The Neurochemical Organization of the Claustrum

The claustrum is a large subcortical structure found in all mammals with a cerebral cortex. It is reciprocally and topographically connected with the cerebral cortex. The cerebral cortex itself is composed of multiple cytoarchitectonically and myeoloarchitectonically distinct functional regions. In contrast, when examined in cell or fiber stains the structure of the claustrum is uniform over its entire rostrocaudal and dorsoventral extent. In this review we consider two major questions about the claustrum: first, is there any evidence for neurochemically distinguished compartmental organization? Second, what can neurochemistry reveal about the number of cell types in the claustrum and intraclaustral information processing? We and others have used immunohistochemistry for a number of markers including the calcium-binding proteins parvalbumin, calbindin and calretinin, the synthetic enzyme for nitric oxide, nitric oxide synthase (nNOS), and markers for glutamatergic and GABAergic neurons to delineate the number of cell types and the transmitters used by different cell types. We have also used markers for serotonin (5-HT) to show a direct serotonergic input to the claustrum. Other studies have used receptor binding techniques as well as immunohistochemistry to provide evidence for dopaminergic, cholinergic and adrenergic input to the claustrum. Our results for the cat and the macaque monkey suggest that the neurochemical organization of the claustrum, like its cytoarchitectonic organization, is uniform across functional subdivisions. The results also show that there are multiple neurochemically distinguished neuronal types that are distributed across the entire claustrum. These results suggest a substrate for elaborate intraclaustral information processing. We also compare the results for cat and monkey, animals with a well-developed cerebral cortex, with the results for rodents, animals with much simpler cortical organization.