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Basal ganglia organization in amphibians: Afferent connections to the striatum and the nucleus accumbens

As part of a research program to determine if the organization of basal ganglia (BG) of amphibians is homologous to that of amniotes, the afferent connections of the BG in the anurans Xenopus laevis and Rana perezi and the urodele Pleurodeles waltl were investigated with sensitive tract‐tracing techniques. Hodological evidence is presented that supports a division of the amphibian BG into a nucleus accumbens and a striatum. Both structures have inputs in common from the olfactory bulb, medial pallium, striatopallial transition area, preoptic area, ventral thalamus, ventral hypothalamic nucleus, posterior tubercle, several mesencephalic and rhombencephalic reticular nuclei, locus coeruleus, raphe, and the nucleus of the solitary tract. Several nuclei that project to both subdivisions of the BG, however, show a clear preference for either the striatum (lateral amygdala, parabrachial nucleus) or the nucleus accumbens (medial amygdala, ventral midbrain tegmentum). In addition, the anterior entopeduncular nucleus, central thalamic nucleus, anterior and posteroventral divisions of the lateral thalamic nucleus, and torus semicircularis project exclusively to the striatum, whereas the anterior thalamic nucleus, anteroventral, and anterodorsal tegmental nuclei provide inputs solely to the nucleus accumbens. Apart from this subdivision of the basal forebrain, the results of the present study have revealed more elaborate patterns of afferent projections to the BG of amphibians than previously thought. Moreover, regional differences within the striatum and the nucleus accumbens were demonstrated, suggesting the existence of functional subdivisions. The present study has revealed that the organization of the afferent connections to the BG in amphibians is basically similar to that of amniotes. According to their afferent connections, the striatum and the nucleus accumbens of amphibians may play a key role in processing olfactory, visual, auditory, lateral line, and visceral information. However, contrary to the situation in amniotes, only a minor involvement of pallial structures on the BG functions is present in amphibians. J. Comp. Neurol. 378:16–49, 1997. © 1997 Wiley‐Liss, Inc.