Neuroprotective effects of HTR1A antagonist WAY-100635 on scopolamine-induced delirium in rats and underlying molecular mechanisms

BACKGROUND

Limited surveys have assessed the performance of 5-hydroxytreptamine receptor 1A and its antagonist WAY-100635 in pharmacological manipulations targeting delirium therapies. The purpose of this paper was to assess the central pharmacological activity of WAY-100635 in a rat model of scopolamine-induced delirium and its underlying mechanism.

RESULTS

A delirium rat model was established by intraperitoneal injection of scopolamine and behavioral changes evaluated through open field and elevated plus maze experiments. Concentrations of monoamines in the hippocampus and amygdalae were detected by high performance liquid chromatography. The effect of WAY-100635 on the recovery of rats from delirium was assessed by stereotactic injection of WAY-100635 and its mechanism of action determined by measuring mRNA and protein expression via real time PCR and western blotting methods. The total distance and the number of crossing and rearing in the elevated plus maze test and the time spent in the light compartment in the dark/light test of scopolamine-treated rats were significantly increased while the percentage of time spent in the open arms was decreased, showing the validity of the established delirium rat model. The measurement of the concentrations of noradrenaline, 3,4-dihydroxyphenylacetic acid, the homovanillic acid, 5-hydroxy-3-indoleacetic acid and serotonin concentrations in the cerebrospinal fluid (CSF) of scopolamine-induced delirium rats were significantly increased. The intra-hippocampus and intra-BLA injections of WAY-100635 improved the delirium-like behavior of rats by significantly reducing the expression of NLRP3 inflammasome and the release of IL1-β and IL8 into CSF.

CONCLUSIONS

Taken together, these findings indicate that WAY-100635 may exert a therapeutic effect on post-operative delirium by controlling neurotransmission as well as suppressing neuroinflammation in the central nervous system.

5-HT(1A) receptor antagonist improves behavior performance of delirium rats through inhibiting PI3K/Akt/mTOR activation-induced NLRP3 activity

Postoperative delirium is a common complication that often results in poor outcomes in surgical and elderly patients. Accumulating evidences suggest that the pathophysiology of delirium results from multiple neurotransmitter system dysfunctions. To further clarify the effects of the selective serotonin (5-HT) (1A) antagonist WAY-100635 on the behaviors in scopolamine induced-delirium rats and to explore the molecular mechanism, in this study, we investigated the change of monoamine levels in the cerebrospinal fluid (CSF) and different brain regions using high-performance liquid chromatography and assessed the behavioral retrieval of delirium rats treated with WAY-100635. It was found that 5-hydroxy-3-indoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid, and homovanillic acid concentrations in the CSF of scopolamine-induced delirium rats were significantly increased, among which 5-HIAA was also increased in hippocampus and basolateral amygdala (BLA), and 5-HT(1A) receptor was significantly higher in the hippocampuses and BLA than other brain regions. Furthermore, intrahippocampus and intra-BLA stereotactic injection of WAY-100635 improved the delirium-like behavior of rats. Mechanistically, after WAY-100635 treatment, significant reduction of IL-1β release into CSF and NOD-like receptor family, pyrin domain containing 3 (NLRP3) expression, phosphorylated phosphatidylinositol-3-kinase (PI3K), protein kinase B (AKT), and S6K was observed. Altogether, these results suggest that delirium rats induced by scopolamine may be correlated with an increased cerebral concentration of 5-HT and dopamine neurotransmitters system; the selective 5-HT(1A) antagoniszts can reverse the delirium symptoms at some extent through tendering PI3K/Akt/mammalian target of rapamycin complex 1 (mTOR) activation-induced NLRP3 activity and then reducing IL-1β release.

