiscent of neuronal cells, after exposure to an inductive medium. These induced BMSC-derived neuron-like (BDNL) cells express several neuronal markers, including Microtubule-Associated Protein Tau, Neurofilament M, and Nestin as revealed by immunocytochemistry analysis. To assess whether the induction process has possible functional relevance, we have focused our attention on the expression of neurotransmitter receptors. In particular, we show that the expression of GABA(A) subunits alpha1, beta2/3, epsilon and GABA(B1) mRNAs is greatly enhanced in BMSC by the induction treatment. Similar results were obtained from rat skin fibroblasts subjected to the same induction protocol, with the exception for the GABA(B2) transcript that was expressed only by BMSC and BDNL. The presence of both GABA(B1) and GABA(B2) subunits in BDNL cells suggests that functional GABA(B) receptors might be assembled: we indeed found that a functional GABA(B) receptor, negatively linked to cyclic AMP production, is expressed in BDNL. Therefore, we suggest that BMSC can be converted into cells equipped with appropriate receptors coupled to transduction mechanisms, potentially responding to a specific neurotransmitter.

Local inhibition shapes duration tuning in the inferior colliculus of guinea pigs

Neural tuning to sound durations is a useful filter for the identification of a variety of sounds. Previous studies have shown that the interaction between excitatory and inhibitory inputs plays a role in duration selectivity in echolocating bats. However, this has not been investigated in non-echolocating mammals. In the inferior colliculus (IC) of these mammals, it is recognized that the excitatory responses to sounds are mediated through AMPA and NMDA receptors while the inhibitory input is mediated through gamma-aminobutyric acid (GABA) and glycine receptors. The present study explores the potential interplay between inhibitory and excitatory inputs and its role in the duration selectivity of IC neurons in guinea pigs. It was found that the application of bicuculline (BIC, a GABA A blocker) and/or strychnine (STRY, a glycine blocker) eliminated or reduced duration tuning in most units that were duration tuned (32 out of 39 for BIC, 50 out of 64 for STRY, respectively). The inhibitory input (either by GABA or by glycine) appeared to have a stronger regulating effect on the early excitation mediated by AMPA than on later excitation by NMDA. This is more distinguishable in neurons that show duration selectivity. In conclusion, the inhibitory effect on the early responses appears to be the main contributor for the duration selectivity of the IC in guinea pigs; potential mechanisms for this duration selectivity are also discussed.

Spinal GABAergic Transplants Attenuate Mechanical Allodynia in a Rat Model of Neuropathic Pain

Injury to the spinal cord or peripheral nerves can lead to the development of allodynia due to the loss of inhibitory tone involved in spinal sensory function. The potential of intraspinal transplants of GABAergic cells to restore inhibitory tone and thus decrease pain behaviors in a rat model of neuropathic pain was investigated. Allodynia of the left hind paw was induced in rats by unilateral L5- 6 spinal nerve root ligation. Mechanical sensitivity was assessed using von Frey filaments. Postinjury, transgenic fetal green fluorescent protein mouse GABAergic cells or human neural precursor cells (HNPCs) expanded in suspension bioreactors and differentiated into a GABAergic phenotype were transplanted into the spinal cord. Control rats received undifferentiated HNPCs or cell suspension medium only. Animals that received either fetal mouse GABAergic cell or differentiated GABAergic HNPC intraspinal transplants demonstrated a significant increase in paw withdrawal thresholds at 1 week post-transplantation that was sustained for 6 weeks. Transplanted fetal mouse GABAergic cells demonstrated immunoreactivity for glutamic acid decarboxylase and GABA that colocalized with green fluorescent protein. Intraspinally transplanted differentiated GABAergic HNPCs demonstrated immunoreactivity for GABA and beta-III tubulin. In contrast, intraspinal transplantation of undifferentiated HNPCs, which predominantly differentiated into astrocytes, or cell suspension medium did not affect any behavioral recovery. Intraspinally transplanted GABAergic cells can reduce allodynia in a rat model of neuropathic pain. In addition, HNPCs expanded in a standardized fashion in suspension bioreactors and differentiated into a GABAergic phenotype may be an alternative to fetal cells for cell-based therapies to treat chronic pain syndromes.

G-protein-coupled GABAB receptors inhibit Ca2+ channels and modulate transmitter release in descending turtle spinal cord terminal synapsing motoneurons

Presynaptic gamma-aminobutyric acid type B receptors (GABA(B)Rs) regulate transmitter release at many central synapses by inhibiting Ca(2+) channels. However, the mechanisms by which GABA(B)Rs modulate neurotransmission at descending terminals synapsing on motoneurons in the spinal cord remain unexplored. To address this issue, we characterized the effects of baclofen, an agonist of GABA(B)Rs, on the monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in motoneurons by stimulation of the dorsolateral funiculus (DLF) terminals in a slice preparation from the turtle spinal cord. We found that baclofen depressed neurotransmission in a dose-dependent manner (IC(50) of approximately 2 microM). The membrane time constant of the motoneurons did not change, whereas the amplitude ratio of the evoked EPSPs in response to a paired pulse was altered in the presence of the drug, suggesting a presynaptic mechanism. Likewise, the use of N- and P/Q-type Ca(2+) channel antagonists (omega-conotoxin GVIA and omega-agatoxin IVA, respectively) also depressed EPSPs significantly. Therefore, these channels are likely involved in the Ca(2+) influx that triggers transmitter release from DLF terminals. To determine whether the N and P/Q channels were regulated by GABA(B)R activation, we analyzed the action of the toxins in the presence of baclofen. Interestingly, baclofen occluded omega-conotoxin GVIA action by approximately 50% without affecting omega-agatoxin IVA inhibition, indicating that the N-type channels are the target of GABA(B)Rs. Lastly, the mechanism underlying this effect was further assessed by inhibiting G-proteins with N-ethylmaleimide (NEM). Our data show that EPSP depression caused by baclofen was prevented by NEM, suggesting that GABA(B)Rs inhibit N-type channels via G-protein activation.

Newer N-Phthaloyl GABA Derivatives with Antiallodynic and Antihyperalgesic Activities in Both Sciatic Nerve and Spinal Nerve Ligation Models of Neuropathic Pain

BACKGROUND

There is considerable research evidence supporting a palliative role for gamma-aminobutyric acid (GABA)-ergic neurotransmission and voltage-gated sodium channel blockade in neuropathic pain conditions. Hence, the present study was undertaken to assess the peripheral analgesic, antiallodynic and antihyperalgesic activities of the synthesized structural analogues of GABA.

METHODS

The screening study included acute tissue injury, chronic constriction injury (CCI), and spinal nerve ligation (SNL) models of neuropathic pain.

