GABAB Receptor Agonist Baclofen Research Articles

Effects of co-administration of the GABAB receptor agonist baclofen and a positive allosteric modulator of the GABAB receptor, CGP7930, on the development and expression of amphetamineinduced locomotor sensitization in rats

Several of the behavioral effects of amphetamine (AMPH) are mediated by an increase in dopamine neurotransmission in the nucleus accumbens. However, evidence shows that γ-aminobutyric acid B (GABAB) receptors are involved in the behavioral effects of psychostimulants, including AMPH. Here, we examined the effects of co-administration of the GABAB receptor agonist baclofen and a positive allosteric modulator of the GABAB receptor, CGP7930, on AMPH-induced locomotor sensitization. In a series of experiments, we examined whether baclofen (2.0, 3.0 and 4.0 mg/kg), CGP7930 (5.0, 10.0 and 20.0 mg/kg), or co-administration of CGP7930 (5.0, 10.0 and 20.0 mg/kg) with a lower dose of baclofen (2.0 mg/kg) could prevent the development and expression of locomotor sensitization produced by AMPH (1.0 mg/kg). The results showed that baclofen treatment prevented both the development and expression of AMPH-induced locomotor sensitization in a dose-dependent manner. Furthermore, the positive allosteric modulator of the GABAB receptor, CGP7930, increased the effects of a lower dose of baclofen on AMPH-induced locomotor sensitization under both conditions. These data provide further evidence that GABAB receptor ligands may modulate psychostimulant-induced behaviors.

Abstract 2191: GABA but not baclofen prevents gemcitabine resistance induced by low dose nicotine in pancreatic cancer xenografts.

Abstract Pancreatic ductal adenocarinoma (PDAC) is a leading cause of cancer deaths in developed countries. The nucleoside analog Gemcitabine, which induces apoptosis, is widely used for the therapy of pancreatic cancer. Smoking and alcoholism are risk factors for pancreatic cancer. Nicotine replacement therapy often accompanies chemotherapy while the GABA-B receptor (GABA-B-R) agonist Baclofen has recently been suggested as an effective agent for the treatment of alcohol dependence. Our laboratory has shown that the proliferation and migration of PDAC and pancreatic duct epithelial cells in vitro is regulated by the nicotinic receptor-mediated synthesis and release of stress neurotransmitters that bind to beta-adrenoreceptors (beta-ARs). We have additionally shown that nicotine in the drinking water at a high dose (432 μmole/L) comparable to nicotine exposure in heavy smokers significantly stimulated the growth of PDAC xenografts whereas identical exposure of mice to low dose nicotine (1 μmole/L) reduced gemcitabine-induced apoptosis, thus significantly increasing resistance to gemcitabine. In the current study, we have investigated the potential prevention of nicotine-induced gemcitabine resistance by γ-aminobutyric acid (GABA) and Baclofen in PDAC xenografts. We found that GABA significantly reduced nicotine-induce drug resistance. By contrast, Baclofen failed to reduce nicotine-induced resistance to gemcitabine while even slightly increasing xenograft growth in mice not exposed to nicotine. Investigation of xenograft tissues for the expression levels of the GABA-B-R, intracellular cAMP and signaling proteins associated with cell proliferation, apoptosis and metastasis by immunoassays and western blots revealed effective inhibition of cAMP-dependent signaling in xenografts of mice treated with GABA. By contrast, Baclofen did not inhibit cAMP-dependent signaling and decreased the protein expression of the GABA-R, suggesting downregulation of the receptor. Our findings identify GABA as a promising agent for the prevention of nicotine-induced resistance to gemcitabine in PDAC. On the other hand, our data suggest that treatment of alcohol dependence by Baclofen should be avoided in PDAC patients. Supported by grants RO1CA130888 and RO1CA042829 with the National Cancer Institute. Citation Format: Jheelam Banerjee, Mohammed H. Al-Wadei, Hussein A. N. Al-Wadei, Hildegard M. Schuller. GABA but not baclofen prevents gemcitabine resistance induced by low dose nicotine in pancreatic cancer xenografts. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2191. doi:10.1158/1538-7445.AM2013-2191

