Migration of peptide-immunoreactive local circuit neurons to rat cingulate cortex

The times of origin of neurons immunoreactive for somatostatin (SRIF), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP) were determined using a combined immunohistochemical-autoradiographic technique. 3H-thymidine (3H-dT) was injected into pregnant rats on gestational day 13 (G13), G15, G17, G19, or G21. Animals were killed on postnatal day 30, that is, after the completion of neuronal migration. Sections of the brain including posterior cingulate cortex (area 29), visual cortex (area 17), and somatosensory cortex (area 3) were processed serially for peptide immunoreactivity and autoradiographically for labeling with 3H-dT. SRIF- and CCK-immunoreactive neurons were cogenerated and comigrated according to an inside-to-outside sequence. Accordingly, neurons in layer VI were born on G13, neurons in intermediate layers were born on G15-G17, and neurons in layer II/III were born on G19-G21. In contrast, VIP-positive neurons did not follow such a sequence. Neurons in all layers of cortex were generated at relatively constant rates between G13 and G21. VIP-immunoreactive neurons were the only known subpopulation of neurons that did not migrate into cortex by an inside-to-outside sequence. Thus, the migration of local circuit neurons to cingulate cortex follows patterns that are similar to those discerned in more differentiated cortical areas.