HYPOTHALAMIC REGULATION OF THE PITUITARY-THYROID AXIS IN THE TILAPIA OREOCHROMIS-MOSSAMBICUS

Electrolytic lesioning of the preoptic area resulted in an increase in plasma thyroxine (T-4) and reverse triiodothyronine (rT(3)) 10 days l ater; plasma triiodothyronine (T-3) levels were not affected, so that there was also a significant decrease in the T-3:T-4, but not rT(3):T- 4, ratios. No significant changes in T-4, T-3, or rT(3) levels were ob served in fish with lesions in either the anterior or posterior portio ns of the lateral tuberal nucleus. The pituitary contents of growth ho rmone and the two prolactins were not affected by any lesion. This ind icates that the preoptic area may play a role in the inhibitory regula tion of the pituitary-thyroid axis in Oreochromis mossambicus, presuma bly by way of effects on thyrotropin secretion. (C) 1997 Academic Pres s.