New component of the limbic system: Marginal division of the neostriatum that links the limbic system to the basal nucleus of Meynert.

Abstract

The limbic system refers to a group of connected neural regions that are associated with motivation, learning, and memory. The marginal division (MrD) is a zone located at the caudal border of the neostriatum in mammalian brains that has been shown to be involved in learning and memory. In a previous study, c-fos expression showed functional connections between the MrD, basal nucleus of Meynert (NBM) and limbic system (Shu et al., 1988a, 1999). In the present study, to explore the relationship between these regions, the expression of limbic system-associated membrane protein (LAMP) was investigated using molecular and immunohistochemical methods. Synaptic and functional connections between the MrD and the NBM were studied also using tract tracing, electron microscopic and behavioral methods. LAMP is thought to be a marker of the limbic system and expression of LAMP protein and mRNA was observed in both the MrD and the limbic system. From such results, it is concluded that the MrD is a new component of the limbic system. Fibers from the MrD were observed projecting and synapsing on cholinergic neurons of the NBM. As reduction of learning and memory was induced by lesioning the projection from the MrD to the NBM, it would seem that the MrD modulates the learning and memory function of the NBM. In conclusion, the results of these studies suggest that the MrD is a new component of the limbic system, and there are functional and structural connections between the MrD, NBM and limbic system. The MrD seems to act as a link between the limbic system and the NBM, and plays a role in learning and memory.