Functional modulation of GABAA receptors by cAMP-dependent protein phosphorylation

Biology of the postsynaptic glycine receptor

Glycine is one of the major inhibitory neurotransmitters, and upon binding to its receptor it activates chloride conductances. Receptors are accumulated immediately opposite release sites, at the postsynaptic differentiations, where they form functional microdomains. This review describes recent advances in our understanding of the structure-function relationships of the glycine receptor, a member of the ligand-gated ion channel superfamily. Following purification of the receptor complex and identification of its integral and peripheral membrane protein components, molecular cloning has revealed the existence of several subtypes of the ligand-binding subunit. This heterogeneity is responsible for the distinct pharmacological and functional properties displayed by the various receptor configurations that are differentially expressed and assembled during development. This review also focuses on the molecular aspects of glycinergic synaptogenesis, highlighting gephyrin, the peripheral component of the receptor. The role of this cytoplasmic protein in anchoring and maintaining the channel complex in postsynaptic clusters is discussed. The glycine receptor recently moved into the spotlight as a paradigm in the approach to cell biology of the formation of the postsynaptic membrane.

Activation and deactivation rates of recombinant GABA(A) receptor channels are dependent on alpha-subunit isoform

The role of subunit composition in determining intrinsic maximum activation and deactivation kinetics of GABA(A) receptor channels is unknown. We used rapid ligand application (100-micros solution exchange) to examine the effects of alpha-subunit composition on GABA-evoked activation and deactivation rates. HEK 293 cells were transfected with human cDNAs encoding alpha1beta1gamma2- or alpha2beta1gamma2-subunits. Channel kinetics were similar across different transfections of the same subunits and reproducible across several GABA applications in the same patch. Current rise to peak was at least twice as fast for alpha2beta1gamma2 receptors than for alpha1beta1gamma2 receptors (reflected in 10-90% rise times of 0.5 versus 1.0 ms, respectively), and deactivation was six to seven times slower (long time constants of 208 ms versus 31 ms) after saturating GABA applications. Thus alpha-subunit composition determined activation and deactivation kinetics of GABA(A) receptor channels and is therefore likely to influence the kinetics and efficacy of inhibitory postsynaptic currents.

The role of phosphorylation in the activity and mercury modulation of GABA-induced currents in rat neurons

The role of protein kinase A (PKA) and protein kinase C (PKC) in the function and modulation by mercury chloride of the GABA(A) receptor-chloride channel complex was studied with rat dorsal root ganglion cells using the whole-cell patch clamp technique. When added to the internal pipette solutions, both KT 5720, a selective PKA inhibitor, and calphostin C, a selective PKC inhibitor, increased the maximal current and shifted the EC50 for GABA in the direction of higher GABA concentrations. GABA-activated currents were decreased by the addition of 5 mM cAMP to the internal pipette solution, and by external perfusion of 100 nM phorbol 13-myristate 13-acetate. Mercury chloride potentiation of GABA-activated currents was blocked by internal application of 5 mM cAMP. PKA in the recording pipette abolished the mercury chloride potentiation of GABA-activated currents. In contrast, 0.56 microM KT 5720, but not calphostin C, in the internal pipette solution enhanced the effect of mercury chloride. In conclusion, both PKA and PKC negatively regulate the activity of the GABA(A) receptor-channel complex probably through phosphorylation of the receptor, and the PKA system underlies the mechanism of mercury chloride potentiation of GABA-activated currents.

D5 dopamine receptors enhance Zn2+-sensitive GABA(A) currents in striatal cholinergic interneurons through a PKA/PP1 cascade

Cholinergic interneurons have been implicated in striatally mediated associative learning. In classical conditioning paradigms, conditioned stimuli trigger a transient suppression of neuronal activity that is dependent upon an intact dopaminergic innervation. Our hypothesis was that this suppression reflected dopaminergic enhancement of sensory-linked GABAergic input. As a test, the impact of dopamine on interneuronal GABA(A) receptor function was studied by combined patch-clamp recording and single-cell reverse transcription PCR. Activation of D5 dopamine receptors reversibly enhanced a Zn2+-sensitive component of GABA(A) currents. Although dependent upon protein kinase A (PKA) activation, the modulation was blocked by protein phosphatase 1 (PP1) inhibition, suggesting it was dependent upon dephosphorylation. These results establish a novel mechanism by which intrastriatally released dopamine mediates changes in GABAergic signaling that could underlie the initial stages of associative learning.

