Expression of the prodynorphin gene in the developing and adult cerebral cortex of the rat: an in situ hybridization study.

A population of cortical neurons contains the opioid peptide dynorphin; the laminar distribution of these neurons in the adult cerebral cortex and their patterns of development are not well known. We have utilized in situ hybridization techniques to localize prodynorphin mRNA-containing neurons. Rats aged from embryonic day (E) 15 through postnatal day (P) 90 were used. Prenatal animals did not show any labeling in the cerebral cortex. By P4, prodynorphin was expressed in a small number of cortical neurons for the first time. The autoradiographic signal was restricted to perikarya. In the frontoparietal cortex, labeled neurons first appeared in layer V and the upper part of layer VI. Subsequently, from P11 onward, the band expanded in an "inside-out" sequence to include layers IV through II. In the posterior cingulate cortex and in the insular and perirhinal cortices, prodinorphin mRNA containing-neurons were located preferentially in layer V. In all cortical areas analyzed, a progressive increase in the packing density of neurons expressing prodynorphin mRNA was observed until P14; it decreased slightly thereafter.