On the basal ganglia of amphibians: dopaminergic mesostriatal projections.

Recent studies have revealed a rather dense dopaminergic innervation of basal forebrain areas in anurans and urodeles. The aim of the present study is not only to extend these observations to the third order of amphibians, i.e. the Gymnophiona, but also to determine the cells of origin of the dopaminergic projection to the basal forebrain by means of combined tract-tracing/transmitter-immunohistochemical techniques. Immunohistochemistry with antibodies against tyrosine hydroxylase (TH) and dopamine (DA) demonstrated some remarkable differences in the distribution of DA fibers within the basal forebrain. Whereas in anurans the nucleus accumbens receives a major DA input, in urodeles the DA input to the striatum prevails. In gymnophionans, both basal forebrain structures receive an equally dense DA innervation. The double-labeling studies revealed also differences in the distribution of the cells of origin of the DA striatal input. Although in anurans and urodeles DA cells in both the posterior tubercle and the midbrain tegmentum contribute to this projection, the mesostriatal projection of urodeles is more elaborated than that of anurans.