RESULTS

All of the tested compounds sup-pressed the acetic acid-induced writhing response significantly in comparison to the control. In particular, compound JVP-8 was observed to be the most active compound with percent inhibition greater than that of the standard drug aspirin (97.8% inhibition of writhing response as against 97.0% shown by aspirin). In neuropathic pain studies, compound JVP-5 (100 mg/kg i.p.) emerged as the most active compound affording maximum protection against dynamic allodynia and mechanical hyperalgesia in the CCI model, and against spontaneous pain and mechanical hyperalgesia in SNL rats.

CONCLUSION

In this study, we have demonstrated that combining phthalimide pharmacophore with GABA has evolved compounds effective for the treatment of neuropathic pain.

A single amino acid in the second transmembrane domain of GABA rho receptors regulates channel conductance

GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA rho subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA rho subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch rho1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA rho2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch rho2A and rho2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2' was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA rho2 subunit to the proline residue present in the rho1 subunit reduced the channel conductance to a level similar to that of the wild type rho1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the rho1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.

Activation of pedunculopontine tegmental PKA prevents GABAB receptor activation-mediated rapid eye movement sleep suppression in the freely moving rat

The pedunculopontine tegmental (PPT) GABAergic system plays a crucial role in the regulation of rapid eye movement (REM) sleep. I recently reported that the activation of PPT GABA(B) receptors suppressed REM sleep by inhibiting REM-on cells. One of the important mechanisms for GABA(B) receptor activation-mediated physiological action is the inhibition of the intracellular cAMP-dependent protein kinase A (cAMP-PKA) signaling pathway. Accordingly, I hypothesized that the PPT GABA(B) receptor activation-mediated REM sleep suppression effect could be mediated through inhibition of cAMP-PKA activation. To test this hypothesis, a GABA(B) receptor selective agonist, baclofen hydrochloride (baclofen), cAMP-PKA activator, Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine (SpCAMPS), and vehicle control were microinjected into the PPT in selected combinations to determine effects on sleep-waking states of chronically instrumented, freely moving rats. Microinjection of SpCAMPS (1.5 nmol) induced REM sleep within a short latency (12.1 +/- 3.6 min) compared with vehicle control microinjection (60.0 +/- 6.5 min). On the contrary, microinjection of baclofen (1.5 nmol) suppressed REM sleep by delaying its appearance for approximately 183 min; however, the suppression of REM sleep by baclofen was prevented by a subsequent microinjection of SpCAMPS. These results provide evidence that the activation of cAMP-PKA within the PPT can successfully block the GABA(B) receptor activation-mediated REM sleep suppression effect. These findings suggest that the PPT GABA(B) receptor activation-mediated REM sleep regulating mechanism involves inactivation of cAMP-PKA signaling in the freely moving rat.

Effect of chronic administration of ethanol on the regulation of tyrosine kinase phosphorylation of the GABAA receptor subunits in the rat brain

One of the many pharmacological targets of ethanol is the GABA inhibitory system, and chronic ethanol (CE) is known to alter the polypeptide levels of the GABA(A )receptor subunits in rat brain regions. In the present study, we investigated the regulation of the tyrosine kinase phosphorylation of the GABA(A) receptor alpha(1)-, beta(2)- and gamma(2)-subunits in the rat cerebellum, cerebral cortex and hippocampus following chronic administration of ethanol to the rats. We observed either down-regulation or no change in the tyrosine kinase phosphorylation of the alpha(1) subunit, whereas there was an up-regulation or no change in the case of beta(2)- and gamma(2)-subunits of the GABA(A) receptors depending on the brain region following chronic administration of ethanol to the rats. These changes reverted back to the control level following 48 h of ethanol-withdrawal. These results suggest that tyrosine kinase phosphorylation of GABA(A )receptors may play a significant role in ethanol dependence.

Capillary electrophoretic analysis of gamma-aminobutyric acid and alanine in tea with in-capillary derivatization and fluorescence detection

The aim of this study was to investigate an in-capillary derivatization capillary electrophoresis (CE) technique that was performed to determine the concentration of gamma-aminobutyric acid (GABA) and alanine (Ala) in tea after being derivatized with o-phthaldialdehyde/2-mercaptoethanol (OPA/2-ME) to form fluorescence-labeled products. The conditions of labeled derivatization and CE separation were optimized and then applied to real sample analysis. The labeled derivatization with 20 mM OPA and 26.67 mM 2-ME (mol ratio=0.75) at pH 10 offered the most sensitive detection, and the separation with 30 mM sodium tetraborate buffer (pH 10.0) under 21 kV achieved good selectivity within 14 min. The detections were linear in the range of 0.05-5 microM with correlation coefficients (R2) of 0.9995 and 0.9964 and with detection limits of 0.004 and 0.02 microM for GABA and Ala, respectively. The recoveries were 94.22% (3.58% RSD) and 93.54% (6.46% RSD) for five determinations of GABA and Ala, respectively. This method is a fast, convenient, sensitive, and eco-friendly way to determine the GABA and Ala in tea samples from different manufacturing processes.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Design and synthesis of anticonvulsants from a combined phthalimide–GABA–anilide and hydrazone pharmacophore

Two series of pharmacophoric hybrids of phthalimide-GABA-anilides/hydrazones were designed and synthesized and evaluated for their anticonvulsant and neurotoxic properties. The structures of the synthesized compounds were confirmed by the use of their spectral data besides elemental analysis. Initial anticonvulsant screening was performed using intraperitoneal (i.p.) maximal electroshock-induced seizure (MES), subcutaneous pentylenetetrazole (scPTZ), subcutaneous strychnine (scSTY), and intraperitoneal picrotoxin (ipPIC)-induced seizure threshold tests. All of the compounds were ineffective in the MES test. Most of the compounds were found to be effective in the scSTY and ipPIC models and very few compounds showed protection in the scPTZ model.

Quantitative trait transcripts for nicotine resistance in Drosophila melanogaster

Although most genetic association studies are performed with the intention of detecting nucleotide polymorphisms that are correlated with a complex trait, transcript abundance should also be expected to associate with diseases or phenotypes. We performed a scan for such quantitative trait transcripts in adult female heads of the fruit fly (Drosophila melanogaster) that might explain variation for nicotine resistance. The strongest association was seen for abundance of ornithine aminotransferase transcripts, implicating detoxification and neurotransmitter biosynthesis as mediators of the quantitative response to the drug. Subsequently, genetic analysis and metabolite profiling confirmed a complex role for ornithine and GABA levels in modification of survival time upon chronic nicotine exposure. Differences between populations from North Carolina and California suggest that the resistance mechanism may be an evolved response to environmental exposure.

