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 re port we show, by Western-blotting, that cultures of adult rat melanotrophs, but not adenohypophyseal cells, express tyrosine hydroxylase, Immunocytoch emical analyses confirmed the existence of a subpopulation of melanotrophs expressing tyrosine hydroxylase, Bromocryptine (2.5 x 10(-7) M), a D2 dopam ine 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-regulati on at 3 days was dose-dependent (ED50 around 2 x 10(-9) M). This decrease w as reversed by sulpiride, a D2 dopamine antagonist. The cell number was sli ghtly increased by bromocryptine treatment. These data suggest tyrosine hyd roxylase expression in melanotrophs being under tonic inhibitory control by dopamine innervation in vivo. (C) 2000 Elsevier Science Inc.