EFFECTS OF CASTRATION ON LUTEINIZING-HORMONE SECRETION AND RESPONSE TO GONADOTROPIN-RELEASING-HORMONE IN SHEEP INFECTED WITH TRYPANOSOMA-CONGOLENSE

The effects of trypanosomiasis on the endocrine function of the hypoth alamo-pituitary-gonadal ards were investigated before and after castra tion of Scottish Blackface rams infected with Trypanosoma congolense a nd uninfected controls, Blood samples were collected at 15-min interva ls for 6 h before and at 10, 20, 40, 60, 80, 100 and 120 min after inj ection of synthetic gonadotrophin-releasing hormone (GnRH, 20 mu g iv) 2 days before infection and 26 and 54 days after infection, with cast ration being performed 28 days after infection. Mean luteinizing hormo ne (LH) pulse amplitude was higher (3.3 +/- 0.2 vs 2.6 +/- 0.3 ng/ml) and mean plasma testosterone concentration was lower (4.1 +/- 0.6 vs 7 .6 +/- 1.2 nmol/l) in infected vs control rams 26 days after infection (p < 0.05). Mean plasma LH concentration and pulse amplitude increase d in both groups after castration but both were significantly lower in infected compared to control rams (6.6 +/- 1.5 and 13.0 +/- 2.2 ng/ml , p < 0.01; 7.7 +/- 0.9 and 11.6 +/- 0.9 ng/ml, p < 0.001), respective ly. However, LH responses to exogenous GnRH were similar in infected a nd control rams at each stage of the experiment; suggesting that the s maller increase in plasma LH after castration in infected rams was not caused by reduced responsiveness of the pituitary to GnRH but by alte rations in GnRH secretion by the hypothalamus or its transport to the adenohypophysis. These results also demonstrate that impairment of tes tosterone secretion within 4 weeks of T, congolense infection in sheep may be due to testicular rather than pituitary effects.