The heterogeneity of γ-aminobutyric acid type A (GABA A ) receptors contributes to the diversity of neuronal inhibition in the regulation of information processing. Although most GABA A receptors are ...located synaptically, the small population of α 5 GABA A receptors is largely expressed extrasynaptically. To clarify the role of the α 5 GABA A receptors in the control of behavior, a histidine-to-arginine point mutation was introduced in position 105 of the murine α 5 subunit gene, which rendered the α 5 GABA A receptors diazepam-insensitive. Apart from an incomplete muscle relaxing effect, neither the sedative, anticonvulsant, nor anxiolytic-like activity of diazepam was impaired in α 5 (H105R) mice. However, in hippocampal pyramidal cells, the point mutation resulted in a selective reduction of α 5 GABA A receptors, which altered the drug-independent behavior. In line with the role of the hippocampus in certain forms of associative learning, trace fear conditioning, but not delay conditioning or contextual conditioning, was facilitated in the mutant mice. Trace fear conditioning differs from delay conditioning in that the conditioned and unconditioned stimulus are separated by a time interval. Thus, the largely extrasynaptic α 5 GABA A receptors in hippocampal pyramidal cells are implicated as control elements of the temporal association of threat cues in trace fear conditioning.
Benzodiazepine tranquilizers are used in the treatment of anxiety disorders. To identify the molecular and neuronal target mediating the anxiolytic action of benzodiazepines, we generated and ...analyzed two mouse lines in which the α2 or α3 GABAA(γ-aminobutyric acid type A) receptors, respectively, were rendered insensitive to diazepam by a knock-in point mutation. The anxiolytic action of diazepam was absent in mice with the α2(H101R) point mutation but present in mice with the α3(H126R) point mutation. These findings indicate that the anxiolytic effect of benzodiazepine drugs is mediated by α2 GABAAreceptors, which are largely expressed in the limbic system, but not by α3 GABAAreceptors, which predominate in the reticular activating system.
Overactivity of the dopaminergic system in the brain is considered to be a contributing factor to the development and symptomatology of schizophrenia. Therefore, the GABAergic control of dopamine ...functions was assessed by disrupting the gene encoding the α3 subunit of the GABA A receptor. α3 knockout (α3KO) mice exhibited neither an obvious developmental defect nor apparent morphological brain abnormalities, and there was no evidence for compensatory up-regulation of other major GABA A -receptor subunits. Anxiety-related behavior in the elevated-plus-maze test was undisturbed, and the anxiolytic-like effect of diazepam, which is mediated by α2-containing GABA A receptors, was preserved. As a result of the loss of α3 GABA A receptors, the GABA-induced whole-cell current recorded from midbrain dopamine neurons was significantly reduced. Spontaneous locomotor activity was slightly elevated in α3KO mice. Most notably, prepulse inhibition of the acoustic startle reflex was markedly attenuated in the α3KO mice, pointing to a deficit in sensorimotor information processing. This deficit was completely normalized by treatment with the antipsychotic D2-receptor antagonist haloperidol. The amphetamine-induced hyperlocomotion was not altered in α3KO mice compared with WT mice. These results suggest that the absence of α3-subunit-containing GABA A receptors induces a hyperdopaminergic phenotype, including a severe deficit in sensorimotor gating, a common feature among psychiatric conditions, including schizophrenia. Hence, agonists acting at α3-containing GABA A receptors may constitute an avenue for an effective treatment of sensorimotor-gating deficits in various psychiatric conditions. haloperidol sensorimotor gating
Prepulse inhibition (PPI) refers to the phenomenon in which a low-intensity prepulse stimulus attenuates the reflexive response to a succeeding startle-eliciting pulse stimulus. The hippocampus, ...among other structures, is believed to play an important role in the modulation of PPI expression. In α5(H105R) mutant mice, the expression of the α5 subunit-containing GABAA receptors in the hippocampus is reduced. Here, we report that PPI was attenuated, and spontaneous locomotor activity was increased in α5(H105R) mutant mice. These effects were apparent in both genders. Thus, α5 subunit-containing GABAA receptors, which are located extrasynaptically and are thought to mediate tonic inhibition, are important regulators of the expression of PPI and locomotor exploration. Post-mortem analyses of schizophrenia brains have consistently revealed structural abnormalities of a developmental origin in the hippocampus. There may be a possibility that such abnormalities include disturbance of α5 GABAA receptor function or distribution, given that schizophrenia patients are known to exhibit a PPI deficit. Our data further highlight that the potential use of α5-selective inverse agonists to treat hippocampal-related mnemonic dysfunction needs to be considered against the possibility that such compounds may be adversely associated with deficient sensorimotor gating.
