Ahuja M, et al (2016). Distinct Nrf2 Signaling Mechanisms of Fumaric Acid Esters and Their Role in Neuroprotection against 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Induced Experimental Parkinson's-Like Disease. J Neurosci. 8;36(23):6332-51. PMID: 27277809
Kaidery NA, et al., (2013) Targeting Nrf2-mediated gene transcription by extremely potent synthetic triterpenoids attenuate dopaminergic neurotoxicity in the MPTP mouse model of Parkinson's disease. Antioxid Redox Signal.18(2):139-57. PMID: 22746536
Ahuja M, et al (2016). Distinct Nrf2 Signaling Mechanisms of Fumaric Acid Esters and Their Role in Neuroprotection against 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Induced Experimental Parkinson's-Like Disease. J Neurosci. 8;36(23):6332-51. PMID: 27277809
Kaidery NA, et al., (2013) Targeting Nrf2-mediated gene transcription by extremely potent synthetic triterpenoids attenuate dopaminergic neurotoxicity in the MPTP mouse model of Parkinson's disease. Antioxid Redox Signal.18(2):139-57. PMID: 22746536
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.