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HPLC-ECD is a perfect sensitive and specific way of measuring dopamine and serotonin. But it all depends whether you plan to do it in vivo (microdialysis) [then measuring the released 'transmitters' in the extracellular spaces is possible] or in selected areas of brain homogenate [then, it is better to measure the transmitters' metabolites to relate the measured metabolites' levels to the 'activity' of the targeted neurons]. In brain homogenate, you measure the intra- as well as extra-cellular levels of the transmitter at the same time, without distinction. So, you would not know what exactly was released. Hence, metabolites will reflect the released transmitter after been exposed to the extracellular degrading enzymes (e.g. COMT). Then, measurements using HPLC-ECD is indispensable (at levels in pmol/mg tissue or tissue protein).
Anyway, it all depends on why you are measuring these transmitters.
If you consider selectivity and sensitivity, HPLC coupled to electrochemical detection would be method of choice for the analysis of neurotransmitters in rat/mice brain. Concentration levels up to 50 pmol/L and lower are achievable with only 5 uL sample volume injected. So it can be used in both micro dialysis experiments as well as the analysis of brain tissue (homogenates). See for example: