The hippocampus plays a major role in both spatial and non-spatial, declarative memory. Further, the hippocampus serves as a temporary memory store, while long-term memories reside in neocortical structures. Little is known of what the functional neuronal organization, if any, may be in either the hippocampus or neocortex. Using immediate-early genes (IEG's; Zif268/Homer 1a) as indicators of neuronal activity, we have recently shown a cluster-type organization in the CA1/CA3 fields of the hippocampus in animals engaged in a spatial task (Nakamura, et al. (2010). Neurosci., 166, 994-1007). The present study was aimed at investigating whether a similar type functional organization exists in the hippocampus for a non-spatial, declarative memory task.
C57BL/6J mice were food deprived at 80% of normal body weight and trained in a sequential order discrimination task. Five sets of 5 odours were presented in the same sequence daily, followed after each set by a probe trial in which two odours were presented randomly and the animal had to choose the earliest of the two odours for a food (Kellogg, frosted flakes) reward. Control animals were presented with a similar sequence, however, in the probe trial, both food cups were baited (i.e., no memory demand). The experimental group of mice reached criterion (80% correct responses for four consecutive days) in the sequential task in 30±4 d (mean ±SD). The mice were then isolated for 24 h. On the day of the experiment, they were presented with 10 sets of trials with probes following which they were isolated for 1.5 h, following which they were perfused and their brains processed for Zif268 immunohistochemistry. The topographic distribution of Zif268 immunoreactive cells will be determined in both hippocampus and neocortical structures.