Distribution of insoluble cAMP-dependent kinase type RI and RII in the lizard and turtle central nervous system.

cAMP is a universal second messenger. In eucaryotes it acts mainly via protein kinases composed of regulatory (R) and catalytic subunits; their subcellular distribution may differ according to the cell type. In rodent brain, peculiar detergent-insoluble RIalpha aggregates were previously described in neurons of areas related to the limbic system, while RIIbeta is more evenly distributed also in non-nervous cells. It is unclear whether the regional distribution of regulatory subunits is typical of mammalian brain. Western blots and immunohistochemistry showed that in lizard brains a large fraction of the cAMP-dependent protein kinase regulatory isoforms is insoluble, as in mammals. Insoluble RIalpha and RII regulatory isoforms were not evenly distributed but organized in clearly separated aggregates. Numerous RII aggregates were present in almost all brain regions and were found also in non-nervous cells. As shown by immunohistochemistry and equilibrium binding of fluorescently tagged cAMP, RIalpha aggregates were restricted to neurons of some brain regions: telencephalon, particularly medial cortical areas, dorsal ventricular ridge, olfactory pathways, medial hypothalamus and cerebellar granular layer were intensely labelled. A very weak RIalpha labelling was detected in the brainstem reticular formation, in the periaqueductal gray and in the spinal cord dorsal horn. A similar distribution of RIalpha aggregates was also found in turtle brains. Their distribution is reminiscent of that observed in mammals, although with some differences in relative intensity and persistence. The supramolecular organization of the RIalpha isoform may help in establishing homologies and differences between brain areas involved in visceroemotional control.