Evolution of the basal ganglia: new perspectives through a comparative approach

The basal ganglia (BG) have received much attention during the last 3 decad es mainly because of their clinical relevance. Our understanding of their s tructure, organisation and function in terms of chemoarchitecture, compartm entalisation, connections and receptor localisation has increased equally. Most of the research has been focused on the mammalian BG, but a considerab le number of studies have been carried out in nonmammalian vertebrates, in particular reptiles and birds. The BG of the latter 2 classes of vertebrate s, which together with mammals constitute the amniotic vertebrates, have be en thoroughly studied by means of tract-tracing and immunohistochemical tec hniques. The terminology used for amniotic BG structures has frequently bee n adopted to indicate putative corresponding structures in the brain of ana mniotes, i.e. amphibians and fishes, but data for such a comparison were, u ntil recently, almost totally lacking. It has been proposed several times t hat the occurrence of well developed BG structures probably constitutes a l andmark in the anamniote-amniote transition. However, our recent studies of connections, chemoarchitecture and development of the basal forebrain of a mphibians have revealed that tetrapod vertebrates share a common pattern of BG organisation. This pattern includes the existence of dorsal and ventral striatopallidal systems, reciprocal connections between the striatopallida l complex and the diencephalic and mesencephalic basal plate (striatonigral and nigrostriatal projections), and descending pathways from the striatopa llidal system to the midbrain tectum and reticular formation. The connectio nal similarities are paralleled by similarities in the distribution of chem ical markers of striatal and pallidal structures such as dopamine, substanc e P and enkephalin, as well as by similarities in development and expressio n of homeobox genes. On the other hand, a major evolutionary trend is the p rogressive involvement of the cortex in the processing of the thalamic sens ory information relayed to the BG of tetrapods. By using the comparative ap proach, new insights have been gained with respect to certain features of t he BG of vertebrates in general, such as the segmental organisation of the midbrain dopaminergic cell groups, the occurrence of large numbers of dopam inergic cell bodies within the telencephalon itself and the variability in, among others, connectivity and chemoarchitecture. However, the intriguing question whether the basal forebrain organisation of nontetrapods differs e ssentially from that observed in tetrapods still needs to be answered.