Development of tolerance upon repeated administration with the GABAB receptor positive allosteric modulator, COR659, on alcohol drinking in rodents

Background: Recent work has demonstrated that acute administration of the novel positive allosteric modulator of the GABAB receptor, COR659, reduces several alcohol-related behaviors in rodents.Objective: To assess whether COR659 continues to lessen alcohol intake after repeated administration, a fundamental feature of drugs with therapeutic potential.Methods: Male C57BL/6J mice (n = 40) were exposed to daily 2-hour drinking sessions (20% (v/v) alcohol) under the 1-bottle "drinking in the dark" protocol and male Sardinian alcohol-preferring rats (n = 40) were exposed to daily 1-hour drinking sessions under the 2-bottle "alcohol (10%, v/v) vs water" choice regimen. COR659 (0, 10, 20, and 40 mg/kg in the mouse experiment; 0, 5, 10, and 20 mg/kg in the rat experiment) was administered intraperitoneally before 7 consecutive drinking sessions.Results: Alcohol intake in vehicle-treated mice and rats averaged 2.5-3.0 and 1.5-1.6 g/kg/session, respectively, indicative of high basal levels. In both experiments, treatment with COR659 resulted in an initial, dose-related suppression of alcohol intake (up to 70-80% compared to vehicle treatment; P < .0005 and P < .0001 in mouse and rat experiments, respectively). The magnitude of the reducing effect of COR659 on alcohol drinking diminished progressively, until vanishing over the subsequent 2-4 drinking sessions.Conclusion: COR659 effectively reduced alcohol intake in two different rodent models of excessive alcohol drinking. However, tolerance to the anti-alcohol effects of COR659 developed rapidly. If theoretically transposed to humans, these data would represent a possible limitation to the clinical use of COR659.

Pre- and post-synaptic modulation by GABAB receptors of rat neuroendocrine dopamine neurones

The secretion of prolactin from the pituitary is negatively controlled by tuberoinfundibular dopamine (TIDA) neurones. The electrical properties of TIDA cells have recently been identified as a modulatory target of neurotransmitters and hormones in the lactotrophic axis. The role of the GABA_B receptor in this control has received little attention, yet is of particular interest because it may act as a TIDA neurone autoreceptor. Here, this issue was explored in a spontaneously active rat TIDA in vitro slice preparation using whole-cell recordings. Application of the GABA_B receptor agonist, baclofen, dose-dependently slowed down or abolished the network oscillations typical of this preparation. Pharmacological manipulations identify the underlying mechanism as an outward current mediated by G-protein-coupled inwardly rectifying K⁺ -like channels. In addition to this postsynaptic modulation, we describe a presynaptic modulation where GABA_B receptors restrain the release of glutamate and GABA onto TIDA neurones. Our data identify both pre- and postsynaptic modulation of TIDA neurones by GABA_B receptors that may play a role in the neuronal network control of pituitary prolactin secretion and lactation.

Inhibition of sigma-1 receptors substantially modulates GABA and glutamate transport in presynaptic nerve terminals

Sigma-1 receptors (Sig-1Rs) have been implicated in many neurological and psychiatric disorders and are a novel target for the treatment of such disorders. Sig-1R expression/activity deficits are linked to neurodegeneration, whereas the mechanisms mediated by Sig-1R are still unclear. Here, presynaptic [3H]GABA and L-[14C]glutamate transport was analysed in rat brain nerve terminals (synaptosomes) in the presence of the Sig-1R antagonist NE-100. NE-100 at doses of 1 and 10 μM increased the initial rate of synaptosomal [3H]GABA uptake, whereas 50 and 100 μM NE-100 decreased this rate, exerting a biphasic mode of action.Antagonists of GABAA and GABAB receptors, flumazenil and saclofen, respectively, prevented an increase in [3H]GABA uptake caused by 10 μM NE-100. L-[14C]glutamate uptake was decreased by 10-100 μM NE-100. A decrease in the uptake of both neurotransmitters mediated by NE-100 (50-100 μM) may have resulted from simultaneous antagonist-induced membrane depolarization, which was measured using the potential-sensitive fluorescent dye rhodamine 6G. The extracellular level of [3H]GABA was decreased by 1-10 μM NE-100, but that of L-[14C]glutamate remained unchanged. The tonic release of [3H]GABA measured in the presence of NO-711 was not changed by the antagonist, suggesting that NE-100 did not disrupt membrane integrity. The KCl- and FCCP-induced transporter-mediated release of L-[14C]glutamate was decreased by the antagonist; this may underlie the neuroprotective action of the antagonist in hypoxia/ischaemia. NE-100 (10-100 μM) decreased the KCl-evoked exocytotic release of [3H]GABA and L-[14C]glutamate, whereas the induction of the release of both neurotransmitters by the Ca2+ ionophore ionomycin was not affected by the antagonist; therefore, the mitigation of KCl-evoked exocytosis was associated with the NE-100-induced dysfunction of potential-dependent Ca2+ channels. Therefore, the Sig-1R antagonist can specifically act in an acute manner at the presynaptic level through the modulation of GABA and glutamate uptake, transporter-mediated release and exocytosis.

Manganese induced nervous injury by α-synuclein accumulation via ATP-sensitive K(+) channels and GABA receptors

Manganese (Mn) is an environmental pollutant having a toxic effect on Parkinson's disease, with significant damage seen in the neurons of basal ganglia. Hence, Mn pollution is a public health concern. A Sprague-Dawley rat model was used to determine the damage to basal nuclei, and the effect of Mn intake was detected using the Morris water maze test and transmission electron microscopy. The SH-SY5Y cell line was exposed to Mn, and downstream signaling was assessed to determine the mechanism of toxicity. Mn exposure injured neurons, repressing GABAAR receptors and inducing GABABR receptors. The synergistic effect of the GABABR receptor and Kir6.1-SUR1 or Kir6.2-SUR1 was found to be one of the potential factors for the secretion of α-synuclein. The accumulation of α-synuclein regulated downstream factors calmodulin (CAM) cAMP response element-binding protein (CREB), thereby impairing learning and memory. Other genes downstream of CREB, rather than the feedback regulation of CREB, and brain-derived neurotrophic factor might also be involved.

Activation of GABAB receptors results in excitatory modulation of calyx terminals in rat semicircular canal cristae

Previous studies have found GABA in vestibular end organs. However, existence of GABA receptors or possible GABAergic effects on vestibular nerve afferents has not been investigated. The current study was conducted to determine whether activation of GABAB receptors affects calyx afferent terminals in the central region of the cristae of semicircular canals. We used patch-clamp recording in postnatal day 13-18 (P13-P18) Sprague-Dawley rats of either sex. Application of GABAB receptor agonist baclofen inhibited voltage-sensitive potassium currents. This effect was blocked by selective GABAB receptor antagonist CGP 35348. Application of antagonists of small (SK)- and large-conductance potassium (BK) channels almost completely blocked the effects of baclofen. The remaining baclofen effect was blocked by cadmium chloride, suggesting that it could be due to inhibition of voltage-gated calcium channels. Furthermore, baclofen had no effect in the absence of calcium in the extracellular fluid. Inhibition of potassium currents by GABAB activation resulted in an excitatory effect on calyx terminal action potential firing. While in the control condition calyces could only fire a single action potential during step depolarizations, in the presence of baclofen they fired continuously during steps and a few even showed repetitive discharge. We also found a decrease in threshold for action potential generation and a decrease in first-spike latency during step depolarization. These results provide the first evidence for the presence of GABAB receptors on calyx terminals, showing that their activation results in an excitatory effect and that GABA inputs could be used to modulate calyx response properties.NEW & NOTEWORTHY Using in vitro whole cell patch-clamp recordings from calyx terminals in the vestibular end organs, we show that activation of GABAB receptors result in an excitatory effect, with decreased spike-frequency adaptation and shortened first-spike latencies. Our results suggest that these effects are mediated through inhibition of calcium-sensitive potassium channels.

Sex-specific alterations in GABA receptor-mediated responses in laterodorsal tegmentum are associated with prenatal exposure to nicotine

Smoking during pregnancy is associated with deleterious physiological and cognitive effects on the offspring, which are likely due to nicotine-induced alteration in the development of neurotransmitter systems. Prenatal nicotine exposure (PNE) in rodents is associated with changes in behaviors controlled in part by the pontine laterodorsal tegmentum (LDT), and LDT excitatory signaling is altered in a sex and age-dependent manner by PNE. As effects on GABAergic LDT signaling are unknown, we used calcium imaging to evaluate GABA_A receptor- (GABA_A R as well as GABA_A -_ρ R) and GABA_B receptor (GABA_B R)-mediated calcium responses in LDT brain slices from female and male PNE mice in two different age groups. Overall, in older PNE females, changes in calcium induced by stimulation of GABA_A R and GABA_B R, including GABA_A -_ρ R were shifted toward calcium rises. In both young and old males, PNE was associated with alterations in calcium mediated by all three receptors; however, the GABA_A R was the most affected. These results show for the first time that PNE is associated with alterations in GABAergic transmission in the LDT in a sex- and age-dependent manner, and these data are the first to show PNE-associated alterations in functionality of GABA receptors in any nucleus. PNE-associated alterations in LDT GABAergic transmission within the LDT would be expected to alter output to target regions and could play a role in LDT-implicated, negative behavioral outcomes following gestational exposure to smoking. Accordingly, our data provide further supportive evidence of the importance of eliminating the consumption of nicotine during pregnancy.

