Evaluation of the role of noradrenaline precursor on LH release induced by ovarian steroids.

The role of noradrenaline precursor on the release of luteinizing hormone-releasing hormone based on the plasma concentration of luteinizing hormone (LH) was studied in rats. Rats were ovariectomized at 1000h on diestrus day 1(D1) and primed with estradiol immediately after the operation. They received progesterone at 0930h in the next morning (the expected diestrus day 2, D2). Dynamic changes of plasma LH levels were examined in the afternoon of D2. Treatments and results are as follows: 1) Administration of DL-threo-dihydroxyphenylserine (DOPS, 200mg/kg BW, ip) at 0900h on D2, 30 min prior to progesterone treatment, results in 87.5% of rats showing LH surges. 2) Administration of 5-hydroxytryptophan (5-HTP, 100mg/kg BW, ip) 30 min before DOPS completely inhibits the action of DOPS on LH release induced by ovarian steroids. 3) Making a change of the whole working schedule of operation, estrogen was given 5 hours earlier on D1. DOPS and progesterone were given at 0500 hand 0530 h on D2, respectively. It does not induce the LH surge during the whole morning of D2. These results demonstrate that DOPS might just reduce the threshold of the central neurons governing LH release. The time in a fixed lighting schedule was important in determining the LH release induced by ovarian steroids.