Psychotogenic drugs and delirium pathogenesis: the central role of the thalamus

Delirium is thought to be a temporary psychiatric disorder resulting from a reduced central cholinergic transmission, combined with an increased dopaminergic transmission. The cholinergic and the dopaminergic systems interact not only with each other but with glutamatergic and gamma-amino-butyric acid (GABA) pathways. Besides the cerebral cortex, critical anatomical substrates of psychosis pathophysiology would comprise the striatum, the substantia nigra/ventral tegmental area, and the thalamus. The thalamus acts as a filter, allowing only the relevant information to travel to the cortex. Drugs of abuse (e.g. PCP, Ecstasy), as well as psychoactive medications frequently prescribed to hospitalized patients (e.g. benzodiazepines, opioids) could compromise the thalamic gating function, leading to sensory overload and hyperarousal. We propose that drug-induced delirium would result from the transient thalamic dysfunction caused by exposure to medications that interfere with central glutamatergic, GABAergic, dopaminergic and cholinergic pathways at critical sites of action. This model provides directions for future studies in neurophysiology, in vivo brain imaging, and psychopharmacology investigating delirium neuropathophysiology.

Therapeutic potential of positive AMPA modulators and their relationship to AMPA receptor subunits. A review of preclinical data

BACKGROUND

Positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) modulators enhance glutamate transmission via the AMPA receptor by altering the rate of desensitization; alone they have no intrinsic activity. They are the only class of compounds known that may pharmacologically separate AMPA subtypes.

OBJECTIVE

This manuscript will review preclinical work on positive AMPA modulators, with clinical examples where relevant.

RESULTS

The activity of these compounds appears to be determined by the AMPA receptor subunit composition. Studies have shown that splice variant and/or subunit combinations change the desensitization rate of this receptor. Also, these subunits are heterogeneously expressed across the central nervous system. Therefore, the functional outcome of different positive AMPA modulators could indeed be different. The origins of this pharmacological class come from hippocampal long-term potentiation studies, so quite naturally they were first studied in models of short- and long-term memory (e.g., delayed match to sample, maze performance). In general, these agents were procognitive. However, more recent work with different chemical classes has suggested additional therapeutic effects in models of schizophrenia (e.g., amphetamine locomotor activity), depression (e.g., forced swim test), neuroprotection (e.g., NMDA agonist lesions) and Parkinson's disease (e.g., 6-hydroxydopamine lesion).

CONCLUSIONS

In conclusion, positive modulation of AMPA may offer numerous therapeutic avenues for central nervous system drug discovery.

Cholinergic and dopaminergic mechanisms involved in the recovery of circadian anticipation by aniracetam in aged rats

We have reported that repeated administration of aniracetam (100 mg/kg p.o.) for 7 consecutive days recovers mealtime-associated circadian anticipatory behavior diminished in aged rats. The present study examines the mode of action underlying the restoration by aniracetam with various types of receptor antagonists. Coadministration of scopolamine (0.1 mg/kg i.p.) or haloperidol (0.1 mg/kg i.p.) for the last 3 days significantly reduced the restorative effects of aniracetam without affecting the timed feeding-induced anticipatory behavior by each receptor antagonist itself. The other receptor antagonists, mecamylamine (3 mg/kg i.p.), 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX, 1 microg/rat i.c.v.) had no effect on either the basal or aniracetam-elicited circadian anticipation. In contrast, ketanserin (1 mg/kg i.p.) itself recovered the diminished anticipatory behavior as aniracetam did, but it did not alter the restorative effects of aniracetam. Among the receptor antagonists tested, NBQX reduced appetite and haloperidol induced circadian hypoactivity. These results suggest that the food-entrainable circadian oscillations or the temporal regulatory system of behavior is modulated by cholinergic, dopaminergic and serotonergic systems. Furthermore, aniracetam may restore the aging-diminished behavioral anticipation by activating muscarinic acetylcholine (ACh) and/or dopamine (DA) D2 receptors through the enhanced release of ACh and/or DA in the brain.

