Development of vasoactive-intestinal-polypeptide-immunoreactive neurons in the rat occipital cortex: a combined immunohistochemical-autoradiographic study.

The postnatal development of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons, previously labeled with [3H]thymidine on embryonic days E14-E22, has been studied in the rat occipital cortex. Immuno-histochemistry combined with autoradiography showed very little evidence of an "inside-out" pattern of maturation. Most VIP neurons are generated between E17 and E21 and are found in layers II-IV of the cortex, but their position within these layers is not dictated by their date of birth. There is evidence of a temporal maturation since E17 VIP neurons were seen first (at day 7) and E21 last. Peak numbers of VIP neurons were generated on E19. The numbers of VIP-immunoreactive neuronal somata detected in the cortex increased from the first week after birth to the third week and declined thereafter. However VIP-immunoreactive dendrites were still visible, suggesting that VIP levels in the cell bodies were very low, and not that there was a loss of neurons.