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Linking hippocampus-dependent discriminative learning to hippocampal neuronal ensemble separation using Arc/Homer1a FISH imaging

Final Report Summary - IMAGING LEARNING (Linking hippocampus-dependent discriminative learning to hippocampal neuronal ensemble separation using Arc/Homer1a FISH imaging.)

Immediate-early genes (IEGs) are exploited as markers of neuronal activity because they are expressed immediately after structured neuronal activity. IEGs Arc and Homer1a have been used to visualize ensembles of hippocampal neurons active during two distinct behavioral epochs (Arc/Homer1a catFISH) and to revealed contextual specificity of the expression. CA1 and CA3 ensembles are more similar after repeated exploration of the same environment than after exploration of two different environments, resembling behavior of place cells in electrophysiological recordings.
This project examined the effects of behavioral experience on similarity between activated neuronal ensembles in the rat hippocampus and related cortical areas. It focused primarily on the separation (inverse similarity) between activated hippocampal ensembles as a potential mechanism of discriminative learning and cognitive coordination.
The primary goal of the project was establishing the Arc/Homer1a catFISH at the Institute of Physiology in Prague. This objective was fully achieved. A new laboratory of Molecular Imaging has been built and it is now routinely generating catFISH data. Furthermore, new imaging system has been acquired to allow acquisition of extensive image datasets. Since the limiting step of the technique is the time- and labor-consuming confocal image data analysis, a new software tool for automatic analysis of catFISH data has been developed in collaboration with Dr. Janáček, head of the Biomathematics department. This tool is currently tested and compared to the established semi-manual protocol and will provide material for methodic publication pending successful validation.
Systemic administration of non-competitive NMDAR antagonists such as MK-801 (dizocilpine) is used to model schizophrenia in animals and people. MK-801 (0.15 mg/kg) impaired avoidance of a hidden place by rats foraging on a continuously rotating arena (Carousel), when misleading information from the irrelevant spatial frame was present, but not when it was absent. This result suggests that MK-801 compromised the ability to maintain separate coherent spatial representations of room and arena stimuli dissociated by the arena rotation. Importantly, it also eliminated contextual specificity of IEG expression when CA1 ensembles expressing Arc and Homer1a in two different environments were as similar as after repeated exploration of the same environment, whereas ensemble similarity in different environments was markedly lower in saline controls. Lower dose of MK-801 (0.10 mg/kg) showed no such effect on behavior or ensemble similarity, but both doses reduced overall IEG expression. These results suggest that a key aspect of schizophrenic cognitive symptomatology, compromised discrimination between relevant and irrelevant information, which likely contributes to the false sense of reality, can be due to deficit in cognitive coordination of discordant information into coherent subsets. They also demonstrate that this deficit may be straightforwardly modeled in experimental animals. This study has been published in Frontiers in Behavioral Neuroscience (Kubik et al., 2014;
Rats discriminating between two contexts two days after fear conditioning do not show greater separation between CA1 and CA3 ensembles than rats generalizing between the same contexts nine days after the conditioning. Nevertheless, the ensemble similarity was rather low, indicating that separate ensembles may still facilitate context discrimination at the early, but not at the late time point. Lesions of the dentate gyrus (DG) markedly impaired place avoidance on the Carousel but largely spared spatial memory in the water maze supporting the notion that the DG facilitates separation of discordant information into coherent subsets. Interestingly, incremental training completely rescued the avoidance, indicating that "inescapable" stress in the aversively motivated task exacerbated the effect of the lesions. The results of these studies will be prepared for publication pending ongoing stereological analysis of the extent of the lesion and potential collateral damage to other areas, evaluation of the stress response, and catFISH analysis of Arc/Homer1a expression. They also triggered new collaborations with Faculty of Science of the Charles University and Prague Psychiatric Center.