Distribution of GABA, calbindin and nitric oxide synthase in the developing chick entopallium

The distribution of GABA, calbindin and neuronal nitric oxide synthase (nNOS) was analyzed in the developing avian entopallium. The study was carried out in chick embryos from embryonic day (E)8 to hatching postnatal day (P)0, using immunohistochemical methods. At E8, GABA-positive cells were observed in pallial regions. Neither calbindin nor nNOS-immunoreactive cells were observed. At E10, the number of GABA neurons in the prospective entopallium increased and also nNOS cells were observed. Lightly stained nNOS neurons predominated over intensely stained ones. Calbindin immunoreactivity was not observed in the entopallium. At E12, the entopallial complex appeared as the pallial region displaying the highest density of GABA neurons. Also the whole entopallium displayed an intensely stained calbindin neuropil with many embedded stained cells. From E12 on, there was a decrease in the expression of nNOS. At E14-16, both GABA and calbindin-immunoreactive neurons were numerous and homogeneously distributed within the entopallium. The whole entopallium displayed a moderately stained neuropil. From E18 to P0, GABA and nNOS immunoreactivities remained similar to previous stages. At these stages, calbindin immunoreactivity within the entopallium consisted of a moderately stained central region bordered dorsally by a pale stained region. These two areas could correspond to the entopallial core and the perientopallial belt, respectively.