Shaping of IPSCs by endogenous calcineurin activity

Synaptic inhibition, mediated by GABAA receptors, regulates neuronal firing, influences coincidence detection (König et al., 1996), and can synchronize the output of neural circuits (Cobb et al., 1995). Although GABAA receptors can be modulated by phosphorylation, few studies have directly addressed the role of such modulation at synapses, where the nonequilibrium conditions of receptor activation are quite different from those often used to study GABAA receptors in vitro. Here we promoted endogenous phosphorylation by inhibiting specific phosphatases in rat hippocampal neurons and compared the effects on IPSCs with GABAA channel responses in outside-out patches. Brief and saturating GABA pulses (5 msec; 10 mM) activated patch currents resembling the IPSC. Inhibition of calcineurin (protein phosphatase 2B), but not phosphatases 1 or 2A, produced a similar shortening of IPSC and patch responses, as did nonspecific inhibition of dephosphorylation using ATPgammaS or high concentrations of intracellular phosphate. Calcineurin inhibition increased the microscopic ligand unbinding rate, which was measured using the competitive antagonist 2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide, suggesting that the IPSC shortening was partly caused by destabilization of the ligand binding site. Calcineurin inhibition also increased the rate and extent of macroscopic receptor desensitization. These results show that endogenous regulation by kinases and calcineurin can produce substantial changes in the IPSC duration by altering the unbinding and gating kinetics of the GABAA receptor. Dynamic regulation of synaptic inhibition may thus allow for the tuning of circuit behavior at the level of individual inhibitory synapses.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Conserved phosphorylation of the intracellular domains of GABA(A) receptor beta2 and beta3 subunits by cAMP-dependent protein kinase, cGMP-dependent protein kinase protein kinase C and Ca2+/calmodulin type II-dependent protein kinase

All mammalian GABA(A) receptor beta subunits contain a conserved consensus site for phosphorylation by a number of serine/threonine protein kinases. This site corresponds to Serine 410 of the beta2 subunit and Serine 409 of the beta3 subunit, each of which lies within the conserved sequence R-R-R-X-S-L-Q-K, where X = A (beta1, beta2 and beta4) or S (beta3). We have analysed the phosphorylation of the beta2 and beta3 subunits of the murine GABA(A) receptor by expressing the large intracellular domains of these subunits as soluble fusion proteins in E. coli. The intracellular domain of the beta2 subunit was phosphorylated to high stoichiometry by both cAMP- and cGMP-dependent protein kinases, protein kinase C and Ca2+/calmodulin type II-dependent protein kinase in vitro. Site-directed mutagenesis identified Serine 410 as the single site within the beta2 subunit phosphorylated by these four protein kinases. Using similar methodologies, Serine 409 of the beta3 subunit was shown to be a substrate for phosphorylation by these protein kinases. Serine 408 was also seen to be phosphorylated by protein kinase C and Serine 383 was phosphorylated by Ca2+/calmodulin type II-dependent protein kinase. Since beta subunits are believed to be essential for robust GABA(A) receptor expression, these results suggest a critical role for conserved phosphorylated amino acids within the beta subunits in coordinating cellular regulation of GABA(A) receptors via multiple protein kinases.

Differential intracellular regulation of cortical GABA(A) and spinal glycine receptors in cultured neurons

