A novel benzodiazepine inverse agonist, S-8510, as a cognitive enhancer

The effect of age and sex on the expression of GABA signaling components in the human hippocampus and entorhinal cortex

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the nervous system. The GABA signaling system in the brain is comprised of GABA synthesizing enzymes, transporters, GABAA and GABAB receptors (GABA_AR and GABA_BR). Alterations in the expression of these signaling components have been observed in several brain regions throughout aging and between sexes in various animal models. The hippocampus is the memory centre of the brain and is impaired in several age-related disorders. It is composed of two main regions: the Cornu Ammonis (CA1-4) and the Dentate Gyrus (DG), which are interconnected with the Entorhinal Cortex (ECx). The age- and sex-specific changes of GABA signaling components in these regions of the human brain have not been examined. This study is the first to determine the effect of age and sex on the expression of GABA signaling components-GABA_AR α1,2,3,5, β1-3, γ2, GABA_BR R1 and R2 subunits and the GABA synthesizing enzymes GAD 65/67-in the ECx, and the CA1 and DG regions of the human hippocampus using Western blotting. No significant differences were found in GABA_AR α1,2,3,5, β1-3, γ2, GABA_BR R1 and R2 subunit and GAD65/76 expression levels in the ECx, CA1 and DG regions between the younger and older age groups for both sexes. However, we observed a significant negative correlation between age and GABA_AR α1subunit level in the CA1 region for females; significant negative correlation between age and GABA_AR β1, β3 and γ2 subunit expression in the DG region for males. In females a significant positive correlation was found between age and GABA_AR γ2 subunit expression in the ECx and GABA_BR R2 subunit expression in the CA1 region. The results indicate that age and sex do not affect the expression of GAD 65/67. In conclusion, our results show age- and sex-related GABA_A/BR subunit alterations in the ECx and hippocampus that might significantly influence GABAergic neurotransmission and underlie disease susceptibility and progression.

Hippocampal GABAA antagonism reverses the novel object recognition deficit in sub-chronic phencyclidine-treated rats

BACKGROUND

Abnormalities in prefrontal cortical and hippocampal GABAergic function are postulated to be major causes of the cognitive impairment associated with schizophrenia (CIAS). There are conflicting views on whether diminished or enhanced GABAergic activity contributes to the deficit in short-term novel object recognition (NOR) in the sub-chronic phencyclidine (scPCP) rodent model of CIAS. This study assessed the role of GABA_A signaling in the medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC) in NOR in saline (scSAL)- and scPCP-treated rats.

METHODS

The effects of local administration of a GABA_A agonist (muscimol) into the vHPC or mPFC and an antagonist (bicuculline) or a GABA_A/benzodiazepine partial agonist (bretazenil) into the vHPC on NOR in scSAL and scPCP-treated rats were determined.

RESULTS

In scSAL-treated rats, injection of muscimol into the vHPC, but not mPFC, induced a deficit in NOR. The scPCP-induced NOR deficit was significantly reversed by intra-vHPC bicuculline, while intra-vHPC bretazenil produced a non-significant trend for reversal (p = .06). scPCP treatment increased mRNA expression of GABA_A γ₂ in PFC and GABA_A α₅ and GABA_A β₁ in the HPC. However, GABA concentration in the PFC or HPC was not altered.

CONCLUSIONS

These findings indicate that the scPCP-induced NOR deficit can be rescued by reducing GABA_A receptor stimulation in vHPC, indicating that increased vHPC GABA_A inhibition may contribute to the scPCP-induced NOR deficit in rats. These results also indicate that excessive GABA_A receptor signalling in the vHPC has a deleterious effect on NOR in normal rats.