Aniracetam: its novel therapeutic potential in cerebral dysfunctional disorders based on recent pharmacological discoveries

Aniracetam is a pyrrolidinone-type cognition enhancer that has been clinically used in the treatment of behavioral and psychological symptoms of dementia following stroke and in Alzheimer's disease. New discoveries in the behavioral pharmacology, biochemistry and pharmacokinetics of aniracetam provided new indications for this drug in the treatment of various CNS disorders or disease states. This article reviews these new findings and describes the effects of aniracetam in various rodent models of mental function impairment or cerebral dysfunction. Also, several metabolites of aniracetam have been reported to affect learning and memory in animals. It is, therefore, conceivable that major metabolites of aniracetam contribute to its pharmacological effects. The animal models, used in pharmacological evaluation of aniracetam included models of hypoattention, hypovigilance-arousal, impulsiveness, hyperactivity, fear and anxiety, depression, impaired rapid-eye movement sleep, disturbed temporal regulation, behavioral performance, and bladder hyperactivity. These are models of clinical disorders or symptoms that may include personality disorders, anxiety, depression, posttraumatic stress disorder, attention-deficit/hyperactivity disorder, autism, negative symptoms of schizophrenia, and sleep disorders. At present, there is no convincing evidence that promising effects of aniracetam in the animal models will guarantee its clinical efficacy. It is conceivable, however, that clinical trials will demonstrate beneficial effects of aniracetam in the above listed disease states. New findings regarding the mechanism of action of aniracetam, its central target sites, and its effects on signal transduction are also discussed in this review article.

Aniracetam enhances cortical dopamine and serotonin release via cholinergic and glutamatergic mechanisms in SHRSP

Aniracetam, a cognition enhancer, has been recently found to preferentially increase extracellular levels of dopamine (DA) and serotonin (5-HT) in the prefrontal cortex (PFC), basolateral amygdala and dorsal hippocampus of the mesocorticolimbic system in stroke-prone spontaneously hypertensive rats. In the present study, we aimed to identify actually active substances among aniracetam and its major metabolites and to clarify the mode of action in DA and 5-HT release in the PFC. Local perfusion of mecamylamine, a nicotinic acetylcholine (nACh) and N-methyl-D-aspartate (NMDA) receptor antagonist, into the ventral tegmental area (VTA) and dorsal raphe nucleus (DRN) completely blocked DA and 5-HT release, respectively, in the PFC elicited by orally administered aniracetam. The effects of aniracetam were mimicked by local perfusion of N-anisoyl-gamma-aminobutyric acid [corrected] (N-anisoyl-GABA), one of the major metabolites of aniracetam, into the VTA and DRN. The cortical DA release induced by N-anisoyl-GABA applied to the VTA was also completely abolished by co-perfusion of mecamylamine. Additionally, when p-anisic acid, another metabolite of aniracetam, and N-anisoyl-GABA were locally perfused into the PFC, they induced DA and 5-HT release in the same region, respectively. These results indicate that aniracetam enhances DA and 5-HT release by mainly mediating the action of N-anisoyl-GABA that targets not only somatodendritic nACh and NMDA receptors but also presynaptic nACh receptors.

Anxiolytic effects of aniracetam in three different mouse models of anxiety and the underlying mechanism

The anxiolytic effects of aniracetam have not been proven in animals despite its clinical usefulness for post-stroke anxiety. This study, therefore, aimed to characterize the anxiolytic effects of aniracetam in different anxiety models using mice and to examine the mode of action. In a social interaction test in which all classes (serotonergic, cholinergic and dopaminergic) of compounds were effective, aniracetam (10-100 mg/kg) increased total social interaction scores (time and frequency), and the increase in the total social interaction time mainly reflected an increase in trunk sniffing and following. The anxiolytic effects were completely blocked by haloperidol and nearly completely by mecamylamine or ketanserin, suggesting an involvement of nicotinic acetylcholine, 5-HT2A and dopamine D2 receptors in the anxiolytic mechanism. Aniracetam also showed anti-anxiety effects in two other anxiety models (elevated plus-maze and conditioned fear stress tests), whereas diazepam as a positive control was anxiolytic only in the elevated plus-maze and social interaction tests. The anxiolytic effects of aniracetam in each model were mimicked by different metabolites (i.e., p-anisic acid in the elevated plus-maze test) or specific combinations of metabolites. These results indicate that aniracetam possesses a wide range of anxiolytic properties, which may be mediated by an interaction between cholinergic, dopaminergic and serotonergic systems. Thus, our findings suggest the potential usefulness of aniracetam against various types of anxiety-related disorders and social failure/impairments.