GABA receptor crosstalk

The inhibitory neurotransmitter GABA activates both ionotropic type A GABA receptors (GABAARs) and metabotropic GABAB receptors (GABABRs). Two independent studies in rat brain slices now show that activation of postsynaptic GABABRs enhances signalling through extrasynaptic GABAARs. Activity-dependent changes in GABA concentration activate postsynaptic GABABRs leading to slow inhibition, whereas high-affinity GABAARs at extrasynaptic sites are activated tonically by ambient GABA. This tonic inhibitory tone has been implicated in synaptic integration, anxiety-related behaviours and seizure susceptibility. Tao et al. show that activation of postsynaptic GABABRs enhances GABAAR-mediated currents caused by exogenous GABA or the GABABR agonist baclofen in dentate gyrus granule cells (DGGCs) but not in CA1 pyramidal neurons or cortical layer 2/3 pyramidal neurons. Connelly et al. report similar crosstalk in thalamocortical neurons of the ventrobasal thalamus and cerebellar granule cells as well as in DGGCs.

Impaired hypotensive responses induced by intrathecally injected drugs in fructose-fed rats

Blood pressure responses to intrathecal (i.t.) injection of neurochemicals were examined in the fructose-fed rat, an experimental model of metabolic syndrome.Sprague-Dawley rats receiving either tap water or water containing 10% fructose during 8 weeks were anesthetized with sodium pentobarbital. The endocannabinoid anandamide (100 nmol; i.t.) decreased mean blood pressure in control rats (-21.2 ± 6.3 mm Hg), but had no effect in fructose-fed animals. Similarly, calcitonin gene-related peptide (CGRP; 0.125 nmol; i.t.) decreased mean blood pressure in control, but not in treated rats. The high fructose diet did not cause significant changes in the pressor effects of i.t. administered noradrenaline (100 nmol) and N-methyl-d-aspartate (30 nmol). The nitric oxide donor sodium nitroprusside (500 nmol, i.t.) induced a brief hypotension followed by a sustained increase in mean blood pressure in control rats; however, this drug only produced pressor effects in fructose-fed animals. The GABAA-receptor agonist muscimol (8.8 nmol, i.t.) and the GABAB-receptor agonist baclofen (100 nmol, i.t.) decreased mean blood pressure 30-35 mm Hg, both in control and in fructose-fed rats. Fructose potentiated the pressor effect of i.v. injected noradrenaline, but did not modify the hypotensive responses to i.v. administered sodium nitroprusside and acetylcholine.These results could suggest that, in pentobarbital-anesthetized rats, fructose feeding could alter spinal mechanisms of regulation of preganglionic sympathetic nerve activity. It is proposed that the spinal cord could be involved in the sympathetic dysfunction associated with the metabolic syndrome.

Activation of GABAA receptors in the medial prefrontal cortex produces an anxiolytic-like response.

There has been increasing evidence that the γ-aminobutyric acid (GABA)ergic system is involved in the neurobiology of anxiety. The present study aimed to investigate the role of GABAergic systems in the modulation of anxiety in the medial prefrontal cortex (mPFC) of rats using the elevated plus maze test. Rats were anaesthetised with a mixture of ketamine and xylazine, and then special cannulae were inserted stereotaxically into the mPFC. After 5-7 days of recovery, the effects of intra-mPFC administration of GABAergic agents were studied. Bilateral injection of the GABAA receptor agonist muscimol (0.25, 0.5 and 1 μg/rat) produces an anxiolytic-like effect, shown by significant increases in the percentage of open-arm time (%OAT) and percentage of open-arm entries (%OAE). Intra-mPFC administration of the GABAA receptor antagonist bicuculline (0.25, 0.5 and 1 μg/rat) produces significant anxiogenic-like behaviour. However, intra-mPFC injection of the GABAB receptor agonist baclofen (0.05, 0.1 and 0.2 μg/rat) and the GABAB receptor antagonist CGP35348 (5, 10 and 15 μg/rat) did not alter %OAT and %OAE significantly. The results of the present study demonstrate that the GABAergic system of the mPFC modulates anxiety-related behaviours of rats through GABAA receptors.

