Multi-Color Channelrhodopsin-Assisted Circuit Mapping of Auditory Inputs to a Novel Class of Stellate Cells in the Mouse Inferior Colliculus
Final Report Abstract
The inferior colliculus (IC) is the hub of the ascending auditory system, as it is a nearly obligatory processing center for the output of the auditory brainstem. Although the IC plays important roles in speech processing, sound localization, and other auditory computations, the identity of individual cell types and the organization of neural circuits in the IC has remained largely unknown. To better understand how sounds are processed in the IC, it is important to identify the classes of neurons that make up the IC and determine how they function within neural circuits. In this project, by using a combination of genetic, anatomical, and physiological methods, we identified a novel class of stellate cells that are labeled in Vasoactive Intestinal Peptide (VIP)-IRES-Cre mice (cf. Goyer et al. 2019). VIP neurons share a common set of molecular, neurochemical, morphological, and physiological features: They are glutamatergic, have a sustained firing pattern, have a stellate morphology, represent ~ 20% of stellate cells in the IC, and have spiny dendrites. Via axonal tract tracing studies, we found that VIP neurons project to auditory thalamus, auditory brainstem, the periaqueductal gray, and superior colliculus. Thus, VIP neurons are well-positioned to influence auditory computations in a number of brain regions. Using Channelrhodopsin assisted circuit mapping (CRACM), we found that VIP neurons receive input from the contralateral IC and the contralateral DCN. Commissural inputs could be excitatory or inhibitory, or a combination of both. Excitatory commissural inputs were mediated by AMPA and NMDA receptors, while inhibitory inputs were mediated by GABA-A receptors. EPSPs evoked by optical stimulation of DCN afferents were excitatory, surprisingly slow and had no NMDA receptor contribution. Activation of DCN afferents also elicited feedforward inhibition (FFI), which limited EPSP duration, uncovering a novel circuit motif in the IC. Together, these results represent a critical step toward determining how defined neural circuits in the IC support sound processing. Having identified VIP neurons as a distinct neuron type in the IC, we turned to two-color CRACM experiments in vitro to identify how inputs from the DCN and contralateral IC combine with FFI to shape the output of VIP neurons. These experiments are a critical step toward defining how IC neurons integrate diverse streams of inputs and influence sound processing in the IC. Pilot experiments showed that a red-shifted opsin like ChrimsonR is a suitable candidate for long range CRACM experiments in the auditory pathway. Yet, current generation red-shifted opsins have considerable co-activation by blue light in slice experiments, confounding the results and calling for additional methods to tease apart synaptic activation from different sources on one neuron. This project was severely affected by the COVID-19 / corona pandemic in 2020, leading to a premature termination of the project.
Publications
- Optogenetic Circuit Mapping of Commissural and Ascending Projections to VIP Neurons in the Inferior Colliculus of Mice (2018)
David Goyer, Marina A. Silveira, Alexander P. George, Peter T. Malinski, Michael T. Roberts
- 2019 eLife digest: Making sound connections
Goyer et al.
(See online at https://doi.org/10.7554/eLife.43770.002) - A Feedforward Inhibitory Circuit that Limits EPSP Duration in the Inferior Colliculus of Mice (2019)
David Goyer, Marina A Silveira, Alexander P George, Nichole L Beebe, Brett R Schofield, Michael T Roberts
- A novel class of inferior colliculus principal neurons labeled in vasoactive intestinal peptide-Cre mice. (2019) eLife 2019;8:e43770
David Goyer, Marina A Silveira, Alexander P George, Nichole L Beebe, Ryan M Edelbrock, Peter T Malinski, Brett R Schofield, Michael T Roberts
(See online at https://doi.org/10.7554/eLife.43770) - Synaptic integration and sound processing by VIP neurons in the inferior colliculus of mice (2019)
David Goyer, Marina A Silveira, Alexander P George, Nichole L Beebe, Brett R Schofield, Michael T Roberts
- Long-range Channelrhodopsin-assisted Circuit Mapping of Inferior Colliculus Neurons with Blue and Red-shifted Channelrhodopsins (2020) J. Vis. Exp. (156), e60760
David Goyer & Michael T. Roberts
(See online at https://doi.org/10.3791/60760) - Sound processing by VIP neurons in the mouse inferior colliculus (2020)
David Goyer, Marina A Silveira, Alexander P George, Nichole L Beebe, Brett R Schofield, Michael T Roberts
