J. Endocrinol. Invest. 14: 147-150,1991 SHORT COMMUNICATION Effect of gestational mastectomy on postpartum gonadotropin releasing hormone and thyrotropin releasing hormone-induced luteinizing hormone and prolactin response in first lactation holstein cattle L.C. Holt, A.M. Akers, C.J. Denbow, F.C. Gwazdauskas and W.E. Vinson Department of Dairy Science, College of Agriculture and Life Sciences, Virginia Agricultural Experiment Station, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 , USA. ABSTRACT. First lactation Holstein cows were di- vided into two treatment groups to evaluate thy- rotropin releasing hormone (TRH, 0.25 )..I.g/kg body weight) and gonadotropin releasing hormone (GnRH; 200 )..I.g) induced secretion of prolactin (PRL) and luteinizing hormone (LH) on days 7 and 16 postpartum. Disregarding treatment, LH re- sponse was greater (p <0.01) on day 16 than day 7 postpartum (7.5 ± 0.3 ng/ml on day 7 vs 10.2 ± 0.3 ng/ml serum on day 16). Mastectomized cattle had similar time for initiation of LH increase, but peak concentrations were achieved later. Peak PRL INTRODUCTION A sexually inactive period follows parturition in dairy cows prior to the establishment of normal estrous cyclicity (1). At parturition, pituitary content and plas- ma LH concentrations are low « 1 ng/ml) and in- crease gradually 8 to 10 days postpartum (2), with the first preovulatory peak at approximately 17 days postpartum (3). Although administration of GnRH to normally cycling cows results in plasma LH peaks within 2 to 4 h, pituitary responsiveness is not fully restored until 8 to 10 days postpartum (2, 4). PRL is believed to be essential for mammary growth, lactogenesis and the maintenance of lactation in most ruminants (5). High concentrations of serum PRL have been indicated in the inhibition of ovarian activity in humans and the ewe (6). In dairy cattle the suckling of calves resulted in elevated serum PRL Key-words: Luteinizing hormone, prolactin, GnRH, TRH, mastectomy. Correspondence. Dr. F.C. Gwazdauskas, Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA. Received May 16, 1989; accepted December 6, 1990. 147 concentrations were reached 12 to 15 min after in- jection and returned to baseline within 2.5 h in both groups. However, intact cows had higher (p <0.01) mean serum PRL than the mastectomized cows for 1 h following injection. Peak PRL concentration was 83.3 ± 17.6 ng/ml for mastectomized cows vs 128.0 ± 24.7 ng/ml for intact cows. It appears that udder removal allows for greater pituitary responsiveness to GnRH but diminishes PRL response to TRH sug- gesting the mammary gland differentially affects pi- tuitary secretion of LH and PRL. concentrations (7). However, several researchers have found no difference in basal PRL concentra- tions between suckled and milked cows (1,8). In lactating dairy cows TRH causes an acute tran- sient release of PRL into the blood (9). However, to our knowledge the effect of TRH on PRL secretion in mastectomized dairy cattle has not been evalu- ated. The purpose of this experiment was to evaluate the effect of GnRH and TRH injection to assess the ef- fect of lactation condition on serum LH and PRL re- sponse in intact and mastectomized first-lactation Holstein cattle on days 7 and 16 postpartum. MATERIALS AND METHODS Mastectomies were performed using procedures similar to Short et aI., (10) on individual heifers paired by breeding date under xylazine hy- drochloride between days 92 and 124 of gestation. Prophylactic penicillin treatment was administered for 10 days following surgery. All animals were housed in a tie stall barn with access to a loafing area for 4 h daily. After parturition (between