Increased Sensitivity of Dopaminergic Inhibition of Luteinizing Hormone Release in Immature and Castrated Female Rats

Apomorphine, a dopamine receptor-stimulating drug, at a dose of 5 mg/kg BW was highly effective in reducing serum LH levels in chronically ovariectomized rats. Continued treatment of the same rats with a depot form of this drug (dibutyrylapomorphine (ZK 48241)) twice daily for 10 days resulted in depressed LH levels for a period of 3 days. By day 9, serum LH levels were normal and could not be depressed by another injection of apomorphine (5 mg/kg BW). The first injection of apomorphine had no effect on serum FSH levels; however, chronic treatment with ZK 48241 caused serum FSH to rise throughout the remainder of the 10-day period. Serum prolactin levels were depressed by the acute apomorphine treatment as well as during the period of ZK 48241 treatment. Apomorphine as well as piribedil (another dopamine receptor-stimulating drug) at doses of 1 or 10 mg/kg BW completely prevented the occurrence of premature preovulatory-type LH peaks when injected at noon into 15-day old female rats. Serum FSH remained un...