Postsynaptic GABABReceptors Enhance Extrasynaptic GABAAReceptor Function in Dentate Gyrus Granule Cells

Ambient GABA in the brain tonically activates extrasynaptic GABA(A) receptors, and activity-dependent changes in ambient GABA concentration can also activate GABA(B) receptors. To investigate an interaction between postsynaptic GABA(B) and GABA(A) receptors, we recorded GABA(A) currents elicited by exogenous GABA (10 μm) from dentate gyrus granule cells (DGGCs) in adult rat hippocampal slices. The GABA(B) receptor agonist baclofen (20 μm) enhanced GABA(A) currents. This enhancement was blocked by the GABA(B) receptor antagonist CGP 55845 and intracellular solutions containing the GTP analog GDP-β-s, indicating that baclofen was acting on postsynaptic GABA(B) receptors. Modulation of GABA(A) currents by postsynaptic GABA(B) receptors was not observed in CA1 pyramidal cells or layer 2/3 cortical pyramidal neurons. Baclofen reduced the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) but did not alter sIPSC amplitude or kinetics. Thus, GABA(A) receptors activated at synapses were not modulated by postsynaptic GABA(B) receptors. In contrast, tonic GABA currents and currents activated by the GABA(A) receptor δ subunit-selective agonist THIP (10 μm) were potentiated by baclofen. Our data indicate that postsynaptic GABA(B) receptors enhance the function of extrasynaptic GABA(A) receptors, including δ subunit-containing receptors that mediate tonic inhibition in DGGCs. The modulation of GABA(A) receptor function by postsynaptic GABA(B) receptors is a newly identified mechanism that will influence the inhibitory tone of DGGCs when GABA(B) and GABA(A) receptors are both activated.

Comparison of the effects of the GABAB receptor positive modulator BHF177 and the GABAB receptor agonist baclofen on anxiety-like behavior, learning, and memory in mice

γ-Aminobutyric acid B (GABAB) receptor activation is a potential therapeutic approach for the treatment of drug addiction, pain, anxiety, and depression. However, full agonists of this receptor induce side-effects, such as sedation, muscle relaxation, tolerance, and cognitive disruption. Positive allosteric modulators (PAMs) of the GABAB receptor may have similar therapeutic effects as agonists with superior side-effect profiles. The present study behaviorally characterized N-([1R,2R,4S]-bicyclo[2.2.1]hept-2-yl)-2-methyl-5-(4-[trifluoromethyl]phenyl)-4-pyrimidinamine (BHF177), a GABAB receptor PAM, in mouse models of anxiety-like behavior, learning and memory. In addition, the effects of BHF177 were compared with the agonist baclofen. Unlike the anxiolytic chlordiazepoxide, baclofen (0.5, 1.5, and 2.5 mg/kg, intraperitoneally) and BHF177 (10, 20, and 40 mg/kg, orally) had no effect on anxiety-like behavior in the elevated plus maze, light/dark box, or Vogel conflict test. Baclofen increased punished drinking in the Vogel conflict test, but this effect may be attributable to the analgesic actions of baclofen. At the highest dose tested (2.5 mg/kg), baclofen-treated mice exhibited sedation-like effects (i.e., reduced locomotor activity) across many of the tests, whereas BHF177-treated mice exhibited no sedation-like effects. BHF177 exhibited pro-convulsion properties only in mice, but not in rats, indicating that this effect may be species-specific. At doses that were not sedative or pro-convulsant, baclofen and BHF177 had no selective effects on fear memory retrieval in contextual and cued fear conditioning or spatial learning and memory in the Barnes maze. These data suggest that BHF177 has little sedative activity, no anxiolytic-like profile, and minimal impairment of learning and memory in mice.

