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The main fast-acting inhibitory receptors in the mammalian brain are γ-aminobutyric acid type-A (GABAA) receptors for which neurosteroids, a subclass of steroids synthesized de novo in the brain, constitute a group of endogenous ligands with the most potent positive modulatory actions known. Neurosteroids can act on all subtypes of GABAA receptors, with a preference for δ-subunit-containing receptors that mediate extrasynaptic tonic inhibition. Pathological conditions characterized by emotional and motivational disturbances are often associated with perturbation in the levels of endogenous neurosteroids. We studied the effects of ganaxolone (GAN)-a synthetic analog of endogenous allopregnanolone that lacks activity on nuclear steroid receptors-on the mesolimbic dopamine (DA) system involved in emotions and motivation. A single dose of GAN in young mice induced a dose-dependent, long-lasting neuroplasticity of glutamate synapses of DA neurons ex vivo in the ventral tegmental area (VTA). Increased α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate ratio and rectification of AMPA receptor responses even at 6 days after GAN administration suggested persistent synaptic targeting of GluA2-lacking AMPA receptors. This glutamate neuroplasticity was not observed in GABAA receptor δ-subunit-knockout (δ-KO) mice. GAN (500 nM) applied locally to VTA selectively increased tonic inhibition of GABA interneurons and triggered potentiation of DA neurons within 4 h in vitro. Place-conditioning experiments in adult wild-type C57BL/6J and δ-KO mice revealed aversive properties of repeated GAN administration that were dependent on the δ-subunits. Prolonged neuroadaptation to neurosteroids in the VTA might contribute to both the physiology and pathophysiology underlying processes and changes in motivation, mood, cognition, and drug addiction. [ABSTRACT FROM AUTHOR]

Muscimol has been regarded as a universal agonist for all γ-aminobutyric acid type A receptor (GABAA-R) subtypes. However, brain regional distribution of muscimol's high-affinity binding sites greatly differs from those of other binding sites of the GABAA-R. To test whether behavioral effects of muscimol correlated with the density of high-affinity [3H]muscimol binding, we examined several GABAA-R subunit gene-modified mouse lines: α1, α4, or δ-knockouts (KO), α4+δ-double KO, and Thy1.2 promoter-driven α6 transgenic mice (Thy1α6). We determined the high-affinity [3H]muscimol binding in brain sections by quantitative autoradiography and sedative/ataxic effects induced in vivo by muscimol using a constant speed rotarod. α4-KO mice had reduced [3H]muscimol binding in the caudate-putamen, thalamus, and hippocampus, and were less sensitive to the behavioral impairment by muscimol. Similarly, δ-KO mice also had reduced binding to forebrain regions and a lower behavioral sensitivity to muscimol than their wild-type controls. In contrast, α1-KO mice had unaltered behavioral sensitivity to muscimol and unaltered [3H]muscimol binding, even though previous studies have demonstrated dramatically reduced binding to various other GABAA-R sites in these mice. Finally, Thy1α6 mice exhibited increased behavioral sensitivity to muscimol, and to another direct GABA-site agonist gaboxadol, and increased [3H]muscimol binding in the cerebral cortex and hippocampus. Thus, the differences in sedative and motor-impairing actions of muscimol in various mouse models correlated with the level of forebrain high-affinity [3H]muscimol binding. These data suggest that a small special population of GABAA-Rs, most likely extrasynaptic non-α1-containing receptors, strongly contributes to the in vivo pharmacological effects of muscimol. [ABSTRACT FROM AUTHOR]

