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Recently, the afferent and efferent connections of the components of the amphibian basal ganglia, I.e., the striatum and the nucleus accumbens, have been studied in the brain of adult anurans and urodeles (Marin et aI., 1995, 1996, '997a,b). Significant differences in connections between the nucleus accumbens and the striatum were demonstrated, suggesting the presence of functional subdivisions. Moreover, those studies strongly supported the notion that a common pattern of connectivity exists among tetrapod vertebrates. The aim of the present study was to establish the temporal sequence of appearance of striatal and accumbal connections in amphibians so that the comparison with other amniotes allows to gain more insight into the evolution of the basal ganglia of vertebrates. To reach this goal, the anuran Xenopus laevis has been selected because of the existence of an accurate timetable of development (Nieuwkoop and Faber, , 967) and the existence of recent hodological data of basal ganglia connections in the adult brain of this species. The methodology used in this study is based on the neuronal transport of low molecular weight (3kD) biotinylated dextranamines (BOA). Shortly, at appropriate times, embryos and larvae anesthetized in MS222 were cooled to a body temperature of 40C and perfused transcardially with iced Ringer's solution. The brains were then removed and placed in fresh iced Ringer's solution. Applications of BDA were made unilaterally within the striatum (Fig.1) or the nucleus accumbens. After '5-24 h at 150C in continuously oxygenated Ringer's solution the brains were fixed in 4% parformaldehyde. Following crioprotection, the brains were cut on a freezing microtome and free-floating sections were reacted with avidin-biotin complex for BOA visualization. The detection of peroxidase activity was based on DAB-nickel as chromogen. After mounting the sections, the distribution of labeled cells and fibers in the brains of the tadpoles was studied. At late embryonic stages, the basal forebrain of Xenopus receives inputs from cells located in the amygdala, the thalamus, the suprachiasmatic nucleus, the posterior tubercle, the raphe nucleus and the rhombencephalic reticular formation. During the premetamorphosis, the distinct connections of the striatum and the nucleus accumbens start to be patent. Due to the bidirectional transport of BOA, both afferent cells and efferent fibers are distinguishable at these stages. Thus, afferent cells to the straitum are more numerous in the same areas observed in the embryos (mainly in the central dorsal thalamus, Fig. 2) but also new sources of inputs were detected in the mesencephalic and isthmic tegmentum, the parabrachial nucleus and the nucleus of the solitary tract. The efferent fibers leaving the telencephalon via de lateral forebrain bundle form a conspicuous terminal plexus in the mesencephalic tegmentum (Fig. . .~. TO "

Embryonic and larval development of the basal ganglia connections in the brain of Xenopus laevis.