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Layer 6 corticocollicular neurons, in contrast, are nonpyramidal, regular-spiking neurons with elongated somata and thin, but densely branching dendrites ( Slater et al., 2013 Zurita et al., 2018). Layer 5 corticocollicular neurons are morphologically and physiologically distinct from layer 5 neurons which project intracortically ( Games and Winer, 1988 Llano and Sherman, 2009 Rock and Apicella, 2015). Layer 5 corticocollicular neurons are large pyramidal neurons with thick apical dendrites that rhythmically burst when depolarized ( Slater et al., 2013). A subset of the corticocollicular projection, primarily from layer 5, targets contralateral IC ( Andersen et al., 1980 Druga et al., 1997 Budinger et al., 2000a Bajo et al., 2007 Schofield, 2009). Across multiple species, neurons from two cortical layers project to the IC: those from layer 5 and a smaller proportion from deep layer 6 ( Games and Winer, 1988 KГјnzle, 1995 Coomes et al., 2005 Bajo et al., 2007 Schofield and Motts, 2009 Slater et al., 2013). However, little is known about corticocollicular circuitry. Descending cortical projections are responsible for shaping auditory information, particularly in the inferior colliculus (IC), which is the main integration site for acoustic information before reaching the forebrain (for review, see Winer, 2006 Suga, 2008 Bajo and King, 2012 Stebbings et al., 2014). The latter connection provides a hyperdirect link between acoustic sensation and descending control, thus demonstrating a novel mechanism for rapid вЂњonlineвЂќ modulation of sensory perception.ДЄs we navigate the environment, complex arrays of sounds must be selected, enhanced, or suppressed to guide behavior. Specifically, we observed that layer 5 corticocollicular neurons integrate information across cortical lamina and receive direct thalamic input. Here we show, using a range of optical techniques, that mouse auditory corticocollicular neurons from different layers are embedded into different cortical and thalamic networks. The projection from the auditory cortex to the inferior colliculus is a massive, yet poorly understood, pathway emanating from two distinct cortical layers. SIGNIFICANCE STATEMENT Descending projections from the cortex play a critical role in shaping the response properties of sensory neurons. Furthermore, the direct connection between the auditory thalamus and layer 5 corticocollicular neurons reveals a novel and rapid link connecting ascending and descending pathways. These data demonstrate that layer 5 and 6 corticocollicular neurons receive distinct sets of cortical and thalamic inputs, supporting the hypothesis that they have divergent roles in modulating the inferior colliculus. Finally, a new low-calcium/synaptic blockade approach to separate direct from indirect inputs using laser photostimulation was validated. In addition, all layer 5 corticocollicular neurons received direct and large thalamic inputs from channelrhodopsin-2-labeled thalamocortical fibers, whereas such inputs were less common in layer 6 corticocollicular neurons. Layer 5 corticocollicular neurons were found to vertically integrate supragranular excitatory and inhibitory input to a substantially greater degree than their layer 6 counterparts. Here, laser scanning photostimulation glutamate uncaging and photoactivation of channelrhodopsin-2 were used to probe the local and long-range network differences between preidentified layer 5 and layer 6 auditory corticocollicular neurons from male and female mice in vitro. However, little is known about the cortical and thalamic networks within which corticocollicular neurons are embedded. The auditory corticocollicular projection is massive and heterogeneous, with axons emanating from cortical layers 5 and 6, and plays a key role in directing plastic changes in the inferior colliculus. Long-range descending projections from the auditory cortex play key roles in shaping response properties in the inferior colliculus.

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