Site-specific activation of dopamine and serotonin transmission by aniracetam in the mesocorticolimbic pathway of rats

The effects of aniracetam on extracellular levels of dopamine (DA), serotonin (5-HT) and their metabolites were examined in five brain regions in freely moving stroke-prone spontaneously hypertensive rats (SHRSP) using in vivo microdialysis. Basal DA release in SHRSP was uniformly lower in all regions tested than that in age-matched control Wistar Kyoto rats. 3,4-Dihydroxyphenylacetic acid and homovanillic acid levels were altered in the basolateral amygdala, dorsal hippocampus and prefrontal cortex of SHRSP. While basal 5-HT release decreased in the striatum and increased in the basolateral amygdala, there was no associated change in 5-hydroxyindoleacetic acid levels. Systemic administration of aniracetam to SHRSP enhanced both DA and 5-HT release with partly associated change in their metabolite levels in the prefrontal cortex, basolateral amygdala and dorsal hippocampus, but not in the striatum and nucleus accumbens shell, in a dose-dependent manner (30 and/or 100 mg/kg p.o.). Microinjection (1 and 10 ng) of aniracetam or its metabolites (N-anisoyl-GABA and 2-pyrrolidinone) into the nucleus accumbens shell produced no turning behavior. These findings indicate that SHRSP have a dopaminergic hypofunction throughout the brain and that aniracetam elicits a site-specific activation in mesocorticolimbic dopaminergic and serotonergic pathways in SHRSP, possibly via nicotinic acetylcholine receptors in the ventral tegmental area and raphe nuclei. The physiological roles in the aniracetam-sensitive brain regions may closely link with their clinical efficacy towards emotional disturbances appearing after cerebral infarction.

Impulsivity and AMPA receptors: aniracetam ameliorates impulsive behavior induced by a blockade of AMPA receptors in rats

The study aimed to ascertain the involvement of central AMPA receptors in impulsive behaviors of aged rats and to examine the effects of aniracetam. Premature response in the two-lever choice reaction task was assessed as an index of impulsivity. Intracerebroventricular injection of 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX), an AMPA receptor antagonist, dose-dependently (10.1-1009 ng/rat) increased only premature response without altering responding speed and choice accuracy 30 min after the injection. Aniracetam (30 mg/kg p.o.), a positive allosteric modulator of AMPA receptors, or AMPA (55.9 ng/rat, co-injected with NBQX) completely restored the NBQX-induced increase in impulsivity. These results indicate that AMPA receptors are tonically involved in the regulation of impulsivity.

Group II metabotropic glutamate receptors are a common target of N-anisoyl-GABA and 1S,3R-ACPD in enhancing ACh release in the prefrontal cortex of freely moving SHRSP

Aniracetam is a therapeutically useful cognition enhancer for treating various neuropsychiatric symptoms occurring after cerebral infarction. We recently reported that local perfusion of its major metabolites N-anisoyl-GABA and p-anisic acid, but not aniracetam itself, enhanced acetylcholine (ACh) release with a delayed onset in cerebral regions of stroke-prone spontaneously hypertensive rats (SHRSP). In this study, we examined the possible involvement of metabotropic and ionotropic glutamate (mGlu and AMPA) receptors in the N-anisoyl-GABA-induced ACh release using brain in vivo microdialysis. Basal ACh release in SHRSP was commonly lower in the nucleus reticularis thalami, dorsal hippocampus and prefrontal cortex than that in age-matched Wistar Kyoto rats. The delayed ACh release in the prefrontal cortex of SHRSP was completely blocked by MCPG, a group I and II mGlu receptor antagonist, and MCCG, a group II-selective mGlu receptor antagonist. In contrast, it was largely unaffected by AIDA, a group I-selective mGlu receptor antagonist, or by YM90K, an AMPA receptor antagonist. 1S,3R-ACPD, a preferential group II mGlu receptor agonist, enhanced ACh release with a similar latency and the effect was antagonized by MCCG, whereas AMPA induced a prompt ACh release. These results indicate that N-anisoyl-GABA and 1S,3R-ACPD share a common mechanism mediated by group II mGlu receptors in enhancing ACh release. The findings suggest a possible mechanism for aniracetam's clinical efficacy in stroke patients with cholinergic deficits.

