Dependencia de los sistemas de memoria al ciclo luz-oscuridad en la expresión de estrategias adaptativas. Segunda parte

RESUMEN

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ABSTRACT

Abstract In the first part of this work we reviewed the hippocampus and striatum anatomy and function in the context of the memory systems. In this second part we describe the anatomic and physiologic basis of the memory systems represented by the amygdala and prefrontal cortex (PFC) and their participation in the expression of strategies for the solution of specific problems. Amygdaloid formation is divided in three principal regions, the baso-lateral nucleus, the superficial nucleus, and the centromedial nucleus. Amygdala is highly connected with several regions of the brain including hippocampus, striatum and PFC. Amygdala has been implicated in the processing, storing and retrieval of emotional information. Another function proposed for the amygdala is to modulate the activity of structures such as the hippocampus, the striatum and the cerebral cortex. The participation of the amygdala has been shown in different tasks such as the Morris water maze, the radial maze, the passive avoidance task, and the freezing behavior among others. In some of these studies it has been shown that the activation of the amygdala enhances the acquisition of the task. When the amygdala is activated pharmacologically it is able to enhance the acquisition of hippocampus or striatum related tasks. In these context, the efficiency of the amygdala activation depends on the synchrony, the precise time, at which it occurs in relation to the event the subject is learning. This is, either immediately before, during or immediately after learning. In support of this enhancing role of the amygdala, some electrophysiological studies have shown that the activation of the amygdala facilitates the development of LTP in the hippocampus while its lesion decreases it. On the other hand, it has also been shown that the amygdala activation increases c-Fos expression in both, the hippocampus and the striatum. In summary, the amygdaloid formation has been proposed as an enhancer of learning, representing the emotional component of the response to the environment. PFC is the other structure involved in the generation of strategies. It has been related with the correct functioning of higher functions such as memory, attention, emotion, anticipation and planning. It has been called the central executor for its fundamental role as a coordinator of past, present information and future performance. It is been proposed as responsible for the so called working memory, that allows to put together different kinds of information at the same time, giving the chance of comparing, selecting and generating a goaloriented behavior. Working memory has been studied with many different techniques, however electrophysiological experiments have shown interesting aspects of its functioning. Recording cells from the PFC of monkeys, Goldman-Rakic showed that these cells remain firing in a short period of time when visual information should be retained to be used in ulterior comparison task. This cell activity suggests that these neurons would be responsible for the maintenance of information in our "mind" a short period of time. These results have been replicated in humans by using real time imaging techniques as fMRI and PET. Again, during the periods of retention of the information, the activity on prefrontal areas increase until such information is used. Besides working memory, anticipation is another important function regulated by the PFC. Several studies have shown that the activity of prefrontal cortex increases before the performance, it seems like the prefrontal cortex predicts the actions in the environment and readily generates a strategy to efficiently act in response. PFC is connected reciprocally with the hippocampus, the striatum and the amygdala, the relation between these structures is under heavy investigation. Regarding the hippocampus, some interaction has been observed, and it has been proposed an interaction between these structures for the long term consolidation of memory. As for the striatum, the relationship with PFC has been studied preferentially with the ventral striatum or nucleus accumbens with respect to reinforcement of behavior. We understand poorly the relationship with the dorsal striatum. The relation between amygdala and PFC, on the other hand, has been studied in relation to the expectancy of the reinforcement. This is defined as the representation in the mind of the reinforcement and the association of that representation with the conditions under which it was delivered. In simple words, this is a way to explain how is that a subject prefers a specific reinforcer over another. It has been shown that lesions of the basolateral amygdala as well as PFC interfere with the expectancy of reinforcement. The function of the amygdala in this case is to provide the emotional component related to the presence of the reinforcement. An extensive literature has addressed the question of circadian variations in the release of neurotransmitters. For example, the diurnal variations in the release of acetylcholine in the hippocampus and PFC. The binding for acetylcholine, serotonin and norepinephrine to glutamatergic hippocampal cells is different depending on the light-dark cycle, suggesting that the modulation of the hippocampus by these neurotransmitters is different depending on the presence or absence of light. In this review, we have devoted special interest to the influence of the light dark cycle on these mnemonic systems and on goaloriented behaviors. We analyze selected papers from the available literature on circadian rhythms and memory, emphasizing the hippocampus role. We believe that the study of this relationship (brain/light-dark cycle) could be a useful tool to understand how the environment influences behavior. On this topic, there's evidence that the learning of a task may be different depending on the part of the day when it was learned. For example, it has been shown in humans that when subjects are submitted to explicit or implicit task the performance is different depending on the hour of the day, being better during the light for the explicit memory and better during the dark for the implicit memory. Studies in rats trained in fear conditioning tasks, showed that subjects learn the task easily when they are trained during the light phase of the cycle and the learned behavior showed a higher resistance to extinction. Conclusión. When a subject is confronted with a specific problem, he/she can find the solution by using different strategies. The expression of one of those strategies depends on the interaction of the different memory systems, these systems process and storage different kinds of information, and this information is useful to generate and exhibit a given strategy. The memory systems are constantly under the influence of the environment, one critical component of this environment is the lightdark cycle, which apparently is modulating the activity of these structures. As a result of the influence of the light-dark cycle on these structures, the behavior of the subject would be modulated as well. All these interaction just for the sake of adaptation, survival, and reproduction in this rotating and translating world.