Protein kinase C-delta in rat brain: association with sensory neuronal hierarchies

Originally characterized as the calcium- and phospholipid-dependent protein kinases, the protein kinases C include at least eight separate isoforms, some of which are calcium-independent and all of which are highly enriched in brain. Of the calcium-independent isoforms, the delta subspecies of protein kinase C has the most restricted complement of lipid activators and substrate specificity, suggesting that it may have a unique role in cell signalling pathways. Using immunocytochemistry, we report that the distribution of protein kinase C-delta immunoreactivity in rat brain is also restricted, being present in all sensory systems. Moreover, it is found in alternating hierarchies of sensory pathways: in all sensory systems except auditory, it is found in first- and third-order neurons, while in the auditory system, it is found in second- and fourth-order neurons. Thalamocortical systems are intensely immunoreactive, including barrel fields of the rat parietal cortex. Outside of sensory systems, protein kinase C-delta is present in cerebellum within longitudinal stripes in Purkinje neurons, and in the caudate-putamen, it appears to be associated with the striosome (patch) compartment. In contrast to all other protein kinase C isoforms, protein kinase C-delta is absent from hippocampus. These findings suggest that protein kinase C-delta may have a unique role in signal transduction in the central nervous system (CNS), especially in sensory systems.