GAMMA-ISOFORM-SELECTIVE CHANGES IN PKC IMMUNOREACTIVITY AFTER TRACE EYEBLINK CONDITIONING IN THE RABBIT HIPPOCAMPUS

An immunocytochemical examination of the rabbit hippocampus was done t o determine which of the Ca2+-dependent protein kinase C (PKC) isoform s (PKC alpha, -beta I, -beta II, or -gamma) are involved in associativ e learning. The hippocampally dependent trace eyeblink conditioning ta sk was used for behavioral training, and pseudoconditioned and naive a nimals served as controls. Significant increases (P < 0.05) in stainin g intensity were found with antibodies reactive with the catalytic or the regulatory domain of PKC gamma in conditioned animals compared wit h naive and pseudoconditioned subjects at a 24-h post-conditioning tim e point. The increase was found in CA1 and CA3 pyramidal cell bodies, in apical dendrites and the proximal part of the basilar dendrites, an d in cell bodies of dentate granule cells. In contrast, no conditionin g-specific changes were found for PKC alpha, -beta I, or -beta II in h ippocampal neurons. The increase in PKC gamma immunoreactivity (ir) wa s significantly less (P < 0.05) in poor learners than in good learners . The correlation between the degree of PKC gamma-ir and the total num ber of conditioned responses across training sessions was both positiv e and significant. These results suggest that PKC gamma is the major C a2+-dependent PKC isoform involved in hippocampal neurons during acqui sition of associative memories. Immunoblots revealed no conditioning-i nduced increase in the total amount or translocation of PKC gamma at t he 24-h time point, and no proteolytic PKC fragments were observed. In agreement with the Western blot data, PKC activity did not differ amo ng naive, pseudoconditioned, and trace conditioned animals. The condit ioning-induced increase in antibody binding to the gamma-isoform must therefore be due to an increased access to the antigenic site(s) as a result of alteration in the tertiary structure of PKC gamma or in quat ernary interactions of PKC gamma in situ. (C) 1997 Wiley-Liss, Inc.