ULTRASTRUCTURAL-LOCALIZATION OF DENDRITIC MESSENGER-RNA IN ADULT-RAT HIPPOCAMPUS

An ultrastructural examination of mRNA within adult rat CA1 hippocampa l dendrites was conducted using two different methods. The messages fo r the alpha and beta forms of the calcium-calmodulin-dependent protein kinase it were localized in ultra-cryosections using silver-intensifi ed gold detection of isoform-specific oligonucleotide probes. Labeling for both isoforms was observed within the cell bodies and proximal de ndrites of pyramidal neurons, but only the alpha form was observed in more distal dendrites. Unfortunately, the morphological preservation o f the tissue was not sufficient to determine the localization of label ing relative to subcellular features such as denritic spines. To addre ss this issue, a preembedding peroxidase-based method was developed, r esulting in better preservation of the neuropil. The total population of polyadenylated [poly(A)] mRNA was localized in hippocampus using a biotinylated poly(dT) probe. Poly(A) mRNA was present in the nucleus a nd throughout the cell body of all hippocampal cells and within isolat ed dendrites and glial processes within the neuropil. Within pyramidal neurons, labeling was distributed in a longitudinal pattern in proxim al apical dendrites. More distally, the amount of labeling diminished, and smaller foci of labeling were observed, particularly near the pla sma membrane. Concentrated labeling was present at the base of dendrit ic spines and, less frequently, near synapses onto the dendritic shaft . These results suggest that dendritic mRNA is found in the vicinity o f postsynaptic sites and provide additional evidence that local protei n synthesis may play an important role in establishing and maintaining synaptic specializations.