(-)-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.