IN-SITU HYBRIDIZATION FOR SOMATOSTATIN MESSENGER-RNA IN THE ADULT-RAT- CINGULATE, INSULAR, PREPIRIFORM, PERIRHINAL, ENTORHINAL, AND RETROSPLENIAL CORTICAL REGIONS

The expression of somatostatin mRNA within the allocortex of the rat w as examined by in situ hybridization with an alkaline phosphatase labe led probe. We sought to determine whether parcellation of the allocort ex could be based upon the number and laminar location of the hybridiz ed cells and to contrast the allocortical features with those of the i socortical areas. The cingulate region was characterized by intense, m oderate, and faint cells, small to medium in size throughout the lamin ae. The retrosplenial region demonstrated a somewhat stratified appear ance with an abundance of cells expressing somatostatin mRNA in the up per portion of the composite layer II-IV and also in the upper portion of layer VI. The insular region displayed more heterogeneity. The dis tribution of the cells hybridized for somatostatin mRNA formed distinc tive configurations within the insular region (dorsal and ventral agra nular insular areas) with no obvious generality. The perirhinal area r esembled the ventral agranular insular area, and the cell distribution of the entorhinal and prepiriform areas displayed a common characteri stic in that the primary axis of the perikarya of somatostatin mRNA ex pressing cells within the lower layers were oriented at almost every p ossible angle. The conclusion of the investigation is that in situ hyb ridization for somatostatin mRNA provides a means by which the areal b oundaries within the allocortex may be drawn.