1,2,3,4-Tetrahydro-2-methyl-4,6,7-isoquinolinetriol inhibits tyrosine hydroxylase activity in rat striatal synaptosomes.

1,2,3,4-tetrahydro-2-methyl-4,6,7-isoquinolinetriol (TMIQ), a tetrahydroisoquinoline derivative of adrenaline, was tested for potency as an analog of the dopamine depleting agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in assays of tyrosine hydroxylase (TH) activity in the striatal synaptosome preparation. TMIQ inhibited TH activity with an IC50 (4 x 10(-6)M) similar to that found for MPTP (IC50 1 x 10(-6)M). TH inhibitions produced by IC50 concentrations of TMIQ were reversed by monoamine oxidase (MAO)-A or MAO-B inhibitors (clorgyline or deprenyl), or the dopamine reuptake blocker nomifensine, or excess cofactor (6R)-5,6,7,8-tetrahydro-L-biopterin. TMIQ did not appear to act at the presynaptic D2 sulpiride sensitive autoreceptor for dopamine synthesis modulation. These in vitro data are consistent with earlier findings that TMIQ acts as a dopamine depleting agent, and with the possibility that TMIQ may have a degree of MPTP-like activity in vivo.