Basal ganglia organization in amphibians: development of striatal and nucleus accumbens connections with emphasis on the catecholaminergic inputs.
To broaden our insight into the organization of the basal ganglia of amphibians, the development of the connections of the striatum and the nucleus accumbens was studied by means of tract-tracing techniques based on the transport of biotinylated dextran amines. In a number of experiments, these techniques were combined with tyrosine hydroxylase immunohistochemistry to identify the sources of catecholaminergic inputs to the striatum and the nucleus accumbens. Already at late embryonic stages, the basal telencephalon receives inputs from cells located in the amygdala, the thalamus, the suprachiasmatic nucleus, the raphe nucleus, and the rhombencephalic reticular formation. At these stages, the rostral part of the posterior tubercle seems to be the only source of the dopaminergic input to the basal telencephalon. During premetamorphosis, not only a differentiation between connections of the striatum and the nucleus accumbens could be made, but new sources of inputs were also detected in the mesencephalic and isthmic tegmentum, the parabrachial nucleus, and the nucleus of the solitary tract. Double-labeling experiments revealed that, at these stages, in addition to the posterior tubercle, cells within the mesencephalic tegmentum, the locus coeruleus, and the solitary tract nucleus contribute to the catecholaminergic innervation of the basal forebrain. During prometamorphic stages, a gradual increase occurs in the number of cells that project to the basal telencephalon. At the beginning of the metamorphic climax, the organization of the basal ganglia afferents largely resembles the pattern observed in juveniles and adults. Remarkably, during larval stages, the cells that contribute to the dopaminergic innervation of the basal forebrain show a rostrocaudal gradient in time of appearance. Moreover, the dopaminergic fibers reach the striatum earlier than the nucleus accumbens, and they precede markedly the development of the efferent connections of both brain structures. These developmental aspects are easily correlated with the situation in amniotes; therefore, the notion that amphibians share an essentially similar pattern of basal ganglia organization with other tetrapods is further strengthened.