Dopaminergic Input to and Reception in the Auditory Midbrain

In anurans acoustic communication mediates reproductive behavior and fitness, making the auditory system a robust model for studying how biologically relevant sounds are processed. Neuromodulators, such as dopamine, can significantly alter auditory neural processing via modulation of action potential initiation and regulation of response properties in several species such as mice, songbirds and fish. Recently, dopamine has been shown to modulate neuronal response properties in the torus semicircularis (torus) of bullfrogs (Rana catesbeiana). The torus is the anuran homolog of the mammalian inferior colliculus. Because the torus receives input from all ascending and descending auditory processing centers and is known to play a crucial role in the analysis of behaviorally relevant sounds, sources of dopamine input and which receptors receive it are of interest. To this end, a precursor in the dopamine synthesis pathway, tyrosine hydroxylase, and dopamine receptors were visualized within the bullfrog brain via immunohistochemistry. Results indicate a potential of seven dopaminergic sources to the torus suprachiasmatic nucleus, dorsal hypothalamus, ventromedial thalamic nucleus, posterior thalamic nucleus, central thalamic nucleus, anteroventral tegmental nucleus, and the ventromedial border of the solitary. The presence of axonal tracts and synaptic boutons positive for tyrosine hydroxylase within all nuclei of the torus indicate dopamine transmission to and reception the torus. The dopamine is received by dopamine 2 receptors in all subnuclei of the torus. These findings fill critical gaps in our knowledge about the neural basis of signal recognition and the role of dopamine in this process.