Spencer J. Tye

Evidence for a Significant Role of α3-Containing GABAA Receptors in Mediating the Anxiolytic Effects of Benzodiazepines

The GABAA receptor subtypes responsible for the anxiolytic effects of nonselective benzodiazepines (BZs) such as chlordiazepoxide (CDP) and diazepam remain controversial. Hence, molecular genetic data suggest that α2-rather than α3-containing GABAA receptors are responsible for the anxiolytic effects of diazepam, whereas the anxiogenic effects of an α3-selective inverse agonist suggest that an agonist selective for this subtype should be anxiolytic. We have extended this latter pharmacological approach to identify a compound, 4,2′-difluoro-5′-[8-fluoro-7-(1-hydroxy-1-methylethyl)imidazo[1,2-á]pyridin-3-yl]biphenyl-2-carbonitrile (TP003), that is an α3 subtype selective agonist that produced a robust anxiolytic-like effect in both rodent and non-human primate behavioral models of anxiety. Moreover, in mice containing a point mutation that renders α2-containing receptors BZ insensitive (α2H101R mice), TP003 as well as the nonselective agonist CDP retained efficacy in a stress-induced hyperthermia model. Together, these data show that potentiation of α3-containing GABAA receptors is sufficient to produce the anxiolytic effects of BZs and that α2 potentiation may not be necessary.

Orexin Receptor Antagonists Differ from Standard Sleep Drugs by Promoting Sleep at Doses That Do Not Disrupt Cognition

Selective orexin receptor antagonism promotes sleep at doses lower than those that impair attention and memory in contrast to the GABA receptor–positive allosteric modulators currently in use. Sleep Without the After Effects Currently available treatments for insomnia can produce a number of central nervous system–based cognitive side effects, including the potential to impair memory and attention. Recently, selective dual orexin receptor antagonists, such as suvorexant and almorexant, have been shown to promote sleep onset and maintenance in clinical trials for patients with insomnia. In new work, Uslaner and colleagues compared sleep-promoting doses to the cognitive-impairing doses for an orexin receptor antagonist, DORA-22, versus sleep drugs currently in use: zolpidem, diazepam, or eszopiclone. At doses that produced equivalent amounts of sleep in rat and rhesus monkey, zolpidem, diazepam, and eszopiclone significantly disrupted attention and memory, whereas DORA-22 promoted sleep at doses that did not exert measurable effects on cognition. Furthermore, when compared to the other insomnia treatments that modulate γ-aminobutyric acid (GABA) receptor function, the authors saw greater separation for orexin receptor antagonism between doses that promoted sleep and doses that reduced expression of a hippocampal gene involved in synaptic plasticity called Arc. These findings suggest that dual orexin receptor antagonists might provide an effective treatment for insomnia with a greater therapeutic margin for sleep versus cognitive disturbances compared to the GABAA-positive allosteric modulators currently available. Current treatments for insomnia, such as zolpidem (Ambien) and eszopiclone (Lunesta), are γ-aminobutyric acid type A (GABAA)–positive allosteric modulators that carry a number of side effects including the potential to disrupt cognition. In an effort to develop better tolerated medicines, we have identified dual orexin 1 and 2 receptor antagonists (DORAs), which promote sleep in preclinical animal models and humans. We compare the effects of orally administered eszopiclone, zolpidem, and diazepam to the dual orexin receptor antagonist DORA-22 on sleep and the novel object recognition test in rat, and on sleep and two cognition tests (delayed match to sample and serial choice reaction time) in the rhesus monkey. Each compound’s minimal dose that promoted sleep versus the minimal dose that exerted deficits in these cognitive tests was determined, and a therapeutic margin was established. We found that DORA-22 has a wider therapeutic margin for sleep versus cognitive impairment in rat and rhesus monkey compared to the other compounds tested. These data were further supported with the demonstration of a wider therapeutic margin for DORA-22 compared to the other compounds on sleep versus the expression of hippocampal activity–regulated cytoskeletal-associated protein (Arc), an immediate-early gene product involved in synaptic plasticity. These findings suggest that DORAs might provide an effective treatment for insomnia with a greater therapeutic margin for sleep versus cognitive disturbances compared to the GABAA-positive allosteric modulators currently in use.

