Effects of medial septal or unilateral hippocampal inactivations on reference and working spatial memory in rats

Abstract

The memory performances of rats receiving a reversible inactivation of either the medial septum or one side of the ventral hippocampus were compared in a radial arm maze task allowing the assessment of both working and reference memory. After pre-surgery training, rats were chronically equipped with bilateral cannulae into the ventral hippocampus and a single cannula into the medial septum. Following post-surgery retraining, animals received a series of test trials during which they received saline or lidocaine injections in either the medial septum or one side of the ventral hippocampus. Lidocaine injections in either structure resulted in both reference and working memory deficits. However, animals were more impaired after septal injections than after unilateral hippocampal injections. This result suggests that the septo-hippocampal formation acts as a functionally homogeneous structure essential for spatial processing.

Metabolic Activation Pattern of Distinct Hippocampal Subregions during Spatial Learning and Memory Retrieval

“…Different neuronal ensembles in each location would contribute either to encode specific episodic features or create the contextual framework to bind them. Interestingly, a finer analysis within the hippocampus reveals that different subregions contribute in a heterogeneous manner to these neuronal networks, confirming the suggestion that the hippocampus participates in acquisition of spatial information not as a unitary structure but as an area divided in different functional components along its longitudinal axis (Hughes, 1965;Jung et al, 1994;Lepage et al, 1998;Moser and Moser, 1998;Poucet and Buhot, 1994;Stevens and Cowey, 1973). Indeed, recent studies provide support for the concept of functional heterogeneity along the anteroposterior axis within the hippocampus that may depend on distinctive information conveyed to separate parts of the hippocampus by inputs from different part of the entorhinal cortex (Hargreaves et al, 2005;Steffenach et al, 2005).…”

“…In this context, the notion that separate components of this structure could be engaged and differentially recruited at different stages of the learning process and on recall has been suggested (Nadel and Moscovitch, 1998) but not extensively explored. Previous evidence, notably provided by the description of place cells (Jung et al, 1994;Poucet and Buhot, 1994), suggested that the hippocampus does not participate in learning and memory tasks as a unitary structure but appears to display regional specialization. Based on the morphologic organization of afferent and efferent connections from the septum to the temporal lobe as well as on the results of lesion studies (Hughes, 1965;Stevens and Cowey, 1973), Moser and Moser (1998) have proposed that the anterior third of the hippocampus is functionally distinct from the posterior two-thirds.…”

Neonatal Damage of the Ventral Hippocampus Impairs Working Memory in the Rat

“…One possible explanation for the discrepancy between our results and previous findings is that the hippocampal lesions in our study were small and restricted to the ventral portion. However, some studies of the small reversible lesions (e.g., lidocaine infusions) of the ventral hippocampus showed that these lesions can also disrupt performance on spatial tasks (Floresco et al 1997;Poucet and Buhot 1994). Another possibility is that the hippocampus is not critical for the performance of delayed alternation tasks that are relatively non-spatial or that involve relatively short delays (lasting for seconds in our tests rather than minutes or hours).…”

Neuron–glia metabolic coupling and plasticity

“…What is less clear, however, is whether such tasks are accomplished by the hippocampus as a uniform computational unit, or whether information processing occurs in discrete steps distributed throughout distinct subregions with evolving temporal patterns. Previous evidence from electrophysiology, partial hippocampal lesion studies and, more recently, human brain imaging have suggested the existence of functional specialization within the hippocampus (Jung et al, 1994;Lepage et al, 1998;Moser and Moser, 1998;Poucet and Buhot, 1994). In addition, studies in rodents have shown that markers of activity (in particular metabolic markers such as 2-deoxyglucose 2-DG), evolve spatially and temporally over the .…”