Descending pathways from the basal ganglia to the optic tectum in a frog, Rana pipiens

In amniotes the basal ganglia influences the optic tectum by two disynaptic pathways. A ventral pathway from the basal ganglia to the optic tectum with a relay in the substantia nigra/ventral tegmental area exists in all amniotes studied. A dorsal pathway from the basal ganglia to the optic tectum with a relay in an enkephalinergic pretectal area is present only in birds and some reptiles. Evidence exists suggesting that both pathways are present in anurans. This study was undertaken in a frog, Rana pipiens, to determine if both pathways exist and if so, to see if the neurochemical content of the pathways was the same as in amniotes. Three fluorescent tracers, DiI, Fluoro-Ruby and Fluoro-Gold, were used to elucidate the basal ganglia-optic tectum pathways. Fluoro-Ruby, mainly an anterograde tracer, was injected in the ventral lateral telencephalon while Fluoro-Gold, a retrograde tracer was injected into the optic tectum. DiI was injected in the ventral lateral telencephalon, the optic tectum, and the dorsal and ventral mesencephalon. Immunohistochemistry was used on serial sections of the "tracer" brains to determine neurochemical content to the cell groups involved in the pathways. The results revealed a ventral pathway from the dorsal striatum through the posterior tuberculum, a suspected homologue to the amniote substantia nigra/ventral tegmental area, to the optic tectum. Although a pathway from the dorsal mesencephalon to the optic tectum was observed, no pathway from the telencephalon to the dorsal mesencephalon was found. Due to the weakness of negative findings and problems with the tracer experiments, a strong case for the absence of the dorsal pathway cannot be made. In addition, a recent study has demonstrated strong evidence for the presence of the dorsal pathway. If this is the case then the presence of both pathways appears to be a primitive characteristic with the loss of the dorsal pathway being a derived characteristic in mammals and some reptiles.