Differential immunoreactivity for alpha-actinin-2, an N-methyl-D-aspartate-receptor/actin binding protein, in hippocampal interneurons.

Recent studies have demonstrated that hippocampal interneurons possess distinct cytoskeletal and cell-signaling proteins in comparison to hippocampal principal cells; however, little is known about the differences in the actin cytoskeleton between these two populations. This study examined the immunoreactivity of alpha-actinin-2, an actin binding/N-methyl-D-aspartate-receptor linking protein, in the rat hippocampal formation using double-labelling immunofluorescence. Alpha-actinin-2 immunoreactivity is seen throughout the hippocampus with heavy labeling observed in the dendrites of granule cells, in CA2 pyramidal cells and in presumed interneuronal somata throughout the dentate gyrus and CA1. All the cells with heavy somatic alpha-actinin-2 immunoreactivity in the dentate gyrus and CA1 were GABAergic interneurons labeled by glutamate decarboxylase (99%). Examination of the neurochemical marker content of the alpha-actinin-2 immunoreactive interneurons revealed that the majority of this population was neuropeptide-Y-positive and a minority was positive for calretinin. Fluid percussion head trauma did not result in significant alterations of alpha-actinin-2 immunoreactivity in hippocampal interneurons. The developmental profile of alpha-actinin-2 immunoreactivity showed the presence of alpha-actinin-2 in the hippocampus at P1, labeling of interneurons by P7 and the adult staining pattern seen by P21. This study demonstrates that principal cells and interneurons are differentially immunoreactive for alpha-actinin-2, and that alpha-actinin-2 staining is restricted to a subpopulation of interneurons. Each of the three classes of cytoskeletal elements have been shown to be differentially expressed in hippocampal interneurons and principal cells, suggesting that the cytoskeleton is a defining feature of neuronal populations. Additionally, the limited expression of alpha-actinin-2 could have important functional implications in N-methyl-D-aspartate receptor localization and modulation.