(-)-Stepholidine enhances K+ depolarization-induced activation of synaptosomal tyrosine 3-monooxygenase from rat striatum

AIM: To study the mechanism of K+ depolarization-induced activation of
synaptosomal tyrosine 3-monooxygenase (TM) in rat striatum and the effect of
(-)-stepholidine (SPD) on this activation.
METHODS: The TM was assayed for DOPA by HPLC-ECD; the activities of
Ca2+/calmodulin (CaM)-dependent protein kinase (PK II) and
Ca2+/phosphoinositide-dependent protein kinase (PKC) were assayed using histidine
as substrate.
RESULTS: The incubation of striatal synaptosomes in K(+)-riched (60 mmol.L-1)
medium resulted in a marked activation of TM. PKC inhibitor polymyxin B (PMB)
completely blocked the activation of K+ 60 mmol.L-1 on TM. Selective D2 receptor
agonist quinpirole (QP), Ca2+ removal from incubation medium and CaM antagonist
W7 failed to affect the activation. However, SPD enhanced the activation of K+ 60
mmol.L-1 on TM. Meanwhile, the incubation in K+ 60 mmol.L-1 also activated PKC.
Neither QP nor SPD affected K+ depolarization-induced activation of PKC.
CONCLUSION: The activation of K+ depolarization on synaptosomal TM is enhanced by
SPD and this activation is mediated by PKC rather than by PK II.