Effects of biogenic amine precursors on ovarian steroid-induced LH release in acutely ovariectomized rats

Effects of catecholamines and indoleamine precursors on the luteinizing hormone (LH) release induced by estradiol (E2) and progesterone (P4) in acutely ovariectomized (Ovx) rats were studied. Rats were Ovx on diestrus day 1 (D1) and received a spaced injection of E2 and P4 on D1 and D2, respectively. One of the following substances: L-beta-3, 4-dihydroxyphenylalanine (L-DOPA), DL-threo-dihydroxyphenylserine (DOPS), or 5-hydroxy-L-tryptophan (5-HTP) was administered intraperitoneally at 0900h on the expected diestrus day 2 (D2). The control group received saline. Serial blood samples were collected in the afternoon of D2 and plasma concentrations of LH were measured by radioimmunoassay. Results indicate that E2 and P4-induced LH release could be facilitated, but not significantly, by DOPS, delayed/inhibited by L-DOPA, and completely inhibited by 5-HTP as compared with saline controls. Exogenous administration of luteinizing hormone releasing hormone (LHRH) induced significantly higher LH release in E2/P4-primed Ovx than in control Ovx rats. Results imply that the classical neurotransmitters in the central nervous system might modify the threshold of LHRH neurons in the hypothalamus.