Evaluation of an osmotic pump for microdialysis sampling in an awake and untethered rat

The feasibility of using an osmotic pump in place of a syringe pump for microdialysis sampling in rat brain was investigated. The use of an osmotic pump permits the rat to be free from the constraints of the standard tethered system. The in vitro flow rates of a microdialysis syringe pump (set at 10.80 μl/h) and the osmotic pump (pump specifications were 11.35 μl/h) with no probe attached were compared, yielding results of 10.87 μl/h ± 1.7% and 10.95 μl/h ± 8.0%, respectively. The average of four flow rate experiments in vivo yielded R.S.D.s less than 10% and an average flow rate of 11.1 μl/h. Following the flow rate studies, in vivo sampling of neurotransmitters was accomplished with the osmotic pump coupled to a microdialysis probe implanted in the brain. Finally, after determination of basal levels of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindole-3-acetic acid (5-HIAA) in the rats, the rats were dosed with benserazide followed by l-3,4-dihydroxyphenylalanine (l-DOPA). The results from the dosing study showed at least a 10-fold increase in compounds in the l-DOPA metabolic pathway (DOPAC and HVA) and a slight or no increase in 5-HIAA (serotonin metabolic pathway.) These results indicate that the osmotic pump is a viable alternative to the syringe pump for use in microdialysis sampling.