Using patch-clamp techniques we studied several aspects of intracellular GABA(A) and glycine Cl- current regulation in cortical and spinal cord neurons, respectively. Activation of PKA with a permeable analog of cyclic AMP (cAMP) produced a potentiation of the Cl- current activated with glycine, but not of the current induced with GABA. The inactive analog was without effect. Activation of PKC with 1 microM PMA reduced the amplitude of the GABA(A) and glycine currents. Internal application of 1 mM cGMP, on the other hand, had no effect on the amplitude of either current. The amplitude of these inhibitory currents changed slightly during 20 min of patch-clamp recording. Internal perfusion of the neurons with 1 microM okadaic acid, a phosphatase inhibitor, induced potentiation in both currents. The amplitude of GABA(A) and glycine currents recorded with 1 mM internal CaCl2 and 10 mM EGTA (10 nM free Ca2+) decayed by less than 30% of control. Increasing the CaCl2 concentration to 10 mM (34 microM free Ca2+) induced a transient potentiation of the GABA(A) current. A strong depression of current amplitude was found with longer times of dialysis. The glycine current, on the contrary, was unchanged by increasing the intracellular Ca2+ concentration. Activation of G proteins with internal FAl4- induced an inhibition of the GABA(A) current, but potentiated the amplitude of the strychnine-sensitive Cl- current. These results indicate that GABA(A) and glycine receptors are differentially regulated by activation of protein kinases, G proteins and Ca2+. This conclusion supports the existence of selectivity in the intracellular regulation of these two receptor types.

Modulation of GABAA receptor function by tyrosine phosphorylation of beta subunits

Protein tyrosine phosphorylation is a key event in diverse intracellular signaling pathways and has been implicated in modification of neuronal functioning. We investigated the role of tyrosine phosphorylation in regulating type A GABA (GABAA) receptors in cultured CNS neurons. Extracellular application of genistein (50 microM), a membrane-permeable inhibitor of protein tyrosine kinases (PTKs), produced a reversible reduction in the amplitude of GABAA receptor-mediated whole-cell currents, and this effect was not reproduced by daidzein (50 microM), an inactive analog of genistein. In contrast, intracellular application of the PTK pp60(c-src) (30 U/ml) resulted in a progressive increase in current amplitude, and this potentiation was prevented by pretreatment of the neurons with genistein. Immunoprecipitation and immunoblotting of cultured neuronal homogenates indicated that the beta2/beta3 subunit(s) of the GABAA receptor are tyrosine phosphorylated in situ. Moreover, genistein (50 microM) was found to be capable of decreasing GABAA currents in human embryonic kidney 293 cells transiently expressing functional GABAA receptors containing the beta2 subunit. Thus, the present work provides the first evidence that native GABAA receptors are phosphorylated and modulated in situ by endogenous PTKs in cultured CNS neurons and that phosphorylation of the beta subunits may be sufficient to support such a modulation. Given the prominent role of GABAA receptors in mediating many brain functions and dysfunctions, modulation of these receptors by PTKs may be important in a wide range of physiological and pathological processes in the CNS.

GABA activity mediating cytosolic Ca2+ rises in developing neurons is modulated by cAMP-dependent signal transduction

In the majority of developing neurons, GABA can exert depolarizing actions, thereby raising neuronal Ca2+. Ca2+ elevations can have broad consequences during development, inducing gene expression, altering neurite outgrowth and growth cone turning, activating enzyme pathways, and influencing neuronal survival. We used fura-2 and fluo-3 Ca2+ digital imaging to assess the effects of inhibiting or activating the cAMP signal transduction pathway on GABA activity mediating Ca2+ rises during the early stages of in vitro hypothalamic neural development. Our experiments stemmed from the finding that stimulation of transmitter receptors shown to either activate or inhibit adenylyl cyclase activity caused a rapid decrease in Ca2+ rises mediated by synaptically released GABA. Both the adenylyl cyclase activator forskolin and the inhibitor SQ-22,536 reduced the Ca2+ rise elicited by the synaptic release of GABA. Bath application of the membrane-permeable cAMP analogs 8-bromo-cAMP (8-Br-cAMP) or 8-(4-chlorophenylthio)-cAMP (0.2-5 mM) produced a rapid, reversible, dose-dependent inhibition of Ca2+ rises triggered by synaptic GABA release. Potentiation of GABAergic activity mediating Ca2+ rises was observed in some neurons at relatively low concentrations of the membrane-permeable cAMP analogs (20-50 microM). In the presence of tetrodotoxin (TTX), postsynaptic Ca2+ rises triggered by the bath application of GABA were only moderately depressed (13%) by 8-Br-cAMP (1 mM), suggesting that the inhibitory effects of 8-Br-cAMP were largely the result of a presynaptic mechanism. The protein kinase A (PKA) inhibitors H89 and Rp-3', 5'-cyclic monophosphothioate triethylamine also caused a large reduction (>70%) in Ca2+ rises triggered by synaptic GABA release. Unlike the short-term depression elicited by activation of the cAMP signal transduction pathway, Ca2+ depression elicited by PKA inhibition persisted for an extended period (>30 min) after PKA inhibitor washout. Postsynaptic depression of GABA-evoked Ca2+ rises triggered by H89 (in the presence of TTX) recovered rapidly, suggesting that the extended depression observed during synaptic GABA release was largely through a presynaptic mechanism. Long-term Ca2+ modulation by cAMP-regulating hypothalamic peptides may be mediated through a parallel mechanism. Together, these results suggest that GABAergic activity mediating Ca2+ rises is dependent on ongoing PKA activity that is maintained within a narrow zone for GABA to elicit a maximal Ca2+ elevation. Thus, neuromodulator-mediated changes in the cAMP-dependent signal transduction pathway (activation or inhibition) could lead to a substantial decrease in GABA-mediated Ca2+ rises during early development.

