Application of long-term microdialysis in circadian rhythm research

Our laboratory has pioneered long-term microdialysis to monitor pineal melatonin secretion in living animals across multiple circadian cycles. There are numerous advantages of this approach for rhythm analysis: (1) we can precisely define melatonin onset and offset phases; (2) melatonin is a reliable and stable neuroendocrine output of the circadian clock (versus behavioral output which is sensitive to stress or other factors); (3) melatonin measurements can be performed extremely frequently, permitting high temporal resolution (10 min sampling intervals), which allows detection of slight changes in phase; (4) the measurements can be performed for more than four weeks, allowing perturbations of the circadian clock to be followed long-term in the same animals; (5) this is an automated process (microdialysis coupled with on-line HPLC analysis), which increases accuracy and bypasses the labor-intensive and error-prone manual handling of dialysis samples; and (6) our approach allows real-time investigation of circadian rhythm function and permits appropriate timely adjustments of experimental conditions. The longevity of microdialysis probes, the key to the success of this approach, depends at least in part on the methods of the construction and implantation of dialysis probes. In this article, we have detailed the procedures of construction and surgical implantation of microdialysis probes used currently in our laboratory, which are significantly improved from our previous methods.