Luteinizing hormone-releasing hormone as a potent stimulator of the thyroidal axis in ranid frogs.
Plasma concentrations of T4, measured by radioimmunoassay, were raised significantly 2 and 4 hr after intravenous injection of synthetic luteinizing hormone-releasing hormone (LHRH) in Rana ridibunda (1 and 10 micrograms on 2 consecutive days) and in Rana esculenta (10 micrograms). A dose of 1 microgram LHRH was not so effective as 50 micrograms synthetic thyrotropin-releasing hormone (TRH) when injected in Rana ridibunda in November. However 10 micrograms LHRH was equipotent to 50 micrograms TRH. In February somewhat less than half of the Rana temporaria group was responsive to LHRH. There is no clear indication that fluctuating plasma T3 concentrations were caused by LHRH or TRH. Preinjection levels of T3 and T4 were higher during the breeding season (April) in R. esculenta (resp. 35.4 +/- 1.4 pg/ml; 744 +/- 134 pg/ml; n = 22) compared to the basal concentrations in the very closely related Rana ridibunda (November) (resp. 15.2 +/- 1.1; 162 +/- 24 pg/ml; n = 28). Four days after removal of the pars distalis plasma T4 concentrations were significantly decreased in Rana esculenta, whereas T3 could stay longer in circulation. T3 and T4 content of the thyroids was not altered by the short-term hypophysectomy. Injection of 10 micrograms LHRH had no influence on plasma T4 nor testosterone concentrations in these frogs, contrary to the sham-ectomized animals in which plasma testosterone remained elevated longer than T4. The results suggest that the stimulatory effect of intravenous injected LHRH on thyroid (and gonadal) activity in the frog is primarily mediated through the hypophysis. They also point to a possible correlation between the gonadal and thyroidal axis.
D000818:Animals; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D007275:Injections, Intravenous / Q000379:methods; D008297:Male; D010902:Pituitary Gland / Q000187:drug effects; D011893:Rana esculenta; D011895:Rana ridibunda; D011896:Rana temporaria; D013739:Testosterone / Q000097:blood; D013961:Thyroid Gland / Q000032:analysis / Q000187:drug effects*; D013973:Thyrotropin-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology; D013974:Thyroxine / Q000032:analysis / Q000097:blood; D014284:Triiodothyronine / Q000032:analysis / Q000097:blood
D014284:Triiodothyronine; D007987:Gonadotropin-Releasing Hormone; D013739:Testosterone; D013973:Thyrotropin-Releasing Hormone; D013974:Thyroxine