Resumen:

Objetivo. Describir las bases anatómicas y fisiológicas de los sistemas de memoria y su participación en la expresión de estrategias para la solución de problemas específicos (estrategias adaptativas). Además, hacemos especial hincapié en la influencia del ciclo luz-oscuridad sobre los sistemas de memoria y la conducta. Desarrollo. En esta primera parte se hace una revisión de la bibliografía con respecto al funcionamiento anatómico-fisiológico del hipocampo, el núcleo estriado y su participación en la expresión de estrategias adaptativas. Después nos enfocamos en las ideas básicas de los sistemas de memoria y en experimentos hechos en ratas intactas y lesionadas, además de estudios electrofisiológicos. En este apartado, tratamos de hacer énfasis en el sistema hipocamapal y estriatal como dos sistemas que almacenan diferentes tipos de información pero que al momento de enfrentarse a una situación determinada, ambos aportan un componente fundamental a la conducta que resolverá el problema. En la segunda parte de esta revisión nos encargaremos de analizar la influencia que ejerce el medio ambiente sobre estas estructuras, y de cómo esta influencia afecta la conducta. Conclusiones. Cuando un sujeto es sometido a un problema determinado, éste puede solucionarlo con diferentes estrategias. La génesis y expresión de las estrategias depende de la interacción de diferentes estructuras cerebrales, entre ellas las estructuras relacionadas con los procesos de memoria. Los diferentes sistemas de memoria procesan y almacenan diferentes tipos de información. Esta información es el sustento que utiliza el cerebro para la generación de las estrategias adaptativas.

Palabras clave: Estrategia; hipocampo; estriado; amígdala; corteza prefrontal; ciclo luz-oscuridad

Abstract:

The ability to abstract, store and recover information from the environment in order to generate new strategies to solve problems is one of the most important qualities of the human brain. We mean by strategy, the sophisticated way to solve a problem. A strategy represents in essence the refinement of a given behavior to solve a problem. A strategy could be generalized to solve different problems. The generation of strategies is subjected to the correct functioning of the brain, meaning, alertness, attention, memory among others brain processes in good stand. In this work we focus on the role of memory in the generation of strategies.

In this context, we focus on the literature concerning to memory systems, to show that different memory systems process and store different kinds of information. Therefore, the generation of a given strategy would require the participation of one system instead of other, or at least, one system would be commanding over the others. A memory system is defined as neural network consisting on a central structure communicated through afferences and efferences with others. The ones conveying information to this central structure would provide information from the internal or external environment to be interpreted and stored; while the ones that receive information from the central structure would execute its commands. Curiously, the role of central structure can be played by one structure "A" that in other conditions was under the control of a structure "B". In this condition, "B" is under the control of "A".

In this review we sought to describe the anatomic and physiologic basis of the memory systems and their participation in the expression of strategies for the solution of specific problems. In this first part, we review the literature concerning to the hippocampus and striatum. Our endeavor was to make a synthesis of the main components of the functional neuroanatomy of memory and of its specific participation in the generation and expression of strategies, and also of the influence of the light-dark cycle on the strategies resulting from the interaction of these structures. In this review we focus mainly on the basic description of memory systems and on the data obtained from intact rats and of others with lesions and subject to electrophysiological experiments.

Many studies reviewed on this first part confront subjects to situations where different solutions can be performed; basically this studies are conducted on mazes were the subject can use different kinds of information for spatial orientation. Depending on the nature of the information available or selected by the subject, investigators may infer the kind of strategy the subject is using to solve the problem. From this background, concepts such as stimulus-stimulus strategy and stimulus-response strategy have been generated. The first one consists of making associations between neutral stimuli, to make a conceptual map that guides the subject toward his/her objective. It has been related with the hippocampus function and it has been classically related to the processing, interpretation, and storage of contexts and events as well as to spatial navigation. We center our attention on studies carried out in mazes, showing that lesions or temporal inactivation of the hippocampus disturb the capacity of orientation by using spatial cues. We also review studies where the expression of spatial strategies is correlated with preferential activation of hippocampus detected with different techniques such as immuno-histochemistry and mycrodialisis in vivo.

The stimulus-response strategy, on the other hand, consists on making associations between a particular stimulus and the immediate consequence of its presence. This kind of strategy has been related with the striatum, particularly with its dorsolateral region. For this section we discuss studies where lesions or inactivation of the dorsolateral striatum were performed, on rats submitted to tasks where the solution could be achieved by using stimu-lus-stimulus or stimulus-response strategy. In subjects with striatal dysfunction the ability to perform using a stimulus-response strategy was disrupted but not the ability to use a stimulus-stimu-lus strategy. In addition, we revise studies where the expression of the stimulus-response strategy is correlated with a preferential activation of the striatum over hippocampus.

We additionally discuss the interaction hippocampus-striatum to solve a spatial task. We make special emphasis in describing the hippocampal and the striatal systems as independent systems that process and store different kinds of information; therefore, they seem to alternate their activity depending on the demand of the environment. This means that if a stimulus-stimulus strategy is required, the hippocampus will govern the response of the subject, increasing its activity that will be over the activity of the striatum. The opposite will occur if a stimulus-response strategy is required. Studies in humans and rats have been performed to understand the interaction between hippocampus and striatum with similar results. Apparently hippocampus appears more active during the first stages of learning, leading behavior and being expressed as stimulus-stimulus strategy. Later, in learning, the hippocampus decreases in activity and the striatum increases, thus becoming the leader structure. This later activation of stria-tum has been related with the phase of learning when the task is mastered and is starting to become a habit.

Finally, we devoted special interest to describe the influence of the light dark cycle over these systems and over the goal-oriented behavior. And as we will see on the second part of this review, the functioning of these structures may be regulated by the light-dark cycle. We will review the influence of the presence or absence of light on neurotransmitters release. We will give evidence indicating that the neurochemical modulation depends greatly on the influence of the light-dark cycle and that it results obviously in a different activity of these structures and hence the behavior.

In conclusion, when a subject is confronted with a specific problem, he/she can find the solution by using different strategies. At present, we can not say which are the mechanisms responsible for the selection of a particular strategy at a given mo-ment, but we can say that the expression of any strategy depends on the activity of structures such as the hippocampus and the striatum. In theory each structure represents a memory system or a fundamental part of a memory system. The interaction of the different memory systems, produce a scenario were each system provides, processes, and stores different information about the environment, and this information is useful to generate and exhibit a given strategy.

On the second part of this review we will focus on the func-tioning and participation of the amygdala and prefrontal cortex, and the influence of the environment on the memory systems.

Key words: Strategy; hippocampus; striatum; amygdala; pre-frontal cortex; light-dark cycle