Convergence of Excitatory and Inhibitory Projections in the Mouse Medial Geniculate Body

Abstract

The medial geniculate body (MGB) is the target of excitatory and inhibitory inputs from several neural sources. Among these, the inferior colliculus (IC) is an important nucleus in the midbrain that acts as a nexus for many auditory pathways and projections, ascending and descending, throughout the rest of the central auditory system and provides both excitatory and inhibitory projections to the MGB. In addition, the thalamic reticular nucleus (TRN) is a major source of inhibition to the MGB, particularly in rodents. Finally, the auditory cortex (AC) is a major source of descending input to the MGB, providing direct excitation and indirect inhibition via the TRN. In our study, we assessed the relative contribution from these excitatory and inhibitory projection sources to the MGB of the auditory system in mice. Using retrograde tract tracing with CTβ -Alexa Fluor 594 injected into the MGB of the mouse, we quantitatively mapped the projections from both the ipsilateral and contralateral IC, the TRN, and the AC to the ipsilateral MGB. Our results indicate significant GABAergic projections from the IC and TRN to the MGB and excitation from the AC that play an overlooked role in shaping auditory processing. These results complement prior studies in other species, which suggests that these pathways are important factors in the regulation of neuronal activity in the auditory forebrain.