Aims/hypothesis
Non-invasive diagnostic tools specific for pancreatic beta cells will have a profound impact on our understanding of the pathophysiology of metabolic diseases such as diabetes. The ...objective of this study was to use molecular imaging probes specifically targeting beta cells on human samples and animal models using state-of-the-art imaging modalities (fluorescence and PET) with preclinical and clinical perspective.
Methods
We generated a monoclonal antibody, 8/9-mAb, targeting transmembrane protein 27 (TMEM27; a surface
N
-glycoprotein that is highly expressed on beta cells), compared its expression in human and mouse pancreas, and demonstrated beta cell-specific binding in both. In vivo imaging was performed in mice with subcutaneous insulinomas overexpressing the human
TMEM27
gene, or transgenic mice with beta cell-specific
hTMEM27
expression under the control of rat insulin promoter (RIP-hTMEM27-tg), using fluorescence and radioactively labelled antibody, followed by tissue ex vivo analysis and fluorescence microscopy.
Results
Fluorescently labelled 8/9-mAb showed beta cell-specific staining on human and mouse pancreatic sections. Real-time PCR on islet cDNA indicated about tenfold higher expression of
hTMEM27
in RIP-hTMEM27-tg mice than in humans. In vivo fluorescence and PET imaging in nude mice with insulinoma xenografts expressing
hTMEM27
showed high 8/9-mAb uptake in tumours after 72 h. Antibody homing was also observed in beta cells of RIP-hTMEM27-tg mice by in vivo fluorescence imaging. Ex vivo analysis of intact pancreas and fluorescence microscopy in beta cells confirmed these findings.
Conclusions/interpretation
hTMEM27 constitutes an attractive target for in vivo visualisation of pancreatic beta cells. Studies in mouse insulinoma models and mice expressing
hTMEM27
demonstrate the feasibility of beta cell-targeted in vivo imaging, which is attractive for preclinical investigations and holds potential in clinical diagnostics.
Benzodiazepines reduce EEG slow‐wave activity in non‐REM sleep by potentiating GABAergic neurotransmission at GABAA receptors via a modulatory binding site. However, the mechanisms of action ...underlying the effects of benzodiazepines on sleep and the sleep EEG are still unknown. Slow waves during sleep are generated by the corticothalamic system and synchronized by the inhibitory GABAergic neurons of the reticular thalamic nucleus. This region contains exclusively α3‐containing GABAA receptors. We investigated the role of these receptors in the mediation of diazepam effects on the sleep EEG by studying point‐mutated mice in which the α3‐GABAA receptor is diazepam‐insensitive α3(H126R). Sleep was recorded for 12 h after i.p. injection of 3 mg/kg diazepam or vehicle at light onset in α3(H126R) and wild‐type controls (n = 13–17 per genotype). The main effect was a marked reduction of slow‐wave activity (EEG power density in 0.75–4.00 Hz) in non‐REM sleep and a concomitant increase in frequencies above 15.00 Hz in non‐REM sleep and waking in both genotypes. Neither effect of diazepam differed significantly between the genotypes. Despite the exclusive expression of α3‐containing GABAA receptors in the reticular thalamic nucleus, these receptors do not seem to be critical for the mediation of the effects of diazepam on the sleep EEG.
Hippocampal GABA(A) receptors containing the alpha 5 subunit have been implicated in the modulation of hippocampal-dependent learning, presumably via their tonic inhibitory influence on hippocampal ...glutamatergic activity. Here, we examined the expression of latent inhibition (LI)--a form of selective learning that is sensitive to a number of manipulations targeted at the hippocampal formation, in alpha 5(H105R) mutant mice with reduced levels of hippocampal alpha 5-containing GABA(A) receptors. A single pre-exposure to the taste conditioned stimulus (CS) prior to the pairing of the same CS with LiCl-induced nausea was effective in reducing the conditioned aversion against the taste CS in wild-type mice--thus constituting the LI effect. LI was however distinctly absent in male alpha 5(H105R) mutant mice. Hence, a partial loss of hippocampal alpha 5 GABA(A) receptors is sufficient to alter one major form of selective learning, albeit this was not seen in the female. This observed phenotype suggests that specific activation of these extrasynaptic GABA(A) receptors may confer therapeutic potential against the failure to show selectivity in learning by human psychotic patients.
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