A 45-year-old woman with bipolar I disorder has experienced increasingly frequent manic and depressive episodes over the past fifteen years. Her first episodes were clearly triggered by major psychosocial stressors, but her most recent episodes appear to occur spontaneously without identifiable precipitants. Her PMHNP notes that the interval between episodes has progressively shortened from years to months. Which of the following best explains the neurobiological phenomenon underlying this pattern of illness progression?