Rescue of Inhibitory Synapse Strength following Developmental Hearing Loss

Inhibitory synapse dysfunction may contribute to many developmental brain disorders, including the secondary consequences of sensory deprivation. In fact, developmental hearing loss leads to a profound reduction in the strength of inhibitory postsynaptic currents (IPSCs) in the auditory cortex, and this deficit persists into adulthood. This finding is consistent with the general theory that the emergence of mature synaptic properties requires activity during development. Therefore, we tested the prediction that inhibitory strength can be restored following developmental hearing loss by boosting GABAergic transmission in vivo. Conductive or sensorineural hearing loss was induced surgically in gerbils prior to hearing onset and GABA agonists were then administered for one week. IPSCs were subsequently recorded from pyramidal neurons in a thalamocortical brain slice preparation. Administration of either a GABAA receptor a1 subunit specific agonist (zolpidem), or a selective GABA reuptake inhibitor (SGRI), rescued IPSC amplitude in hearing loss animals. Furthermore, this restoration persisted in adults, long after drug treatment ended. In contrast, a GABAB receptor agonist baclofen did not restore inhibitory strength. IPSCs could also be restored when SGRI administration began 3 weeks after sensory deprivation. Together, these results demonstrate long-lasting restoration of cortical inhibitory strength in the absence of normal experience. This suggests that in vivo GABAA receptor activation is sufficient to promote maturation, and this principle may extend to other developmental disorders associated with diminished inhibitory function.

A human laboratory pilot study with baclofen in alcoholic individuals

Preclinical and clinical studies show that the GABA(B) receptor agonist baclofen may represent a pharmacotherapy for alcohol dependence (AD). However, the mechanisms by which baclofen affects drinking are not well characterized; thus this pilot study investigated possible baclofen's biobehavioral mechanisms. The design was a double-blind controlled randomized human laboratory pilot study. Fourteen non-treatment seeking alcohol-dependent heavy drinking subjects received either baclofen 10mg t.i.d. or an active placebo (cyproheptadine 2mg t.i.d., to control for sedation) for a 7-day period. At day 8, participants performed an alcohol cue-reactivity (CR) followed by an alcohol self-administration (ASA). Additionally, we explored possible moderators that might guide future larger studies, i.e. anxiety, family history and onset of alcoholism, and D4 dopamine receptor (DRD4) and 5-HTTLPR polymorphisms. The main results were a significant effect of baclofen for increasing stimulation (p=.001) and sedation (p<.01). Furthermore, when drinking during the ASA and the 2 days before was analyzed as a composite variable, there was a significant effect of baclofen to reduce alcohol consumption (p<.01). As for the exploratory analyses, baclofen's effects to increase alcohol sedation and to reduce alcohol consumption were limited to those individuals with DRD4 ≥7 repeats (DRD4L). Yet, baclofen's effects on alcohol consumption were also moderated by 5-HTTLPR LL genotype. In conclusion, baclofen's ability to reduce alcohol drinking may be related to its effects on the biphasic effects of alcohol, but larger studies are needed to confirm these preliminary findings.

Effects of the GABAB receptor agonist baclofen administered orally on normal food intake and intraperitoneally on fat intake in non-deprived rats.

It has been previously reported that the GABA(B) receptor agonist baclofen decreases food intake after oral administration and fat intake after intraperitoneal administration. The aim of the study was to investigate the effects of baclofen (1-4 mg/ kg) administered orally (Experiment 1) on food intake in non-deprived rats (n=6) and intraperitoneally (Experiment 2) on fat intake in non-deprived rats (n=8) that were naïve to baclofen (1st set of trials) and in the same group of rats after they were sub-chronically exposed to baclofen (2nd set of trials). The results from Experiment 1 show that baclofen had no effects on food intake during the 1st set of trials, but the 2 and 4 mg/kg doses significantly increased food consumption during the 2nd set of trials. Baclofen produced sedation during the 1st set of trials, but tolerance occurred to this effect and was not apparent during the 2nd set of trials. These observations suggest that the motor effects may have competed with the hyperphagic effects of baclofen during the 1st set of trials. The data from Experiment 2 show that baclofen had no effects on fat intake during either the 1st or 2nd set of trials. The results of the study thus indicate that orally administrated baclofen increases food intake and intraperitoneal administration has no effect on fat intake in non-deprived rats under the conditions used in this study. These findings may have important implications for research on the use of baclofen in studies concerned with ingestive behaviours.