Neuronal histamine has a prominent role in sleep–wake control and body homeostasis, but a number of studies suggest that histamine has also a role in higher brain functions including drug reward. The present experiments characterized the involvement of histamine and its H3 receptor in ethanol-related behaviors in mice. Male histidine decarboxylase knockout (HDC KO) and control mice were used to study the role of histamine in ethanol-induced stimulation of locomotor activity, impairment of motor coordination, and conditioned place preference (CPP). Male C57BL/6Sca mice were used to study the effects of H3 receptor antagonist in the effects of ethanol on locomotor activity. The HDC KO mice displayed a weaker stimulatory response to acute ethanol than the wild-type (WT) mice. No differences between genotypes were found after ethanol administration on accelerating rotarod. The HDC KO mice showed stronger ethanol-induced CPP than the WT mice. Binding of the GABAA receptor ligand [3H]Ro15-4513 was not markedly changed in HDC KO mouse brain and thus could not explain altered responses in KO mice. Ethanol increased the activity of C57BL/6Sca mice, and H3 receptor antagonist ciproxifan inhibited this stimulation. In CPP paradigm ciproxifan, an H3 receptor inverse agonist potentiated ethanol reward. Histaminergic neurotransmission seems to be necessary for the stimulatory effect of ethanol to occur, whereas lack of histamine leads to changes that enhance the conditioned reward by ethanol. Our findings also suggest a role for histamine H3 receptor in modulation of the ethanol stimulation and reward. [ABSTRACT FROM AUTHOR]

Initial effects of drugs of abuse seem to converge on the mesolimbic dopamine pathway originating from the ventral tegmental area (VTA). Even after a single dose, many drugs of abuse are able to modulate the glutamatergic transmission activating the VTA dopamine neurons, which may represent a critical early stage in the development of addiction. Ligands acting on the benzodiazepine site of the inhibitory γ-aminobutyric acid type A (GABAA) receptors are known to be rewarding in animal models and have abuse liability in humans, but notably little evidence exists on the involvement of the mesolimbic dopamine system in their effects. Here we report that single in vivo doses of benzodiazepine-site agonists, similar to morphine and ethanol, induce a modulation in the glutamatergic transmission of VTA dopamine neurons. This is seen 24 h later as an increase in the ratio between α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated excitatory currents using whole-cell patch-clamp configuration in mouse VTA slices. The effect was due to increased frequency of spontaneous miniature AMPA receptor-mediated currents. It lasted at least 3 days after the injection of diazepam, and was prevented by coadministration of the benzodiazepine-site antagonist flumazenil or the NMDA receptor antagonist dizocilpine. A single injection of the GABAA receptor α1 subunit-preferring benzodiazepine-site ligand zolpidem also produced an increase in the AMPA/NMDA ratio in VTA dopamine neurons. These findings suggest a role for the mesolimbic dopamine system in the initial actions of and on neuronal adaptation to benzodiazepines.Neuropsychopharmacology (2009) 34, 290–298; doi:10.1038/npp.2008.89; published online 18 June 2008 [ABSTRACT FROM AUTHOR]

In mammals, identifying the contribution of specific neurons or networks to behavior is a key challenge. Here we describe an approach that facilitates this process by enabling the rapid modulation of synaptic inhibition in defined cell populations. Binding of zolpidem, a systemically active allosteric modulator that enhances the function of the GABAA receptor, requires a phenylalanine residue (Phe77) in the γ2 subunit. Mice in which this residue is changed to isoleucine are insensitive to zolpidem. By Cre recombinase–induced swapping of the γ2 subunit (that is, exchanging Ile77 for Phe77), zolpidem sensitivity can be restored to GABAA receptors in chosen cell types. We demonstrate the power of this method in the cerebellum, where zolpidem rapidly induces significant motor deficits when Purkinje cells are made uniquely sensitive to its action. This combined molecular and pharmacological technique has demonstrable advantages over targeted cell ablation and will be invaluable for investigating many neuronal circuits. [ABSTRACT FROM AUTHOR]

Compares the development of tolerance to the motor-impairing effects of repeated moderate diazepam doses between mice deficient in the cerebellar granule cell-restricted GABA receptor of alpha6 subunit and their wild-type controls. Complete tolerance in both mouse lines produced by chronic treatment; Tolerance to motor-impairing actions of diazepam.

