Hispidulin alleviated methamphetamine-induced hyperlocomotion by acting at alpha 6 subunit-containing GABA sub(A) receptors in the cerebellum

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 GABA sub(A) receptors, including the alpha sub(6) subunit-containing subtype ( alpha sub(6)GABA sub(A)R) 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 alpha sub(6)GABA sub(A)R antagonist, and mimicked by i.c.b. Ro 15-4513, an alpha sub(6)GABA sub(A)R 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 mu M) binding affinity only at the benzodiazepine binding site of GABA sub(A)Rs (IC sub(50) 0.731.78 mu M) and catecholamine-o-methyl-transferase (COMT) (IC sub(50) 1.32 mu M). 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 alpha sub(6)GABA sub(A)Rs, but not through COMT inhibition or affecting dopamine receptor responsiveness. Thus, selective alpha sub(6)GABA sub(A)R PAMs may have the potential to be a novel treatment for hyperdopaminergic disorders.