Spatial memory and c-fos expression in supramammillary nucleus, anterior cingulated gyrus and entorhinal cortex

logic substrate of learning and spatial memory (Jarrad, Okaichi, Steward & Goldschmidt; 1984; Morris, Schenk, Tweedie & Jarrad, 1990; Neave, Nagle & Aggleton, 1997; O ́Keefe & Nadel, 1978; Olton & Papas, 1979; Olton, 1978; García-Moreno, Santín, Rubio, García & Arias, 1993; Santín, Rubio, Begega & Arias, 1999a; Santín, Rubio, Begega & Arias, 1999b). Following the works by Olton and collegues (Olton & Papas, 1979; Olton & Samuelson, 1976; Olton, 1978), in which two kinds of spatial memory are described (reference memory and working memory) in the radial arm maze a number of authors have studied the cerebral substrate of these two kinds of memory. Reference memory (RM) is trial-independent and is used to learn the general rules required for the performance of a task. The information available for solving reference memory tasks was constant throughout the trials (Frick, Baxter, Markowska, Olton & Price, 1995) and was reinforced by repeated training (Young, Stevens, Converse & Mair, 1996). Working memory (WM) is a temporary memory that is trial-dependent (it is only relevant for one trial) (Frick et al., 1995). These two kinds of memory can be assessed by studying the use of spatial information in the rat (Nagahara, Otto & Gallagher, 1995; Santín et al., 1999a). In spite of advances in research in this area in recent years, the precise role of the different regions of the brain in spatial RM and WM is still unclear. Many studies have demostrated that the prefrontal cortex (PFC) plays a role in short-term spatial memory (Funahashi, Bruce & Goldman-Rakic, 1993; Granon, Vidal, Thinus-Blanc, Changeux, & Poucet, 1994). However, lesions of the medial prefrontal cortex (mPFC) do not appear to produce detrimental effects on the performance of a spatial reference memory task (DeBruin, SánchezSanted, Heinsbroek, Donker & Postmes, 1994). This suggests that the role of the mPFC in learning and memory processes could be restricted to working memory tasks (Granon et al., 1994). The role of the hippocampus in processing information could be mediated by connections with the entorhinal cortex (ENT) which supply it with information from the neocortex (Hardman, Evans, Fellows, Hayes, Rupniak, Barnes & Higgins, 1997; Jones, 1993; Tamamaki & Nojyo, 1993). Several experimental studies have observed this relationship between the ENT and spatial learSpatial memory and c-fos expression in supramammillary nucleus, anterior cingulated gyrus and entorhinal cortex