Surface L-type Ca2+ channel expression levels are increased in aged hippocampus

Age-related increase in L-type Ca2+ channel (LTCC) expression in hippocampal pyramidal neurons has been hypothesized to underlie the increased Ca2+ influx and subsequent reduced intrinsic neuronal excitability of these neurons that lead to age-related cognitive deficits. Here, using specific antibodies against Ca(v)1.2 and Ca(v)1.3 subunits of LTCCs, we systematically re-examined the expression of these proteins in the hippocampus from young (3 to 4month old) and aged (30 to 32month old) F344xBN rats. Western blot analysis of the total expression levels revealed significant reductions in both Ca(v)1.2 and Ca(v)1.3 subunits from all three major hippocampal regions of aged rats. Despite the decreases in total expression levels, surface biotinylation experiments revealed significantly higher proportion of expression on the plasma membrane of Ca(v)1.2 in the CA1 and CA3 regions and of Ca(v)1.3 in the CA3 region from aged rats. Furthermore, the surface biotinylation results were supported by immunohistochemical analysis that revealed significant increases in Ca(v)1.2 immunoreactivity in the CA1 and CA3 regions of aged hippocampal pyramidal neurons. In addition, we found a significant increase in the level of phosphorylated Ca(v)1.2 on the plasma membrane in the dentate gyrus of aged rats. Taken together, our present findings strongly suggest that age-related cognitive deficits cannot be attributed to a global change in L-type channel expression nor to the level of phosphorylation of Ca(v)1.2 on the plasma membrane of hippocampal neurons. Rather, increased expression and density of LTCCs on the plasma membrane may underlie the age-related increase in L-type Ca2+ channel activity in CA1 pyramidal neurons.