Organization of the Claustrum-to-Entorhinal Cortical Connection in Mice

Abstract

The claustrum is a poorly understood subcortical structure reciprocally connected with widespread neocortical regions. We investigated the organization and function of identified claustro-cortical projections in mice using pathway-specific approaches. Anatomical tracing showed that the density of anterior claustrum-to-cortical projection is dependent on the target cortical areas and that the medial entorhinal cortex (MEC) is one of the major projection targets. Novel context exposure activated multiple claustro-cortical pathways and a large fraction of the activated neurons projected to the MEC. Optogenetic silencing of the claustro-MEC pathway during contextual fear learning suppressed subsequent memory retrieval. These results suggest that the dense claustro-MEC pathway is activated by novel context and modulates MEC function in contextual memory.

Input–output organization of the mouse claustrum

“…This suggests that CLA output is almost entirely restricted to the cortex, and previously observed thalamic and striatal projections may have arisen from labeled neurons in layers 5 and 6 of the insular cortex. In addition, we did not see any projections directly into, or from, the overlying insular cortex, as has been noted in some previous studies (Kitanishi & Matsuo, ; Wang et al, ). This is in line with previous work in cat reporting such a lack of connections (Markowitsch, Irle, Bang‐Olsen, & Flindt‐Egebak, ), and suggests that the CLA functions separately from the insula, despite residing within its deepest layers.…”

“…The ACA was selected since retrograde tracer injections here have been shown to strongly label CLA neurons throughout the entire length of the structure, and the distribution of these cells has been confirmed to fall within the boundaries of the CLA as defined by dense parvalbumin (PV) immunoreactivity and Gng2 expression (Mathur et al, ; White et al, , ). Similarly, V1 was selected as it has been shown to receive CLA input, however, some reports suggest that sensory cortical regions may receive segregated inputs from discrete parts of the CLA (Kitanishi & Matsuo, ; White et al, ). Interestingly, all three injections resulted in CLA cell body labeling that was spatially overlapped and evenly distributed along the length of the structure (Figure c).…”

“…Despite extensive work characterizing CLA output, the degree to which these neurons segregate to form target‐specific output populations or collateralize to uniformly innervate the cortex remains unknown. This is important to resolve as some studies suggest the CLA may be organized into modules with distinct input and output (Atlan et al, ; Kitanishi & Matsuo, ; Smith & Alloway, ; White et al, ), however recent findings based on the reconstruction of several individual CLA neurons suggest that they are capable of collateralizing to a large number of cortical targets and may therefore broadly influence cortical function (Wang et al, ). Our multi‐fluorescent retrograde tracing revealed that 23–57% of CLA neurons projecting to either V1, RSP, or ACA, also projected to another target, and a modest fraction (15–32%) projected to all three targets.…”

“…Alternatively, CLA neurons may be defined based on their projection to a particular cortical region using injections of a retrogradely transported Cre‐expressing virus. This approach has been reported using adeno‐associated virus‐6 (AAV6) (Kitanishi & Matsuo, ), however care must be taken in choosing a cortical region that receives input only from CLA, but not overlying insular cortex, to avoid non‐specific labeling. In this study, we found that the posterior retrosplenial cortex (RSP) was ideally suited for this purpose as it receives dense input specifically from the CLA, but not from surrounding structures, allowing for the precise targeting of CLA cells following secondary injections of a Cre‐dependent AAV vector.…”

The claustrum: Considerations regarding its anatomy, functions and a programme for research

“…In recent years, several excellent and comprehensive anatomical studies in the rodent have outlined the afferent and efferent cortical connectivity of the claustrum (see Wang et al, 2017), for example, parahippocampal Kitanishi and Matsuo, 2016;Tomás Pereira et al, 2016;White et al, 2017), somatosensory-motor (Smith and Alloway, 2010) and visuomotor cortices (Smith et al, 2014), prefrontal and entorhinal cortices (Watson et al, 2017). There is a consistent pattern of bilateral cortico-claustral projections, which are weaker (but nonetheless extant), to the ipsilateral hemisphere.…”