Fine-scale tonotopic arrangement in the dorsal cortex of the mouse inferior colliculus studied with two-photon calcium imaging

Background The inferior colliculus (IC) is the largest auditory structure of the midbrain. It receives ascending input from the brainstem but also descending projections from the auditory cortex. While the central nucleus of the IC has received a lot of attention, very little is still known about the function of the dorsal cortex (ICd) of the IC. In the mouse the ICd is located between the cerebral cortex and the cerebellum and is, therefore, accessible with optical methods. We took advantage of this and set out to investigate the fine-scale tonotopic arrangement of the ICd with two-photon imaging. Methods We injected C57BL/6 mice with a viral vector to induce the expression of the genetically encoded calcium indicator GCaMP6m. Injections were targeted either to the dorsal part of the inferior colliculus or layer V of the auditory cortex. Four weeks later we imaged, using a two-photon microscope, populations of ICd neurons or cortico-collicular terminals in anaesthetised mice and characterised their responses to noise bursts and pure tones. Results ICd neurons exhibited strong responses to tones and noise bursts and we found robust evidence for a tonotopic arrangement of the cell body populations that we imaged - the difference in characteristic frequency between pairs of neurons increased as a function of distance, and there was evidence for a frequency gradient. Terminals of cortico-collicular neurons showed only weak acoustically driven responses. Conclusion Using two-photon calcium imaging in the ICd for the first time, we found robust acoustically driven responses and sharp frequency tuning of cell bodies in the ICd as well as evidence for a tonotopic arrangement. The nature of the responses of the cortico-collicular terminals suggests that they have more of a modulatory rather than a driving influence on the activity of ICd neurons.