In vitro expression of tyrosine hydroxylase by a subpopulation of rat melanotrophs is down-regulated by dopamine.

Some rat melanotrophs express in vivo tyrosine hydroxylase mRNA. In this report we show, by Western-blotting, that cultures of adult rat melanotrophs, but not adenohypophyseal cells, express tyrosine hydroxylase. Immunocytochemical analyses confirmed the existence of a subpopulation of melanotrophs expressing tyrosine hydroxylase. Bromocryptine (2.5 x 10(-7) M), a D2 dopamine agonist, down-regulated melanotroph tyrosine hydroxylase expression in a time-dependent manner; initial effect was detected at 15 h and maximum at 3 days treatment (reduction to about 40% of control values). Down-regulation at 3 days was dose-dependent (ED(50) around 2 x 10(-9) M). This decrease was reversed by sulpiride, a D2 dopamine antagonist. The cell number was slightly increased by bromocryptine treatment. These data suggest tyrosine hydroxylase expression in melanotrophs being under tonic inhibitory control by dopamine innervation in vivo.