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

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Abstract
Apomorphine, a dopamine receptor-stimulating drug, at a dose of 5 mg/kg BW body wt was highly effective in reducing serum LH [luteinizing hormone] 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 body wt). The 1st injection of apomorpine had no effect on serum FSH [follicle stimulating hormone] 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 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 body wt completely prevented the occurrence of premature preovulatory-type LH peaks when injected at noon into 15 day old female rats. Serum FSH remained unchanged at high levels. Both doses of piribedil proved ineffective in blocking the preovulatory LH peak in adult proestrous rats. A dopamine receptor mechanism which is inhibitory to LH but not to FSH release apparently is highly sensitive in 15 day old female rats but very insensitive in adult proestrous rats. In chronically ovariectomized adult rats, this dopamine-sensitive receptor is resensitized and can be desensitized by chronic treatment with dopamine receptor stimulating drugs.