Taurine Promotes Retinal Ganglion Cell Survival Through GABAB Receptor Activation

Retinal ganglion cell (RGC) degeneration occurs in numerous retinal diseases, either as a primary process like in glaucoma, or secondary to photoreceptor loss and no efficient compound targeting directly RGC neuroprotection is yet available. We previously described that taurine exerts a direct protective effect on RGCs cultured under serum-deprived conditions. Because taurine was known to have an agonist-like activity for GABA/glycine receptors, we investigated here if the taurine-elicited neuroprotective effect may be mediated through the activation of these receptors using selective antagonist ligands. RGCs were purified, seeded in 96-well plate and maintained in culture during 6 days in vitro. Viable cells were labelled with calcein and densities in full-well area were then automatically counted. Here we show that the protective effect of taurine against RGC loss observed under serum deprivation can be mediated through the GABAB receptor stimulation. Hence, two selective agonists, including baclofen, at this metabotropic GABAB receptor were found to reproduce taurine action by enhancing RGC survival in culture. This study suggests that GABAB receptor stimulation provides direct neuroprotection for RGCs. Accordingly, drugs targeting GABAB receptor may represent a new way for the prevention of RGC degeneration.

Nicotine-induced molecular alterations are modulated by GABAB receptor activity

It has been demonstrated that GABAB receptors modulate nicotine (NIC) reward effect; nevertheless, the mechanism implicated is not well known. In this regard, we evaluated the involvement of GABAB receptors on the behavioral, neurochemical, biochemical and molecular alterations associated with the rewarding effects induced by NIC in mice, from a pharmacological and genetic approach. NIC-induced rewarding properties (0.5 mg/kg, subcutaneously, sc) were evaluated by conditioned place preference (CPP) paradigm. CPP has three phases: preconditioning, conditioning and postconditioning. GABAB receptor antagonist 2-hydroxysaclofen (0.25, 0.5 and 1 mg/kg; intraperitoneally, ip) or the GABAB receptor agonist baclofen (3 mg/kg; ip) was injected before NIC during the conditioning phase. GABAB1 knockout (GABAB1 KO) mice received NIC during the conditioning phase. Vehicle and wild-type controls were employed. Neurochemical (dopamine, serotonin and their metabolites), biochemical (nicotinic receptor α4β2, α4β2nAChRs) and molecular (c-Fos) alterations induced by NIC were analyzed after the postconditioning phase by high-performance liquid chromatography (HPLC), receptor-ligand binding assays and immunohistochemistry, respectively, in nucleus accumbens (Acb), prefrontal cortex (PFC) and ventral tegmental area (VTA). NIC induced rewarding effects in the CPP paradigm and increased dopamine levels in Acb and PFC, α4β2nAChRs density in VTA and c-Fos expression in Acb shell (AcbSh), VTA and PFC. We showed that behavioral, neurochemical, biochemical and molecular alterations induced by NIC were prevented by baclofen. However, in 2-hydroxysaclofen pretreated and GABAB1 KO mice, these alterations were potentiated, suggesting that GABAB receptor activity is necessary to control alterations induced by NIC-induced rewarding effects. Therefore, the present findings provided important contributions to the mechanisms implicated in NIC-induced rewarding effects.

GABAB receptor-dependent bidirectional regulation of critical period ocular dominance plasticity in cats

Gama amino butyric acid (GABA) inhibition plays an important role in the onset and offset of the critical period for ocular dominance (OD) plasticity in the primary visual cortex. Previous studies have focused on the involvement of GABA_A receptors, while the potential contribution of GABA_B receptors to OD plasticity has been neglected. In this study, the GABA_B receptor antagonist SCH50911 or agonist baclofen was infused into the primary visual cortex of cats concurrently with a period of monocular deprivation (MD). Using single-unit recordings we found that the OD shift induced by four days of MD during the critical period was impaired by infusion of the antagonist SCH50911, but enhanced by infusion of the agonist baclofen. In contrast, seven days of MD in adult cats did not induce any significant OD shift, even when combined with the infusion of SCH50911 or baclofen. Together, these findings indicate that an endogenous GABA_B receptor-mediated inhibition contributes to juvenile, but not adult, OD plasticity.

The drug candidate, ADX71441, is a novel, potent and selective positive allosteric modulator of the GABAB receptor with a potential for treatment of anxiety, pain and spasticity

Positive allosteric modulation of the GABA_B receptor is a promising alternative to direct activation of the receptor as a therapeutic approach for treatment of addiction, chronic pain, anxiety, epilepsy, autism, Fragile X syndrome, and psychosis. Here we describe in vitro and in vivo characterization of a novel, potent and selective GABA_B positive allosteric modulator (PAM) N-(5-(4-(4-chloro-3-fluorobenzyl)-6-methoxy-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)-2-fluorophenyl)acetamide (ADX71441). In vitro, Schild plot and reversibility tests at the target confirmed PAM properties of the compound. In mice and rats ADX71441 is bioavailable after oral administration and is brain penetrant. A single dose of ADX71441 had an anxiolytic-like profile in the mouse marble burying test (minimum effective dose; MED 3 mg/kg) as well as in the elevated plus maze test in mice and rats (both MED 3 mg/kg). Also, in mice, acute administration of ADX71441 reduced visceral pain-associated behaviors in the acetic acid-induced writhing test. ADX71441 dose-dependently reduced time on rotarod in rats (MED 10 mg/kg) indicative of muscle-relaxant qualities. ADX71441 reduced locomotor activity in mice (10 mg/kg) and rats (3 mg/kg) after single dose; however, following sub-chronic administration in mice, 30 mg/kg ADX71441 was associated with normal locomotor activity. While acute administration of ADX71441 reduced body temperature in rats and mice (both MED 10 mg/kg), the effect in the former was transient, rapidly returning to normal levels despite high concentrations of the compound remaining in plasma. Thus, the GABA_B PAM ADX71441 represents a valid therapeutic approach for development of novel treatment of anxiety, pain and spasticity.

[Tectal Evoked Potentials during Retinal Baclofen Application in the Carp]

Changes of primary visual center evoked potentials in response to white light and optic nerve electric stimulation were investigated during retinal GABAb-receptors activation with baclofen in dark-adapted carp. It was found, that baclofen - induced b-wave ERG decreasing, was accompanied by a significant amplitude growing as in the evoked potential to light as in the evoked potential to electric nerve stimulation: It is proposed, that light evoked potential changes reflect the increasing of the third retinal neuron responses to light and/or tectal neuron responsiveness enhancement.

Anxiety-like behaviors at the end of the nocturnal period in sP rats with a "history" of unpredictable, limited access to alcohol

Recent research found that exposure of selectively bred, Sardinian alcohol-preferring (sP) rats to multiple alcohol concentrations (10%, 20%, and 30%, v/v), under the 4-bottle "alcohol vs. water" choice regimen, in daily 1-h drinking sessions with an unpredictable time schedule, promoted high intakes of alcohol (≥2 g/kg) when the drinking session occurred over the final hours of the dark phase of the light/dark cycle. The present study investigated whether these high intakes of alcohol (a) were associated with alterations in rats' emotional state (Experiment 1) and (b) were pharmacologically manipulable (Experiment 2). In both experiments, over a period of 12 days, sP rats were initially exposed daily to a 1-h drinking session during the dark phase; time of alcohol exposure was changed each day and was unpredictable to rats. The day after this 12-day drinking phase, rats were (a) exposed to the Social Interaction (SI) test at the 1st or 12th hour of the dark phase with no alcohol available (Experiment 1) or (b) treated with the positive allosteric modulator of the GABAB receptor, GS39783 (0, 25, 50, and 100 mg/kg, intragastrically [i.g.]), and exposed to a drinking session at the 12th hour of the dark phase (Experiment 2). In Experiment 1, rats exposed to the SI test during the 12th hour spent approximately 35% less time in "social" behaviors than rats exposed to the SI test during the 1st hour. No difference in "social" behaviors was observed between alcohol-naive sP rats exposed to the SI test at the 1st and 12th hour. In Experiment 2, all doses of GS39783 selectively reduced alcohol intake. These results suggest that (a) expectation of alcohol availability likely exacerbated the anxiety-like state of sP rats and (b) the GABAB receptor is part of the neural substrate underlying these exceptionally high intakes of alcohol in sP rats.

