Accumulation of L-Dopa in the Median Eminence: An Index of Tuberoinfundibular Dopaminergic Nerve Activity*

A microradioenzymatic assay for dihydroxyphenylalanine (dopa) was used to measure the concentration of this precursor of dopamine (DA) in different regions of the rat brain. The accumulation of dopa 30 min after the injection of 3-hydroxybenzylhydrazine (NSD 1015), an inhibitor of dopa decarboxylase, was measured in the striatum and median eminence to obtain a biochemical index of the activity of nigrostriatal and tuberoinfundibular DA nerves, respectively. Injections of a DA agonist, piribedil, reduced dopa accumulation in the striatum but not in the median eminence. Injections of a DA antagonist, haloperidol, increased dopa accumulation in the striatum 2 and 8 h after injection and in the median eminence 16 h after injection. Three daily injections of estradiol benzoate increased the accumulation of dopa in the median eminence but not in the striatum; the estrogen-induced increase of dopa accumulation in the median eminence was not observed in hypophysectomized rats. The effects of these pharmacological and endocrinological manipulations on estimates of DA nerve activity obtained using the dopa accumulation method are the same as those reported previously using the a-methyltyrosine-induced decline of DA and are consistent with the hypothesis that the activity of tuberoinfundibular DA nerves is regulated, in part, by a hormonal feedback loop. These results indicate that dopa accumulation after NSD 1015 provides a reliable means of estimating the activity of tuberoinfundibular DA nerves. (Endocrinology106: 463, 1980)