GAMMA.-Aminobutyric Acid Specifically Inhibits Progression of Tubular Fibrosis and Atrophy in Nephrectomized Rats

Gamma-aminobutyric acid (GABA) was administered orally to rats for 60 d after excision of five-sixths of their kidney volume. A decrease in renal function parameters was observed in these nephrectomized rats. However, the administration of GABA ameliorated renal dysfunction, and a longer administration period of GABA increased its protective effect. In addition, tubular fibrosis was markedly increased at 10 and 60 d in nephrectomized control rats, while GABA administration for 10 d reduced tubular fibrosis to the normal level. Tubular atrophy was markedly induced by nephrectomy, and was significantly reduced by the administration of GABA at 60 d. Furthermore, the nephrectomized control rats exhibited an increased expression level of transforming growth factor-beta1, where GABA significantly decreased it after administration for 10 d. The expression of fibronectin in the tubuli of rats administrated GABA for 60 d was completely and dose-dependently reduced as compared with nephrectomized control rats. However, the improvement effects in glomeruli were less. We also found that GABAA and GABAB receptors were specifically localized in tubuli. Specific agonists for GABAA and GABAB receptors improved renal function. These results suggest that GABA may have a beneficial effect on renal function in nephrectomized rats by inhibiting fibrosis and atrophy primarily in tubuli, and that it ameliorates losses of renal function in renal failure.

Activation of pedunculopontine tegmental protein kinase A: a mechanism for rapid eye movement sleep generation in the freely moving rat

Cells in the pedunculopontine tegmentum (PPT) play a key role in the generation of rapid eye movement (REM) sleep, but its intracellular signaling mechanisms remain unknown. In the current studies, the role of PPT intracellular protein kinase A (PKA) in the regulation of REM sleep was evaluated by comparing PKA subunit [catalytic (PKA(C alpha)) and regulatory (PKA(RI), PKA(RII alpha), and PKA(RII beta)) types] expression and activity in the PPT at normal, high, and low REM sleep conditions. To compare anatomical specificity, REM sleep-dependent expressions of these PKA subunits were also measured in the medial pontine reticular formation (mPRF), medial prefrontal cortex (mPFC), and anterior hypothalamus (AHTh). The results of these PKA subunit expression and activity studies demonstrated that the expression of PKA(C alpha) and PKA activity in the PPT increased and decreased during high and low REM sleep, respectively. Conversely, PKA(C alpha) expression and PKA activity decreased with high REM sleep in the mPRF. Expression of PKA(C alpha) also decreased in the mPFC and remained unchanged in the AHTh with high REM sleep. These subunit expression and PKA activity data reveal a positive relationship between REM sleep and increased PKA activity in the PPT. To test this molecular evidence, localized activation of cAMP-dependent PKA activity was blocked using a pharmacological technique. The results of this pharmacological study demonstrated that the localized inhibition of cAMP-dependent PKA activation in the PPT dose-dependently suppressed REM sleep. Together, these results provide the first evidence that the activation of the PPT intracellular PKA system is involved in the generation of REM sleep.

Differential paired-pulse responses between the CA1 region and the dentate gyrus are related to altered CLC-2 immunoreactivity in the pilocarpine-induced rat epilepsy model

The epileptic hippocampus shows differential paired-pulse responses between the dentate gyrus and the CA1 region. However, little data are available to explain this phenomenon. In the present study, we identified the relationship between regional differences of paired-pulse response and voltage gated Cl(-) channel 2 (CLC-2)/vesicular GABA transport (VGAT) expression in a pilocarpine-induced rat model. During epileptogenic periods, paired-pulse inhibitions in the dentate gyrus and the CA1 region were markedly reduced. After recurrent seizure onset, paired-pulse inhibition in the dentate gyrus was markedly enhanced, while that in the CA1 region more reduced. Unlike VGAT, CLC-2 immunoreactivity was markedly reduced in the hippocampus during epileptogenic periods and was re-enhanced only in the dentate gyrus after recurrent seizure onset. Linear regression analysis showed an inverse proportional relationship between alterations in CLC-2 immunoreactivity and changes in normalized population spike amplitude ratio within the CA1 region and the dentate gyrus. Therefore, our findings suggest that the regionally specific alterations in CLC-2 immunoreactivity after SE may determine the properties of paired-pulse responses in the hippocampus of the pilocarpine-induced rat epilepsy model.

Differential expression of gamma-aminobutyric acid--a receptor subunits in rat dorsal and ventral hippocampus

Recent data demonstrate weaker gamma-aminobutyric acid (GABA)-ergic inhibition in ventral (VH) compared with dorsal (DH) hippocampus. Therefore, we examined possible differences regarding the GABAA receptors between VH and DH as follows: 1) the expression of the GABAA receptor subunits (alpha1/2/4/5, beta1/2/3, gamma2, delta) mRNA and protein and 2) the quantitative distribution and kinetic parameters of [3H] muscimol (GABAA receptor agonist) binding. VH compared with DH showed: 1) lower levels for alpha1, beta2, gamma2 but higher levels for alpha2 and beta1 subunits in CA1, CA2, and CA3, the differences being more pronounced in CA1 region; in the CA1 region, the mRNA levels of alpha5 were higher, whereas those of alpha4 subunit were slightly lower; in dentate gyrus, the mRNA levels of alpha4, beta3, and delta subunits were significantly lower, presumably suggesting a lower expression of the alpha4/beta3/delta receptor subtype; and 2) lower levels of [3H]muscimol binding, with the lowest value observed in CA1, apparently resulting from weaker binding affinity, insofar as the KD values were higher in VH, whereas the Bmax values were similar between DH and VH. The differences in the subunit expression and the lower affinity of GABAA receptor binding observed predominantly in the CA1 region of VH suggest that the alpha1/beta2/gamma2 GABAA receptor subtype dominates in DH, and the alpha2/beta1/gamma2 subtype prevails in VH. This could underlie the lower GABAA-mediated inhibition observed in VH and, to some extent, explain 1) the higher liability of VH for epileptic activity and 2) the differential involvement of DH and VH in cognitive and emotional processes.

Involvement of GABA and opioid peptide receptors in sevoflurane-induced antinociception in rat spinal cord

AIM

The spinal cord is pivotal in immobility induced by volatile anesthetics because the anesthetics depress the activity of motor neurons in the spinal cord. The aim of this study was to observe the effects of sevoflurane on pain processing at the spinal level.

METHODS

The firing of the gastrocnemius muscle was evoked by electrical stimulation to the ipsilateral hindpaw in rats. The nociceptive C response of electromyography (EMG) was selected to study. The GABAA receptor antagonist bicuculline (0.1 mg/kg) and opioid receptor antagonist naloxone (0.4 mg/kg) were administered intravenously, either in the presence or in the absence of 1.0% sevoflurane.

RESULTS

In rats with transected spinal cord, sevoflurane produced a profound reduction in the C response in a dose- and time-dependent manner. In the presence of 1.0% sevoflurane, the C responses were increased after injections of bicuculline and naloxone.

CONCLUSION

Sevoflurane is a volatile anesthetic that acts directly on the spinal cord to suppress the nociceptive reflex. The sevoflurane-induced suppression of the C response is antagonized by either bicuculline or naloxone. The results suggest that spinal GABAA receptors and opioid peptide receptors are involved in the sevoflurane-induced suppression of spinal nociception.

