Effects of thyrotropin-releasing hormone analogues on body shaking and prolactin levels in estrogen-primed and nonprimed rats.

The effects of a thyrotropin-releasing hormone (TRH) analogue, MK-771, in comparison with TRH and another TRH analogue, DN-1417, on body shaking and prolactin secretion were investigated in estrogen-primed and nonprimed rats. Intraperitoneal injections of TRH (20, 40 mg/kg), MK-771 (0.5-4 mg/kg) or DN-1417 (5-20 mg/kg) elicited marked body shaking in male rats. MK-771-induced shaking did not increase upon daily administration of the drug (0.5 mg/kg, s.c.) for 20 days or after treatment with estradiol benzoate (35 micrograms/kg, s.c.) for a 3 days period. Furthermore, following daily administration of MK-771 (0.5 mg/kg, s.c.) for 20 days, serum prolactin levels remained unchanged in male rats. Behaviorally active doses of TRH (20 mg/kg, i.p.), MK-771 (2 mg/kg, i.p.) and DN-1417 (10 mg/kg, i.p.) also failed to elevate serum prolactin levels in both ovariectomized rats and normal male rats. These drugs did, however, markedly elevate prolactin levels which had been increased in estrogen-primed male rats. The results suggest that TRH, MK-771 and DN-1417, in behaviorally active doses, exert a marked endocrine effect on prolactin secretion in estrogen-primed rats, but not in ovariectomized rats or normal male rats.