Ramification patterns of vasoactive intestinal polypeptide (VIP)-cells in the rat primary visual cortex. An immunohistochemical study.

Vasoactive intestinal polypeptide (VIP)-immunoreactive cells in the primary visual cortex of the rat were classified on the basis of ramification pattern of cell processes. The distribution of cells over cortical layers, and proportions of cell classes relative to total cell numbers were evaluated by means of quantitative methods. Two main types of VIP-positive neurons, the bipolar and the multipolar were distinguished constituting 76% and 24% of the VIP populations, respectively. The axons of vertically oriented bipolars were observed to ramify within a column around the descending dendrite. By contrast, multipolar cells have a non-oriented ramification pattern. The two overlapping axonal systems form the VIP-innervation of the rat visual cortex.