Chronic stimulation of GABAA receptor with muscimol reduces amyloid β protein (25–35)-induced neurotoxicity in cultured rat cortical cells

The present study was performed to examine how the stimulation of gamma-aminobutyric acid (GABA) receptor affects amyloid beta protein (25-35) (Abeta (25-35)), a synthetic 25-35 amyloid peptide, -induced neurotoxicity using cultured rat cortical neurons. Abeta (25-35) produced a concentration-dependent reduction of cell viability, which was significantly reduced by (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801), an N-methyl-d-aspartate (NMDA) receptor antagonist, verapamil, an L-type Ca(2+) channel blocker, and N(G)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor. Pretreatment with muscimol, a GABAA receptor agonist, over a concentration range of 0.1-10microM 24h before the treatment with 10microM Abeta (25-35) showed concentration-dependent inhibition on the Abeta (25-35)-induced neuronal apoptotic death. However, baclofen (1 and 10microM), a GABAB receptor agonist, failed to inhibit the Abeta (25-35)-induced neuronal death. In addition, pretreatment with muscimol (1microM) for 24h inhibited the Abeta (25-35) (10microM)-induced elevation of cytosolic Ca(2+) concentration ([Ca(2+)]c) and glutamate release, generation of reactive oxygen species (ROS), and caspase-3 activity in cultured neurons. These neuroprotective effects of muscimol (1microM) were completely blocked by the simultaneous treatment with 10microM bicuculline, a GABAA receptor antagonist, indicating that the protective effects of muscimol were due to GABAA receptor stimulation. When, however, treated just 15min before the treatment with Abeta (25-35), muscimol (1microM) did not show any protective effect against Abeta (25-35) (10microM)-induced neurotoxicity in cultured neurons. These results suggest that the chronic activation of GABAA receptor may ameliorate Abeta-induced neurotoxicity by interfering with the increase of [Ca(2+)]c, and then by inhibiting glutamate release, generation of ROS and caspase-3 activity.

Palladium-Catalyzed Regioselective Arylation of Imidazo[1,2-b][1,2,4]triazine: Synthesis of an α2/3-Selective GABA Agonist

A convergent, practical, and efficient synthesis of 2',6-difluoro-5'-[3-(1-hydroxy-1-methylethyl)imidazo[1,2-b][1,2,4]triazin-7-yl]biphenyl-2-carbonitrile (1), an orally active GABA(A) alpha(2/3)-selective agonist, is described. The seven-step, chromatography-free synthesis was demonstrated on a multi-kilogram scale and utilized biaryl bromide 6 and imidazotriazine 22 as key intermediates. Biaryl bromide 6 was prepared via a highly selective aromatic bromination. The regioselective condensation of aminotriazine 15 with chloroacetaldehyde provided the desired imidazotriazine intermediate 22. A highly regioselective palladium-catalyzed arylation in the final step highlights the efficiency of the route.

Interactions between ρ and γ2 subunits of the GABA receptor

The gamma-aminobutyric acid type C (GABA(C)) receptor is a ligand-gated chloride channel with distinct physiological and pharmacological properties. Although the exact subunit composition of native GABA(C) receptors has yet to be firmly established, there is general agreement that GABA rho subunits participate in their formation. Recent studies on white perch suggest that certain GABA rho subunits can co-assemble with the GABA(A) receptor gamma2 subunit to form a heteromeric receptor with electrophysiological properties that correspond more closely to the native GABA(C) receptor on retinal neurons than any of the homomeric rho receptors. In the present study we examined the interactions among various perch GABA rho and gamma2 subunits. When co-expressed in Xenopus oocytes, the gamma2 subunit co-immunoprecipitated with Flag-tagged perch rho1A, rho1B, and rho2B subunits, but not with the Flag-tagged perch rho2A subunit. Immunocytochemical studies indicated that the membrane surface expression of the gamma2 subunit was detected only when it was co-expressed with perch rho1A, rho1B, or rho2B subunit, but not with the perch rho2A subunit or when expressed alone. In addition, co-immunoprecipitation of perch rho1B and gamma2 subunits was also detected in protein samples of the teleost retina. Taken together, these findings suggest that a heteromeric rho(gamma2) receptor could represent one form of GABA(C) receptor on retinal neurons.

Novel role of brain stem pedunculopontine tegmental adenylyl cyclase in the regulation of spontaneous REM sleep in the freely moving rat

Physiological activation of kainate receptors and GABA(B) receptors within the pedunculopontine tegmentum (PPT) is involved in regulation of rapid-eye-movement (REM) sleep. Because these two types of receptors may also directly and/or indirectly activate the intracellular cyclic adenosine monophosphate (cAMP) signaling pathway, we hypothesized that this signaling pathway may be involved in the PPT to regulate spontaneous REM sleep. To test this hypothesis, four different doses (0.25, 0.50, 0.75, and 1.0 nmol) of a specific adenylyl cyclase (AC) inhibitor, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22536), were microinjected bilaterally (100 nl/site) into the PPT, and the effects on REM sleep in freely moving chronically instrumented rats were quantified. By comparing alterations in the patterns of REM sleep after control injections of vehicle or one of the four different doses of SQ22536, the contributions made by each dose of SQ22536 to REM sleep were evaluated. The results demonstrated that the local microinjection of AC inhibitor SQ22536 into the PPT decreased the total amount of REM sleep for 3 h and increased slow-wave sleep (SWS) for 2 h in a dose-dependent manner. This reduction in REM sleep was due to increased latency and decreased frequency of REM sleep episodes. These results provide evidence that inhibition of AC within the PPT can successfully reduce REM sleep. These findings suggest that activation of the cAMP-signaling pathway within the cholinergic cell compartment of the PPT is an intracellular biochemical/molecular step for generating REM sleep in the freely moving rat.

In Vivo Demonstration of a Late Depolarizing Postsynaptic Potential in CA1 Pyramidal Neurons

Previous studies have shown that GABA can have a depolarizing and excitatory action through GABAA receptors in mature CNS neurons in vitro. However, it remains unknown whether this occurs under physiological conditions. In this study, using intracellular recording and staining in vivo technique, we show a late depolarizing postsynaptic potential (L-PSP) in CA1 pyramidal neurons of adult Wistar rats under halothane anesthesia. This L-PSP was elicited in ∼70% of the recorded neurons on stimulation of the Schaffer collaterals or the contralateral commissural path. The size of L-PSP was linearly correlated to the decay time constant but not the rising slope of the initial excitatory PSP (EPSP). Intravenous administration of the N-methyl-d-aspartate (NMDA) receptor blocker MK-801 and the GABAA receptor blocker picrotoxin significantly reduced the size of the L-PSP. The spine density and apical dendritic branching length of the neurons that displayed L-PSPs was significantly greater than those that do not. These results indicate that NMDA receptor and GABAA receptor-mediated depolarizing postsynaptic potentials can be revealed in CA1 pyramidal neurons of adult rats in vivo, supporting the physiological relevance of GABAA-mediated depolarization in normal neuronal information processing. The difference in electrophysiological properties and morphological features between neurons that display the L-PSP and the other neurons suggest that they might represent two different subtypes of CA1 pyramidal neurons.

