Practical Aspects of in Vivo Detection of Neuropeptides by Microdialysis Coupled Off-Line to Capillary LC with Multistage MS

A method using capillary liquid chromatography-triple-stage mass spectrometry (LC-MS3) to determine endogenous opioid peptides in microdialysis samples collected in vivo was developed, validated, and applied to measurements in the rat striatum. Peptides in dialysate rapidly degraded when stored at room temperature or -80 degrees C. Adding acetic acid to a final concentration of 5% stabilized the peptides for 5 days allowing storage of fractions and off line measurements which proved more convenient and reliable than previously used on-fine methods. Study of the effect of dialysis flow rate from 0.2 to 2 mu L/min and column inner diameter (i.d.) from 25 to 75 pm on the relative signal obtained for peptides revealed that lowest flow rates and smallest column i.d. gave the highest relative signal. The method was tested for 10 different neuropeptides and limits of detection (LODs) were from 0.5 to 60 pM (4 mu L samples) for most. beta-Endorphin had an LOD of 5 nM when detected directly, but it could be quantitatively determined by detecting a characteristic peptide produced by tryptic digestion with an LOD of 3 pM. This approach may prove useful for other huge neuropeptides as well. The method was used to determine met-enkephalin, leu-enkephalin, dynorphin A(1-8), and beta-endorphin in vivo. Endomorphin I and 2 were below the detection limit of the method in vivo. Quantitative determination of leuenkephalin using external calibration was verified by standard addition experiments. The improvements over previous approaches using capillary LC-MSn make in vivo neuropeptide monitoring more practical and feasible for a variety of neuropeptides.