The dopaminergic innervation of the pigeon caudolateral forebrain: immunocytochemical evidence for a 'prefrontal cortex' in birds?

The dopaminergic (DA) innervation of the caudal telencephalon of the pigeon was investigated with an antiserum against glutaraldehyde-conjugated dopamine. It was found that the DA-like fibers were distributed within the Paleostriatum augmentatum and the dorsal Archistriatum in a dense meshwork of fibers, while most of the remaining part of the caudal forebrain was innervated by dopaminergic axons which were coiled up like baskets around unlabelled neurons. Within the basket-type innervated structures, the Neostriatum caudolaterale (Ncl) could be distinguished by the high density of its dopaminergic fibers. Retrograde tracer injections into Ncl revealed afferents from the Area ventralis tegmentalis (AVT) and the n. tegmenti pedunculo-pontinus pars compacta (TPc). Since large numbers of DA-like perikarya could be detected in AVT and TPc, it is supposed that these two structures constitute the main source of the dopaminergic innervation of the Ncl. Previous studies had suggested that the Ncl represents an avian equivalent to the mammalian prefrontal cortex. The present results reveal an organization similar to that of the mesocortico-prefrontal system and would thus strengthen this hypothesis.