Baclofen reduces GABAA receptor responses in acutely dissociated neurons of bullfrog dorsal root ganglia

The effect of baclofen on the function of the gamma-aminobutyric acidA (GABAA) receptor was examined in acutely dissociated neurons of bullfrog dorsal root ganaglia (DRG) by using the whole-cell voltage-clamp method. Baclofen (0.1-100 microM) depressed the inward currents produced by GABA (100 microM) and muscimol (100 microM). Baclofen shifted the concentration-response curve for GABA (1 microM-1 mM) downward. Baclofen decreased the maximum response (Vmax) to GABA without changing the apparent dissociation constant (Kd), suggesting a noncompetitive antagonism. The effect of baclofen on the GABA current was blocked by antagonists for the GABAB receptor; the rank order of potency was P-[3-Aminopropyl]-P-diethoxymethylphosphinic acid (CGP 55845A) > > 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P- benzyl-phosphinic acid (CGP 35348) > saclofen > > phaclofen. Baclofen produced an irreversible depression of the GABA current in neurons dialyzed with an internal solution containing guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S, 100 microM). Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP beta S, 100 microM) blocked the inhibitory effect of baclofen on the GABA current. Forskolin (10 microM) and dibutyryl N6, 2'-O-dibutyryladenosine 3':5'-cyclic monophophate (db-cyclic AMP) (200 microM) depressed the GABA current. N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9, 40 microM) and N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 50 microM), protein kinase A (PKA) inhibitors, reduced the depressant effect of baclofen on the GABA current. The baclofen-induced depression of the GABA current was blocked by PKI(5-24), a specific PKA inhibitor, but not by PKC(19-36), a specific protein kinase C (PKC) inhibitor. We suggest that GABAB receptors regulate the GABAA receptor function through a G-protein linked to the adenylyl cyclase-PKA pathway in bullfrog DRG neurons.

Effect of PKG activation on recombinant GABAA receptors

The effect of cGMP-dependent protein kinase (PKG) on recombinant human alpha 1 beta 2 gamma 2L GABAA receptors expressed in Xenopus oocytes was studied using the two-electrode voltage-clamp technique. The cGMP analog 8BrcGMP (1 mM) produced an increase in GABA-gated chloride currents. Intracellular injection of the PKG inhibitor peptide, PKGI, prevented the 8BrcGMP-mediated increase in the GABA response indicating that 8BrcGMP enhances GABAA receptor function via activation of PKG. Previous studies have shown that PKG phosphorylates a fusion protein corresponding to the intracellular loop of the beta 1 subunit [McDonald and Moss, J. Biol. Chem., 269 (1994) 18111-18117]. In the present study, site-directed mutagenesis of this phosphorylation site (beta 2ser410) failed to eliminate the effects of 8BrcGMP on the GABA response. These results suggest that there may be other sites on the receptor which are regulated by PKG or that PKG phosphorylates other proteins which may influence GABAA receptor function.