In vivo pharmacological characterization of AC-3933, a benzodiazepine receptor partial inverse agonist for the treatment of Alzheimer's disease

GABAergic neurons are known to inhibit neural transduction and therefore negatively affect excitatory neural circuits in the brain. We have previously reported that 5-(3-methoxyphenyl)-3-(5-methyl-1,2,4-oxadiazol-3-yl)-1,6-naphthyridin-2(1H)-one (AC-3933), a partial inverse agonist for the benzodiazepine receptor (BzR), reverses GABAergic inhibitory effect on cholinergic neurons, and thus enhances acetylcholine release from these neurons in rat hippocampal slices. In this study, we evaluated AC-3933 potential for the treatment of Alzheimer's disease, a disorder characterized by progressive decline mainly in cholinergic function. Oral administration of AC-3933 (0.01-0.03mg/kg) resulted in the amelioration of scopolamine-induced amnesia, as well as a shift in electroencephalogram (EEG) relative power characteristic of pro-cognitive cholinergic activators, such as donepezil. In addition, treatment with AC-3933 even at the high dose of 100mg/kg p.o. produced no seizure or anxiety, two major adverse effects of BzR inverse agonists developed in the past. These findings indicate that AC-3933 with its low risk for side effects may be useful in the treatment of Alzheimer's disease.

PWZ-029, an inverse agonist selective for α₅ GABAA receptors, improves object recognition, but not water-maze memory in normal and scopolamine-treated rats

Inverse agonism at the benzodiazepine site of α(5) subunit-containing GABA(A) receptors is an attractive approach for the development of putative cognition-enhancing compounds, which are still far from clinical application. Several ligands with binding and/or functional selectivity for α(5) GABA(A) receptors have been synthesized and tested in a few animal models. PWZ-029 is an α(5) GABA(A) selective inverse agonist whose memory enhancing effects were demonstrated in the passive avoidance task in rats and in Pavlovian fear conditioning in mice. In the present study we investigated the effects of PWZ-029 administration in novel object recognition test and Morris water maze, in normal and scopolamine-treated rats. All the three doses of PWZ-029 (2, 5 and 10 mg/kg) improved object recognition after the 24-h delay period, as shown by significant differences between the exploration times of the novel and old object, and the respective discrimination indices. PWZ-029 (2 mg/kg) also successfully reversed the 0.3 mg/kg scopolamine-induced deficit in recognition memory after the 1-h delay. In the Morris water maze test, PWZ-029 (5, 10 and 15 mg/kg) did not significantly influence swim patterns, either during five acquisition days or during the treatment-free probe trial. PWZ-029 (2, 5 and 10 mg/kg) also proved to be ineffective in the reversal of the 1mg/kg scopolamine-induced memory impairment in the water maze. The present mixed results encourage use of a variety of tests and experimental conditions in order to increase the predictability of preclinical testing of selective α(5) GABA(A) inverse agonists.

RO4938581, a novel cognitive enhancer acting at GABAA alpha5 subunit-containing receptors

RATIONALE

GABAA alpha5 subunit-containing receptors are primarily expressed in the hippocampus and their role in learning and memory has been demonstrated recently by both genetic and pharmacological approaches.

OBJECTIVES

The objective of the study is to evaluate the cognitive effects of a novel GABAA alpha5 receptor inverse agonist, RO4938581 in rats and monkeys.

MATERIALS AND METHODS

The in vitro profile was determined using radioligand binding and electrophysiological assays for the GABAA alpha1, alpha2, alpha3, and alpha5 receptors. Long-term potentiation (LTP) was performed in mouse hippocampal slices. Cognitive effects were assessed in rats in the delayed match to position (DMTP) task and the Morris water maze. In monkeys, the object retrieval task was used. Pro-convulsant and anxiogenic potentials were evaluated in mice and rats. In vivo receptor occupancy was determined using [3H]-RO0154513.

