Tetrabenazine, an amine-depleting drug, also blocks dopamine receptors in rat brain

Tetrabenazine (TBZ) is used in the treatment of hyperkinetic movement disorders. Its effect is thought to be mediated by depletion of dopamine (DA) stores. We studied other possible mechanisms of action of this drug. TBZ decreased DA concentration in rat striatum and nucleus accumbens in a dose-dependent manner with an IC50 of approximately 1.2 mg.kg-1. Maximal depletion was obtained within 30 min with only partial recovery at 8 hr. TBZ induced (at 40 mg . kg-1) 5- to 8-fold increases in 3,4-dihydroxyphenylacetic acid and homovanillic acid concentrations in both brain regions. Unlike reserpine, TBZ completely abolished the apomorphine-induced inhibition of DA synthesis under conditions in which this effect is mediated by presynaptic DA receptors. Both TBZ (5 mg . kg-1) and reserpine (5 mg . kg-1) depleted, at 1 hr, striatal DA content by approximately 90%. However, TBZ, but not reserpine, significantly stimulated in vivo tyrosine hydroxylase activity. TBZ also inhibited [3H]spiperone binding in the striatum with Ki = 2.1 X 10(-6) M. In rats, with unilateral destruction of the nigrostriatal pathway with 6-hydroxydopamine, pretreatment with TBZ significantly reduced the number of rotations induced by apomorphine. Finally, in rats treated with either TBZ (5 mg . kg-1) or reserpine (5 mg . kg-1), Prolactin levels significantly increased as compared to control values. TBZ, but not reserpine, blocked apomorphine inhibition of Prolactin secretion. We conclude that, in addition to depleting monoamines, TBZ also blocks both presynaptic and postsynaptic DA receptors in rat brain.