Cerebellar α6 -subunit-containing GABAA receptors: a novel therapeutic target for disrupted prepulse inhibition in neuropsychiatric disorders.

BACKGROUND AND PURPOSE: The pathophysiological role of α6 -subunit-containing GABAA receptors, which are mainly expressed in cerebellar granule cells, remains unclear. Recently, we demonstrated that hispidulin, a flavonoid isolated from a local herb that remitted a patient's intractable motor tics, attenuated methamphetamine-induced hyperlocomotion in mice as a positive allosteric modulator (PAM) of cerebellar α6 GABAA receptors. Here, using hispidulin and a selective α6 GABAA receptor PAM, the pyrazoloquinolinone Compound 6, we revealed an unprecedented role of cerebellar α6 GABAA receptors in disrupted prepulse inhibition of the startle response (PPI), which reflects sensorimotor gating deficits manifested in several neuropsychiatric disorders. EXPERIMENTAL APPROACH: PPI disruptions were induced by methamphetamine and NMDA receptor antagonists in mice. Effects of the tested compounds were measured in Xenopus oocytes expressing recombinant α6 ß3 γ2S GABAA receptors. KEY RESULTS: Hispidulin given i.p. or by bilateral intracerebellar (i.cb.) injection rescued PPI disruptions induced by methamphetamine, ketamine, MK-801 and phencyclidine. Intracerebellar effects of hispidulin were mimicked by Ro15-4513 and loreclezole (two α6 GABAA receptor PAMs), but not by diazepam (an α6 GABAA receptor-inactive benzodiazepine) and were antagonized by furosemide (i.cb.), an α6 GABAA receptor antagonist. Importantly, Compound 6 (i.p.) also rescued methamphetamine-induced PPI disruption, an effect prevented by furosemide (i.cb.). Both hispidulin and Compound 6 potentiated α6 ß3 γ2S GABAA receptor-mediated GABA currents. CONCLUSIONS AND IMPLICATIONS: Positive allosteric modulation of cerebellar α6 GABAA receptors rescued disrupted PPI by attenuating granule cell activity. α6 GABAA receptor-selective PAMs are potential medicines for treating sensorimotor gating deficits in neuropsychiatric disorders. A mechanistic hypothesis is based on evidence for cerebellar contributions to cognitive functioning including sensorimotor gating.

Hispidulin, a constituent of Clerodendrum inerme that remitted motor tics, alleviated methamphetamine-induced hyperlocomotion without motor impairment in mice.

ETHNOPHARMACOLOGICAL RELEVANCE: Previously, we found a patient with an intractable motor tic disorder that could be ameliorated by the ground leaf juice of Clerodendrum inerme (CI). Furthermore, the ethanol extract of CI leaves effectively ameliorated methamphetamine-induced hyperlocomotion (MIH) in mice, an animal model mimicking the hyper-dopaminergic status of tic disorders/Tourette syndrome, schizophrenia, or obsessive-compulsive disorder. Here, we for the first time identified a constituent able to reduce MIH from the CI ethanol extract that might represent a novel lead for the treatment of such disorders. MATERIALS AND METHODS: The ethanol extract of CI was sub-divided into n-hexane, dichloromethane, n-butanol and water fractions. Using MIH alleviation as a bioassay, active compounds were identified in these fractions using silica gel chromatography, recrystallization and proton NMR spectroscopy. RESULTS: The dichloromethane and n-hexane fractions were active in the bioassay. Further subfractionation and re-crystallization resulted in an active compound that was identified to be hispidulin by proton NMR spectroscopy. Hispidulin significantly alleviated MIH in mice at doses that did not affect their spontaneous locomotor activity or performance in the rotarod test, a measure for motor coordination. CONCLUSIONS: Hispidulin is a flavonoid that has been isolated from several plants and reported to have anti-oxidative, anti-inflammatory and anti-cancer activities. Here, we for the very first time found that hispidulin can also alleviate MIH at doses that did not impair motor activity, suggesting a therapeutic potential of hispidulin in hyper-dopaminergic disorders.

Clerodendrum inerme Leaf Extract Alleviates Animal Behaviors, Hyperlocomotion, and Prepulse Inhibition Disruptions, Mimicking Tourette Syndrome and Schizophrenia.

Previously, we found a patient with intractable motor tic disorder, a spectrum of Tourette syndrome (TS), responsive to the ground leaf juice of Clerodendrum inerme (CI). Here, we examined the effect of the ethanol extract of CI leaves (CI extract) on animal behaviors mimicking TS, hyperlocomotion, and sensorimotor gating deficit. The latter is also observed in schizophrenic patients and can be reflected by a disruption of prepulse inhibition of acoustic startle response (PPI) in animal models induced by methamphetamine and NMDA channel blockers (ketamine or MK-801), based on hyperdopaminergic and hypoglutamatergic hypotheses, respectively. CI extract (10-300 mg/kg, i.p.) dose-dependently inhibited hyperlocomotion induced by methamphetamine (2 mg/kg, i.p.) and PPI disruptions induced by methamphetamine, ketamine (30 mg/kg, i.p.), and MK-801 (0.3 mg/kg, i.p.) but did not affect spontaneous locomotor activity, rotarod performance, and grip force. These results suggest that CI extract can relieve hyperlocomotion and improve sensorimotor gating deficit, supporting the therapeutic potential of CI for TS and schizophrenia.