Effects of sodium nitroprusside, a nitric oxide donor, on γ-aminobutyric acid concentration in the brain and on picrotoxin-induced convulsions in combination with phenobarbitone in rats

The concentrations of nitric oxide (NO), the neuronal messenger molecule, and gamma-aminobutyric acid (GABA), the inhibitory neurotransmitter, and the activity of gamma-aminobutyric acid transaminase (GABA-T), the enzyme involved in the degradation of GABA, were measured in the brain of rats treated with graded doses (1.25, 2.5, 5.0 mg/kg) of sodium nitroprusside (SNP), the donor of NO. The effect of SNP was tested alone and in combination with phenobarbitone (PB), the GABA potentiating antiepileptic drug, against picrotoxin (PCT) (5 mg/kg)-induced convulsions in rats. The results of these studies showed that NO released from SNP (2.5 mg/kg) had a potential to inhibit GABA-T activity resulting in an increase in the concentration of GABA in the brain. Thus, SNP (2.5 mg/kg) was able to inhibit PCT-induced convulsions and was able to produce an additive anticonvulsant action with PB. However, a much greater increase in the concentration of NO by 5.0 mg/kg of SNP did not change the activity of GABA-T and the concentration of GABA, and promoted the convulsant action of PCT. These results suggest that a moderate increase in the concentration of NO following the administration of its donor SNP (2.5 mg/kg) results in an enhancement of the concentration of GABA in the brain and in an inhibition of PCT-induced convulsions independently and additively with PB and that a marked increase in NO concentration after the administration of a larger dose of SNP (5.0 mg/kg) results in proconvulsant action.

Elevated voltage gated Cl− channel expression enhances fast paired-pulse inhibition in the dentate gyrus of seizure sensitive gerbil

The present study was performed to determine whether the voltage gated Cl- channel (CLC) expression is altered in the hippocampus of seizure sensitive (SS) gerbils, and to identify the strong fast paired-pulse inhibition in the dentate gyrus of SS gerbils is associated with altered CLC expression. In the hippocampal proper and the granule cell layer of the dentate gyrus of the SS gerbils, strong CLC-2 immunoreactivity was detected, as compared with seizure resistant (SR) gerbils. In addition, CLC-3 immunoreactivity was observed in the CA1-3 pyramidal cells, and the granule cell and the molecular layer of the dentate gyrus in the SS gerbils, whereas its immunoreactivity was rarely detected in the SR gerbils. However, CLC-3 immunoreactivity in the mossy fiber was reduced, as compared with SR gerbils. Moreover, infusion of the potential CLC inhibitor (4,4'-diisothiocyanostibene-2,2'-disulfanic acid, DIDS) reduced fast paired-pulse inhibition in the dentate gyrus of SS gerbils, although evoked responses in the dentate gyrus between SR and SS gerbils were similarly detected. These findings suggest that enhancement of CLC expression in the dentate gyrus of SS gerbils may be one of the compensatory responses for reduced GABA(A) receptor-mediated fast postsynaptic inhibitory potentials.

Repeated administration of the GABAB receptor agonist CGP44532 decreased nicotine self-administration, and acute administration decreased cue-induced reinstatement of nicotine-seeking in rats

Acute administration of gamma-aminobutyric acid B (GABAB) receptor agonists decreased nicotine, cocaine, ethanol, and heroin self-administration. GABAB receptor agonists also decreased cue-induced cocaine craving or seeking in humans and animals, respectively. The present study investigated the effects of repeated subcutaneous administration of the GABAB receptor agonist CGP44532 on nicotine- and food-maintained responding under a fixed ratio 5 schedule of reinforcement. The second part of the study determined whether contingent presentation of previously nicotine-associated cues reinstated extinguished nicotine-seeking behavior, and whether acute subcutaneous CGP44532 administration affected cue-induced reinstatement of extinguished nicotine-seeking behavior. The results indicated that repeated administration of 0.25 mg/kg CGP44532 selectively decreased nicotine self-administration compared to food-maintained responding during the first 7 days of treatment. Repeated administration of 0.5 mg/kg/day CGP44532 nonselectively decreased both nicotine- and food-maintained responding. Contingent presentation of previously nicotine-associated cues reinstated extinguished nicotine-seeking behavior. Further, acute CGP44532 administration (0.125 and 0.25 mg/kg) decreased cue-induced reinstatement of nicotine-seeking behavior. In summary, the present results indicated that 0.25 mg/kg/day CGP44532 selectively decreased nicotine self-administration compared to food-maintained responding, and acute administration of CGP44532 (0.125 and 0.25 mg/kg) dose-dependently decreased cue-induced reinstatement of nicotine-seeking behavior.

Pharmacology of GABAC receptors: responses to agonists and antagonists distinguish A- and B-subtypes of homomeric rho receptors expressed in Xenopus oocytes

GABA(C) receptors, expressed predominantly in vertebrate retina, are thought to be formed mainly by GABA rho subunits. Five GABA rho subunits have been cloned from white perch retina, four of which form functional homooligomeric receptors when expressed in Xenopus oocytes. These rho subtypes, classified as rho1A, rho1B, rho2A and rho2B receptors based on amino acid sequence alignment, exhibit distinct temporal and pharmacological properties. To examine further the pharmacological properties associated with the various rho receptor subtypes, we investigated the effects of a selective GABA(C) receptor antagonist, TPMPA, on the GABA-mediated activity of receptors formed in Xenopus oocytes by the four GABA rho subunits. In addition, we recorded the activation profiles of beta-alanine, taurine, and glycine, three amino acids that modulate neuronal activity in various parts of the CNS and are purported to be rho receptor agonists. TPMPA effectively inhibited GABA-elicited responses on A-type receptors, whereas B-type receptors exhibited a relatively low sensitivity to the drug. A-type and B-type receptors also displayed distinctly different reactions to agonists. Both taurine and glycine-activated the B-type receptors, whereas these agents had no detectable effect on A-type receptors. Similarly, beta-alanine evoked large responses from B-type receptors, but was far less effective on A-type receptors. These results indicate that, in addition to the characteristic response properties identified previously, there is a pattern of pharmacological reactions that further distinguishes the A- and B-subtypes of GABA rho receptor.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Electrophysiological Properties of Mouse Horizontal Cell GABAAReceptors

