Homology of the mesopallium in the adult chicken identified by gene expression of the neocortical marker cholecystokinin

• CCK mRNA is a gene marker for layers 2/3 of mammalian neocortex.
• CCK mRNA is expressed in the mesopallium of adult chicken brains.
• The mesopallium is probably comparable to layers 2/3 of mammalian neocortex.
• CCK mRNA is expressed in layers 5/6 in mammals and in the arcopallium in birds.

Studies of gene expression and fiber connections have suggested that the primary visual (entopallium) and auditory (field L2) centers in the avian telencephalon are homologous to layer 4 of extrastriate and auditory neocortices of mammals, respectively. In addition, it has been proposed that the arcopallium contains neurons homologous to layers 5/6 and that the mesopallium may be homologous to superficial neocortical layers, but gene expression evidence for the latter is lacking in adult birds. In the present study using adult chickens we have examined the gene expression of cholecystokinin (CCK) mRNA, a selective marker for layers 2/3 of mammalian neocortex. CCK mRNA was expressed in neurons of the entire mesopallium, but not in any part of the nidopallium. Together with hodological evidence of connections between the mesopallium and the two primary sensory areas, our results are consistent with the suggestion that the mesopallium is comparable to certain superficial layers of mammalian neocortex.