The early onset of proTRH gene expression in anterior pituitary (AP) cells in culture and its regulation by dexamethasone (DEX) were investigated. AP cells derived from 15-day-old rats were cultured for up to 4 days in the presence or absence of 10(-7) M DEX. TRH peptide levels, which could be detected only after 3 days of culture in control cells, were detectable after 1 day in DEX-treated cells. Levels rose from undetectable (< 35 fmol/well/0.2 x 10(6) cells) to 121 +/- 11 fmol/well in control cells and from 59 +/- 3 to 2978 +/- 88 fmol/well in DEX-treated cells (Day 1 to Day 4; means +/- SEM, n = 6). ProTRH mRNA levels as analyzed by in situ hybridization showed an excellent correlation with TRH peptide levels: mRNA was already detectable on Day 1 in DEX-treated cells and on Days 2-3 in control cells. DEX stimulated proTRH mRNA levels as determined by Northern blot analysis within 4 h. The half-life of proTRH mRNA was calculated based on a first-order decay model by measuring mRNA levels after addition of 5 micrograms/ml actinomycin D with or without DEX. The t1/2 of proTRH mRNA in control cells was 13.1 +/- 2.8 h and was not influenced by DEX treatment (12.5 +/- 2.8 h). Since DEX stimulated proTRH mRNA levels acutely without any increase in mRNA stability, we propose that DEX expedites proTRH gene expression in our AP cell culture system by acting at the transcriptional level.