PARCELLATION OF CORTICAL AREAS BY IN-SITU HYBRIDIZATION FOR SOMATOSTATIN MESSENGER-RNA IN THE ADULT-RAT - FRONTAL, PARIETAL, OCCIPITAL, ANDTEMPORAL REGIONS

The expression of somatostatin mRNA within the neocortex of the rat wa s examined by in situ hybridization with an alkaline phosphatase-label ed probe. We sought to determine whether parcellation of the neocortex could be based upon the number and laminar location of the hybridized cells. Our investigation demonstrated that the boundaries of the neoc ortical areas can be determined by the distribution pattern of neurons expressing somatostatin mRNA. Few hybridized cells were located withi n layer IV, and this sparsity of cells within their wide granular laye r marked the primary sensory areas. The occipital region was stratifie d, with insensely labeled cells in layers II/III and VI and faintly la beled cells in layer V. The parietal region carried a similar stratifi cation, but more space between intensely labeled cells in layers III a nd V and between layers V and VI gave the region a three-tiered appear ance. The temporal region displayed intensely labeled cells dispersed throughout layers III and VI and many in layer V as well as those fain tly labeled without any breaks between the laminae. The distribution o f the cells hybridized for somatostatin mRNA formed two configurations within the frontal region. It was difficult to identify any laminatio n in the first area, whereas the second area demonstrated a stratifica tion reminiscent of the parietal region, but with only two tiers. The conclusion of the investigation is that in situ hybridization for soma tostatin mRNA provides an exceptional means by which the areal boundar ies within the neocortex may be drawn.