Metabolomic Approaches to Defining the Role(s) of GABAρ Receptors in the Brain

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) acts through various types of receptors in the central nervous system. GABAρ receptors, defined by their characteristic pharmacology and presence of ρ subunits in the channel structure, are poorly understood and their role in the cortex is ill-defined. Here, we used a targeted pharmacological, NMR-based functional metabolomic approach in Guinea pig brain cortical tissue slices to identify a distinct role for these receptors. We compared metabolic fingerprints generated by a range of ligands active at GABAρ and included these in a principal components analysis with a library of other metabolic fingerprints obtained using ligands active at GABAA and GABAB, with inhibitors of GABA uptake and with compounds acting to inhibit enzymes active in the GABAergic system. This enabled us to generate a metabolic "footprint" of the GABAergic system which revealed classes of metabolic activity associated with GABAρ which are distinct from other GABA receptors. Antagonised GABAρ produce large metabolic effects at extrasynaptic sites suggesting they may be involved in tonic inhibition.

GABAB receptors are expressed in human aortic smooth muscle cells and regulate the intracellular Ca(2+) concentration

The aim of this study was to investigate the expression of GABAB receptors, a subclass of receptors to the inhibitory neurotransmitter gamma-aminobutyric acid (GABAB), in human aortic smooth muscle cells (HASMCs), and to explore if altering receptor activation modified intracellular Ca(2+) concentration ([Ca(2+)]i) of HASMCs. Real-time PCR, western blots and immunofluorescence were used to determine the expression of GABABR1 and GABABR2 in cultured HASMCs. Immunohistochemistry was used to localize the two subunits in human left anterior descending artery (LAD). The effects of the GABAB receptor agonist baclofen on [Ca(2+)]i in cultured HASMCs were demonstrated using fluo-3. Both GABABR1 and GABABR2 mRNA and protein were identified in cultured HASMCs and antibody staining was also localized to smooth muscle cells of human LAD. 100 μM baclofen caused a transient increase of [Ca(2+)]i in cultured HASMCs regardless of whether Ca(2+) was added to the medium, and the effects were inhibited by pre-treatment with CGP46381 (selective GABAB receptor antagonist), pertussis toxin (a Gi/o protein inhibitor), and U73122 (a phospholipase C blocker). GABAB receptors are expressed in HASMCs and regulate the [Ca(2+)]i via a Gi/o-coupled receptor pathway and a phospholipase C activation pathway.

Modulation of the arcuate nucleus-medial preoptic nucleus lordosis regulating circuit: a role for GABAB receptors

Estradiol rapidly activates a microcircuit in the arcuate nucleus of the hypothalamus (ARH) that is needed for maximal female sexual receptivity. Membrane estrogen receptor-α complexes with and signals through the metabotropic glutamate receptor-1a stimulating NPY release within the ARH activating proopiomelanocortin (POMC) neurons. These POMC neurons project to the medial preoptic nucleus (MPN) and release β-endorphin. Estradiol treatment induces activation/internalization of MPN μ-opioid receptors (MOR) to inhibit lordosis. Estradiol membrane action modulates ARH gamma-aminobutyric acid receptor-B (GABAB) activity. We tested the hypothesis that ARH GABAB receptors mediate estradiol-induced MOR activation and facilitation of sexual receptivity. Double-label immunohistochemistry revealed expression of GABAB receptors in NPY, ERα and POMC expressing ARH neurons. Approximately 70% of POMC neurons expressed GABAB receptors. Because estradiol initially activates an inhibitory circuit and maintains activation of this circuit, the effects of blocking GABAB receptors were evaluated before estradiol benzoate (EB) treatment and after at the time of lordosis testing. Bilateral infusions of the GABAB receptor antagonist, CGP52432, into the ARH prior to EB treatment of ovariectomized rats prevented estradiol-induced activation/internalization of MPN MOR, and the rats remained unreceptive. However, in EB-treated rats, bilateral CGP52432 infusions 30 min before behavior testing attenuated MOR internalization and facilitated lordosis. These results indicated that GABAB receptors were located within the lordosis-regulating ARH microcircuit and are necessary for activation and maintenance of the estradiol inhibition of lordosis behavior. Although GABAB receptors positively influence estradiol signaling, they negatively regulate lordosis behavior since GABAB activity maintains the estradiol-induced inhibition.

Differentiating enantioselective actions of GABOB: a possible role for threonine 244 in the binding site of GABA(C) ρ(1) receptors

Designing potent and subtype-selective ligands with therapeutic value requires knowledge about how endogenous ligands interact with their binding site. 4-Amino-3-hydroxybutanoic acid (GABOB) is an endogenous ligand found in the central nervous system in mammals. It is a metabolic product of GABA, the major inhibitory neurotransmitter. Homology modeling of the GABA(C) ρ(1) receptor revealed a potential H-bond interaction between the hydroxyl group of GABOB and threonine 244 (T244) located on loop C of the ligand binding site of the ρ(1) subunit. Using site-directed mutagenesis, we examined the effect of mutating T244 on the efficacy and pharmacology of GABOB and various ligands. It was found that mutating T244 to amino acids that lacked a hydroxyl group in their side chains produced GABA insensitive receptors. Only by mutating ρ(1)T244 to serine (ρ(1)T244S) produced a GABA responsive receptor, albeit 39-fold less sensitive to GABA than ρ(1)wild-type. We also observed changes in the activities of the GABA(C) receptor partial agonists, muscimol and imidazole-4-acetic acid (I4AA). At the concentrations we tested, the partial agonists antagonized GABA-induced currents at ρ(1)T244S mutant receptors (Muscimol: ρ(1)wild-type, EC(50) = 1.4 μM; ρ(1)T244S, IC(50) = 32.8 μM. I4AA: ρ(1)wild-type, EC(50) = 8.6 μM; ρ(1)T244S, IC(50) = 21.4 μM). This indicates that T244 is predominantly involved in channel gating. R-(-)-GABOB and S-(+)-GABOB are full agonists at ρ(1)wild-type receptors. In contrast, R-(-)-GABOB was a weak partial agonist at ρ(1)T244S (1 mM activates 26% of the current produced by GABA EC(50) versus ρ(1)wild-type, EC(50) = 19 μM; I(max) 100%), and S-(+)-GABOB was a competitive antagonist at ρ(1)T244S receptors (ρ(1)wild-type, EC(50) = 45 μM versus ρ(1)T244S, IC(50) = 417.4 μM, K(B) = 204 μM). This highlights that the interaction of GABOB with T244 is enantioselective. In contrast, the potencies of a range of antagonists tested, 3-aminopropyl(methyl)phosphinic acid (3-APMPA), 3-aminopropylphosphonic acid (3-APA), S- and R-(3-amino-2-hydroxypropyl)methylphosphinic acid (S-(-)-CGP44532 and R-(+)-CGP44533), were not altered. This suggests that T244 is not critical for antagonist binding. Receptor gating is dynamic, and this study highlights the role of loop C in agonist-evoked receptor activation, coupling agonist binding to channel gating.

Positive allosteric modulation of the GABA(B) receptor by GS39783 attenuates the locomotor stimulant actions of ethanol and potentiates the induction of locomotor sensitization

Acute ethanol-induced locomotor stimulation and ethanol-induced locomotor sensitization are two behavioral assays thought to model the rewarding effects of ethanol. Recent evidence suggests that GS39783, a GABA(B) positive allosteric modulator, may be effective at reducing both the rewarding and reinforcing effects of several drugs of abuse, including ethanol. The goal of this study was to determine if GS39783 was capable of altering acute ethanol-induced stimulation, and the induction and expression of ethanol-induced locomotor sensitization, without effecting basal locomotion levels. Several doses of GS39783 (ranging from 0 to 100 mg/kg, depending on experiment) were tested on adult male DBA/2J mice in four experiments using 3-day basal locomotion and acute ethanol stimulation paradigms, and 14-day induction and expression of ethanol sensitization paradigms. The results of experiment 1 are in agreement with current literature, suggesting that 30 mg/kg doses of GS39783 and lower do not alter basal locomotor activity. In experiment 2, we found that GS39783 significantly decreased acute ethanol stimulation, but only at the 30 mg/kg dose, supporting our hypothesis and other publications suggesting that GABA(B) receptors modulate acute ethanol stimulation. Contrary to our hypothesis, GS39783 did not alter the expression of locomotor sensitization. Additionally, repeated administration of GS39783 in conjunction with ethanol unexpectedly potentiated ethanol-induced locomotor sensitization. Further study of GS39783 is warranted as it may be a more tolerable treatment for alcoholism than full agonists, due to its behavioral efficacy at doses that lack sedative side effects. Our results add to current literature suggesting that the GABA(B) receptor system is indeed involved in the modulation of ethanol-induced locomotor stimulation and sensitization.

