The purported dopamine agonist DPI inhibits [3H]noradrenaline release from rat cortical slices but not [3H]dopamine and [14C]acetylcholine release from rat striatal slices in-vitro

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Abstract
The effects of the purported dopamine (DA) receptor agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) upon the in-vitro K+-induced release of [3H]DA and [14C]acetylcholine from rat neostriatal slices, and of [3H]noradrenaline from rat neocortical slices have been investigated and compared with those of the DA receptor agonist TL-99 and the α-adrenoceptor agonist clonidine, respectively. The rapid decomposition of the catechol compounds DPI and TL-99 in the Krebs-Ringer bicarbonate superfusion medium was shown to be inhibited by both the chelating agent EDTA and the reducing agent ascorbic acid. The results suggest that in-vitro DPI is unable to stimulate striatal DA receptors, whereas it is effective in stimulating cortical α2-adrenoceptors (EC50 = 61 nm). It is concluded that DPI should be considered as a mixed α1/α2-adrenoceptor agonist and that the designation of DPI as a DA receptor agonist should be abandoned.

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