Hispidulin alleviated methamphetamine-induced hyperlocomotion by acting at α6 subunit-containing GABAA receptors in the cerebellum
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Hispidulin is a flavonoid we isolated from Clerodendrum inerme, an herb that effectively remitted a case of intractable motor tic disorders. Hispidulin was shown to be a positive allosteric modulator (PAM) of GABAA receptors, including the α6 subunit-containing subtype (α6GABAAR) that is predominantly expressed in cerebellar granule cells and insensitive to diazepam.
We explored the action mechanism(s) of hispidulin using hyperdopaminergic mouse models induced by methamphetamine and apomorphine, based on the hyperdopaminergic nature of tic disorders.
Hispidulin significantly inhibited methamphetamine-induced hyperlocomotion (MIH) at i.p. doses without affecting apomorphine-induced hyperlocomotion and stereotypy behaviors or having significant benzodiazepine-like effects (BZLE), including sedation, anxiety, and motor impairment. When given by intracerebellar (i.c.b.) microinjection, hispidulin also alleviated MIH and this effect was prevented by i.c.b. coadministration of furosemide, an α6GABAAR antagonist, and mimicked by i.c.b. Ro 15-4513, an α6GABAAR PAM. Conversely, i.c.b. diazepam did not affect MIH while it reduced MIH at i.p. doses having significant BZLE. In a screening assay for 92 neurotransmitter receptors/degradation enzymes/transporters, hispidulin displayed significant (>50 % inhibition of radiolabeled ligand binding at 10 μM) binding affinity only at the benzodiazepine binding site of GABAARs (IC50 0.73∼1.78 μM) and catecholamine-o-methyl-transferase (COMT) (IC50 1.32 μM). OR-486, a more potent COMT inhibitor than hispidulin, did not affect MIH.
It is suggested that hispidulin alleviates MIH via acting as a PAM of cerebellar α6GABAARs, but not through COMT inhibition or affecting dopamine receptor responsiveness. Thus, selective α6GABAAR PAMs may have the potential to be a novel treatment for hyperdopaminergic disorders.
KeywordsHispidulin Flavonoid GABAA receptor Cerebellum Dopamine Tourrette syndrome
α6 Subunit-containing GABAA receptors
Apomorphine-induced stereotypy behaviors
Positive allosteric modulator
This study was mainly supported by the National Research Program for Biopharmaceuticals (NSC 100-2325-B002-050, NSC 101-2325-B002-048, NSC 102-2325-B002-047, MOST 103-2325-B002-037 and MOST 104-2325-B002-010) and the research grants (MOST 103-2321-B002-035 to LCC; NSC 102-2320-B038-019-MY3 to WJH) from the National Science Council (the Ministry of Science and Technology), Taiwan, as well as by the Innovative Research Grant from National Health Research Institutes, Taiwan (NHRI-EX104-10251NI to LCC). We thank the support from Behavior Core, Neurobiology and Cognitive Center, National Taiwan University.
Compliance with ethical standards
Conflict of interest
All authors declare that they have no conflict of interest.
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