ストレス反応と学習・記憶 : 末梢のアドレナリンが中枢に及ぼす影響を中心に

ストレス社会とも呼ばれる現代社会において、ストレスにいかに対処していくかということが重要な課題である。ストレス場面においては、われわれの学習や記憶の機能にも影響が及ぶ。このようなときに記憶の働きが高まることは、つねに良い結果をもたらすとは限らず、PTSDのような臨床的な障害につながる場合もありうる。本論では最初に、歴史的に異なる学問領域で研究されてきたストレスと情動という2つの用語の関係を整理した上で、ストレスや情動にともない、記憶のはたらきが高まる場合があることを指摘した。そして、こうした場面で記憶が増強されるメカニズムには、末梢身体に生じた強い興奮性の反応が関わっているらしい。末梢の興奮が中枢機能に影響を及ぼす上で、媒介的役割を果たすと考えられてきたのが、末梢を循環するアドレナリンであり、迷走神経などの求心性繊維である。アドレナリンが迷走神経繊維の受容体に直接作用するという従来の説に加えて、本論では、アドレナリンによる心臓血管系機能の活性化が、圧受容器から発した求心性の神経線維を経由して、脳へとフィードバックされる可能性を指摘した。これら2つの機能系が同時並行的に機能することで、中枢へのフィードバック効果をより強く持続的なものにしているかもしれない。そのフィードバックの結果として、情動興奮が高まるだけでなく、学習や記憶といった側面にまで影響が及ぶことがあるらしい。つまり、ストレスや情動にともなって生じた末梢身体反応が、中枢の情動機能に加えて、学習や記憶の機能を高める作用を及ぼす可能性があると推定できた。

Generally, our bodies show a variety of changes in autonomic functions when confronted with stressful stimuli. In addition, it is well known that memory functions can be enhanced in emotionally arousing situations. In this paper, the definition of "stress" and the relationship between stress and "emotion" are discussed by focusing on each of the following four aspects of stress: (1) stress as a stimulus-stressor, (2) stress as a response-strain, (3) stress as a process that includes cognitive appraisal and coping, and (4) stress as having motivational properties for coping. Next, the author points out that there are some types of stress-related phenomena that enhance human memory, such as "flashbulb memory" and posttraumatic stress disorder (PTSD). These phenomena might be related to some of the bodily reactions that occur as responses to acute stressful events. Thus far, it has been observed that some of these bodily responses to emotionally arousing stimuli, namely, peripherally released epinephrine, could influence both learning and memory functions. However, epinephrine released peripherally cannot fully pass the blood-brain barrier (BBB) to act directly on the memory system in the brain. Therefore, it is necessary to seek alternative candidates for mechanisms that mediate the enhancing effects that some bodily responses occurring in stressful conditions have on memory functions. In this paper, two possibilities are discussed. First, as other authors have pointed out, the β-adrenergic system might play a major role in the afferent feedback from peripheral bodily functions to the brain. Peripherally released epinephrine could stimulate the vagal afferent nerves by binding the β-adrenergic receptors located on these nerves, and these nerves in turn could affect brain activity. Second, as the author has recently postulated, peripherally released epinephrine could activate the cardiovascular functions and, then, the baroreceptors located in the arteries, which are sensitive to the changes in the blood pressure level, could be stimulated. As a result, such receptors affect the brain activity via afferent nerves originating from these receptors. These two mechanisms might operate simultaneously and coordinate to intensify and prolong the and utility of the application of these findings to clinical treatments and education are also discussed.