Organization of afferents to the striatopallidal systems in the fire-bellied toad Bombina orientalis

Abstract

The cerebral hemispheres of amphibians display paired dorsal and ventral striatum (commonly referred to as striatum proper and nucleus accumbens, respectively). Each striatal region is proposed to be closely associated with a pallidal structure located caudal to it to form a striatopallidal system. In the present study, afferents to the dorsal and ventral striatopallidal systems of the fire-bellied toad (Bombina orientalis) were investigated using the neuronal tracer biocytin. A quantitative analysis of the topographical distribution of afferent neurons from the thalamus and posterior tubercle/ventral tegmentum was emphasised. The main results show that inputs to the two striatopallidal systems originate from distinct dorsal thalamic nuclei, with dorsal and ventral striatopallidal afferent neurons favouring strongly the lateral/central and anterior thalamic nuclei, respectively. However, afferent neuron distribution in the dorsal thalamus does not differ in the rostrocaudal axis of the brain. Afferent neurons from the posterior tubercle and ventral tegmentum, on the other hand, are organised topographically along the rostrocaudal axis. About 85% of afferent neurons to the dorsal striatopallidal system are located rostrally in the posterior tubercle, while 75% of afferent neurons to the ventral striatopallidal system are found more caudally in the ventral tegmentum. This difference is statistically significant and confirms the presence of distinct mesostriatal pathways in an amphibian. These findings demonstrate that an amphibian brain displays striatopallidal systems integrating parallel streams of sensory information potentially under the influence of distinct ascending mesostriatal pathways.