Protein kinase C regulates a vesicular class of calcium channels in the bag cell neurons of Aplysia

Protein kinase C (PKC) acutely increases calcium currents in Aplysia bag cell neurons by recruiting calcium channels different from those constitutively active in the plasma membrane. To study the mechanism of PKC regulation we previously identified two calcium channel α₁-subunits expressed in bag cell neurons. One of these, BC-α(1A), is localized to vesicles concentrated primarily in somata and growth cones. We used antibodies to BC-α(1A) to analyze its expression in the bag cell neurons of juvenile Aplysia at a developmental stage at which PKC-sensitive calcium currents have previously been shown to be low. We find that vesicular BC- α(1A) staining is generally reduced in juvenile bag cell neurons but that its expression level can vary among juvenile animals. In 17 bag cell clusters examined, the percentage of neurons that displayed punctate αBCα(1A) staining ranged from 0 to 85%. Sampling of calcium currents from cells of the same clusters by whole cell patch-clamp techniques revealed that the PKC- sensitive calcium current density is significantly correlated with the degree of vesicular staining. In contrast, no correlation of basal calcium current levels with aBCα(1A) staining was found. These results strongly suggest that BCα(1A), a member of the ABE-subfamily of calcium channels, carries the PKC- sensitive calcium current in bag cell neurons. They are consistent with a model in which PKC recruits channels from the vesicular pool to the plasma membrane.