Substance P suppresses GABAA receptor function via protein kinase C in primary sensory neurones of bullfrogs

1. The effects of substance P (SP) and related tachykinins on the function of gamma-aminobutyric acid-A (GABAA) receptors were examined in acutely dissociated neurones of bullfrog dorsal root ganglia (DRG) by using whole-cell voltage-clamp techniques. 2. Application of SP (10 nM to 1 microM) depressed inward currents produced by GABAA receptor activation (IGABA). Neurokinin A (NKA) and neurokinin B (NKB) also depressed IGABA; the rank order of agonist potency was SP > NKA > NKB. Spantide ([D-Arg1, D-Trp7,9,Leu11]SP) and L-703,606, NK1 receptor antagonists, blocked the SP-induced depression of IGABA. 3. SP irreversibly depressed IGABA, when neurones were intracellularly dialysed with GTP gamma S. Intracellular application of GDP beta S prevented the SP-induced depression of IGABA. Pertussis toxin (PTX) did not block the inhibitory effect of SP on IGABA. 4. The depression of IGABA produced by SP was inhibited by H-7 and PKC(19-36), protein kinase C (PKC) inhibitors, but not by H-9 and HA-1004, protein kinase A inhibitors. IGABA was suppressed by application of sn-1,2-dioctanoyl glycerol (DOG), a PKC activator. 5. It is concluded that activation of neurokinin-1 (NK1) receptors downregulates the function of the GABAA receptor of primary sensory neurones through a PTX-insensitive G-protein. PKC may be involved in the transduction pathway of the tachykinin-induced inhibition of the GABAA receptor.

Noradrenergic potentiation of cerebellar Purkinje cell responses to GABA: cyclic AMP as intracellular intermediary

Norepinephrine and the beta-adrenergic receptor agonist, isoproterenol, have been shown to potentiate the amplitude of GABAA receptor-mediated whole-cell current responses in Purkinje cells acutely dissociated from the rat cerebellum. However, the steps leading from the activation of beta-adrenergic receptors to the modulation of GABAA receptor remain to be delineated. This study tested the hypothesis that a sequelae of intracellular intermediaries involving the cyclic AMP second messenger system serves as the subcellular link to promote this heteroreceptor interaction. Exposure to cholera toxin, but not to pertussis toxin, increased the amplitude of GABA-activated current responses in acutely dissociated Purkinje cells. Intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) also resulted in a time- and dose-dependent augmentation of the response to GABA. while guanosine 5'-O-(2-thiodiphosphate) blocked the norepinephrine-mediated facilitation. A positive modulation of the current response to GABA was observed following intracellular delivery of cyclic AMP or the catalytic subunit of the cyclic AMP-dependent protein kinase. Furthermore, the norepinephrine-induced potentiation of the GABA-activated current response was prevented in the presence of the Rp isomer of cyclic AMP, the regulatory subunit of cyclic AMP-dependent protein kinase and an inhibitor of cyclic AMP-dependent protein kinase. These findings led to the formulation of a working model in which activation of the beta-adrenergic receptor triggers a Gs-protein-mediated transduction cascade in cerebellar Purkinje cells which activates adenylate cyclase, resulting in a rise in intracellular levels of cyclic AMP, increased phosphorylating activity by cyclic AMP-dependent protein kinase and, ultimately, a potentiation of GABAA receptor function.

Upregulation of GABAA current by astrocytes in cultured embryonic rat hippocampal neurons

Embryonic rat hippocampal neurons were cultured on poly-D-lysine (PDL) or a monolayer of postnatal cortical astrocytes to reveal putative changes in neuronal physiology that involve astrocyte-derived signals during the first 4 d of culture, GABA-induced Cl- current (IGABA) was quantified using outside-out and whole-cell patch-clamp recordings beginning at 30 min, when cells had become adherent. The amplitude and density (current normalized to membrane capacitance) of IGABA in neurons grown on astrocytes became statistically greater than that recorded in neurons grown on PDL after 2 hr in culture (HIC). Although the current density remained unchanged in neurons on astrocytes, that in neurons on PDL decreased and became statistically lower beginning after 2 HIC. The differences in amplitude and density of IGABA in the two groups of neurons were maintained during the 4 d experiment. The upregulation effect of astrocytes on neuronal IGABA required intimate contact between the neuronal cell body and underlying astrocytes. Suppression of spontaneous Cac2+ elevations in astrocytes by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid that was loaded intracellularly decreased their modulatory effects on IGABA. IGABA in all cells was blocked completely by bicuculline and exhibited virtually identical affinity constants, Hill coefficients, and potentiation by diazepam in the two groups. Outside-out patch recordings revealed identical unitary properties of IGABA in the two groups. More channels per unit of membrane area could explain the astrocyte enhancement of IGABA. The results reveal that cortical astrocytes potentiate IGABA in hippocampal neurons in a contact-dependent manner via a mechanism involving astrocyte Cac2+ elevation.

