NEUROTRANSMITTER ORGANIZATION AND CONNECTIVITY OF THE BASAL GANGLIA IN VERTEBRATES - IMPLICATIONS FOR THE EVOLUTION OF BASAL GANGLIA

The basal ganglia in modem mammals, birds and reptiles (i.e. modem amn iotes) are very similar in connections and neurotransmitters, suggesti ng that the evolution of the basal ganglia in amniotes has been very c onservative. For example, the basal ganglia in all amniotes possess a dorsal striatum containing two main populations of projection neurons, substance P-containing (SP+) and enkephalin-containing (ENK+) neurons , which have major projections to the dorsal pallidum and the tegmentu m (ventral tegmental area and substantia nigra, or VTA/SN). The VTA/SN , in turn, has a major dopaminergic (DA+) projection to the striatum i n all amniotes. In this paper, we review these data on the basal gangl ia in amniotes and note points of similarity and difference in the fun ctional circuitry of the basal ganglia among amniotes. In addition, we review recent findings on the neurotransmitter organization and conne ctivity of the basal ganglia in amphibians and fishes, with the goal o f assessing whether a basal ganglia showing the same basic features as in amniotes is observed in anamniotes. Published data indicate that i n at least two groups of fishes (cartilaginous fishes and lungfishes) and apparently in amphibians, the basal ganglia is present and consist s of a distinct striatum and pallidum. The striatum of amphibians, car tilaginous fishes, and lungfishes contain SP+ and ENK+ neurons that se em to project to the pallidum as well as to a brainstem cell group tha t appears comparable to the VTA/SN of amniotes. Data for ray-finned fi shes also suggest the presence of a striatum containing SP+ and ENK+ n eurons that projects to VTA/SN-like brainstem cell group. In the basal ganglia of ray-finned fishes, however, a distinct pallidum had not be en identified. Finally, the brainstem cell group receiving striatal in put in all anamniotes contains DA+ neurons that seem to project to the striatum. The present analysis suggests that a rudimentary basal gang lia was already present in the brain of the ancestral jawed vertebrate s. This rudimentary basal ganglia likely consisted of a striatum and a pallidum, and the striatum probably already possessed the same basic connections and some of the same basic cell types as the basal ganglia of modem jawed vertebrates.