Harmaline effects on tetrabenazine depression of avoidance responding in rats

Efficacy of Vesicular Monoamine Transporter 2 Inhibition and Synergy with Antipsychotics in Animal Models of Schizophrenia

Antipsychotic medications function by blocking postsynaptic dopaminergic signaling in the central nervous system. Dopamine transmission can also be modulated presynaptically by inhibitors of vesicular monoamine transporter 2 (VMAT2), which inhibit loading of dopamine into presynaptic vesicles. Here we investigated the combination of these mechanisms in animal models of schizophrenia and weight gain (a primary side effect of antipsychotics). When dosed alone, the highly selective VMAT2 inhibitor RRR-dihydrotetrabenazine (RRR-DHTBZ, also known as [+]-α-HTBZ) elicited efficacy comparable to conventional antipsychotics in prepulse inhibition and conditioned avoidance models without eliciting weight gain. In combination experiments, synergy was observed: subthreshold doses of RRR-DHTBZ and risperidone or olanzapine produced robust efficacy, and in dose response experiments, RRR-DHTBZ increased the antipsychotic potency in the efficacy models but did not affect weight gain. The combinations did not affect plasma compound concentrations. The synergy is consistent with VMAT2 inhibition blocking the counterproductive presynaptic stimulation of dopamine by antipsychotics. These results suggest a therapeutic strategy of adding a VMAT2 inhibitor to lower the antipsychotic dose and reduce the side effect burden of the antipsychotic while maintaining and potentially enhancing its therapeutic effects. SIGNIFICANCE STATEMENT: Antipsychotics are often necessary and life-changing medications that reduce psychotic symptoms; however, these benefits come with a high side effect burden. This study shows that combining these postsynaptic dopaminergic modulators with a presynaptic dopamine modulator (vesicular monoamine transporter 2 [VMAT2] inhibitor) potentiates efficacy synergistically in animal models of schizophrenia without potentiating weight gain. Our data suggest that adding a VMAT2 inhibitor may be a viable therapeutic strategy for reducing antipsychotic side effects by lowering antipsychotic dose while maintaining therapeutic efficacy.

At low doses, harmaline increases forelimb tremor in the rat

A behavioral preparation especially sensitive to low-dose drug effects on fine motor behavior in rats was used to assess the tremorogenic effects of harmaline, an indole alkaloid and beta-carboline derivative. Rats that were trained to press downward on a force transducer for water reinforcement were initially administered harmaline (0.5 and 1.0 mg/kg) in an acute dosing regime. Immediately following the day of initial acute exposure to 1 mg/kg, 3 consecutive days at this dose ensued, providing for a 4-day, repeated-dosing analysis. Harmaline did not significantly suppress task engagement during either acute or repeated dosing. Acute administration of harmaline dose-dependently increased power in the high-frequency (10-25 Hz) band of the power spectrum (tremor) without affecting overall forelimb force output. Upon continued administration, tremor remained significantly elevated above vehicle values. Harmaline also slowed the rats' licking frequency, an effect that did not diminish with repeated dosing. Harmaline increased the durations of individual responses during acute dosing and continued to exert this effect with repeated dosing. The effects reported in the present study may represent low-dose harmaline-induced alterations in the olivo-cerebellar system.