Memória de reconhecimento e localização de objetos em peixe-zebra (Danio rerio) : influência da sinalização glutamatérgica e respostas endócrinas

Abstract

Memory and learning allow animals to adapt and modify their behavior towards new experiences. Some factors may result in memory deficits such as: neurodegenerative diseases, changes due to intoxication and the use of drugs. Zebrafish can be used to model complex human behavioral traits such as learning and memory. This study aims to establish a protocol for evaluating the object recognition memory or object location tasks in zebrafish. We evaluated novel object recognition memory and analyzed the exploration time of novel and familiar objects in the training and test sessions. There was a preference of the animal to explore the new object in comparison to the familiar object (61% of exploration time). We also evaluated the object location task and measured the exploration time of each object in the familiar and novel object locations. There was a preference of the animal to explore the object in the novel location in comparison to the object in the familiar location (63% of exploration time). We also evaluated the effect of the non-competitive NMDA receptor antagonist MK-801 on the object recognition and object location memory in zebrafish. In this regimen treatment, control (water only) and treated animals (5 μM MK-801) presented a significant preference in exploring the familiar object in comparison to the new object (66 and 68% of exploration time, respectively); however, 10 μM MK-801-treated animals did not show differences in the exploration time of the objects. In the object location task, the animals treated with the 5 or 10 μM MK-801 did not show a significant preference for the familiar or novel location whereas the control group had a higher preference in exploring the object in the familiar location (64% of exploration time). Therefore, it is possible to suggest that 5 μM MK-801 impaired the memory formation in an object location task, which is in agreement with previous studies demonstrating the cognitive deficit induced by MK-801 treatment in aversive and spatial memory. Considering the different responses of the control group between original task and in the regimen treatment, we evaluated the impact of habituation on cortisol levels of animals in three different protocols: 1) habituated at the experiment apparatus for 3 days (Condition 1 – C1), 2) habituated at the experiment apparatus for 3 days plus treatment tank exposure at fourth day (Condition 2 - C2), 3) habituated at the treatment tank exposure and experiment apparatus for 3 days and exposed to treatment tank again at fourth day (Condition 3 – C3). The results showed higher levels of cortisol in animals submitted to C2 and C3 conditions compared to animals submitted to C1. These results demonstrated that the animals submitted to treatment tank exposure have a different performance in object recognition and object location memory due to stress responses. Therefore, these tasks are prone to evaluate memory in physiological and pathological conditions, but its use is limited to perform pharmacological studies due to sensitivity to stress caused by manipulation.