GABA-induced currents have been characterized in isolated horizontal cells from lower vertebrates but not in mammalian horizontal cells. Therefore horizontal cells were isolated after enzymatical and mechanical dissociation of the adult mouse retina and visually identified. We recorded from horizontal cell bodies using the whole cell and outside-out configuration of the patch-clamp technique. Extracellular application of GABA induced inward currents carried by chloride ions. GABA-evoked currents were completely and reversibly blocked by the competitive GABAAreceptor antagonist bicuculline (IC50= 1.7 μM), indicating expression of GABAAbut not GABACreceptors. Their affinity for GABA was moderate (EC50= 30 μM), and the Hill coefficient was 1.3, corresponding to two GABA binding sites. GABA responses were partially reduced by picrotoxin with differential effects on peak and steady-state current values. Zinc blocked the GABA response with an IC50value of 7.3 μM in a noncompetitive manner. Furthermore, GABA receptors of horizontal cells were modulated by extracellular application of diazepam, zolpidem, methyl 6,7-dimethoxy-4-ethyl-β-carboxylate, pentobarbital, and alphaxalone, thus showing typical pharmacological properties of CNS GABAAreceptors. GABA-evoked single-channel currents were characterized by a main conductance state of 29.8 pS and two subconductance states (20.2 and 10.8 pS, respectively). Kinetic analysis of single-channel events within bursts revealed similar mean open and closed times for the main conductance and the 20.2-pS subconductance state, resulting in open probabilities of 44.6 and 42.7%, respectively. The ratio of open to closed times, however, was significantly different for the 10.8-pS subconductance state with an open probability of 57.2%.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Role of γ-Aminobutyric Acid (GABA) and Metabotropic Glutamate Receptors in Nicotine Reinforcement: Potential Pharmacotherapies for Smoking Cessation

Previous work indicated a role for GABA and glutamate in the reinforcing effects of drugs of abuse. The present studies assessed the effects of GABAergic and glutamatergic manipulations on the reinforcing effects of nicotine as assessed by intravenous nicotine self-administration. Male Wistar rats were allowed to self-administer either of two nicotine doses under a fixed ratio or a progressive ratio schedule of reinforcement. The effects of a glutamatergic compound on nicotine self-administration in male DBA/2J mice were also explored. Finally, to assess for nonspecific effects of the drug manipulations, the effects of all test compounds on responding maintained by a food reinforcer were investigated. The pharmacological manipulations used were: gamma-vinyl-GABA (vigabatrin or GVG), an irreversible inhibitor of GABA transaminase, the GABAB receptor agonists (-)baclofen and CGP44532, and the metabotropic glutamate receptor 5 (mGluR5) antagonist MPEP. GVG, CGP44532, and (-)baclofen dose-dependently decreased nicotine self-administration on the fixed-ratio schedule, but also decreased food-maintained responding. Furthermore, CGP44532 decreased breakpoints for nicotine and food at identical doses under the progressive-ratio schedule. MPEP dose-dependently decreased nicotine self-administration with no effect on food-maintained responding in rats. MPEP also decreased nicotine self-administration in the mice. These results demonstrate that activation of GABAB receptors or blockade of mGluR5 decreased nicotine self-administration. Although there was some selectivity for the effects of the GABAergic manipulations, there was clear selectivity of the effects of MPEP on nicotine- versus food-maintained responding. Thus, compounds that increase GABAergic neurotransmission and antagonists at mGluR5 have potential as anti-smoking medications for humans.

Acute desensitization of presynaptic GABAB-mediated inhibition and induction of epileptiform discharges in the neonatal rat hippocampus

The consequences of sustained activation of GABA(B) receptors on GABA(B)-mediated inhibition and network activity were investigated in the neonatal rat hippocampus using whole-cell and extracellular field recordings. GABA(B)-mediated presynaptic control of gamma-aminobutyric acid (GABA) release progressively diminished with time in spite of the continued presence of the agonist (100 microM baclofen, 15 min), indicating acute desensitization of presynaptic GABA(B)-mediated inhibition on GABAergic terminals. By contrast, neither GABA(B)-mediated inhibition of glutamate release nor postsynaptic GABA(B)-mediated inhibition seemed to produce this desensitization. Efficacy of presynaptic GABA(B) receptors was still reduced by 49% 30 min after baclofen washout, suggesting a long timeframe for recovery from desensitization. The 15-min baclofen application was followed by a dramatic modification of the spontaneous network activity, with the occurrence of epileptiform events called ictal-like discharges (ILDs). Extracellular field recordings confirmed the epileptic nature of the discharges that could be recorded up to 4 h after baclofen washout. ILDs did not occur when the GABA(B) receptor antagonist CGP35348 was coapplied with baclofen. This indicates that ILD induction is a consequence of the sustained activation of GABA(B) receptors and the correlated changes in GABA(B)-mediated inhibition. Furthermore, ILDs were also induced when blocking with CGP35348 an amount of GABA(B) receptors that exactly mimicked the loss of inhibition obtained with desensitization. These results show that presynaptic GABA(B)-mediated inhibition of GABA release acutely and specifically desensitizes following a sustained application of the GABA(B) receptor agonist baclofen. Conditions that induce desensitization of the GABA(B)-mediated responses also trigger persistent epileptiform discharges in the neonatal rat hippocampus.

Differential distribution of the KCl cotransporter KCC2 in thalamic relay and reticular nuclei

In the thalamus of the rat the reversal potential of GABA-induced anion currents is more negative in relay cells than in neurones of the reticular nucleus (nRt) due to different chloride extrusion mechanisms operating in these cells. The distribution of KCl cotransporter type 2 (KCC2), the major neuronal chloride transporter that may underlie this effect, is unknown in the thalamus. In this study the precise regional and ultrastructural localization of KCC2 was examined in the thalamus using immunocytochemical methods. The neuropil of all relay nuclei was found to display intense KCC2 immunostaining to varying degrees. In sharp contrast, the majority of the nRt was negative for KCC2. In the anterior and dorsal part of the nRt, however, KCC2 immunostaining was similar to relay nuclei and parvalbumin and calretinin were found to colocalize with KCC2. At the ultrastructural level, KCC2 immunoreactivity was mainly located in the extrasynaptic membranes of thick and thin dendrites and the somata of relay cells but was also found in close association with asymmetrical synapses formed by cortical afferents. Quantitative evaluation of KCC2 distribution at the electron microscopic level demonstrated that the density of KCC2 did not correlate with dendritic diameter or synaptic coverage but is 1.7 times higher on perisynaptic membrane surfaces than on extrasynaptic membranes. Our data demonstrate that the regional distribution of KCC2 is compatible with the difference in GABA-A reversal potential between relay and reticular nuclei. At the ultrastructural level, abundant extrasynaptic KCC2 expression will probably play a role in the regulation of extrasynaptic GABA-A receptor-mediated inhibition.

GABAergic function in Alzheimer's disease: evidence for dysfunction and potential as a therapeutic target for the treatment of behavioural and psychological symptoms of dementia

Alzheimer's disease (AD) is characterized by disruptions in multiple major neurotransmitters. While many studies have attempted to establish whether GABA is disrupted in AD patients, findings have varied. We review evidence for disruptions in GABA among patients with AD and suggest that the variable findings reflect subtypes of the disease that are possibly manifested clinically by differing behavioural symptoms. GABA, the major inhibitory neurotransmitter, has long been a target for anxiolytics, hypnotic sedatives, and anticonvulsants. We review the clinical use of GABAergic agents in treating persons with AD symptoms. While newer generation GABAergic medications are now available, they have yet to be evaluated among patients with AD.

