An immunohistochemical approach to lungfish telencephalic organization

The brains of lungfishes have received little attention in comparative neuroanatomy, in spite of the strategic position of these fishes in phylogeny, in that they are currently recognized as the closest living relatives of tetrapods. The neglect has probably been due to the difficulty of obtaining these unique animals, which comprise (along with the coelacanths) the only extant representatives of the lobe-finned fishes. Several previous studies have established the basic anatomy of the telencephalon in lungfishes, but many aspects of the intrinsic organization of the main telencephalic subregions have remained unclear. The immunohistochemical localization of diverse neurotransmitters, neuromodulators, and transcription factors expressed by genes involved in brain regionalization has served to clarify many aspects of the organization of the telencephalon in lungfishes. Here we describe the main immunohistochemical features of the telencephalon in two lungfish species, Protopterus dolloi and Neoceratodus forsteri. Our analysis highlights the common traits shared by lungfishes and tetrapods. These include four pallial regions, distinct striatal and pallidal components of the basal ganglia, specific regionalization of the septum, and the presence of three amygdaloid regions. In general, the use of immunohistochemistry in the study of the telencephalon of lungfishes reveals that this structure is notably more complex than previously thought and that it possesses all major subregions recognized in amphibians and amniotes.