Neurogliaform cells of amygdala: a source of slow phasic inhibition in the basolateral complex

Synaptic inhibition in the amygdala actively participates in processing emotional information. To improve the understanding of interneurons in amygdala networks it is necessary to characterize the GABAergic cell types, their connectivity and physiological roles. We used a mouse line expressing a green fluorescent protein (GFP) under the neuropeptide Y (NPY) promoter. Paired recordings between presynaptic NPY-GFP-expressing (+) cells and postsynaptic principal neurons (PNs) of the basolateral amygdala (BLA) were performed. The NPY-GFP+ neurons displayed small somata and short dendrites embedded in a cloud of highly arborized axon, suggesting a neurogliaform cell (NGFC) type. We discovered that a NPY-GFP+ cell evoked a GABA(A) receptor-mediated slow inhibitory postsynaptic current (IPSC) in a PN and an autaptic IPSC. The slow kinetics of these IPSCs was likely caused by the low concentration and spillover of extracellular GABA. We also report that NGFCs of the BLA fired action potentials phase-locked to hippocampal theta oscillations in anaesthetized rats. When this firing was re-played in NPY+-NGFCs in vitro, it evoked a transient depression of the IPSCs. Presynaptic GABA(B) receptors and functional depletion of synaptic vesicles determined this short-term plasticity. Synaptic contacts made by recorded NGFCs showed close appositions, and rarely identifiable classical synaptic structures. Thus, we report here a novel interneuron type of the amygdala that generates volume transmission of GABA. The peculiar functional mode of NGFCs makes them unique amongst all GABAergic cell types of the amygdala identified so far.

GABAB receptor activation attenuates the stimulant but not mesolimbic dopamine response to ethanol in FAST mice

Neural processes influenced by γ-aminobutyric acid B (GABA(B)) receptors appear to contribute to acute ethanol sensitivity, including the difference between lines of mice bred for extreme sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effect of ethanol. One goal of the current study was to determine whether selection of the FAST and SLOW lines resulted in changes in GABA(B) receptor function, since the lines differ in sensitivity to the GABA(B) receptor agonist baclofen and baclofen attenuates the stimulant response to ethanol in FAST mice. A second goal was to determine whether the baclofen-induced reduction in ethanol stimulation in FAST mice is associated with an attenuation of the mesolimbic dopamine response to ethanol. In Experiment 1, the FAST and SLOW lines were found to not differ in GABA(B) receptor function (measured by baclofen-stimulated [(35)S]GTPγS binding) in whole brain or in several regional preparations, except in the striatum in one of the two replicate sets of selected lines. In Experiment 2, baclofen-induced attenuation of the locomotor stimulant response to ethanol in FAST mice was not accompanied by a reduction in dopamine levels in the nucleus accumbens, as measured by microdialysis. These data suggest that, overall, GABA(B) receptor function does not play an integral role in the genetic difference in ethanol sensitivity between the FAST and SLOW lines. Further, although GABA(B) receptors do modulate the locomotor stimulant response to ethanol in FAST mice, this effect does not appear to be due to a reduction in tonic dopamine signaling in the nucleus accumbens.

