THE MESSENGER-RNA LEVELS OF GTH I-BETA, GTH II-BETA AND GH IN RELATION TO TESTICULAR DEVELOPMENT AND TESTOSTERONE TREATMENT IN PITUITARY-CELLS OF MALE TILAPIA

The gonadotropin (GtH) II beta mRNA levels in pituitary glands of male tilapia hybrids were proportional to the gonadosomatic index (GSI), w hile GtH I beta was highest in fish with GSI of 0.56, but was lower in fish with more developed testes. In pituitary cells of immature fish, only GtH I beta mRNA responded to testosterone (T) exposure (12-72h), rising nearly three-fold compared to the control cells. The stimulato ry effect of T on GtH I beta in these fish was also seen to be dose-re lated, but doses of 100 nM and above were ineffective. In pituitary ce lls of mature fish, both transcript levels rose on exposure to 0.01-0. 1 nM T, but at higher concentrations, the GtH I beta fell to control l evels, while those of GtH II beta continued to increase. Growth hormon e (GH) mRNA levels were not affected by any of the T treatments. These results suggest that T may differentially control expression of the t wo GtH beta subunits; low levels in immature fish stimulating the I be ta but not the II beta mRNA, while higher levels, in maturing fish, st imulate the II beta but not the I beta transcript.