Evidence for Layer-Specific Connectional Heterogeneity in the Mouse Auditory Corticocollicular System.

The auditory cortex (AC) sends long-range projections to virtually all subcortical auditory structures. One of the largest and most complex of these-the projection between AC and inferior colliculus (IC; the corticocollicular pathway)-originates from layer 5 and deep layer 6. Though previous work has shown that these two corticocollicular projection systems have different physiological properties and network connectivities, their functional organization is poorly understood. Here, using a combination of traditional and viral tracers combined with in vivo imaging in both sexes of the mouse, we observed that layer 5 and layer 6 corticocollicular neurons differ in their areas of origin and termination patterns. Layer 5 corticocollicular neurons are concentrated in primary AC, while layer 6 corticocollicular neurons emanate from broad auditory and limbic areas in the temporal cortex. In addition, layer 5 sends dense projections of both small and large (>1 µm2 area) terminals to all regions of nonlemniscal IC, while layer 6 sends small terminals to the most superficial 50-100 µm of the IC. These findings suggest that layer 5 and 6 corticocollicular projections are optimized to play distinct roles in corticofugal modulation. Layer 5 neurons provide strong, rapid, and unimodal feedback to the nonlemniscal IC, while layer 6 neurons provide heteromodal and limbic modulation diffusely to the nonlemniscal IC. Such organizational diversity in the corticocollicular pathway may help to explain the heterogeneous effects of corticocollicular manipulations and, given similar diversity in corticothalamic pathways, may be a general principle in top-down modulation.SIGNIFICANCE STATEMENT We demonstrate that a major descending system in the brain is actually two systems. That is, the auditory corticocollicular projection, which exerts considerable influence over the midbrain, comprises two projections: one from layer 5 and the other from layer 6. The layer 6 projection is diffusely organized, receives multisensory inputs, and ends in small terminals; while the layer 5 projection is derived from a circumscribed auditory cortical area and ends in large terminals. These data suggest that the varied effects of cortical manipulations on the midbrain may be related to effects on two disparate systems. These findings have broader implications because other descending systems derive from two layers. Therefore, a duplex organization may be a common motif in descending control.

A 4-week morning light treatment reduces amygdala reactivity and clinical symptoms in adults with traumatic stress.

Trauma leads to mental health problems including posttraumatic stress disorder (PTSD), depression, and anxiety. New treatments are needed for traumatic stress that can overcome barriers to care while targeting underlying biological mechanisms of the pathology. Morning light treatment has potential as a novel intervention for traumatic stress. We conducted a randomized clinical trial testing 3 doses of a 4-week morning light treatment in people with traumatic stress to evaluate brain mechanisms underlying the treatment. Forty-six participants completed a baseline week followed by a 4-week morning light treatment (15, 30 or 60 mins each morning). Functional magnetic resonance imaging was conducted at pre- and post-treatment using an emotional faces task to probe the amygdala, based on prior work showing direct effects of light on the amygdala and the role of amygdala in traumatic stress. Clinician-rated symptoms and self-reported symptoms were also assessed at pre- and post-treatment. No group differences were observed in left amygdala reactivity, but right amygdala reactivity reduced only in the 30 and 60 min groups with medium effect sizes. Clinical symptoms reduced in all groups with medium to large effect sizes. Self-reported depression and anxiety scores reduced more in the 60 min than in the 15 min group (p = .02). The results suggest that 4 weeks of morning light treatment of at least 30 min per day can reduce amygdala reactivity and symptoms of traumatic stress. Morning light treatment should be further explored as a potential treatment for traumatic stress, given it is relatively safe, acceptable, accessible and scalable.