CHRONIC ACTIVATION OF PROTEIN-KINASE-C BY PHORBOL ESTER REDUCES CALCIUM-CHANNEL EXPRESSION IN CHICK SYMPATHETIC NEURONS

Chronic activation of protein kinase C (PKC) has been implicated in re gulation of Ca2+ entry responsible for normal development of transmitt er properties in cultured sympathetic neurons. The idea that PKC alter s the expression of Ca2+ channels was tested using phorbol 12,13-dibut yrate (PDB) which activates PKC and also supports survival of chick sy mpathetic neurons in the absence of nerve growth factor (NGF). Whole c ell voltage-clamp showed that neurons supported by PDB for 2 days had significantly lower Ca2+ current density (0.243 +/- 0.025 pA/mu m(2)) than those supported by NGF (0.356 +/- 0.033 pA/mu m(2)). [I-125]omega -Conotoxin GVIA binding showed that PDB-supported neurons had signific antly lower maximum binding (617 +/- 223 fmol/mg protein) compared wit h those supported by NGF (1099 +/- 192 fmol/mg protein). These results support the conclusion that chronic activation of PKC limits the expr ession of N-type Ca2+ channels. A reduction in Ca2+ channel number is consistent with, and could account for the mature type Ca2+ handling a nd transmitter release properties seen in sympathetic neuro-effector p reparations, sympathetic neurons co-cultured with their targets, and n eurons supported by PDB. (C) 1997 Elsevier Science Ireland Ltd.