The morphology and distribution of peptide-containing neurons in the adult and developing visual cortex of the rat. II. Vasoactive intestinal polypeptide.

Using immunocytochemistry, we have examined the morphology and distribution of vasoactive intestinal polypeptide-like immunoreactive neurons in the visual cortex of albino rats whose ages were closely spaced in time between the first postnatal day and adulthood. In the adult, immunoreactive neurons were located in layers II to VI but were concentrated in layers II and III. All labelled neurons had the morphological characteristics of cortical non-pyramidal cells with the majority being of the bipolar variety as described in Golgi preparations. Some multipolar forms were also present. Vasoactive intestinal polypeptide-immunoreactivity appeared to develop in postnatal life. Labelled cells were first seen in layers V and VI at day 4. During the subsequent few days, some labelled cells were observed in the more superficial layers and by day 8 they were predominantly present in layers II and III. Although the distribution of immunoreactive cells at this time resembled that of adult animals, their morphology displayed immature features. The size and extent of their dendritic branching appeared to increase considerably during the second and third weeks and their morphological maturation was attained by the middle of the fourth postnatal week.