Basal ganglia organization in amphibians: efferent connections of the striatum and the nucleus accumbens.

As a further step in unraveling the organization of the basal ganglia of amphibians, the efferent connections of the striatum and the nucleus accumbens have been studied in the brains of the anurans, Rana perezi and Xenopus laevis, and the urodele, Pleurodeles waltl, by using biotinylated or fluorescent dextran amines as anterograde tracers. A common pattern of efferent connections was observed in both groups of amphibians, but those in the anurans were more elaborate. Striatal efferent fibers were found to reach the lateral and medial amygdala, the anterior and posterior entopeduncular nuclei, several thalamic nuclei, the dorsomedial posterior tubercle, the pretectum, the optic tectum, the torus semicircularis, the pontomesencephalic reticular formation, and the caudal brainstem. Efferent fibers of the nucleus accumbens project to the medial amygdala, the preoptic area, the ventral hypothalamic nucleus, the dorsomedial posterior tubercle, the medial tegmental area, the pontomesencephalic reticular formation, and the raphe. In addition, the study has revealed the existence of intrinsic connections within the ventral telencephalic wall, suggesting a possible further compartmentalization of the amphibian basal forebrain. In conclusion, the results of the present study corroborate the notion that the basal ganglia of amphibians share many features with their presumed homologues in amniotes.