Regulation of ionotropic receptors by protein phosphorylation

The regulation of synaptic signal transduction is of central importance to our understanding of normal and abnormal nervous system function. One mechanism by which signal transduction can be affected is the modification of cellular sensitivity by alterations of transmembrane receptor properties. For G-protein coupled receptors, protein phosphorylation is intimately involved in many stages of receptor regulation. This appears to be true for ionotropic receptors as well. Evidence of a role for protein kinase and protein phosphatase activity in the multi-staged ionotropic receptor regulation cascade is presented and a comparison to G-protein coupled receptor regulation is considered.

Receptor stimulation causes slow inhibition of IRK1 inwardly rectifying K+ channels by direct protein kinase A-mediated phosphorylation

Strongly rectifying IRK-type inwardly rectifying K+ channels are involved in the control of neuronal excitability in the mammalian brain. Whole-cell patch-clamp experiments show that cloned rat IRK1 (Kir 2.1) channels, when heterologously expressed in mammalian COS-7 cells, are inhibited following the activation of coexpressed serotonin (5-hydroxytryptamine) type 1A receptors by receptor agonists. Inhibition is mimicked by internal perfusion with GTP[gamma-S] and elevation of internal cAMP concentrations. Addition of the catalytic subunits of protein kinase A (PKA) to the internal recording solution causes complete inhibition of wild-type IRK1 channels, but not of mutant IRK1(S425N) channels in which a C-terminal PKA phosphorylation site has been removed. Our data suggest that in the nervous system serotonin may negatively control IRK1 channel activity by direct PKA-mediated phosphorylation.

8-Bromo-cAMP mimics beta-adrenergic sensitization of GABA responses to ethanol in cerebellar Purkinje neurons in vivo

Previous studies in our laboratory indicated that electrophysiological responses of cerebellar Purkinje neurons to GABA were not routinely potentiated by ethanol (EtOH), and the potentiation was not large when it occurred. In the presence of beta-adrenergic agonists, such as isoproterenol, however, GABA inhibitions became sensitive to potentiation by EtOH in nearly every Purkinje neuron tested. beta-adrenergic receptor activation alone also modulates (potentiates) GABA responses on Purkinje neurons, and this has been reported to be mediated by a cAMP second messenger system. Herein, we report that the membrane-permeable cAMP analog, 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP), but not the membrane-impermeable cAMP, can also modulate GABA responses and that EtOH potentiates this facilitatory action of 8-Br-cAMP. These effects are not likely caused by adenosine receptor mechanisms, because this 8-bromoadenosine mediated modulation and sensitization was observed in the presence of systemic theophylline. These data suggest that the beta-adrenergic modulation and sensitization to EtOH of cerebellar Purkinje neuron GABA responses occur via a cAMP second messenger mechanism.

Reduction of cerebellar GABAA responses by interleukin-1 (IL-1) through an indomethacin insensitive mechanism

Recently, a role of IL-1 in the central nervous system has been described, principally a fever-inducing effect in the hypothalamus through a prostaglandin second messenger system. IL-1 has also been shown to potentiate gamma-aminobutyric acid (GABAA) responses in embryonic chick neurones. This study describes the investigation of the effect of IL-1 on GABAA responses within the in vitro rat cerebellar slice, a preparation containing intact neuronal circuitry. Stimulation of the area of passage of paralleled fibres produced a pure GABAA inhibition of the spontaneous firing of Purkinje cells. 5 and 10 ng/ml IL-1 produced a reduction in the duration of inhibition 10 min after beginning perfusion of IL-1. This effect reversed within 15 min of washing out the IL-1. 10 ng/ml IL-1 also reduced the effects of exogenously-applied GABA (0.1 mM) with the same time course. In the presence of 1 uM indomethacin, there was no change in the effect of the IL-1. It can therefore be concluded that the reduction in cerebellar GABAA responses by IL-1 is not mediated by the indomethacin-sensitive prostaglandin second messenger system.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

gamma-aminobutyric acidA receptor regulation: heterologous uncoupling of modulatory site interactions induced by chronic steroid, barbiturate, benzodiazepine, or GABA treatment in culture