GABAergic synaptic transmission triggers action potentials in thalamic reticular nucleus neurons

GABAergic neurons in the thalamic reticular nucleus (TRN) form powerful inhibitory connections with several dorsal thalamic nuclei, thereby controlling attention, sensory processing, and synchronous oscillations in the thalamocortical system. TRN neurons are interconnected by a network of GABAergic synapses, but their properties and their role in shaping TRN neuronal activity are not well understood. Using recording techniques aimed to minimize changes in the intracellular milieu, we show that synaptic GABA(A) receptor activation triggers postsynaptic depolarizations in mouse TRN neurons. Immunohistochemical data indicate that TRN neurons express very low levels of the Cl(-) transporter KCC2. In agreement, perforated-patch recordings show that intracellular Cl(-) levels are high in TRN neurons, resulting in a Cl(-) reversal potential (E(Cl)) significantly depolarized from rest. Additionally, we find that GABA(A) receptor-evoked depolarizations are amplified by the activation of postsynaptic T-type Ca(2+) channels, leading to dendritic Ca(2+) increases and the generation of burst firing in TRN neurons. In turn, GABA-evoked burst firing results in delayed and long-lasting feedforward inhibition in thalamic relay cells. Our results show that GABA-evoked depolarizations can interact with T-type Ca(2+) channels to powerfully control spike generation in TRN neurons.

Dopamine and serotonin modulate human GABAρ1 receptors expressed in Xenopus laevis oocytes

GABAρ1 receptors are highly expressed in bipolar neurons of the retina and to a lesser extent in several areas of the central nervous system (CNS), and dopamine and serotonin are also involved in the modulation of retinal neural transmission. Whether these biogenic amines have a direct effect on ionotropic GABA receptors was not known. Here, we report that GABAρ1 receptors, expressed in X. laevis oocytes, were negatively modulated by dopamine and serotonin and less so by octopamine and tyramine. Interestingly, these molecules did not have effects on GABA(A) receptors. 5-Carboxamido-tryptamine and apomorphine did not exert evident effects on any of the receptors. Schild plot analyses of the inhibitory actions of dopamine and serotonin on currents elicited by GABA showed slopes of 2.7 ± 0.3 and 6.1 ± 1.8, respectively, indicating a noncompetitive mechanism of inhibition. The inhibition of GABAρ1 currents was independent of the membrane potential and was insensitive to picrotoxin, a GABA receptor channel blocker and to the GABAρ-specific antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl phosphinic acid (TPMPA). Dopamine and serotonin changed the sensitivity of GABAρ1 receptors to the inhibitory actions of Zn(2+). In contrast, La(3+) potentiated the amplitude of the GABA currents generated during negative modulation by dopamine (EC(50) 146 μM) and serotonin (EC(50) 196 μM). The functional role of the direct modulation of GABAρ receptors by dopamine and serotonin remains to be elucidated; however, it may represent an important modulatory pathway in the retina, where GABAρ receptors are highly expressed and where these biogenic amines are abundant.

Inhibitory effect of asiatic acid on acetylcholinesterase, excitatory post synaptic potential and locomotor activity

The asiatic acid, a triterpenoids isolated from Centella asiatica was used to delineate its inhibitory effect on acetylcholinesterase (AChE) properties, excitatory post synaptic potential (EPSP) and locomotor activity. This study is consistent with asiatic acid having an effect on AChE, a selective GABA(B) receptor agonist and no sedative effect on locomotor.

GABA(B) receptor-mediated ERK1/2 phosphorylation via a direct interaction with Ca(V)1.3 channels

Neuronal L-type Ca(2+) channels play pivotal roles in regulating gene expression, cell survival, and synaptic plasticity. The Ca(V)1.2 and Ca(V)1.3 channels are 2 main subtypes of neuronal L-type Ca(2+) channels. However, the specific roles of Ca(V)1.2 and Ca(V)1.3 in L-type Ca(2+) channel-mediated neuronal responses and their cellular mechanisms are poorly elucidated. On the basis of our previous study demonstrating a physical interaction between the Ca(V)1.3 channel and GABA(B) receptor (GABA(B)R), we further examined the involvement of Ca(V)1.2 and Ca(V)1.3 in the GABA(B)R-mediated activation of ERK(1/2), a kinase involved in both CREB activation and synaptic plasticity. After confirming the involvement of L-type Ca(2+) channels in baclofen-induced ERK(1/2) phosphorylation, we examined a specific role of Ca(V)1.2 and Ca(V)1.3 channels in the baclofen effect. Using siRNA-mediated silencing of Ca(V)1.2 or Ca(V)1.3 messenger, we determined the relevance of each channel subtype to baclofen-induced ERK(1/2) phosphorylation in a mouse hippocampal cell line (HT-22) and primary cultured rat neurons. In the detailed characterization of each subtype using HEK293 cells transfected with Ca(V)1.2 or Ca(V)1.3, we found that GABA(B)R can increase ERK(1/2) phosphorylation and Ca(V)1.3 channel activity through direct interaction with Ca(V)1.3 channels. These results suggest a functional interaction between Ca(V)1.3 and GABA(B)R and important implications of Ca(V)1.3/GABA(B)R clusters for translating synaptic activity into gene expression alterations.

[Treatment of insomnia. Pharmacological approaches and their limitations]

Far more people complain of inadequate sleep than of true insomnia warranting prescription of a hypnotic drug. The number of available hypnotics has fallen markedly in recent years. Numerous brain areas, transmitters and receptors are involved in sleep. Currently, the main hypnotics (benzodiazepine derivatives and the so-called 4Z group. Zolpidem, Zopiclone, EsZopiclone and Zaleplon) increase GABAergic transmission by acting on components of chloride channels, thereby inducing Cl entry to neurons and resulting in their hyperpolarisation. This pre-eminence of GABAergic transmission should not make us ignore other important transmitters and their receptors as potential targets for new hypnotic drugs; these include histamine (and H1 receptors), dopamine (D1 and D2), norepinephrine (alpha1), serotonin (5HT2), glutamate (NMDA), acetylcholine (nicotinic), hypocretin (OX1 and OX2), melatonin (MLT1 and MLT2), prostaglandin E2 (EP), prostaglandin D2 (DP1), and endocannabinoids (CB1). Knowledge of the pharmacodynamic, pharmacokinetic and clinical characteristics of current hypnotic drugs has allowed us to establish the profile of an ideal hypnotic for the future.

The GABA-B positive modulator GS39783 decreases psychostimulant conditioned-reinforcement and conditioned-reward

Baclofen, a γ-amino-butyric-acid (GABA)(B) receptor agonist, can reduce cue-enhanced cocaine-seeking in rats and attenuate cue-evoked craving in cocaine addicts. However, baclofen also has sedative effects that might interfere with its efficacy in reducing cocaine's rewarding effects. The present study aimed at comparing the effects of baclofen with the GABA(B) -receptor positive allosteric modulator GS39783 on psychostimulant conditioned cues. Two identically trained groups of male Lister-Hooded rats were baselined on a new responding for a light stimulus previously paired with cocaine self-administration. One group was treated with the GABA(B) -receptor positive allosteric modulator GS39783 (0, 10, 30, 100 mg/kg, i.p.), the other with baclofen (0, 0.6, 1.25, 1.9, 2.5 mg/kg, i.p.). In another series of experiments, male Wistar rats received GS39783 (0, 10, 30, 100 mg/kg, i.p.) or baclofen (1.25 mg/kg) prior to the expression of a conditioned place preference (CPP) to amphetamine (2 mg/kg i.p.). Both GS39783 (30 and 100 mg/kg) and baclofen (2.5 mg/kg) significantly decreased responding for the cocaine cue; however, only GS39783 (30 mg/kg) reduced lever pressing responding without interfering with locomotor activity. Both GS39783 (30 and 100 mg/kg) and baclofen (1.25 mg/kg), significantly blocked the expression of amphetamine CPP without affecting locomotor activity. These findings suggest that GABA(B) positive allosteric modulators can modulate discrete and contextual psychostimulant conditioned stimuli in a manner dissociable from unwanted sedative effects and may offer a novel therapeutic approach to treat cravings and relapse to drug-taking triggered by stimuli associated with psychostimulant use.

The GABA&#946; receptor as a target for antidepressant drug action

Preclinical and clinical data suggest that a modification in GABA(B) receptor expression and function may contribute to the symptoms of major depression and the response to antidepressants. This includes laboratory animal experiments demonstrating that antidepressants modify brain GABA(B) receptor expression and function and that GABA(B) receptor antagonists display antidepressant potential in animal models of this condition. Clinical and post-mortem studies reveal changes in GABAergic transmission associated with depression as well as depression-related changes in GABA(B) subunit expression that are localized to the cortical depression network. Detailed in this review are the preclinical and clinical data implicating a role for the GABA(B) receptor system in mediating symptoms of this disorder and its possible involvement in the response to antidepressants. Particular emphasis is placed on clinical and post-mortem studies, including previously unpublished work demonstrating regionally-selective modifications in GABA(B) receptor subunit expression in brain samples obtained from depressed subjects. Together with the earlier preclinical studies, these new data point to a role for the GABA(B) system in major depression and support the antidepressant potential of GABA(B) receptor antagonists.