Peripheral GABAB agonists stimulate gastric acid secretion in mice

1 We characterized the effects of intravenous GABA and preferential GABAA (muscimol), GABAB (R-baclofen and SKF-97541) and GABAC agonists (imidazole-4-acetic acid) on gastric acid secretion in urethane-anesthetized mice implanted with a gastric cannula, and determined the role of vagal cholinergic mechanisms, and gastrin and somatostatin by using peptide immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. 2 The selective GABA(B) agonists R-baclofen (0.1-3 mg kg(-1), i.v.) and SKF-97541 (0.01-0.3 mg kg(-1), i.v.) induced a dose-related stimulation of gastric acid secretion. SKF-97541 was about 10 times more potent than R-baclofen stimulating gastric acid secretion. Neither GABA (0.1-100 mg kg(-1), i.v.) nor muscimol (0.1-3 mg kg(-1)) nor imidazole-4-acetic acid (0.1-10 mg kg(-1)) affected basal gastric acid secretion. 3 Stimulatory effects of SKF-97541 (0.1 mg kg(-1), i.v.) were blocked by the selective GABAB antagonist, 2-hydroxysaclofen, cholinergic blockade with atropine, subdiaphragmatic vagotomy or gastrin immunoneutralization. 4 Somatostatin immunoneutralization or SSTR2 blockade with PRL-2903 enhanced the secretory response to SKF-97541 (0.1 mg kg(-1), i.v.) by 78 and 105%, respectively. 5 In SSTR2 knockout mice, SKF-97541 (0.1 mg kg(-1), i.v.) increased basal gastric acid secretion by 48%. Neither GABA nor muscimol nor imidazole-4-acetic acid modified basal gastric acid secretion in SSTR2 knockout mice. 6 These results indicate that, in mice, stimulation of GABAB receptors increases gastric acid secretion through vagal- and gastrin-dependent mechanisms. Somatostatin implication might be secondary to the release of gastrin and the increase in gastric luminal acidity.

Chronic NGF treatment of rat nociceptive DRG neurons in culture facilitates desensitization and deactivation of GABAA receptor-mediated currents

1 The present study tested the hypothesis that nerve growth factor (NGF) could affect presynaptic inhibition mediated by GABAA (GABA-sensitive ionotropic receptors) receptors on the afferents of nociceptive dorsal root ganglia (DRG) neurons, thus reducing the filtering of central nociceptive signals. 2 To investigate this issue, small-diameter, nociceptive DRG neurons were cultured for 48-72 h either in the normal medium or in the presence of NGF (50 ng ml(-1)). After 15 min washout, cells were patch clamped with Cs+ containing electrodes to block GABAB (GABA-sensitive metabotropic receptors) receptor-activated currents. 3 Chronically treated DRG neurons showed no difference in the peak amplitude of GABA-induced currents. However, NGF-treated cells exhibited increased fading of the response to continuous GABA application, with faster desensitization onset, decreased residual current at the end of agonist application and slower recovery from desensitization. Moreover, the deactivation phase after brief agonist pulses was also accelerated. 4 Unlike responses to GABA, chronic NGF treatment had no effect on the desensitization process to the excitatory transmitter ATP, as no difference in peak amplitude, fast and slow time constants of current decay was found. 5 Experimental tests indicated that the observed effects on GABA currents were not a reactive process triggered by washing out NGF after its long application. Acutely applied NGF did not change GABAA receptor-mediated responses. 6 NGF-treated neurons showed decreased sensitivity to the antagonist picrotoxin. The action of pentobarbitone, midazolam, bicuculline or gabazine was, however, unchanged. 7 These observations suggest that the modulation of GABAA receptor function of DRG nociceptors by NGF may contribute to the algogenic action of this neurotrophin.

Studies on the mechanisms of action of picrotoxin, quercetin and pregnanolone at the GABA rho 1 receptor

1. The mechanisms of action of antagonists of the gamma-aminobutyric acid C (GABA(C)) receptor picrotoxin, quercetin and pregnanolone were studied. 2. Ionic currents (chloride), mediated through human homomeric GABA rho(1) receptors expressed in Xenopus oocytes, were recorded by two-electrode voltage clamp. 3. Dose-response (D-R) curves and kinetic measurements of GABA rho(1) currents were carried out in the presence or absence of antagonists. Use-dependent actions were also evaluated. 4. Picrotoxin, quercetin and pregnanolone exerted noncompetitive actions. 5. IC(50) values measured at the EC(50) for GABA (1 microM) were as follows: picrotoxin 0.6+/-0.1 microM (Hill coefficient n=1.0+/-0.2); quercetin 4.4+/-0.4 microM (n=1.5+/-0.2); pregnanolone 2.1+/-0.5 microM (n=0.8+/-0.1). 6. These antagonists produced changes only in the slope of the linear current-voltage relationships, which was indicative of voltage-independent effects. 7. The effect of picrotoxin on GABA rho(1) currents was use-dependent, strongly relied on agonist concentration and showed a slow onset and offset. The mechanism was compatible with an allosteric inhibition and receptor activation was a prerequisite for antagonism. 8. The effect of quercetin was use-independent, showed relatively fast onset and offset, and resulted in a slowed time course of the GABA-evoked currents. 9. The effect of pregnanolone was use-independent, presented fast onset and a very slow washout, and did not affect current activation. 10. All the antagonists accelerated the time course of deactivation of the GABA rho(1) currents.

Intracerebroventricular injection of muscimol, baclofen or nipecotic acid stimulates food intake in layer-type, but not meat-type, chicks

This study was designed to compare the effects of muscimol (GABA(A) agonist), baclofen (GABA(B) agonist) and nipecotic acid (GABA uptake inhibitor) on food intake in two chicken strains (meat-type and layer-type chicks). The intracerebroventricular (ICV) injection of all GABA agents induced hyperphagia in layer-type chicks. However, in broiler chicks, there were similar tendencies with muscimol and nipecotic acid but not significantly different. Conversely, ICV injection of baclofen depressed feeding of broiler chicks. These results suggest that there are some differences in central GABAergic systems between these strains of chicks, but GABAergic systems have an important role in the regulation of food intake in neonatal chicks.