The possible effect of atipamezole, a potent and specific α2-adrenoceptor antagonist, on the development and expression of d-amphetamine-induced behavioural sensitization was evaluated in mice. Male (C57Bl/6J) mice were given daily doses of d-amphetamine (2 mg/kg). In addition, groups of mice received injections of atipamezole (0.3 or 1 mg/kg) 20 min before d-amphetamine or vehicle administration. Idazoxan (1 mg/kg) was used in some experiments to extend the results to other α2-adrenoceptor antagonists. Challenge doses of d-amphetamine were administered to the mice on days 7–9 to evaluate the effects of α2-adrenoceptor antagonists on the d-amphetamine sensitization, evidenced by increased locomotor activation. Mice treated repeatedly with d-amphetamine developed strong locomotor sensitization that was reduced by pretreatment with α2-adrenoceptor antagonists. Acute atipamezole at both doses attenuated the expression of d-amphetamine-induced sensitization. Atipamezole at 1 mg/kg alone had no effect on locomotor activity, but the lower dose (0.3 mg/kg) increased locomotor activity after repeated administration. These results indicate that α2-adrenoceptor antagonists modulate the actions of d-amphetamine in a manner not explicable by their enhancing actions on noradrenaline and dopamine release, and may thus provide a novel approach to the treatment of motor complications caused by dopaminergic agents, such as dyskinesias, and perhaps also drug dependence. [ABSTRACT FROM AUTHOR]

Abstract Locomotor stimulation and behavioral sensitization induced by acute and repeated treatment with alcohol, cocaine or morphine were studied in the alcohol-preferring AA (Alko, Alcohol), alcohol-avoiding ANA (Alko, Non-Alcohol) rats and non-selected Wistar rats. Daily treatment with alcohol (ethanol, 0.4 or 1.0 g/kg, IP) for 6 days had no effect on locomotor activity either in the AA or ANA rats. Acute cocaine (5, 10 or 20 mg/kg, IP) produced a locomotor stimulation in the animals of all lines studied, and there was no difference in this effect between the AA and ANA rats. During a 4-day repeated cocaine treatment, the AA rats became sensitized with the 10 mg/kg dose, while the ANA rats did not show any sensitization with this dose. With the 20 mg/kg cocaine dose, in addition to locomotor stimulation, the rats of all lines studied showed stereotyped behavior, which response was enhanced during repeated treatment. Morphine-induced locomotor stimulation was larger in the AA rats than in the ANA or Wistar rats both with 1.0 and 3.0 mg/kg doses and only the AA rats were sensitized during 4-day treatment with the 1 mg/kg dose. With the 3.0 mg/kg morphine dose, only the AA rats showed a weak sensitization evident only during the initial 30 min after morphine injection. As the drug-induced behavioral sensitization is an important factor in the development of drug addiction, it is possible that mechanisms underlying the enhanced susceptibility of the AA rats to morphine- and cocaine-induced sensitization contribute to the high intake of alcohol and other abused drugs by these animals. [ABSTRACT FROM AUTHOR]

Reports that the alpha6 gene of alcohol-non-tolerant (ANT) rats is expressed at wild-type levels but carries a point mutation generating an arginine-to-glutamine substitution at position 100. Suggestion that cerebellar motor control may be a distinct behavioral correlate of the alpha6-subunit-containing GABAA (gamma-aminobutyric acid) receptor subtype; Mutation in the alpha6 subunit of ANT rats; Analysis of the alpha6 subunit gene in ANT and control rats.

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABA A receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of α1, α6 and β2 subunit mRNA expression, the expression of β3, γ2 and δ subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [ 3 H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [ 3 H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of α6 subunit mRNA present. Diazepam-sensitive [ 3 H]Ro 15-4513 binding was decreased only by 40%, contrasting >90% decrease in α1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the α1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of α5 (and/or α2 or α3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates α1, α6 and β2 subunits resulting in altered GABA A receptor pharmacology. [ABSTRACT FROM AUTHOR]