RESULTS

RO4938581 is a potent inverse agonist at the GABAA alpha5 receptor, with both binding and functional selectivity, enhancing hippocampal LTP. RO4938581 reversed scopolamine-induced working memory impairment in the DMTP task (0.3-1 mg/kg p.o.) and diazepam-induced spatial learning impairment (1-10 mg/kg p.o.). RO4938581 improved executive function in monkeys (3-10 mg/kg p.o.). Importantly, RO4938581 showed no anxiogenic and pro-convulsive potential. RO4938581 dose-dependently bound to GABAA alpha5 receptors and approximately 30% receptor occupancy was sufficient to produce enhanced cognition in the rat.

CONCLUSIONS

The data further support the potential of GABAA alpha5 receptors as a target for cognition-enhancing drugs. The dual binding and functional selectivity offers an ideal profile for cognition-enhancing effects without the unwanted side effects associated with activity at other GABAA receptor subtypes.

GABA(A) receptor subtypes as targets for neuropsychiatric drug development

The main inhibitory neurotransmitter system in the brain, the gamma-aminobutyric acid (GABA) system, is the target for many clinically used drugs to treat, for example, anxiety disorders and epilepsy and to induce sedation and anesthesia. These drugs facilitate the function of pentameric A-type GABA (GABA(A)) receptors that are extremely widespread in the brain and composed from the repertoire of 19 subunit variants. Modern genetic studies have found associations of various subunit gene polymorphisms with neuropsychiatric disorders, including alcoholism, schizophrenia, anxiety, and bipolar affective disorder, but these studies are still at their early phase because they still have failed to lead to validated drug development targets. Recent neurobiological studies on new animal models and receptor subunit mutations have revealed novel aspects of the GABA(A) receptors, which might allow selective targeting of the drug action in receptor subtype-selective fashion, either on the synaptic or extrasynaptic receptor populations. More precisely, the greatest advances have occurred in the clarification of the molecular and behavioral mechanisms of action of the GABA(A) receptor agonists already in the clinical use, such as benzodiazepines and anesthetics, rather than in the introduction of novel compounds to clinical practice. It is likely that these new developments will help to overcome the present problems of the chronic treatment with nonselective GABA(A) agonists, that is, the development of tolerance and dependence, and to focus the drug action on the neurobiologically and neuropathologically relevant substrates.

Characterization of a new synthetic isoflavonoid with inverse agonist activity at the central benzodiazepine receptor

Research aimed at developing selective drugs acting on gamma-aminobutyric acid (GABA)A receptors introduced compounds from diverse chemical classes unrelated to the 1,4-benzodiazepines, including flavonoids. These studies also revealed the potential use of inverse agonists as cognition-enhancing agents. Here we report pharmacological properties of the novel synthetic isoflavonoid 2-methoxy-3,8,9-trihydroxy coumestan (PCALC36). PCALC36 displaced [3H]flunitrazepam binding to rat brain synaptosomes with an IC50 of 13.8 microM. Scatchard analysis of the effect of PCALC36 showed a concentration-dependent reduction of the Bmax of [3H]flunitrazepam, without a marked change in Kd. This effect could be reversed by diluting and washing the preparation. Addition of 20-microM GABA shifted to the right the inhibition curve of PCALC36 on [3H]flunitrazepam binding (IC50 ratio of 0.68), which is characteristic for inverse agonists. PCALC36 produced little change in the GABAergic tonic currents recorded by whole-cell patch clamp in cultured rat hippocampal neurones, but it caused a 20% reduction in miniature inhibitory postsynaptic current amplitude and completely antagonised the full (direct) agonist midazolam in a quickly reversible manner. The data suggest that the coumestan backbone can be useful for developing novel ligands at the GABAA receptor.

