IN VIVO MONITORING OF NEUROTRANSMITTER SEROTONIN AND DOPAMINE IN THE STRIATUM OF FREELY-MOVING RATS WITH ONE MINUTE TEMPORAL RESOLUTION BY ONLINE MICRODIALYSIS COUPLED WITH CAPIILARY HPLC

Dopamine and serotonin are widely studied monoamine neurotransmitters which are heavily involved in many physiological functions and pathological conditions. One of the methods used to investigate the normal or abnormal functioning of the monoaminergic systems is to monitor the extracellular dopamine and serotonin: normal and altered resting levels and neurotransmission process by using microdialysis coupled with high performance liquid chromatography (HPLC) system. Traditional microdialysis/HPLC systems are usually done offline with low 5-30 minutes temporal resolution. Here we developed an online microdialysis/HPLC system with one minute temporal resolution to monitor dopamine and serotonin in the striatum of freely-moving rats. \ud One minute HPLC separations of dopamine and serotonin were achieved by using columns packed with sub-2 µm particles, ultrahigh pressures and elevated temperatures. Parameters of the HPLC-electrochemical detection system were optimized towards high speed and high sensitivity. On-column preconcentration was taken into account in the overall optimization and it was found that column diameter played a key role in the optimization process. To control zone dispersion during solute transportation, capillary tubing (75 µm inside diameter, 70 cm long) was used as microdialysis probe inlet and outlet. In vitro assessment by step function showed that the solute dispersion was well within one minute (46.4 s from -2σ to 2σ). In vivo monitoring of basal levels and fast changes of the extracellular serotonin in the striatum of freely-moving rats was performed. The longest monitoring lasted for 16.7 hours with one minute temporal resolution. The superior temporal resolution revealed dynamic details in response to stimulation by 120 mM K+ and fluoxetine intake. In vivo monitoring of the dopaminergic system with one minute temporal resolution was carried out with slightly altered HPLC conditions. Dopamine maximum release and stimulus K+ concentration showed exponential correlation before the threshold of spreading depression, 100 mM K+. Repetitive dopamine maximum concentration with a 4.5 ± 0.3 min time interval during the 20 minutes 120 mM K+ administration each represented a neuron depolarization process. Perfusing dexamethasone, a neuroprotective drug, enabled the increase of dopamine basal levels and the reduction of the frequency of complete neuron depolarization under the same stimulation conditions