An indirect basal ganglia pathway in anuran amphibians?

The mammalian subthalamic nucleus (STN) is a glutamatergic cell group within the indirect pathway of the basal ganglia. It receives input from the external globus pallidus (GP) and in turn projects to the internal GP and the substantia nigra pars reticulata (SNr). While the direct pathway from striatum to SNr is well established in anurans, it is unknown whether they possess an indirect pathway including a STN homologue. The subthalamic region comprises the dorsocaudal suprachiasmatic nucleus (dcSC), the posterior entopeduncular nucleus (EP), and the ventral part of the ventral thalamus (vVM/VL). In the fire-bellied toad Bombina orientalis we investigated whether one of these areas match the criteria established for the mammalian STN. We delineated the SNr in the midbrain tegmentum by labeling the striatonigral terminal field by means of GABA-, substance P-, and enkephalin immunohistochemistry and striatal tracer injections. Subsequently, we used double fluorescence tracing with injections into the SNr and GP to stain different parts of the indirect pathway. Confocal laser scan analysis revealed that dcSC, EP, and vVM/VL contain retrogradely labeled neurons projecting to the SNr, contacted by anterogradely labeled terminals arising in the GP. Immunohistochemical stainings with antibodies against glutamate and the glutamate transporters EAAC1 and vGluT2 demonstrated that the investigated nuclei contain glutamatergic neurons. Our results suggest that all regions in the subthalamic region fulfill our morphological criteria, except the connection back to the GP. An indirect basal ganglia pathway seems to be present in anuran amphibians, although we cannot exclusively delineate an STN homologue.