Neuropeptide Y, GABA and circadian phase shifts to photic stimuli

Circadian rhythms can be phase shifted by photic and non-photic stimuli. The circadian clock, anatomically defined as the suprachiasmatic nucleus (SCN), can be phase delayed by light during the early subjective night and phase advanced during the late subjective night. Non-photic stimuli reset the clock when presented during the subjective day. A possible pathway for the non-photic resetting of the clock is thought to originate from the intergeniculate leaflet, which conveys information to the SCN through the geniculohypothalamic tract and utilizes among others neuropeptide Y (NPY) and GABA as neurotransmitters. Photic and non-photic stimuli have been shown to interact during the early and late subjective night. Microinjections of NPY or muscimol, a GABA(A) receptor agonist, into the region of the SCN can attenuate light-induced phase shifts during the early and late subjective night. The precise mechanism for these interactions is unknown. In the current study we investigate the involvement of a GABAergic mechanism in the interaction between NPY and light during the early and late subjective night. Microinjections of NPY significantly attenuated light-induced phase delays and inhibited phase advances (P<0.05). The administration of bicuculline during light exposure, before NPY microinjection did not alter the ability of NPY to attenuate light-induced phase delays and block photic phase advances. These results indicate that NPY attenuates photic phase shifts via a mechanism independent of GABA(A) receptor activation. Furthermore it is evident that NPY influences circadian clock function via differing cellular pathways over the course of a circadian cycle.

Partial hippocampal kindling increases GABAB receptor-mediated postsynaptic currents in CA1 pyramidal cells

In previous studies, we showed that partial hippocampal kindling decreased the efficacy of the presynaptic GABAB receptors on both GABAergic and glutamatergic terminals of CA1 neurons in hippocampal slices in vitro. In this study, GABAB receptor-mediated inhibitory postsynaptic currents (GABAB-IPSCs) were assessed by whole-cell recordings in CA1 pyramidal neurons in hippocampal slices of male Long-Evans rats. The peak GABAB-IPSC evoked by a brief train of supramaximal stratum radiatum stimuli (20 pulses of 300 Hz) in the presence of picrotoxin (0.1 mM) and kynurenic acid (1 mM) was larger in neurons of kindled (65.9 +/- 5.2 pA, N=42 cells) than control (45.8 +/- 4.8 pA, N=32 cells) rats (P<0.01). Adding GABA uptake blocker nipecotic acid (1 mM) or GABAB receptor agonist baclofen (0.01 mM) in the perfusate induced outward currents that were blocked by GABAB receptor antagonist CGP 55845A (1 microM). The peak outward current induced by nipecotic acid was larger in neurons of the kindled (55.4 +/- 5.7 pA, N=30) than the control group (39.8 +/- 4.5 pA, N=28) (P<0.05). However, the magnitude of the baclofen-induced current was not different between kindled (90.8 +/- 6.9 pA, N=29) and control (87.2 +/- 5.9 pA, N=21) groups (P>0.05). We concluded that partial hippocampal kindling increased GABAB-IPSCs in hippocampal CA1 pyramidal cells via multiple presynaptic mechanisms.

Biphasic action of midazolam on GABAA receptor-mediated responses in rat sacral dorsal commissural neurons

The effect of the benzodiazepine agonist midazolam on gamma-aminobutyric acid(A) (GABA(A)) receptor-mediated currents was investigated in neurons acutely dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. Midazolam displayed a biphasic effect on GABA responses. Low concentrations of midazolam (1nM-10 microM) reversibly potentiated GABA (3 microM)-activated Cl(-) currents (I(GABA)) in a bell-shaped manner, with the maximal facilitary effect at 0.1 microM; whereas at higher concentrations (above 10 microM), midazolam had an antagonistic effect on I(GABA). Our further study indicated that midazolam changed GABA(A) receptor affinity to GABA and the effects of midazolam on I(GABA) were voltage-independent. The benzodiazepine receptor antagonist, flumazenil, abolished the facilitary effect of low concentrations of midazolam rather than the antagonism of I(GABA) induced by high doses of midazolam. In addition, activation of protein kinase C prevented the inhibitory effect of midazolam at higher concentrations, but did not influence the effect of midazolam at low concentrations. These results indicate that midazolam interacts with another distinct site other than the central benzodiazepine receptors on GABA(A) receptors as an antagonist at higher concentrations in SDCN neurons.

Synaptic strength between motoneurons and terminals of the dorsolateral funiculus is regulated by GABA receptors in the turtle spinal cord

The role of GABAA and GABAB receptors in modulation of excitatory synaptic transmission between motoneurons and terminals from dorsolateral funiculus (DLF) was studied in in vitro spinal cord slices of adult turtles. Muscimol--a GABAA receptor agonist--depressed the monosynaptic excitatory postsynaptic potential (EPSP) induced by stimulation of the DLF and shortened its duration. The input resistance and the membrane time constant also were strongly reduced. The input membrane resistance, the amplitude, and the half-width of the EPSP were reduced at the same rate in the presence of muscimol. Bicuculline--a GABAA receptor antagonist--increased the EPSPs amplitude and the input membrane resistance. The EPSP amplitude ratio elicited by a paired-pulse protocol did not change significantly. Our results suggest that muscimol acts mainly by activation of postsynaptic GABAA receptors located on the motoneuron and the synaptic strength on motoneurons may be modulated by tonic activation of postsynaptic GABAA receptors. Baclofen--a GABAB receptor agonist--also depressed DLF-motoneuron synaptic transmission. However, it did not affect the falling phase of the EPSPs or the motoneuron membrane time constant but induced a small decrement in input resistance. In the presence of baclofen, the amplitude ratio produced by a paired-pulse protocol increased significantly. This suggests that baclofen decreased the synaptic strength by inhibition of neurotransmitter release from the DLF terminals via activation of presynaptic GABAB receptors.

Stimulation by surangin B of endogenous amino acid release from synaptosomes

The effect of surangin B, an insecticidal natural product coumarin, on presynaptic release of endogenous amino acids was investigated using a purified synaptosomal fraction isolated from mouse brain. Surangin B stimulated the release of glutamic acid (GLU), gamma-aminobutyric acid (GABA), serine, alanine and the aminosulfonic acid taurine from synaptosomes at micromolar concentrations. In all cases, these responses were reduced by removing calcium from the saline and surangin B-evoked release of GLU, GABA, aspartic acid (ASP) and alanine was significantly inhibited by the sodium channel blocker tetrodotoxin. Rotenone (a complex I inhibitor) and carbonyl cyanide chlorophenylhydrazone (CCCP; an uncoupler), were more potent releasers of amino acids from synaptosomes than surangin B, however, carboxin (a complex II-selective inhibitor), was extremely weak to ineffective in this regard. The stimulatory effect of surangin B and complex III-selective inhibitors on release of GLU, GABA, ASP and alanine by synaptosomes was significantly reduced by N,N,N',N'-tetramethyl-p-phenylenediamine, suggesting that blockade of complex III in intraterminal mitochondria is an important effect of this coumarin. Our results demonstrate that surangin B, in common with CCCP and inhibitors of complex I and III, cause release of both neurotransmitter and non-neurotransmitter amino acids from nerve endings in vitro. However, in contrast to most classical agents which interfere selectively with mitochondrial function, the release of endogenous amino acids from synaptosomes by surangin B also involves a moderate extracellular calcium ion-dependent component and relies partially on sodium ion entry into the nerve ending.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.