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Effects of Trypanosoma congolense infection in rams on the pulsatile secretion of LH and testosterone and responses to injection of GnRH

Changes in pulsatile secretion of LH and testosterone and responses to exogenous GnRH were assessed at different stages of Trypanosoma congolense infection in rams. Jugular blood samples were collected every 15 min for 6 h followed by immediate injection of GnRH (20 µg i.v.) and further sample collection after 10, 20, 40, 60, 80, 100 and 120 min. This sampling and injection regimen was performed 5 days before infection (day – 5) and 23 and 52 days after infection. T. congolense infection increased (P<0.05) the mean plasma LH concentration over 6 h on day 23 (3.2 ± 0.2 ng ml^–1) and decreased (P < 0.05) the mean LH concentration on day 52 (1.2 ± 0.2 ng ml^–1, P < 0.05) compared with day – 5 values (2.0 ± 0.2 ng ml^–1). Trypanosome infection induced a rapid decline in plasma testosterone concentration from a mean of 7.5 ± 1.4 nmol l^–1 on day – 5 over 6 h to 3.6 ± 0.4 nmol l^–1 (P<0.05) on day 23 and 1.7 ±0.3 nmol l^–1 (P< 0.001) on day 52. The observed decline in plasma LH concentration in infected rams was not associated with reduced sensitivity of the pituitary to GnRH or its ability to release LH, as the LH response to exogenous GnRH was not impaired throughout the period of infection. However, the testosterone response to GnRH-induced LH stimulation was depressed on both days 23 and 52 after infection. It was concluded that the decline in plasma LH concentration in infected rams was caused by reduced GnRH stimulation of the pituitary, whereas the decline in plasma testosterone was partly caused by reduced sensitivity of the Leydig cells to circulating LH.