Acute cocaine induced deficits in cognitive performance in rhesus macaque monkeys treated with baclofen

Acute and/or chronic exposure to cocaine can affect cognitive performance, which may influence rate of recovery during treatment. Effects of the GABA-B receptor agonist baclofen were assessed for potency to reverse the negative influence of acute, pre-session, intravenous (IV) injection of cocaine on cognitive performance in Macaca mulatta nonhuman primates. Animals were trained to perform a modified delayed match to sample (DMS) task incorporating two types of trials with varying degrees of cognitive load that had different decision requirements in order to correctly utilize information retained over the delay interval. The effects of cocaine (0.2, 0.4, and 0.6 mg/kg, IV) alone and in combination with baclofen (0.29 and 0.40 mg/kg, IV) were examined with respect to sustained performance levels. Brain metabolic activity during performance of the task was assessed using PET imaged uptake of [(18) F]-fluorodeoxyglucose. Acute cocaine injections produced a dose-dependent decline in DMS performance selective for trials of high cognitive load. The GABA-receptor agonist baclofen, co-administered with cocaine, reversed task performance back to nondrug (saline IV) control levels. Simultaneous assessment of PET-imaged brain metabolic activity in prefrontal cortex (PFC) showed alterations by cocaine compared to PFC metabolic activation in nondrug (saline, IV) control DMS sessions, but like performance, PFC activation was returned to control levels by baclofen (0.40 mg/kg, IV) injected with cocaine. The results show that baclofen, administered at a relatively high dose, reversed the cognitive deficits produced by acute cocaine intoxication that may have implications for use in chronic drug exposure.

GABAB-mediated rescue of altered excitatory–inhibitory balance, gamma synchrony and behavioral deficits following constitutive NMDAR-hypofunction

Reduced N-methyl-D-aspartate-receptor (NMDAR) signaling has been associated with schizophrenia, autism and intellectual disability. NMDAR-hypofunction is thought to contribute to social, cognitive and gamma (30–80 Hz) oscillatory abnormalities, phenotypes common to these disorders. However, circuit-level mechanisms underlying such deficits remain unclear. This study investigated the relationship between gamma synchrony, excitatory–inhibitory (E/I) signaling, and behavioral phenotypes in NMDA-NR1neo−/− mice, which have constitutively reduced expression of the obligate NR1 subunit to model disrupted developmental NMDAR function. Constitutive NMDAR-hypofunction caused a loss of E/I balance, with an increase in intrinsic pyramidal cell excitability and a selective disruption of parvalbumin-expressing interneurons. Disrupted E/I coupling was associated with deficits in auditory-evoked gamma signal-to-noise ratio (SNR). Gamma-band abnormalities predicted deficits in spatial working memory and social preference, linking cellular changes in E/I signaling to target behaviors. The GABAB-receptor agonist baclofen improved E/I balance, gamma-SNR and broadly reversed behavioral deficits. These data demonstrate a clinically relevant, highly translatable neural-activity-based biomarker for preclinical screening and therapeutic development across a broad range of disorders that share common endophenotypes and disrupted NMDA-receptor signaling.

Baclofen Modulates the Expression and Release of Neurotrophins in Schwann-Like Adipose Stem Cells

Neurotrophins are a group of polypeptides that specifically influence neuronal activity during development and adult life in the central and peripheral nervous system (PNS). In particular, Schwann cells (SC) in the PNS exert a neurotrophic role following up-regulation of several growth factors, including nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF). Also SC-like cells derived from adipose tissue (dASC), which have molecular and functional properties similar to SC, can produce and secrete NGF and BDNF. Interestingly, gamma-aminobutyric acid (GABA) and its receptors have been also suggested as modulators of development and myelination in PNS. Therefore, it was interesting to investigate whether the stimulation of the GABA-B receptor may regulate the expression of neurotrophins in SC and dASC. Our findings demonstrated that the specific GABA-B receptors agonist baclofen influences the expression and the secretion NGF and BDNF. In particular, 2 and 24h of baclofen exposure lead to increased neurotrophins expression in both SC and dASC, as measured by western blot. Moreover, enzyme-linked immunosorbent assay showed that also the levels of released neurotrophins were modified after baclofen treatments. The possibility to modulate the neurotrophic potential of adult stem cell, acting on functional GABAergic receptors, could represent a novel pharmacological approach to improve nerve regeneration.

