Lambert, M. A. | bioRxiv Science

bioRxiv2019

The anatomical boundary of the rat claustrum

The claustrum is a subcortical nucleus that exhibits dense connectivity across the neocortex. Considerable recent progress has been made in establishing its genetic and anatomical characteristics, however a core, contentious issue that regularly presents in the literature pertains to the rostral extent of its anatomical boundary. The present study addressed this issue in the rat brain.\n\nUsing a combination of immunohistochemistry and neuronal tract tracing, we have examined the expression profiles of several genes that have previously been identified as exhibiting a differential expression profile in the claustrum relative to the surrounding cortex. The expression profiles of parvalbumin, crystallin mu (Crym), and guanine nucleotide binding protein (G protein), gamma 2 (Gng2) were assessed immunohistochemically alongside, or in combination with cortical anterograde, or retrograde tracer injections. Retrograde neuronal tracer injections into various thalamic nuclei were used to further establish the rostral border of the claustrum.\n\nExpression of all three markers delineated a nuclear boundary that extended considerably (~500 m) beyond the anterior horn of the neostriatum. Cortical retrograde and anterograde neuronal tracer injections, respectively, revealed distributions of cortically-projecting claustral neurons and cortical efferent inputs to the claustrum that overlapped with the gene marker-derived claustrum boundary. Finally, retrograde tracer injections centred in nucleus reuniens, whilst including the rhomboid, mediodorsal and centromedial nuclei, revealed that insular cortico-thalamic projections encapsulated a claustral area, with strongly diminished cell label, that corresponded to the claustrum.

neuroscience↗

bioRxiv2017

Demarcating the anatomical structure of the claustrum: piecing together a new functional puddle

BackgroundA major obstacle in the understanding of the functional anatomy of the claustrum has been, and continues to be, the contradiction surrounding its anatomical boundary and, in particular, its rostral (i.e. anterior to striatum), extent. In a recent review we highlighted gene expression-based evidence from the mouse brain which lends weight to the idea that the anatomical boundary of the claustrum does in fact extend rostral to the anterior apex of the striatum. In light of this evidence, in the present study, we have examined the expression of two genes that have previously been identified as having differential expression in the mouse claustrum, with the aim of: 1, establishing the true neuroanatomical boundaries of the rat claustrum; and 2, determining the efficacy of claustral marker expression in the histological verification of claustral electrode placement following electrophysiological recordings in awake behaving rats.\n\nMethodsThe expression profiles of two genes, crystallin mu (Crym) and guanine nucleotide binding protein (G protein), gamma 2 (Gng2) were assessed immunohistochemically in five male rats (one Wistar Kyoto (WK) and four Lister hooded (LH)). Prior to histological analysis, two of the rats had undergone surgical implantation of tetrodes targeting the putative rostral claustrum.\n\nResultsIn results that are consistent with those we have previously described in the mouse brain, expression of Crym in the rat brain was highly attenuated, or absent, in the claustrum, demarcating a nuclear boundary that extended considerably beyond the anterior apex of the striatum. In concordance, enriched expression of Gng2 was found in the claustrum, again extending equivalently rostral to the anterior apex of the striatum. The expression of claustral marker genes is a highly effective tool in the verification of electrophysiological electrode placement. Electrophysiological recordings from within this Gng2 and Crym-defined boundary of the rostral claustrum support previous reports of a spatial map in the rostral claustrum.\n\nConclusionsIt is now clear that the anatomical boundary of the rat claustrum does in fact extend into more frontal regions of the brain than has previously been asserted. The importance of these findings are considered in the context of the regional specificity of claustral function, anatomical connectivity and electrophysiological properties.

neuroscience↗