The mRNA levels of GtH Iβ, GtH IIβ and GH in relation to testicular development and testosterone treatment in pituitary cells of male tilapia

The gonadotropin (GtH) IIβ mRNA levels in pituitary glands of male tilapia hybrids were proportional to the gonadosomatic index (GSI), while GtH Iβ 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 Iβ mRNA responded to testosterone (T) exposure (12-72h), rising nearly three-fold compared to the control cells. The stimulatory effect of T on GtH Iβ in these fish was also seen to be dose-related, but doses of 100 nM and above were ineffective. In pituitary cells of mature fish, both transcript levels rose on exposure to 0.01-0.1 nM T, but at higher concentrations, the GtH Iβ fell to control levels, while those of GtH IIβ continued to increase. Growth hormone (GH) mRNA levels were not affected by any of the T treatments. These results suggest that T may differentially control expression of the two GtH β subunits; low levels in immature fish stimulating the Iβ but not the IIβ mRNA, while higher levels, in maturing fish, stimulate the IIβ but not the Iβ transcript.