Second messenger pathways in the honeybee brain: immunohistochemistry of protein kinase A and protein kinase C.

As in other model systems for investigating mechanisms of learning, second messenger regulated protein phosphorylation are implicated in processes of associative learning in the honeybee Apis mellifera. Since the cAMP-dependent phosphorylation via protein kinase A (PKA) and the Ca2+/phospholipid-dependent phosphorylation by protein kinase C (PKC) contribute to different aspects of associative olfactory learning in the honeybee, the localization of PKA and PKC in the neuronal circuitry is of general interest. The presented immunohistological study compares the distribution of PKA and PKC in honeybee brain focusing on the antennal lobes and the mushroom bodies, neuropiles implicated in olfactory learning. While PKA, is found in all neuropiles and somata throughout the honeybee brain, the Ca2+/phospholipid-dependent PKC is concentrated in the antennal lobes and the mushroom bodies. In the antennal lobes, the primary neuropiles of olfactory information processing, PKAII is localized in sensory neurons and interneurons. In contrast, the PKC-immunolabeling is exclusively due to interneurons with a characteristically strong labeling of the central area of the antennal lobes. The mushroom bodies show the highest PKC- and PKAII-immunostaining of the brain. PKAII and PKC are expressed at different levels in different subsets of the mushroom body intrinsic Kenyon cells. This different distribution of PKAII and PKC in the antennal lobes and the mushroom bodies, at least in part, accounts for the different roles of PKAII and PKC mediated phosphorylation in olfactory learning.