Biology of Reproduction, Vol 13, 223-227, Copyright © 1975 by Society for the Study of Reproduction

Effect of L-dopa on Nocturnal Prolactin Surges During Pseudopregnancy

¹ Department of Physiology and Biophysics University of Louisville Health Sciences Center Louisville, Kentucky 40201

Rats were made pseudopregnant by cervical stimulation during late proestrus or early estrus, and were cannulated via the right carotid artery several days later. A pattern of prolactin secretion during pseudopregnancy was found in which there were two daily surges of prolactin. Prolactin levels in the plasma reached a peak between 0100-0500 h (nocturnal peak) and 1700-1900 h (diurnal peak). Injection of L-dihydroxyphenylalanine (L-dopa) intraperitoneally at 0030 h on days 5-8 of pseudopregnancy initially suppressed prolactin secretion, compared to control pseudopregnant rats injected with the vehicle. This inhibition of prolactin was followed by a surge of prolactin between 0300-1700 h. When two injections of L-dopa were given at 0030 and 0230 h, prolactin levels were 10-15 percent of the controls at 0100 and 0300 h. However, at 0500-0700 h, there was a significant rebound effect; the concentration of prolactin in the plasma was then 12 times higher in rats which were treated with L-dopa than in controls. In none of the treated groups was there a significant change in the duration of pseudopregnancy. Thus, temporary inhibition of the nocturnal prolactin surge did not alter maintenance of pseudopregnancy.