Ion channel modulators that enhance acetylcholine release: potential therapies for Alzheimer’s disease

Enhancing the release of acetylcholine (ACh) in the brain is one approach to increasing neuronal activity, restoring central cholinergic tone and improving attention and cognition. ACh release is modulated by both ligand-gated (gamma-amino butyric acid A receptors/benzodiazepine [GABA(A)/BDZ], nicotinic-acetylcholine and serotonin, 5-HT3) and voltage-gated (calcium and potassium) ion channels. Of the ligand-gated channel modulators, the BDZ receptor (BDZR) inverse agonists (beta-CCM, ZK 93426) enhance activity-dependent release in animals, whereas S-8510, a partial inverse agonist, and the BDZR antagonist, flumazenil, show enhancement regardless of the behavioural state of the animal. Some of these agents have undergone limited clinical evaluation for Alzheimer's disease (AD) (ZK 93426, flumazenil, S-8510), but their potential anxiogenic liability makes their therapeutic use uncertain until more clinical data are available. Within the group of nicotinic agonists, ABT-418, though less potent than nicotine and epibatidine in promoting ACh release in vitro, was clinically evaluated based on its in vivo profile. Its lack of oral bioavailability has limited its acceptability, though transdermal administration has been used to circumvent this deficiency. Serotonin 5-HT3 receptor modulators have not been advanced for clinical evaluation for the treatment of AD. Among the voltage-gated ion channel modulators affecting L- or N-type calcium channels, nefiracetam, a nootropic agent, also increased ACh release in animal studies. It is currently undergoing clinical evaluation for AD, though a need for more potent and brain selective calcium channel blockers exists. Potassium channel modulators have been the most studied ACh release enhancing agents and several of these compounds (4-AP, 3,4-DAP, linopirdine) have been clinically evaluated. In AD patients, 4-AP, an A-type K+ channel blocker, elicited inconsistent and unremarkable effects. Linopirdine, whose enhancement of ACh release correlates with its ability to block M-type K+ channels, also produced disappointing clinical results, which may have been related to its suboptimal pharmacokinetic profile. Further work in this series has provided a compound (DMP 543) that should be a more reliable indicator of whether a blocker of this ion channel can activate the cholinergic system in man. This agent is currently undergoing clinical evaluation for AD.

Fluoxetine disrupts the integration of anxiety and aversive memories

Anxiety disorders may result from an overexpression of aversive memories. Evidence suggests that the hippocampal cholinergic system could be the point of convergence of anxiety and memory. We propose that clinically effective anxiolytics may exert their effect by interfering with this integration mechanism. To assess anxiety and aversive memory, we used the shock-probe burying test. A reduction in anxiety in this test is indicated by decreased burying, whereas impaired cognition is reflected by an increased number of probe-contacts and/or reduced retention latency. Both an aversive stimulus and the memory of that stimulus significantly increased hippocampal acetylcholine (ACh) levels (Experiment 1). In fact, the memory of the event seemed to be more important than the event itself since the aversive memory induced a greater increase in hippocampal ACh. Injections (i.p.) of fluoxetine (Prozac) reduced burying behavior, while not affecting probe contacts or retention latency (Experiment 2). Although injections of fluoxetine did not affect basal hippocampal ACh efflux (Experiment 3), fluoxetine abolished the increase in ACh induced by the aversive stimulus and the memory of that stimulus (Experiment 4), emphasizing the significance of aversive memories in anxiety disorders. These actions may be mediated by a decrease in the event-related enhancement in cholinergic neurotransmission through M1 cholinergic receptors (Experiment 5). Therefore, anxiety disorders may stem from an unopposed formation of aversive memories and clinically effective anxiolytics hinder the association between emotional and cognitive processing. This reduces the emotional impact of aversive memories, thereby opposing consequent anxiety.

