Reelin and cognition

The cognitive function of the human brain refers to the mental manipulation of all the information acquired by the sensory system. A more physiological definition of cognition by Ulric Neisser states, ⋯the term cognition refers to all processes by which the sensory input is transformed, reduced, elaborated, stored, recovered, and used. It is our unique cognitive ability acquired from our distinct experience that ultimately makes us who we are; as Rene Descartes said, I think, therefore I am. The authors have no intention or expertise to embark on a philosophical discussion of cognition in this chapter; we rather safely rely on our assertion that normal cognition requires proper function of the central nervous system, which constitutes hundreds of millions of neurons and supporting cells and is correctly wired during development and, equally as important, rewired in response to our postnatal experiences. Although much of our knowledge on the actions of Reelin comes from investigation of embryonic CNS development, recent studies have indicated that Reelin signaling also plays an important role in synaptic function in the postnatal brain (Weeber et al., 2002; Beffert et al., 2005; Ramos-Moreno et al., 2006). Reelin-expressing neurons are widely distributed in the adult brain (Pesold et al., 1998; Martinez-Cerdeno et al., 2002; Abraham and Meyer, 2003; Roberts et al., 2005; Ramos-Moreno et al., 2006), at a period long after the decrease in number of Cajal-Retzius cells (Marin-Padilla, 1998; Meyer et al., 1998; Sarnat and Flores-Sarnat, 2002). The functional significance of the persistent expression of Reelin after the completion of cell migration remains enigmatic in most brain regions. However, in the hippocampus, Reelin and its lipoprotein receptors are required for proper function of synaptic transmission and plasticity, and this signaling system profoundly modifies mammalian learning and memory behavior (Weeber et al., 2002; Beffert et al., 2005; D'Arcangelo, 2005). This chapter will focus on existing experimental evidence supporting the requirement of Reelin for normal synaptic function in the hippocampus as well as mammalian cognitive ability.