GABA(A) receptors in the dorsal raphé nucleus of mice: escalation of aggression after alcohol consumption

RATIONALE

The dorsal raphé nucleus (DRN), the origin for serotonin (5-HT) in forebrain areas, has been implicated in the neural control of escalated aggression. Gamma aminobutyric acid type-A (GABA(A)) and type-B (GABA(B)) receptors are expressed in the DRN and modulate 5-HT neuronal activity, and both play a role in the behavioral effect of alcohol.

OBJECTIVE

The purpose of this study is to examine the interaction between drugs acting on GABA receptors in the DRN and alcohol in their effects on aggressive behaviors.

METHOD

Male CFW mice, housed with a female, were trained to self-administer ethanol (1.0 g/kg) or water via an operant conditioning panel in their home cage. Immediately after they drank either ethanol or water, the animals were microinfused with a GABAergic drug into the DRN, and their aggressive behaviors were assessed 10 min later. Muscimol (0.006 nmol), a GABA(A) receptor agonist, escalated alcohol-heightened aggression but had no effect in the absence of ethanol. This effect of muscimol was prominent in the animals that showed alcohol-heightened aggression, but not the animals that reduced or did not change aggressive behavior after ethanol infusion compared to water. On the other hand, the GABA(B) agonist baclofen (0.06 nmol) increased aggressive behavior similarly in both water and ethanol conditions. Antagonists of the GABA(A) and GABA(B) receptors, bicuculline (0.006 nmol) and phaclofen (0.3 nmol) respectively, did not suppress heightened-aggressive behavior induced by ethanol self-administration.

CONCLUSION

GABA(A) receptors in the DRN are one of the neurobiological targets of alcohol-heightened aggression. Activation of the GABA(B) receptors in the DRN also produced escalated aggression, but that is independent of the effect of alcohol.

Type B GABA receptors contribute to the restoration of balance during vestibular compensation in mice

Following unilateral vestibular damage (UVD), vestibular compensation restores both static and dynamic vestibular reflexes. The cerebellar cortex provides powerful GABAergic inhibitory input to the vestibular nuclei which is necessary for compensation. Metabotropic GABA type B (GABA(B)) receptors in the vestibular nuclei are thought to be involved. However, the contribution of GABA(B) receptors may differ between static and dynamic compensation. We tested static and dynamic postural reflexes and gait in young mice, while they compensated for UVD caused by injection of air into the vestibular labyrinth. The effects of an agonist (baclofen), an antagonist (CGP56433A) and a positive allosteric modulator (CGP7930) of the GABA(B) receptor were evaluated during compensation. Static postural reflexes recovered very rapidly in our model, and baclofen slightly accelerated recovery. However, CGP56433A significantly impaired static compensation. Dynamic reflexes were evaluated by balance-beam performance and by gait; both showed significant decrements following UVD and performance improved over the next 2 days. Both CGP56433A and baclofen temporarily impaired the ability to walk on a balance beam after UVD. Two days later, there were no longer any significant effects of drug treatments on balance-beam performance. Baclofen slightly accelerated the recovery of stride length on a flat surface, but CGP7930 worsened the gait impairment following UVD. Using immunohistochemistry, we confirmed that GABA(B) receptors are abundantly expressed on the vestibulospinal neurons of Deiters in mice. Our results suggest that GABA(B) receptors contribute to the compensation of static vestibular reflexes following unilateral peripheral damage. We also conclude that impairment of the first stage of compensation, static recovery, does not necessarily result in an impairment of dynamic recovery in the long term.

Repeated ethanol administration modifies the temporal structure of sucrose intake patterns in mice: effects associated with behavioral sensitization

Neuroadaptations supporting behavioral sensitization to abused drugs are suggested to underlie pathological, excessive motivation toward drugs and drug-associated stimuli. Drug-induced sensitization has also been linked to increased appetitive responses for non-drug, natural reinforcers. The present research investigated whether ethanol (EtOH)-induced neural changes, inferred from psychomotor sensitization, can modify consumption and intake dynamics for the natural reinforcer, sucrose. The effects of EtOH-induced sensitization in mice on the temporal structure of sucrose intake patterns were measured using a lickometer system. After sensitization, sucrose intake dynamics were measured for 1 hour daily for 7 days and indicated more rapid initial approach and consumption of sucrose in EtOH-sensitized groups; animals showed a shorter latency to the first intake bout and an increased number of sucrose bottle licks during the initial 15 minutes of the 1-hour sessions. This effect was associated with increased frequency and size of bouts. For the total 1-hour session, sucrose intake and bout dynamics were not different between groups, indicating a change in patterns of sucrose intake but not total consumption. When sensitization was prevented by the gamma-aminobutyric acid B receptor agonist, baclofen, the increased rate of approach and consumption of sucrose were also prevented. Thus, EtOH-induced sensitization, and not the mere exposure to EtOH, was associated with changes in sucrose intake patterns. These data are consistent with current literature suggesting an enhancing effect of drug-induced sensitization on motivational processes involved in reinforcement.

Therapeutics for alcoholism: what's the future?

As with other addictions, human alcoholism is characterised as a chronically relapsing condition. Consequently, the therapeutic goal is the development of clinically effective, safe drugs that promote high adherence rates and prevent relapse. These products can then be used in conjunction with psychosocial approaches. In this review, preclinical studies are highlighted that indicate the mechanism of action of currently used anti-craving medications or demonstrate the potential of novel pharmacological agents for the treatment of alcohol use disorders. While current pharmacological strategies are far from ideal, there are a number of candidate molecules that may ultimately be developed into therapeutic agents. In addition, prescribing clinicians should also consider strategies such as combinations of various drugs to aid in the regulation of aberrant alcohol consumption.

Receptor activation involving positive allosteric modulation, unlike full agonism, does not result in GABAB receptor desensitization

Allosteric modulators act more physiologically than orthosteric ligands, targeting only endogenously activated receptors and not their whole population, which is why they are expected to produce less side effects and tolerance. To inspect the role of the positive allosteric modulator GS39783 in GABAB receptor desensitization, we examined receptor function and cell surface expression in a recombinant GABAB cell line and in primary neuronal cultures upon persistent treatments with GABAB agonists, and combinations of agonists and GS39783. The potency of GABA to inhibit 7beta-forskolin-induced cAMP formation in recombinant cells decreased after the exposure to a saturating GABA concentration, but not after a combination of a low GABA concentration and GS39783, that activated the receptor to the same extent. Concordantly, a significant decrease of cell surface receptors was found after GABA-induced desensitization, unlike after the combined treatment with GABA and GS39783. Similar observations regarding receptor function were found in primary neurons for baclofen-induced inhibition of spontaneous Ca2+ oscillations. However, the cell surface receptor density remained unaffected upon baclofen-induced desensitization in the primary neurons, possibly due to different mechanisms of desensitization in the neurons and the recombinant cell line. These findings indicate that the degree of occupancy of the orthosteric site determines desensitization rather than the degree of receptor activation. In summary, our results conform to predictions that positive allosteric modulators have less propensity for the development of tolerance due to receptor desensitization than classical agonists.

Fentanyl treatment reduces GABAergic inhibition in the CA1 area of the hippocampus 24 h after acute exposure to the drug

The effect of in vivo fentanyl treatment on synaptic transmission was studied in the CA1 area of the rat hippocampus. Animals were treated either with saline or fentanyl (4 x 80 microg/kg, s.c./15 min). Intracellular in vitro recordings were obtained, 24 h after treatment, from CA1 pyramidal neurons. No difference in pyramidal neuron basic membrane properties or postsynaptic membrane excitability was observed between neurons from saline- and fentanyl-treated animals. The peak amplitude of fast (f-) and slow (s-) components of IPSPs elicited in standard ACSF and the peak amplitude and rate of rise of isolated f- and s-IPSPs elicited in the presence of antagonists (CNQX, 10 microM; AP-5, 10 microM; CGP 55845, 1 microM; and bicuculline methochloride, 10 microM), in response to various stimulus intensities, was smaller in fentanyl-treated animals. Conversely, the rising slope of excitatory responses was similar in neurons from saline- and fentanyl-treated animals. Furthermore, in fentanyl-treated animals, lower stimulus strengths were required to elicit subthreshold excitatory responses of the same amplitude suggesting that acute exposure to fentanyl increases susceptibility of pyramidal neurons to presynaptic stimulation. GABA immunohistochemistry revealed lower GABA content in processes and neuronal somata suggesting diminished GABA release onto pyramidal neurons. We conclude that acute in vivo exposure to fentanyl is sufficient to induce long-lasting reduction in GABA-mediated transmission, rather, than enhanced excitatory transmission or modulation of the intrinsic excitability of pyramidal neurons. These findings provide evidence regarding the mechanisms involved in the early stages of tolerance development towards the analgesic effects of opioids.