Effects of hippocampal injections of a novel ligand selective for the alpha 5 beta 2 gamma 2 subunits of the GABA/benzodiazepine receptor on Pavlovian conditioning

Benzodiazepine pharmacology has led to greater insight into the neural mechanisms underlying learning and anxiety. The synthesis of new compounds capable of modulating responses produced by these receptors has been made possible by the development of an isoform model of the GABA(A)/benzodiazepine receptor complex. In the current experiment, rats were pretreated with several concentrations of the novel ligand RY024 (an alpha 5 beta 2 gamma 2 -selective benzodiazepine receptor inverse agonist) in the hippocampus and were trained in a Pavlovian fear conditioning paradigm. RY024 independently produced fear-related behavior prior to training and, at the highest concentration, decreased the strength of conditioning observed 24 h after training. These data provide further evidence for the involvement of hippocampal GABA(A)/benzodiazepine receptors in learning and anxiety.

Imaging the GABA-benzodiazepine receptor subtype containing the alpha5-subunit in vivo with [11C]Ro15 4513 positron emission tomography

There is evidence of marked variation in the brain distribution of specific subtypes of the GABA-benzodiazepine receptor and that particular subtypes mediate different functions. The alpha5-containing subtype is highly expressed in the hippocampus, and selective alpha5 inverse agonists (which decrease tonic GABA inhibition) are being developed as potential memory-enhancing agents. Evidence for such receptor localization and specialization in humans in vivo is lacking because the widely used probes for imaging the GABA-benzodiazepine receptors, [11C]flumazenil and [123I]iomazenil, appear to reflect binding to the alpha1 subtype, based on its distribution and affinity of flumazenil for this subtype. The authors characterized for positron emission tomography (PET) a radioligand from Ro15 4513, the binding of which has a marked limbic distribution in the rat and human brain in vivo. Competition studies in vivo in the rat revealed that radiolabeled Ro15 4513 uptake was reduced to nonspecific levels only by drugs that have affinity for the alpha5 subtype (flunitrazepam, RY80, Ro15 4513, L655,708), but not by the alpha1 selective agonist, zolpidem. Quantification of [11C]Ro15 4513 PET was performed in humans using a metabolite-corrected plasma input function. [11C]Ro15 4513 uptake was relatively greater in limbic areas compared with [11C]flumazenil, but lower in the occipital cortex and cerebellum. The authors conclude that [11C]Ro15 4513 PET labels in vivo the GABA-benzodiazepine receptor containing the alpha5 subtype in limbic structures and can be used to further explore the functional role of this subtype in humans.

Density and pharmacology of alpha5 subunit-containing GABA(A) receptors are preserved in hippocampus of Alzheimer's disease patients

The anatomical localization and pharmacology of alpha5 subunit-containing GABA type-A receptors in the human hippocampal formation of Alzheimer's disease patients were studied with an alpha5 receptor selective ligand, [3H]L-655,708 and compared to age-matched human controls. Autoradiographic analyses revealed a heterogeneous distribution of [3H]L-655,708 binding sites in CA1-CA3 areas with high levels in stratum oriens, stratum pyramidale and stratum radiatum contrasting with low levels in stratum lacunosum. The highest quantity of alpha5 receptors was found in the molecular layer of the dentate gyrus. This pattern of expression was identical in both hippocampus from control and Alzheimer's disease subjects. Quantitative studies demonstrated that the number of [3H]L-655,708 binding sites is well preserved in Alzheimer's disease with only a moderate reduction (25-30%) in the CA1 subfield and entorhinal cortex. Furthermore, saturation and competition experiments with [3H]L-655,708 and representative benzodiazepine site ligands revealed that alpha5 receptors in Alzheimer's hippocampus have an alpha5beta2/3gamma2 pharmacology and structure as in control human brain.Overall, the data reported here provide evidence for a specific expression and relative sparing of alpha5 subunit-containing gamma-aminobutyric acid type-A receptors in the hippocampus of Alzheimer's patients.

