Characteristics of Dopamine Uptake and 3,4-Dihydroxyphenylacetic Acid (DOPAC) Formation in the Dopaminergic Terminals of the Neurointermediate Lobe of the Pituitary Gland

The kinetics of dopamine (DA) uptake were studied in synaptosomal preparations of the neurointermediate lobe (NIL) of the pituitary gland, a region containing the terminals of the tuberoinfundibular dopaminergic neurons. Lineweaver-Burk plots of the initial velocity of DA uptake versus the concentration of DA yielded a single straight line in the NIL. The Km values in the NIL of the rat and the steer were 2.2+/-0.5 x 10(-6) M and 2.0+/-0.1 x 10(-6) M, respectively. The uptake was predominantly into dopaminergic terminals since preincubation with desipramine did not affect the Vmax or Km of DA uptake. Observed uptake was predominantly due to transport across the neuronal membrane and not into storage granules, since reserpine caused only a small decrease in uptake. The concentration of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the NIL was 3.6 ng/mg protein. The ratio of DA to DOPAC in the NIL (3.73) was similar to that obtained in the medial basal hypothalamus, another region innervated by the tuberoinfundibular dopaminergic neurons. The kinetics of DA uptake in the nerve terminals of the NIL are similar to those observed in the DA terminals in the median eminence. The affinity of uptake in the terminal fields of the tuberoinfundibular system is considerably lower than in terminals of the mesolimbic and nigrostriatal dopaminergic terminals.