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.