Activation of the reticulothalamic cholinergic pathway by the major metabolites of aniracetam

The aim of the study was to further investigate the effects of aniracetam, a cognition enhancer, and its metabolites on the brain cholinergic system. We measured choline acetyltransferase activity and acetylcholine release using in vivo brain microdialysis in stroke-prone spontaneously hypertensive rats (SHRSP). The enzyme activity in the pons-midbrain and hippocampus, and basal acetylcholine release in the nucleus reticularis thalami were lower in SHRSP than in age-matched Wistar Kyoto rats, indicating central cholinergic deficits in SHRSP. Repeated treatment of aniracetam (50 mg/kg p.o. x 11 for 6 days) preferentially increased the enzyme activity in the thalamus, whereas decreased it in the striatum. Among the metabolites of aniracetam, local perfusion of N-anisoyl-gamma-aminobutyric acid (GABA, 0.1 and/or 1 microM) and p-anisic acid (1 microM) into the nucleus reticularis thalami, dorsal hippocampus and prefrontal cortex of SHRSP produced a significant but delayed increase of acetylcholine release. We failed, however, to find any effect of aniracetam itself. A direct injection of N-anisoyl-GABA (1 nmol) into the pedunculopontine tegmental nucleus of SHRSP enhanced the release in the nucleus reticularis thalami. Thus, these data prove that aniracetam can facilitate central cholinergic neurotransmission via both metabolites. Based on its pharmacokinetic profile, N-anisoyl-GABA may contribute to the clinical effects of aniracetam, mainly by acting on the reticulothalamic cholinergic pathway.

Apomorphine-induced hypoattention in rats and reversal of the choice performance impairment by aniracetam

Aging-, disease- and medication-related imbalance of central dopaminergic neurons causes functional impairment of cognition and neuropsychological delirium in humans. We attempted to develop a new delirium model using the direct dopamine agonist, apomorphine, and a choice reaction performance task performed by middle-aged rats. The psychological properties of the model were assessed by determining behavioral measures such as choice reaction time, % correct and % omission. Apomorphine (0.03-0.3 mg/kg s.c.) produced a dose-dependent impairment of task performance. The dose of 0.1 mg/kg prolonged choice reaction time, decreased % correct and increased % omission, indicating that rats had attentional deficits and a reduced arousal or vigilance but no motor deficits or reduced food motivation. This psychological and behavioral impairment of performance resembled that of clinically defined delirium. In this model, the cholinomimetic, aniracetam (10 mg/kg p.o.), reversed the performance impairment induced by apomorphine. Its two metabolites, 2-pyrrolidinone (10 and 30 mg/kg p.o.) and N-anisoyl-gamma-aminobutyric acid (GABA, 10 mg/kg p.o.), effectively reversed the performance impairment as the intact drug did. Another pyrrolidinone derivative, nefiracetam (10 and 30 mg/kg p.o.), tended to worsen the apomorphine effect. The cholinesterase inhibitor, tacrine (10 mg/kg p.o.), markedly worsened all of the behavioral measures. Neuroleptics, haloperidol (0.025 mg/kg s.c.), tiapride (30 mg/kg p.o.) and sulpiride (10 and 30 mg/kg p.o.), antagonized the apomorphine effect. The present results suggest that apomorphine-induced behavioral disturbances in the choice reaction performance task seems to be a useful delirium model and aniracetam may improve delirium through the action of 2-pyrrolidinone and N-anisoyl-GABA, presumably by facilitating dopamine release in the striatum by acting as an AMPA or metabotropic glutamate receptor agonist.