Differences in the kinetics of dopamine uptake in synaptosome preparations of the median eminence relative to other dopaminergically inervated brain regions.

A comparison of the kinetics of dopamine uptake was made in synaptosomal preparations of the median eminence, striatum and olfactory tubercle. Double reciprocal Lineweaver-Burk plots of the initial velocity versus the concentration of dopamine yielded a single straight line in all three areas. The Michaelis constant (Km) in the median eminence (1.8 +/- 0.9 X 10(-6) M) was significantly higher (p < 0.05) than in the striatum (4.2 +/- 0.8 X 10(-8) M) or olfactory tubercle (6.3 +/- 1.9 X 10(-8) M). Uptake in the median eminence appeared to be predominantly into dopaminergic terminals since preincubation with desipramine did not affect the maximum velocity (Vmax) of dopamine uptake. Observed uptake was predominantly due to transport across the neuronal membrane and not into storage granules, since reserpine only caused a small decrease in uptake. The low affinity of dopamine uptake in median eminence synaptosomes is consistent with the neurosecretory nature of these terminals, whereas, in the striatum and olfactory tubercle, high affinity reuptake is consistent with the role of dopamine as a neurotransmitter.