Prolonged administration of anxiolytic, sedative, and anticonvulsant drugs that act through the GABAA receptor (GABAAR) can evoke tolerance and dependence, suggesting the existence of an endogenous mechanism(s) for altering the ability of such agents to interact with the GABAAR. Uncoupling appears to be one such mechanism. This is a decrease in the allosteric interactions between the benzodiazepine (BZD) recognition site and other agonist or modulator sites on the GABAAR, as measured by potentiation of [3H]flunitrazepam ([3H]FNZ) binding. To investigate the mechanism(s) of uncoupling, neuronal cultures were treated chronically with 3 alpha-hydroxy-5 beta-pregnan-20-one (pregnanolone), pentobarbital, flurazepam, or GABA, then tested for enhancement of [3H]FNZ binding by these substances. The results indicate that BZDs, barbiturates, and steroids, as well as GABA itself, are capable of inducing both heterologous and homologous uncoupling. Surprisingly, different chronic drug treatments produce different patterns of homologous and heterologous uncoupling. Chronic exposure to pregnanolone, GABA, flurazepam or pentobarbital induces complete uncoupling of barbiturate-BZD site interactions, partial uncoupling of GABA-BZD site interactions, but different amounts of uncoupling of steroid-BZD site interactions. In addition, the EC50 for pregnanolone-induced homologous uncoupling (1.7 microM) is over an order of magnitude greater than that for heterologous uncoupling of GABA and BZD sites (82 nM). Moreover, heterologous uncoupling by pregnanolone is inhibited by the GABA site antagonist SR-95531, whereas homologous uncoupling by pregnanolone is resistant to SR-95531. Therefore, there are at least two distinct ways in which GABAAR modulatory site interactions can be regulated by chronic drug treatment.

Assembly and cell surface expression of heteromeric and homomeric gamma-aminobutyric acid type A receptors

The ability of differing subunit combinations of gamma-aminobutyric acid type A (GABAA) receptors produced from murine alpha 1, beta 2, and gamma 2L subunits to form functional cell surface receptors was analyzed in both A293 cells and Xenopus oocytes using a combination of molecular, electrophysiological, biochemical, and morphological approaches. The results revealed that GABAA receptor assembly occurred within the endoplasmic reticulum and involved the interaction with the chaperone molecules immunoglobulin heavy chain binding protein and calnexin. Despite all three subunits possessing the ability to oligomerize with each other, only alpha 1 beta 2 and alpha 1 beta 2 gamma 2L subunit combinations could produce functional surface expression in a process that was not dependent on N-linked glycosylation. Single subunits and the alpha 1 gamma 2L and beta 2 gamma 2L combinations were retained within the endoplasmic reticulum. These results suggest that receptor assembly occurs by defined pathways, which may serve to limit the diversity of GABAA receptors that exist on the surface of neurons.

Modulation of amino acid-gated ion channels by protein phosphorylation

The major excitatory and inhibitory amino acid receptors in the mammalian central nervous system are considered to be glutamate, gamma-aminobutyric acid type A (GABAA), and glycine receptors. These receptors are widely acknowledged to participated in fast synaptic neurotransmission, which ultimately is responsible for the control of neuronal excitability. In addition to these receptors being regulated by endogenous factors, including the natural neurotransmitters, they also form target substrates for phosphorylation by a number of protein kinases, including serine/threonine and tyrosine kinases. The process of phosphorylation involves the transfer of a phosphate group(s) from adenosine triphosphate to one or more serine, threonine, or tyrosine residues, which are invariably found in an intracellular location within the receptor Phosphorylation is an important means of receptor regulation since it represents a covalent modification of the receptor structure, which can have important implications for ion channel function. This chapter reviews the current molecular and biochemical evidence regarding the sites of phosphorylation for both native neuronal and recombinant glutamate, GABAA and glycine receptors, and also reviews the functional electrophysiological implications of phosphorylation for receptor function.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.