Effects of synthetic thyrotropin-releasing hormone (TRH) and various doses of thyroxin (T-4) on prolactin (PRL)-producing cells and thyrotropic cells in the pituitary were investigated in adult mate and female Rana ridibunda frogs. Animals were given 200 mug TRH once a week for 4 weeks and 0.2-0.5 mg T-4 during 3 days per week for a period of 2 weeks by injections in the groin. PRL-producing cells and thyrotropic cells were identified with tight microscopical and electron microscopical immunocytochemical methods, using rabbit anti-PRL and rabbit anti-thyroid stimulating hormone (TSH) as primary antibodies. TRH caused cytological changes in both cell types, which were consistent with increased synthesis and release of both PRL and TSH. Treatment with 0.5 mg T-4 activated both cell types less than TRH treatment did, whereas 0.2 and 0.4 mg T-4 caused inactivation of both cell types. In conclusion, mammalian TRH is effective on both types of frog pituitary cells. Our study suggests that T-4 has a positive rather than a negative effect when concentrations above a certain threshold are given. (C) 2004 Elsevier GmbH. All rights reserved.