Effects of intraperitoneal administration of the GABAB receptor positive allosteric modulator 2,6-di tert-butyl-4-(2-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) on food intake in non-deprived rats

γ-Aminobutyric acid-(B) (GABA(B)) receptor positive allosteric modulators (PAMs) act on an allosteric site on the GABA(B) receptor to potentiate the effects of GABA and GABA(B) receptor agonists. It has previously been demonstrated that the GABA(B) receptor agonist baclofen increases food intake in non-deprived rats. The aim of this study was to investigate whether the GABA(B) receptor PAM 2,6-di tert-butyl-4-(2-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) would (i) increase food intake, and (ii) potentiate the hyperphagic effects of baclofen in rats. In Experiment 1, the effects of intraperitoneal (i.p.) administration of CGP7930 (1, 6 and 12 mg/kg) was investigated on food intake in non-deprived male Wistar rats. The 12 mg/kg dose of CGP7930 significantly increased cumulative food intake 30, 60 and 120 min (P<0.05, in each case) after administration. The 1 and 6 mg/kg doses were without effect. In Experiment 2, the effects of pretreatment with CGP7930 (6 mg/kg; i.p.) 5 min prior to administration of baclofen (2mg/kg, i.p.) was investigated on 30min cumulative food intake in non-deprived male Wistar rats. Baclofen (2mg/kg) significantly increased food intake compared with vehicle treatment (P<0.01). CGP7930 (6 mg/kg) had no effect on feeding. However, pretreatment with CGP7930 (6 mg/kg) significantly potentiated the hyperphagic effects of baclofen (2mg/kg) (P<0.01). These findings show that CGP7930 increases food intake and enhances the hyperphagic effects of baclofen, and are consistent with in vitro studies that suggest that it potentiates the effects of endogenous GABA and GABA(B) receptor agonists by allosteric modulation of the GABA(B) receptor.

Evidence for involvement of nitric oxide and GABAB receptors in MK-801- stimulated release of glutamate in rat prefrontal cortex

Systemic administration of NMDA receptor antagonists elevates extracellular glutamate within prefrontal cortex. The cognitive and behavioral effects of NMDA receptor blockade have direct relevance to symptoms of schizophrenia, and recent studies demonstrate an important role for nitric oxide and GABA(B) receptors in mediating the effects of NMDA receptor blockade on these behaviors. We sought to extend those observations by directly measuring the effects of nitric oxide and GABA(B) receptor mechanisms on MK-801-induced glutamate release in the prefrontal cortex. Systemic MK-801 injection (0.3 mg/kg) to male Sprague-Dawley rats significantly increased extracellular glutamate levels in prefrontal cortex, as determined by microdialysis. This effect was blocked by pre-treatment with the nitric oxide synthase inhibitor L-NAME (60 mg/kg). Reverse dialysis of the nitric oxide donor SNAP (0.5-5 mM) directly into prefrontal cortex mimicked the effect of systemic MK-801, dose-dependently elevating cortical extracellular glutamate. The effect of MK-801 was also blocked by systemic treatment with the GABA(B) receptor agonist baclofen (5 mg/kg). In combination, these data suggest increased nitric oxide formation is necessary for NMDA antagonist-induced elevations of extracellular glutamate in the prefrontal cortex. Additionally, the data suggest GABA(B) receptor activation can modulate the NMDA antagonist-induced increase in cortical glutamate release.

Regulation of GABA and Glutamate Release from Proopiomelanocortin Neuron Terminals in Intact Hypothalamic Networks

