Posthatching development of preproenkephalin mRNA-expressing cell populations in the pigeon telencephalon

Enkephalin peptides are highly expressed in the vertebrate telencephalon. Our previous investigations in the pigeon and in the chicken [26] suggested that the cellular distribution of these peptides is conserved in phylogenetically 'old' telencephalic regions (e.g. the basal ganglia), while it has species-specific organizations in areas (e.g. dorsomedial forebrain and bulbus olfactorius) that are likely to play important roles in species-specific behaviors. In the present study, we investigated the posthatching development of preproenkephalin (PPE) mRNA-containing cells in the pigeon forebrain using in situ hybridization histochemistry. These cells are densely distributed in the paleostriatal complex (corresponding to the mammalian caudate-putamen) at hatching, and their density progressively decreases during the first 9 days posthatching, when it is similar to that of adult pigeons. In the dorsomedial forebrain (corresponding to the mammalian hippocampus), PPE mRNA-expressing cells are present at hatching, and their density reaches a peak around the 6th day posthatching. In the bulbus olfactorius, the first PPE mRNA-containing cells are observed after 9 days posthatching. The developmental profile of PPE mRNA expression in these areas of the pigeon telencephalon shows remarkable similarities with the development of enkephalinergic cells in corresponding brain areas of mammals. As in the mammalian caudate-putamen, the developmental expression of enkephalin peptides in the paleostriatal complex is likely to be related to neuronal withdrawal from the mitotic cycle. The developmental pattern of expression of PPE mRNA in the dorsomedial forebrain suggests that enkephalin peptides contribute to the maturation of the behavioral functions of this area.