Presynaptic metabotropic glutamate and GABAB receptors

Glutamate and GABA, the two most abundant neurotransmitters in the mammalian central nervous system, can act on metabotropic receptors that are structurally quite dissimilar from those targeted by most other neurotransmitters/modulators. Accordingly, metabotropic glutamate receptors (mGluRs) and GABA(B) receptors (GABA(B)Rs) are classified as members of family 3 (or family C) of G protein-coupled receptors. On the other hand, mGluRs and GABA(B)Rs exhibit pronounced and partly unresolved differences between each other. The most intriguing difference is that mGluRs exist as multiple pharmacologically as well as structurally distinct subtypes, whereas, in the case of GABA(B)Rs, molecular biologists have so far identified only one structurally distinct heterodimeric complex whose few variants seem unable to explain the pharmacological heterogeneity of GABA(B)Rs observed in many functional studies. Both mGluRs and GABA(B)Rs can be localized on axon terminals of different neuronal systems as presynaptic autoreceptors and heteroreceptors modulating the exocytosis of various transmitters.

Functional asymmetry of the conserved cystine loops in alphabetagamma GABA A receptors revealed by the response to GABA activation and drug potentiation

Ligand-gated ion channels respond to specific neurotransmitters by transiently opening an integral membrane ion-selective pore, allowing ions to move down their electrochemical gradient. A distinguishing feature of all members of the ligand-gated ion channel superfamily is the presence of a 13-amino acid disulfide loop (Cys-loop) in the extracellular ligand-binding domain. Structural data derived from the acetylcholine receptor place this loop at the interface between the ligand-binding domain and the transmembrane pore-forming domain where it is ideally located to participate in coupling ligand binding to channel opening. We have introduced specific mutations into a conserved motif at the mid-point of the Cys-loop of the GABA A receptor subunits alpha1, beta2 and gamma2S where the sequence reads aromatic, proline, aliphatic (ArProAl motif). Receptors carrying a mutation in the Cys-loop of one of their subunits were expressed in L929 cells and responses to both GABA and drugs were assessed using the whole-cell patch clamp technique. Drug potentiation and direct activation were significantly enhanced by mutations in this Cys-loop but these effects were subunit-dependent. Currents in response to agonists were larger when mutations were carried in the alpha and beta subunits but not in the gamma subunit. In contrast, potentiation of current responses by diazepam, etomidate and pentobarbital were all enhanced when mutations were carried in the alpha and gamma subunits, but not the beta subunit. Since the disruption of interactions mediated through the ArProAl motif enhances the mutant receptor's response to both agonist and drugs we suggest that this motif in the Cys-loop of the wild-type receptor participates in interactions that create activation barriers to conformational changes during channel gating.

Effects of GABAB receptor antagonist, agonists and allosteric positive modulator on the cocaine-induced self-administration and drug discrimination

Preclinical and clinical findings indicate that a GABA(B) receptor agonist baclofen decreases cocaine use. The present study investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phoshinic acid (SKF 97541) and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) in cocaine-and food-maintained responding under a fixed ratio 5 schedule of reinforcement in male Wistar rats. The effects of the GABA(B) receptor ligands on cocaine (10 mg/kg)-induced discriminative stimulus in a two-lever, water-reinforced fixed ratio 20 task and on basal locomotor activity were also assessed. Baclofen (2.5-5 mg/kg), SKF 97541 (0.1-0.3 mg/kg) and CGP 7930 (30-100 mg/kg) decreased the cocaine (0.5 mg/kg/injection)-maintained responding; SCH 50911 (3-10 mg/kg) was inactive in this respect. Baclofen (5 mg/kg) and SKF 97541 (0.3 mg/kg), but not CGP 7930 or SCH 50911 attenuated the food-maintained responding. The inhibitory effects of the GABA(B) receptor agonists and the modulator were blocked by SCH 50911. SKF 97541 (0.1 mg/kg) or CGP 9730 (30-100 mg/kg) did not produce a significant shift in the cocaine (1.25-10 mg/kg) dose-response curve in a drug discrimination procedure, while baclofen (1.5 mg/kg) or SCH 50911 (10 mg/kg) attenuated the effects of separate doses of cocaine. Baclofen (5 mg/kg) and CGP 7930 (100 mg/kg) significantly reduced basal horizontal activity. We found that pharmacological stimulation of GABA(B) receptors by direct agonists or allosteric positive modulation reduces cocaine reinforcement while this property of cocaine is not related to tonic activation of GABA(B) receptors. The GABA(B) receptor stimulation-induced reduction of cocaine reinforcement was separated from its discriminative stimulus effects. Moreover, a dissociation between effects of direct GABA(B) receptor agonists and a GABA(B) allosteric positive modulator on cocaine vs. food-maintained responding was demonstrated.

GABAergic drugs for the treatment of migraine

Within the last decades significant progress has been made in the understanding of the underlying pathophysiological mechanisms of migraine. There is a general agreement now that migraine is not only a vascular phenomenon but also a genetically determined heterogenic ion-channelopathy resulting in cortical-spreading-depression-like events, the temporary impairment of antinociceptive structures of the brainstem and the activation of the trigeminal-vascular system. The development and use of drugs targeting ion-channels and subsequently reducing cortical excitability appears as a promising avenue for both the acute treatment of migraine and migraine prevention. This review summarizes the current knowledge and evidence for GABAergic drugs in the treatment of migraine.

Effect of GABAergic substances on firing rate and thermal coefficient of hypothalamic neurons in the juvenile chicken

The goal of the study is to investigate the GABAergic action on firing rate (FR) and temperature coefficient (TC) on hypothalamic neurons in the juvenile chicken. Extracellular recordings were obtained from 37 warm-sensitive, 32 cold-sensitive and 56 temperature-insensitive neurons in brain slices to determine the effect of GABA(A)-receptor agonist muscimol, GABA(A)-receptor antagonist bicuculline, GABA(B)-receptor agonist baclofen and GABA(B)-receptor antagonist CGP 35348. Muscimol and baclofen in equimolar concentrations (1 microM) significantly inhibited FR of the neurons, regardless of their type of thermosensitivity. In contrast, bicuculline, as well as CGP 35348 (10 microM) increased FR of the majority of the neurons. The TC of most chick hypothalamic neurons could not be estimated during muscimol application because FR was completely inhibited. GABA(B)-receptor agonist specifically increased TC. This effect was restricted to cold-sensitive neurons, which were determined in a high number. The TC was significantly increased (p<0.05) by baclofen and significantly decreased (p<0.05) by CGP 35348. The effects of muscimol and baclofen on FR and TC were prevented by co-perfusion of the appropriate antagonists bicuculline and CGP 35348. The results suggest that the fundamental mechanisms of GABAergic influence on temperature sensitive and insensitive neurons in the chicken PO/AH are conserved during evolution of amniotes.

GABAB receptor activation inhibits dopamine D1 receptor-mediated facilitation of [3H]GABA release in substantia nigra pars reticulata

GABA(B) receptors inhibit and dopamine D1 receptors stimulate the release of GABA from striatal terminals in the pars reticulata of the substantia nigra. Here we have studied the interaction between both classes of receptors by exploring the effect of GABA(B) receptors upon the stimulation of depolarization-induced [(3)H]GABA release induced by the activation of D1 receptors in slices of the pars reticulata of the rat substantia nigra. The activation of GABA(B) receptors with baclofen (100 microM) inhibited by 48+/-8% the evoked [(3)H]GABA release in normal slices but did not modify the release in slices from reserpine-treated rats, indicating that the inhibition was dependent on endogenous dopamine. The inhibitory effect of baclofen was also abolished by the D1 receptor antagonist SCH 23390 (1 microM), indicating a D1 receptor-dependence of the baclofen inhibition. Baclofen dose-dependently inhibited (IC(50)=3.6 microM) the stimulation of release induced by the D1 agonist SKF 38393 (1 microM). Baclofen also blocked the stimulation of release induced by forskolin but not that induced by 8-Br-cAMP, indicating that the inhibitory effect was exerted before cAMP synthesis. N-ethylmaleimide (NEM), a selective inactivator of PTX-sensitive G-proteins, abolished the baclofen inhibition of the SKF 38393-induced stimulation of the release without affecting the stimulation induced by the D1 agonist, suggesting that the baclofen effect was mediated by Galpha(i/o) proteins. These results might have relevance in the control motor disorders associated with D1 receptor supersensitivity.

Activation of GABA(B) receptors reverses spontaneous gating deficits in juvenile DBA/2J mice

RATIONALE

Gamma-amino-butyric acid (GABA)(B) receptors play a key role in the pathophysiology of psychotic disorders. We previously reported that baclofen, the prototypical GABA(B) agonist, elicits antipsychotic-like effects in the rat paradigm of prepulse inhibition (PPI) of the startle, a highly validated animal model of schizophrenia.

