Genesis of GABA-immunoreactive neurons in the ferret visual cortex

The pattern of neurogenesis of GABA-immunoreactive neurons in the ferret primary visual cortex was determined using immunohistochemical and 3H-thymidine autoradiographic techniques. Neurons in the visual cortex of the ferret undergo their final cell division during a period extending from embryonic day 20 (E20) to postnatal day 14 (P14) and follow an inside-out pattern of neuronal production (Jackson et al., 1984) similar to that observed in other mammals. Earlier-generated neurons are found at deeper cortical positions in the adult than are those generated later. Layer I is an exception to this rule, since neurons destined for this layer are produced at both the beginning and end of neurogenesis. In this study, the pattern of neurogenesis of GABA-immunoreactive neurons is compared to the pattern observed for nonimmunoreactive neurons. The overall pattern of cortical neurogenesis (inside-out pattern) is similar for GABA-immunoreactive neurons and neurons that are not GABA-immunoreactive. However, the GABA-immunoreactive neurons born on a given day of development are more broadly distributed across the radial axis of the adult cortex than are nonimmunoreactive neurons generated on the same day. GABA-immunoreactive neurons generated later in neurogenesis are, on average, slightly smaller than those generated early. If GABA-immunoreactive neurons in the visual cortex are interneurons, then these findings suggest that interneurons follow the same pattern of neurogenesis as do projecting neurons in the visual cortex.