Basal ganglionic pathways to the tectum: studies in reptiles

Abstract

Relations between the basal ganglia and the tectum were investigated in two different orders of reptiles: turtles (Chrysemys scripta) and crocodilians (Caiman crocodilus). In both species, efferents from the paleostriatal complex, a telencephalic region considered comparable to the mammalian basal ganglia on the basis of topographic, histochemical, and hodological criteria, were found to project to a prominent pretectal cell group called the dorsal nucleus of the posterior commissure (nDCP). Cells within nDCP, in turn, were found to project extensively upon the optic tectum. This paleostriatal-pretectal-tectal pathway is comparable to a previously described paleostriatal-pretectal-tectal channel in birds that involves a relay in the pretectal nucleus, spiriformis lateralis (SpL). Neither the presently described paleostriatal-pretectal-tectal channel of reptiles nor that previously described in birds, however, appears comparable to the superficially similar basal ganglionic-nigral-superior collicular pathway of mammals. Rather, data from the present experiments indicate the existence of a second paleostriatal channel to the tectum, one which does appear comparable to the basal ganglionic-nigral-superior collicular pathway of mammals. This second paleostriatal channel to the tectum, relayed via a tegmental cell group termed the substantia nigra in turtles and the tegmentipedunculopontine complex in caiman, is of much lesser prominence in reptiles than the paleostriatal-pretectal-tectal channel. The present results indicate the existence of at least two separate systems by which the basal ganglia in reptiles can influence the midbrain roof. These two channels, particularly the prominent paleostriatal-pretectal-tectal pathway, may represent major routes by which the basal ganglia influence motor functions in reptiles. Further, although previous research had only indicated the existence of a paleostriatal-pretectal-tectal pathway in birds and a basal ganglionic-nigral-collicular channel in mammals, existing data are consistent with the hypothesis that both presently described pathways in reptiles exist in birds and mammals, though only one of the two may be prominent in mammals.