OBJECTIVES

We studied the role of GABA(B) receptors in the spontaneous PPI deficits displayed by DBA/2J mice.

MATERIALS AND METHODS

We tested the effects of baclofen (1.25-5 mg/kg, intraperitoneal [i.p.]) in DBA/2J and C57BL/6J mice, in comparison to the antipsychotic drugs haloperidol (1 mg/kg, i.p.) and clozapine (5 mg/kg, i.p.). Furthermore, we investigated the expression of GABA(B) receptors in the brain of DBA/2J and C57BL/6J mice by quantitative autoradiography.

RESULTS

Baclofen dose-dependently restored PPI deficit in DBA/2J mice, in a fashion similar to the antipsychotic clozapine (5 mg/kg, i.p.). This effect was reversed by pretreatment with the GABA(B) antagonist SCH50211 (50 mg/kg, i.p.). In contrast, baclofen did not affect PPI in C57BL/6J mice. Finally, quantitative autoradiographic analyses assessed a lower GABA(B) receptor expression in DBA/2J mice in comparison to C57BL/6J controls in the prefrontal cortex and hippocampus but not in other brain regions.

CONCLUSIONS

Our data highlight GABA(B) receptors as an important substrate for sensorimotor gating control in DBA/2J mice and encourage further investigations on the role of GABA(B) receptors in sensorimotor gating, as well as in the pathophysiology of psychotic disturbances.

Effects of GABAB receptor agents on cocaine priming, discrete contextual cue and food induced relapses

In the present study we investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phosphinic acid (SKF 97541), and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl)-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) on cocaine seeking behavior. The effects of the above drugs on the reinstatement of responding induced by natural reinforcer (food) were also studied. Male Wistar rats were trained to self-administer either cocaine (0.5 mg/kg/infusion) or food (sweet milk) and responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p.), a discrete contextual cue, or a contingent presentation of food. SCH 50911 (3-10 mg/kg) dose-dependently attenuated responding on the previously cocaine-paired lever during both reinstatement conditions, with slightly greater efficacy at reducing conditioned cue reinstatement. At the same time, it failed to alter reinstatement of food-seeking behavior. Baclofen (1.25-5 mg/kg) and SKF 97541 (0.03-0.3 mg/kg) attenuated cocaine- or food-seeking behavior; the effect of the drug appeared more effective for cocaine-seeking than food-seeking. CGP 7930 (10-30 mg/kg) reduced cocaine seeking without affecting food-induced reinstatement on reward seeking. Our results indicate that tonic activation of GABA(B) receptors is required for cocaine seeking behavior in rats. Moreover, the GABA(B) receptor antagonist SCH 50911 was effective in reducing relapse to cocaine at doses that failed to alter reinstatement of food-seeking behavior (present study), basal locomotor activity, cocaine and food self-administration (Filip et al., submitted for publication), suggesting its selective effects on motivated drug-seeking behavior. The potent inhibitory responses on cocaine seeking behavior were also seen following the GABA(B) receptor agonists or the allosteric positive modulator, however, doses of baclofen and SKF 97541 that inhibited cocaine-seeking were only threefold lower of those that inhibited food-seeking. In addition, the direct GABA(B) receptor agonists and the allosteric positive modulator cause decreases in cocaine or food self-administration (Filip et al., submitted for publication), indicating their nonspecific effects on relapse to drug-seeking and drug-taking behavior. In conclusion, the GABA(B) receptor antagonist SCH 50911 seems to be viable treatment for reducing cocaine craving and preventing relapse, while the GABA(B) receptor allosteric positive modulator CGP 7930 may hold the highest promise for attenuating cue-evoked relapses to cocaine as well as the direct rewarding properties of cocaine.

Syntheses and optimization of new GS39783 analogues as positive allosteric modulators of GABA B receptors

The optimization of GS39783 into potent, selective, and safe positive allosteric modulators of GABA(B) receptors is presented.

The ameliorating effects of LiuWei Dihuang Wang on cycloheximide-induced impairment of passive avoidance performance in rats

The ameliorating effects of aqueous and ethanolic extracts of LiuWei Dihuang Wang (LDW(W) and LDW(E)) after single, 1-week or 2-week consecutive treatment on the cycloheximide-induced amnesia by using the passive avoidance task in rats were studied. After single treatment, LDW(W) and LDW(E) (1 and 2g/kg) significantly prolonged the shortened step-through latency induced by CXM and their potency was equal. LDW(W) at 1g/kg after 1-week consecutive treatment or at 0.1g/kg after 2-week consecutive treatment almost completely reversed CXM-induced amnesia. LDW(W) at any dose alone after single, 1-week or 2-week consecutive treatment did not influence the step-through latency in the training trial in rats. Furthermore, muscarinic antagonist scopolamine, peripheral cholinergic antagonist scopolamine methylbromide, serotonin precursor 5-hydroxytryptamine and serotonin releaser p-chloroamphetamine could block the ameliorating effects of LDW(W). GABA(A) receptor antagonist bicuculline and GABA(B) receptor agonist baclofen also blocked the ameliorating effects of LDW(W). These results suggest that the ameliorating effects of LDW whose potency were parallel to treatment duration might be related to activating peripheral cholinergic neuronal system and modulating the central nervous system.

A spinal muscarinic M2 receptor-GABAergic disinhibition pathway that modulates peripheral inflammation in mice

Previous data from our laboratories using the mouse air pouch model demonstrated that intrathecal injection of the cholinomimetic drug, neostigmine, produces a significant peripheral anti-inflammatory effect through activation of spinal muscarinic type 2 receptors. This anti-inflammatory effect is mediated by activation of sympathetic preganglionic neurons and subsequent release of adrenomedullary catecholamines. It has been established that adrenomedullary catecholamine release is controlled by sympathetic preganglionic neurons and that these neurons are modulated by GABAergic inhibitory input. To further establish the neurochemical circuitry underlying spinally mediated anti-inflammation, the present study examined whether spinal muscarinic type 2 receptors are associated with this spinal GABAergic pathway. Intrathecal injection of the M(2) receptor agonist, arecaidine but-2-ynyl ester tosylate (ABET) dose-dependently suppressed zymosan-induced leukocyte migration into the air pouch and increased Fos (neuronal activation marker) expression in sympathetic preganglionic neurons of the T7-T11 spinal cord segments (which mainly project to the adrenal medulla), but not in sympathetic preganglionic neurons of the T1-T6 or T12-L2 segments. These effects of arecaidine but-2-ynyl ester tosylate were completely blocked by intrathecal pretreatment with baclofen (a GABA(B)R agonist) but not muscimol (a GABA(A)R agonist). Intrathecal saclofen (a GABA(B)R antagonist), but not bicuculline (a GABA(A)R antagonist), significantly reduced leukocyte migration and increased Fos expression in T7-T11 sympathetic preganglionic neurons. More importantly, this intrathecal saclofen-induced anti-inflammatory effect was completely blocked by adrenalectomy or systemic pretreatment with propranonol (a beta-adrenoceptor antagonist). Collectively, these novel findings suggest that activation of spinal muscarinic type 2 receptors suppress spinal GABA(B) receptor input and that this disinhibition mechanism ultimately leads to the release of adrenal catecholamines and a subsequent reduction in peripheral inflammation.

Reducing effect of the positive allosteric modulator of the GABA(B) receptor, GS39,783, on alcohol self-administration in alcohol-preferring rats

RATIONALE AND OBJECTIVES

The positive allosteric modulator of the GABA(B) receptor, GS39,783, has recently been found to suppress acquisition and maintenance of alcohol drinking behavior in selectively bred Sardinian alcohol-preferring (sP) rats exposed to the standard, homecage two-bottle "alcohol vs water" choice regimen. The present study was designed to extend the characterization of the "anti-alcohol" effects of GS39,783 to oral self-administration of alcohol under an operant procedure.

MATERIALS AND METHODS

Two separate groups of male sP rats were trained to lever-press (on an FR4 schedule) to orally self-administer alcohol (15%, v/v) or sucrose (0.3%, w/v) in daily 30-min sessions. Once lever-pressing behavior reached stable levels, the effect of GS39,783 (0, 25, 50, and 100 mg/kg, i.g.) on responding for alcohol and sucrose was determined.

RESULTS

Pretreatment with GS39,783 resulted in a significant, dose-dependent reduction in responding for alcohol; at the dose of 100 mg/kg GS39,783, the number of lever responses for alcohol was reduced by approximately 50% in comparison to vehicle-treated rats. The effect of GS39,783 on alcohol self-administration was specific, as responding for sucrose was completely unaffected by pretreatment with GS39,783.

CONCLUSIONS

These data demonstrate the capability of GS39,783 to attenuate the reinforcing properties of alcohol in alcohol-preferring rats. These data constitute a further piece of experimental evidence in support of the hypothesized role for the GABA(B) receptor in the control of alcohol drinking and reinforcement.

