| Abstract: | The effects of thyroid hormones (triiodothyronine, T3, and thyroxine, T4) on circulating, stored and gonadotropin releasing hormone (GnRH) induced release of gonadotropin (GtH) have been examined. Sexually regressed fish were immersed for 8 days in 50L tanks containing either T4 (4μg/100ml tank water), T3 (2μg/100ml) or control NaOH (thyroid hormone vehicle, 1 μI of 0.1N/100ml). Immersions of goldfish in thyroid hormones significantly raised plasma thyroid hormone levels. Circulating plasma levels of GtH were not significantly altered by either T3 or T4 immersions. Another similar experiment was performed using midrecrudescent goldfish immersed in T4 or control NaOH, and again, T4 immersion did not alter plasma GtH levels. Thyroid hormone modulation of pituitary GtH content was studied by measuring GtH content in homogenized pituitaries from control, T4 and T3 immersed sexually regressed and recrudescent goldfish. 1986 data (consists mainly of sexually regressed goldfish) showed that T4 immersion in females, and T4 and T3 immersion in males did not affect pituitary GtH content. 1987 data (consists mainly of recrudescent goldfish) showed that, again, T4 immersion did not affect pituitary stored GtH content in either males and females. Thyroid hormone modulation of GnRH induced release of GtH was studied using an in vitro perfusion system. Although test perfusions done with intact pituitaries (neurointermediate lobe not removed) gave GnRH responsive results, a series of perfusions (neurointermediate lobe removed) done with control, T4 and T3 immersed, sexually regressed and midrecrudescent goldfish failed to respond to GnRH. Thus, although it is unclear whether thyroid hormones are able to modulate GnRH-induced release of GtH, these results provide evidence that thyroid hormones do not exert in vivo effects on circulating and stored pituitary GtH. |
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