Enhanced performance of spatial and visual recognition memory tasks by the selective acetylcholinesterase inhibitor E2020 in rhesus monkeys

Abstract Hepatotoxicity limits the clinical utility of the cholinesterase inhibitor tacrine as a palliative therapy for Alzheimer’s disease. The present studies examined the effects of E2020, a selective acetylcholinesterase inhibitor not associated with liver toxicity in man, on cognitive performance in rhesus monkeys using tasks employed previously to evaluate tacrine and other cholinomimetic agents. The ability of E2020 to prevent the induction of a cognitive impairment by the muscarinic receptor antagonist scopolamine was assessed using an automated spatial delayed response task. Coadministration of E2020 (0.5–1.75 mg/kg) caused a dose-dependent reversal of the scopolamine (0.03 mg/kg) induced impairment observed after retention intervals of 10 and 20 s. At the highest dose of E2020 examined (1.75 mg/kg), choice accuracy approached normal control levels. In this dose range, E2020 was well tolerated, but at the higher dose of 2 mg/kg, cholinergic side-effects were apparent. The effect of E2020 on choice accuracy in a visual recognition task was also assessed as this task does not require the use of scopolamine to disrupt performance and beneficial effects of cholinomimetics can therefore be detected at lower doses than in the spatial memory paradigm. In this task, administration of E2020 increased choice accuracy from 59 ± 1% correct to up to 71 ± 2% at doses of 0.03 and 0.05 mg/kg. No observable adverse effects were induced by E2020 in this dose range. The ability of E2020 to improve performance in these cognitive tasks resembles the profile of other cholinesterase inhibitors, including tacrine, that also improve cognitive function in Alzheimer’s disease patients. Because of its more favourable clinical safety profile, E2020 may provide a significantly improved palliative therapy for dementia.

Reversal of cognitive impairment by heptyl physostigmine, a long-lasting cholinesterase inhibitor, in primates

Cholinergic replacement therapy for Alzheimers disease using existing cholinesterase inhibitors is compromised by short duration, meagre benefits restricted to subgroups of patients, and peripheral toxicity. Heptyl physostigmine is a lipophilic carbamate derivative of physostigmine. In rhesus monkeys, heptyl physostigmine (0.2-0.9 mg/kg i.m.) fully reversed a scopolamine-induced cognitive impairment. Following oral administration in squirrel monkeys, heptyl physostigmine (8 mg/kg) induced long-lasting hypothermia (greater than or equal to 4 h), a centrally-mediated cholinergic effect. Erythrocyte acetylcholinesterase activity was inhibited by 86% at the time of peak hypothermia (180 min). Clinical trials with heptyl physostigmine will enable a more rigorous evaluation of cholinomimetic therapy for dementia.

Lack of effect of CCKB receptor antagonists in ethological and conditioned animal screens for anxiolytic drugs.

The effects of the CCKB receptor antagonists L-365,260, CI-988 and L-740,093, a new compound with improved bioavailability and CNS penetration, were assessed for anxiolytic-like effects in three rat anxiolytic screens sensitive to benzodiazepines, the elevated plus maze (EPM), conditioned suppression of drinking (CSD) and conditioned emotional response (CER) tests. In the EPM, L-740,093 (0.1–1.0 mg/kg), L-365,260 (0.00001–10.0 mg/kg), and CI-988 (0.01–1.0 mg/kg) did not increase the time spent on the open arms of the maze or the number of entries onto the open arms. In contrast, the benzodiazepine receptor partial agonist, bretazenil (0.3–10.0 mg/kg), significantly increased both the time spent on the open arms and the number of open arm entries. In the CSD and the CER tests, L-740,093 (0.1–1.0 mg/kg) L-365,260 (0.0001–0.1 mg/kg) and CI-988 (0.01–10.0 mg/kg) failed to increase suppression ratios compared to the vehicle-treated control rats, whereas, the benzodiazepine receptor partial agonist FG 8205 (10.0 mg/kg) (CSD) and bretazenil (0.3–3.0 mg/kg) (CER) both significantly increased suppression ratios compared to vehicle-treated control rats. In addition, L-365,260 (1.0–50.0 mg/kg), CI-988 (0.1–10.0 mg/kg) and diazepam (0.1–1.0 mg/kg) were assessed in a squirrel monkey conflict procedure. Although diazepam significantly increased suppressed lever pressing rates, L-365,260 and CI-988 were without effect. The present findings provide little support for the hypothesis that CCKB receptor antagonists have anti-anxiety effects in animals.