Preserved benzodiazepine receptors in Alzheimer's disease measured with C-11 flumazenil PET and I-123 iomazenil SPECT in comparison with CBF

This study evaluates the regional cerebral blood flow (CBF) with H2(15)O-PET and the distribution of central benzodiazepine receptor (BZR) with C-11 flumazenil (FMZ) by PET and I-123 iomazenil (IMZ) by SPECT in Alzheimer's disease (AD). In AD, whereas the CBF was diminished in the frontal, temporal, parietal, and occipital cortex, the distribution volume of FMZ and delayed activity of IMZ were relatively preserved in these cortices, suggesting that the BZR reduction, reflecting neuronal loss, is less prominent than the CBF suppression. The mini-mental state examination score (MMS) was weakly correlated with the CBF in the parietal cortex but not with BZR. It is speculated that the neuronal density reflected by BZR is less impaired than the neuronal function assessed with blood flow in the association cortex of AD. High correlation was found between the uptake of FMZ and the delayed activity of IMZ. The delayed image of IMZ-SPECT is clinically useful to evaluate the preservation of neuronal density in the affected temoporoparietal association cortex in AD.

Radioimmunoassay for a novel benzodiazepine inverse agonist, S-8510, in human plasma and urine

A radioimmunoassay (RIA) was developed for a new benzodiazepine inverse agonist, 2-(isoxazol-3-yl)-3,6,7,9-tetrahydroimidazo [4,5-d] pyrano [4,3-b] pyridine monophosphate monohydrate (S-8510), in human plasma and urine. For competitive RIA, three amino derivatives of S-8510 were labelled by the Bolton and Hunter method and rabbit antisera were prepared using three immunogens, conjugates of three carboxyl derivatives of S-8510 with bovine serum albumin. All combinations of the labelled antigens and antisera were examined and homologous combinations were selected for the competitive RIA. One of the three homologous combinations had the best selectivity after investigations of cross-reactivity using 12 related compounds and was very sensitive for S-8510. Next, a pretreatment for biological samples was developed using mixed mode solid-phase extraction (SPE) column followed by the RIA (SPE/RIA). The assay recoveries for human plasma and urine were both excellent and the limits of quantitation were extremely low, 80 and 200 pg ml(-1), respectively. Human plasma samples and urine samples after administration of this drug were successfully measured by the SPE/RIA. No cross-reactive metabolites were detected in any fractions after RP-HPLC separation of the plasma samples. The RIA using carefully selected antiserum and labelled antigen was highly specific for unchanged S-8510. To simplify the RIA procedure, a scintillation proximity assay (SPA) using the same labelled antigen and antiserum was developed for analyzing S-8510 in human plasma and found to be very promising.

Effects of S-8510, a novel benzodiazepine receptor partial inverse agonist, on basal forebrain lesioning-induced dysfunction in rats

We investigated the effects of a novel benzodiazepine partial inverse agonist, S-8510 (2-(3-isoxazolyl)-3,6,7,9-tetrahydroimidazo [4,5-d] pyrano [4,3-b] pyridine monophosphate monohydrate), on the impairment of spatial memory, decreased high-affinity choline uptake and acetylcholine release in basal forebrain-lesioned rats. S-8510 (3 and 5 mg/kg, p.o. 30 min before each training session) significantly ameliorated the basal forebrain-lesion-induced impairment of spatial memory in water maze task. In vivo brain microdialysis studies showed that systemic administration of S-8510 at 3 and 10 mg/kg significantly increased the release of acetylcholine in the front-parietal cortex in basal forebrain-lesioned rats. Further, repeated administration of S-8510 (3 and 10 mg kg(-1) day(-1) for 5 days) reversed the decrease in cortical high-affinity choline uptake induced by basal forebrain lesion. Thus, S-8510 improved the spatial memory impairment induced by lesion of the basal forebrain in rats. In addition, it increased acetylcholine release and high-affinity choline uptake from the cortex, a region closely associated with memory, in basal forebrain-lesioned rats. These results indicate that S-8510 has cognition enhancing and cholinergic-activating effects in the basal forebrain-lesioned rats, suggesting that this agent may be useful for the treatment of mild to moderate senile dementia including Alzheimer's disease.