Neural circuits containing pallidotegmental GABAergic neurons are involved in the prepulse inhibition of the startle reflex in mice

BACKGROUND

Prepulse inhibition (PPI) of the startle response is a measure of the inhibitory function and time-linked information processing by which a weak sensory stimulus (the prepulse) inhibits the startle response caused by a sudden intense stimulus. We attempted to clarify the neuronal circuits underlying the control of PPI of the startle reflex in mice.

METHODS

c-Fos immunohistochemistry was used to detect neurons activated by startle pulse and/or prepulse trials. Behavioural pharmacology and tracing studies were also conducted.

RESULTS

The lateral globus pallidus (LGP) was activated by prepulses. Activation of the caudal pontine reticular nucleus (PnC) evoked by the startle pulses was inhibited under PPI conditions. Double-immunostaining revealed that c-Fos-positive cells in the LGP following prepulse trials were GABAergic neurons. Bilateral microinjections of lidocaine into the LGP resulted in an impairment of PPI. Fluoro-gold infusion into the PnC and the pedunculopontine tegmental nucleus (PPTg) retrogradely labeled neurons in the PPTg and LGP, respectively. Microinjections of phaclofen into the PPTg significantly impaired PPI.

CONCLUSIONS

These results suggest that GABAergic neurons in the LGP which project to the PPTg play a crucial role through the activation of GABAB receptors in the regulation of PPI of the startle reflex in mice.

The role of GABA(B) receptors in the regulation of excitatory neurotransmission

GABA(B) receptors are the metabotrophic receptors for GABA. They are members of the G-protein coupled superfamily of receptors but are highly unusual as they are made up of a dimer of 7-transmembrane spanning subunits. The receptors are widely distributed throughout the central nervous system where they act post-synaptically to cause a long-lasting hyperpolarisation through the activation of a potassium conductance. They are also present pre-synaptically where they act as auto and heteroreceptors to inhibit neurotransmitter release. GABA(B) receptors play a complex role in the regulation of excitatory transmission and their activation can have both inhibitory and dis-inhibitory effects. This has profound physiological and behavioural consequences including modification of LTP and memory, regulation of seizure activity and nociception.

Cataleptic effects of gamma-hydroxybutyrate (GHB), its precursor gamma-butyrolactone (GBL), and GABAB receptor agonists in mice: differential antagonism by the GABAB receptor antagonist CGP35348

RATIONALE

Gamma-hydroxybutyrate (GHB) is used to treat narcolepsy but is also abused. GHB has many actions in common with the GABA(B) receptor agonist baclofen.

OBJECTIVE

To further study the role of GABA(B) receptors in the effects of GHB.

MATERIALS AND METHODS

The experiments examined the ability of the GABA(B) receptor antagonist CGP35348 to attenuate GHB-induced catalepsy in comparison with its ability to attenuate the cataleptic effects of GABA(B) receptor agonists.

RESULTS

In C57BL/6J mice, GHB, the GHB precursor gamma-butyrolactone (GBL), and the GABA(B) receptor agonists baclofen and SKF97541 all produced catalepsy but differed in potency (i.e., SKF97541>baclofen>GBL>GHB) and in onset of action. The cataleptic effects of drug combinations were assessed at the time of peak effect of each compound, i.e., 60 min after CGP35348 and 60, 30, 30, and 15 min after baclofen, SKF97541, GHB, and GBL, respectively. At 100 mg/kg, CGP35348 shifted the dose-response curves of baclofen and SKF97541 to the right but not those of GHB and GBL; at 320 mg/kg, CGP35348 shifted the curves of all four compounds to the right.

CONCLUSIONS

The finding that CGP35348 was about threefold less potent to antagonize GHB and GBL than baclofen and SKF97541 is further evidence that the mechanisms mediating the effects of GHB and GABA(B) agonists are not identical. Differential involvement of GABA(B) receptor subtypes, or differential interactions with GABA(B) receptors, may possibly explain why GHB is effective for treating narcolepsy and is abused whereas baclofen is not.

Enhanced activity of GABA receptors inhibits glutamate release induced by focal cerebral ischemia in rat striatum

Cerebral ischemia causes an excess release of glutamate, which can injure neurons. The striatum is one of the important regions vulnerable to hypoxia and ischemia. Using push-pull perfusion technique, we investigated the regulatory role of gamma-aminobutyric acid (GABA) and its receptors in modifying the amount of glutamate in rat striatum with ischemia. Perfusion with exogenous GABA (1 mM) inhibited cerebral ischemia-induced glutamate release by as much as 47%. We further characterized relative roles of subtype receptors of GABA on glutamate release by using pharmacological tools. While baclofen (500 microM), a GABA(B) receptor agonist, suppressed ischemia-induced glutamate release by 52%, GABA(B) receptor antagonist saclofen (500 microM) failed to produce a significant increase of glutamate release. The GABA(A) receptor agonist muscimol (500 microM) also reduced by 38% the release of glutamate induced by cerebral ischemia but the GABA(A) receptor antagonist bicuculline (500 microM) had very little effect. The present study demonstrates that the excessive release of glutamate or the overly activated glutamate receptor, triggered by cerebral ischemia, can be down-regulated by exogenous GABA or by increased activity of GABA receptors, especially the presynaptic GABA(B) receptors, which might be one of the important mechanisms to protect against striatum neuronal damage from over stimulation by excessive glutamate during ischemia.

Genomic and functional conservation of sedative-hypnotic targets in the zebrafish

OBJECTIVES

The zebrafish is an ideally suited vertebrate animal model for large-scale genetic screens and is emerging as a model organism in pharmacological and behavioral research. We investigated the effects of sedative hypnotics commonly used in humans on zebrafish locomotor activity and identified the corresponding genomic and receptor binding targets.

METHODS

We studied radioreceptor binding and behavioral responses to compounds with known sedative hypnotic properties representing multiple pharmacological classes. These included GABAergic hypnotics such as benzodiazepines, barbiturates, and baclofen; alpha-2 adrenergic agonists; and histaminergic H1 antagonists. An automated system was used to quantify behavioral effects. Zebrafish homologs of histamine receptor H1, gamma-amino-n-butyric acid type A (alpha-subunit), and gamma-amino-n-butyric acid type B (1 and 2) receptor genes were identified through translating queries of the zebrafish Zv4 database with human receptor protein sequences. A pilot screen of 154 N-ethyl-N-nitroso-urea-mutagenized F2 families was conducted with pentobarbital, flurazepam and mepyramine.

RESULTS

Radioreceptor binding studies revealed high affinity binding sites for known gamma-amino-n-butyric acid type A, gamma-amino-n-butyric acid type B, and histaminergic ligands. Drug immersion of 5-7-day-old larvae reduced mobility and, in some cases, produced a complete state of unresponsive immobility similar to anesthesia. These effects were dose-dependent and rapidly reversible in water. As established in mammals, (R)-baclofen was more active behaviorally and had higher affinity in binding studies when compared with (S)-baclofen. In this model, (S)-baclofen only partially reduced activity at high dose and blocked (R)-baclofen behavioral hypnotic effects. Genomic sequences with high similarity to the corresponding pharmacological targets were identified, but no mutants were found in the pilot screen.

CONCLUSIONS

These results demonstrate conservation of gene, protein and function for many established sedative hypnotic pathways. The results indicate feasibility of conducting large-scale pharmacogenomic screens to isolate novel proteins modulating susceptibility to hypnotic compounds in a vertebrate system.

A GABA(B) agonist reverses the behavioral sensitization to morphine in rats

RATIONALE

In laboratory animals, repeated administration of drugs of abuse causes sensitization to their stimulant and rewarding effects. Neuroadaptations underlying sensitization could be related to those that contribute to addictive behaviors. An increased understanding of the molecular mechanisms of sensitization could lead to improved treatments for addiction.

OBJECTIVES

Since baclofen (BCF) co-administration has been reported to block the development and the expression of motor sensitization to morphine (MOR), the present study examined the hypothesis that a chronic treatment with BCF alone might reverse and/or prevent MOR-induced sensitization.

METHODS

Rats were first sensitized to MOR (saline or 10 mg/kg MOR i.p.; days 1-10) and then chronically treated with BCF (saline or 2 mg/kg BCF i.p.; days 11-20). Finally, the motility effect of MOR (10 mg/kg i.p.) was assessed 3 and 30 days after the end of BCF treatment. The same rats were again challenged with MOR on day 70, after a further period of saline or MOR treatment (days 51-60).

RESULTS

Behavioral sensitization to MOR was observed in control animals but not in rats chronically treated with BCF (days 23 and 50). Thus, BCF completely reversed MOR-induced sensitization, and its effect was long lasting. However, a previous repeated BCF treatment did not prevent the development of sensitization to MOR both in naive and desensitized rats.

CONCLUSIONS

The present results confirm that gamma-aminobutyric acid (GABA)(B) receptors play an important role in the expression of motor sensitization to MOR and suggest that GABA(B) agonists could be useful for reversing the neuroadaptations related to drug addiction.