DIFFERENTIAL-EFFECTS OF GONADOTROPIN-RELEASING-HORMONE, DOPAMINE AND SOMATOSTATIN AND THEIR 2ND-MESSENGER ON THE MESSENGER-RNA LEVELS OF GONADOTROPIN II-BETA SUBUNIT AND GROWTH-HORMONE IN THE TELEOST FISH, TILAPIA

In cultured pituitary cells of tilapia, gonadotropin-releasing hormone (GnRH; 10 nM 4-24 h), elevation of cyclic AMP (by 10 mu M forskolin o r 0.2 mM 3-isobutyl-1-methylxanthine: IBMX 0.5-36 h) or activation of protein kinase C (PKC; by 12.5 nM tetradecanoyl phorbol-13-acetate: TP A, 0.5-24 h) all increased gonadotropin (GtH) II beta steady state mRN A levels by three- to fourfold. The involvement of PKA and PKC in the GnRH stimulatory effect on both GtH release and GtH II beta mRNA level s was corroborated by use of the PKA and PKC inhibitors, H89 and GF109 203X, respectively (100 nM) which attenuated the GnRH effect. Incubati on with actinomycin D (8 mu M, 4-21 h) after preexposure for 24 h to e ither forskolin (10 mu M) or TPA (12.5 nM), revealed that rates of tra nscript degradation were slower in forskolin-treated cells (T1/2 = 14. 1 h) than in control or TPA-treated cells (T1/2 = 8.47 or 8.38 h), sug gesting a stabilizing effect on the mRNA. Dopamine (DA; 10 mu M, 4-36 h) had no apparent effect oil steady slate mRNA levels of GtH II beta, but reduced GtH release by as much as 75%. Steady slate levels of gro wth hormone (GH) mRNA were not affected by exposure to GnRH (10 nM, 4- 24 h), although GH release was more than doubled. Similarly, activatio n of PKC (by TPA 12.5 nM, 1.5-36 h), which was shown to be essential f or the GnRH-stimulatory effect on GH release, did not alter levels of the GH transcript, but increased GH release by more than fivefold. DA (10 mu M 4-24 h) moderately increased GH transcript levels (160%) with similar kinetics but lower potency than direct elevation of cAMP (by 10 mu M forskolin or 0.2 mM IBMX, 0.5-36 h) which increased transcript levels by more than fourfold. The involvement of PKA in the DA effect was confirmed when the PKA inhibitor H89 (100 nM, 15 min prior to DA exposure) attenuated the DA effect on GK mRNA levels. Exposure of cell s to actinomycin D (8 mu M, 2-16 h) after treatment with forskolin (10 mu M, 24 h) led to a slower rate of transcript degradation than in co ntrol cells (T1/2 = 6.5 h vs. T1/2 = 4.36 h), suggesting that cAMP als o elicits a stabilizing effect on GH mRNA. Somatostatin (100 nM, 0.5-3 6 h) had no clear effect on GH transcript levels, but reduced GH relea se by as much as 90%. These results suggest that activation of either cAMP-PKA or PKC pathways can, possibly by different mechanisms, stimul ate mRNA levels of the GtH II beta gene, but that only the cAMP-PKA pa thway stimulates GH mRNA levels. it would appear therefore that GnRH, although stimulating GH release, does not regulate GH transcription in this fish.