We have investigated the potential neurotoxicity of the
catecholamine depleting agent
1,2,3,4-tetrahydro-2-methyl-4,6,7-isoquinolinetriol (
TMIQ) in SH-SY5Y
neuroblastoma cells.
TMIQ induced a time and dose related inhibition of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium
bromide; thiazoyl blue (MTT) reduction and an increase in
lactate dehydrogenase release. After 72 h
TMIQ (30 μM) significantly (P < 0.05) inhibited MTT reduction, and significantly increased LDH release.
TMIQ cytotoxicity was not prevented by the inclusion of
monoamine oxidase inhibitors (
clorgyline or
deprenyl),
antioxidants (α-
tocopherol or
Trolox C) or the uptake(1) inhibitor
imipramine.
TMIQ also induced a dose dependent stimulation of [(3)H]
noradrenaline (NA) uptake, with maximum at 100 μM and EC(50) of 8 μM. This stimulation of [(3)H]NA uptake was not prevented by the inhibition of
protein kinase C, or activation of adenylate or guanylate cyclases. In addition,
TMIQ significantly (P < 0.05) displaced [(3)H]
nisoxetine binding from the uptake(1) recognition site with a K(i) of 71 ± 8 μM. However, as this interaction occurs at concentrations of
TMIQ well above the EC(50) for [(3)H]NA uptake, it is unlikely to explain
TMIQ stimulated NA uptake. Furthermore,
TMIQ inhibited
potassium evoked [(3)H]NA release from SH-SY5Y cells, with an IC(50) of 490 μM. Thus,
TMIQ is cytotoxic to SH-SY5Y cells. However, the exact mechanism of toxicity requires further investigation, since it appears not to involve
monoamine oxidase bioactivation, and is not mediated through membrane based
free radical damage. Furthermore, although
TMIQ inhibits mitochondrial Complex I (IC(50) = 1.5 mM) with potency apparently greater than
MPTP (2.7 mM), mitochondrial respiration was unaffected. The present studies suggest that the mechanism of toxicity differs from that causing depletion of
catecholamines and inhibition of
tyrosine hydroxylase by
TMIQ described in previous studies.