Evidence against a specific effect of cholinergic drugs on spatial memory in primates

A scopolamine-like delay-dependent impairment in spatial delayed response performance in rhesus monkeys was induced by irrelevant interpolated activity or by using extended retention intervals. Physostigmine readily reversed the effects of scopolamine but had no effect on performance in young monkeys performing an irrelevant distractor task or in monkeys tested using extended retention intervals. Reducing stimulus control did not impair performance and did not alter the dose-response curve for induction of a deficit by scopolamine. Reducing the stimulus presentation time impaired performance across all retention intervals in a way which did not resemble the effect of scopolamine and which disappeared with practice. Our findings do not support the proposal that physostigmine interacts specifically with short-term spatial memory in primates.

Blockade of CCK-B receptors by L-365,260 induces analgesia in the squirrel monkey.

The potential antinociceptive effects of the selective cholecystokinin-B (CCK-B) antagonist L-365,260 were examined in the squirrel monkey tail withdrawal test. Pain threshold was measured in 6 male monkeys by recording the latency to remove the tail from a warm (55 degrees C) water bath. L-365,260 at doses of 100 ng/kg to 100 micrograms/kg significantly elevated tail withdrawal latencies throughout a 2 h test period. These data provide the first evidence that blockade of CCK-B receptors induces analgesia in primates.

Anxiolytic-like and antinociceptive effects of MK-801 accompanied by sedation and ataxia in primates

Anxiolytic-like and antinociceptive activities of the noncompetitive NMDA receptor antagonist MK-801 (dizocilpine) were compared with sedative and ataxic side effects in primates. Administration of MK-801 (0.1 mg/kg) caused taming in cynomolgus monkeys and increased tail withdrawal latencies in squirrel monkeys; both effects were accompanied by sedation and ataxia. These findings are discussed in relation to the possible therapeutic uses of NMDA antagonists and differences in the behavioural effects of such compounds in primate and rodent species.

Failure of d-cycloserine to reverse cognitive disruption induced by scopolamine or phencylidine in primates

The partial glycine agonist d-cycloserine may be of therapeutic use as a cognitive enhancer. We examined the ability of d-cycloserine (3-14 mg/kg i.m.) to reverse cognitive impairments induced by scopolamine (0.03 mg/kg) or PCP (0.25 mg/kg). There was no evidence for a dose-related improvement in visuospatial memory using retention intervals of 2,10 or 20 sec. A possible explanation for these findings is that central glycine function cannot be adequately increased using systemic administration of d-cycloserine in this dose range.

Quantitative electroencephalography within sleep/wake states differentiates GABAA modulators eszopiclone and zolpidem from dual orexin receptor antagonists in rats.

Dual orexin receptor antagonists (DORAs) induce sleep by blocking orexin 1 and orexin 2 receptor-mediated activities responsible for regulating wakefulness. DORAs represent a potential alternative mechanism to the current standard of care that includes the γ-aminobutyric acid (GABA)A receptor-positive allosteric modulators, eszopiclone and zolpidem. This work uses an innovative method to analyze electroencephalogram (EEG) spectral frequencies within sleep/wake states to differentiate the effects of GABAA modulators from DORA-22, an analog of the DORA MK-6096, in Sprague–Dawley rats. The effects of low, intermediate, and high doses of eszopiclone, zolpidem, and DORA-22 were examined after first defining each compound’s ability to promote sleep during active-phase dosing. The EEG spectral frequency power within specific sleep stages was calculated in 1-Hz intervals from 1 to 100 Hz within each sleep/wake state for the first 4 h after the dose. Eszopiclone and zolpidem produced marked, dose-responsive disruptions in sleep stage-specific EEG spectral profiles compared with vehicle treatment. In marked contrast, DORA-22 exhibited marginal changes in the spectral profile, observed only during rapid eye movement sleep, and only at the highest dose tested. Moreover, while eszopiclone- and zolpidem-induced changes were evident in the inactive period, the EEG spectral responses to DORA-22 were absent during this phase. These results suggest that DORA-22 differs from eszopiclone and zolpidem whereby DORA-22 promotes somnolence without altering the neuronal network EEG activity observed during normal sleep.