Functional coupling between substantia nigra and basal ganglia homologues in amphibians.

Neuroanatomical and pharmacological experiments support the existence of a homologue of the mammalian substantia nigra-basal ganglia circuit in the amphibian brain. Demarcation of borders between the striatum and pallidum in frogs, however, has been contentious, and direct evidence of functional coupling between the putative nigral and striatal homologues is lacking. To clarify basal ganglia function in anurans, the authors used expression of immediate-early gene egr-1 as a marker of neural activation in the basal ganglia of túngara frogs (Physalaemus pustulosus). Regional variation in egr-1 mRNA levels distinguished striatal and pallidal portions of the basal ganglia and supported the grouping of the striatopallidal transition zone with the dorsal pallidum. As further evidence for a functional coupling between the dopaminergic cells in the posterior tuberculum (the putative substantia nigra homologue) and the basal ganglia, a positive relationship was demonstrated between the size of the dopaminergic cell population and the neural activation levels within the dorsal pallidum.