Hypothalamic proopiomelanocortin (POMC) neurons and their peptide products mediate important aspects of energy balance, analgesia, and reward. In addition to peptide products, there is evidence that POMC neurons can also express the amino acid transmitters GABA and glutamate, suggesting these neurons may acutely inhibit or activate downstream neurons. However, the release of amino acid transmitters from POMC neurons has not been thoroughly investigated in an intact system. In the present study, the light-activated cation channel channelrhodopsin-2 (ChR2) was used to selectively evoke transmitter release from POMC neurons. Whole-cell electrophysiologic recordings were made in brain slices taken from POMC-Cre transgenic mice that had been injected with a viral vector containing a floxed ChR2 sequence. Brief pulses of blue light depolarized POMC-ChR2 neurons and induced the release of GABA and glutamate onto unidentified neurons within the arcuate nucleus, as well as onto other POMC neurons. To determine whether the release of GABA and glutamate from POMC terminals can be readily modulated, opioid and GABA(B) receptor agonists were applied. Agonists for μ- and κ-, but not δ-opioid receptors inhibited transmitter release from POMC neurons, as did the GABA(B) receptor agonist baclofen. This regulation indicates that opioids and GABA released from POMC neurons may act at presynaptic receptors on POMC terminals in an autoregulatory manner to limit continued transmission. The results show that, in addition to the relatively slow and long-lasting actions of peptides, POMC neurons can rapidly affect the activity of downstream neurons via GABA and glutamate release.

Combinations of intrathecal gamma-amino-butyrate receptor agonists and N-methyl-d-aspartate receptor antagonists in rats with neuropathic spinal cord injury pain

Underlying below-level cutaneous hypersensitivity observed following spinal cord injury (SCI) is a concurrent loss of inhibition with an increase in excitation in the spinal dorsal horn. Thus, a dual pharmacological approach, increasing spinal γ-aminobutyrate (GABA) inhibition and decreasing N-methyl-d-aspartate (NMDA) receptor-mediated excitation, could be more beneficial than either approach alone. The current study evaluated the antinociceptive effects of lumbar intrathecal (i.t.) administration of GABA receptor agonists and NMDA receptor antagonists alone and in combination in rats with neuropathic SCI pain. Rats developed markedly decreased hind paw withdrawal thresholds following an acute thoracic spinal cord compression, indicative of below-level hypersensitivity. Separately, i.t. GABA(A) receptor agonist muscimol and GABA(B) receptor agonist baclofen demonstrated dose-dependent antinociception, whereas i.t. NMDA receptor antagonist ketamine and the endogenous peptide [Ser¹]histogranin, a putative NMDA receptor antagonist, demonstrated no efficacy. The combination of baclofen and ketamine resulted in a supra-additive (synergistic) antinociception whereas the combinations with muscimol were merely additive. Intrathecal pretreatment with the GABA(B) receptor antagonist CGP 35348 prevented the antinociceptive effect of the baclofen and ketamine combination. The data indicate that blocking spinal NMDA receptors alone is not sufficient to ameliorate SCI hypersensitivity, whereas a combined approach, simultaneous activation of spinal GABA(B) receptors and NMDA receptor blockade with ketamine, leads to significant antinociception. By engaging diverse pain modulating systems at the spinal level, combination drug treatment may be a useful approach in treating neuropathic SCI pain.

GABAB receptors involvement in the effects induced by nicotine on anxiety-related behaviour in mice

The aim of the present study was to evaluate the possible involvement of GABA(B) receptors in the anxiolytic- and anxiogenic-like responses induced by nicotine in mice. Animals were exposed to nicotine only once. The acute administration of low (0.05mg/kg, sc) or high (0.8mg/kg, sc) doses of nicotine produced opposite effects in the elevated plus maze test; respectively, anxiolytic- and anxiogenic-like responses. The effect of pretreatment with either the GABA(B) receptor antagonist 2-OH-saclofen (0.25, 0.5 and 1mg/kg; ip) or the GABA(B) receptor agonist baclofen (0.5, 1 and 2mg/kg; ip), was evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine. 2-OH-saclofen completely abolished both nicotine-induced effects (p<0.001) at the highest dose tested, suggesting an involvement of GABA(B) receptors in these behavioural responses. On the other hand, baclofen failed to modify the anxiety-related effects of nicotine. These results suggest that the GABA(B) receptors are involved in the regulation of nicotine-induced anxiety-related behavioural responses in mice, and provide new findings to support a potential pharmaco therapeutic use of GABAergic drugs in the treatment of tobacco addiction.