Effects of Trypanosoma congolense on pituitary and adrenocortical function in sheep: responses to exogenous corticotrophin-releasing hormone

To investigate whether the aberrations in adrenocortical and gonadal activity observed in trypanosomiasis may be induced by the refractoriness of the pituitary to hypothalamic liberins, the responses of the pituitary and adrenal glands and the testes to stimulation with ovine corticotrophin-releasing hormone (oCRH) were studied in rams 23 days (acute phase) and 65 days (chronic phase) after they were infected with Trypanosoma congolense. On both occasions a peak of plasma ACTH was observed within 20 minutes of the injection of CRH but the rate of increase in ACTH and the mean peak values in the infected rams were significantly lower (P<0.001) on day 23 but higher (P<0.05) on day 65 than in the uninfected control rams. Plasma cortisol concentration increased in all the rams after the injection of CRH. The rate of increase in plasma cortisol and the mean peak values were not significantly different between the control and infected rams on day 23 but were significantly (P<0.001) higher in the infected rams on day 65. However, the post peak concentrations of ACTH declined more rapidly in the infected rams than in the controls on both days 23 and 65. The plasma concentration of luteinising hormone (LH) did not change after the injection of CRH, whereas the testosterone levels showed a delayed response and its concentration increased when plasma ACTH and cortisol concentrations declined in both groups. On day 23, there was a greater increase in testosterone in the infected than in the control rams. These results demonstrate that the responsiveness of the pituitary corticotrophs to CRH is depressed during the acute phase and enhanced during the chronic phase of T congolense infection in rams, whereas the adrenal cortisol response is less affected. The results are also consistent with the hypothesis that the modulation of the pituitary-adrenal axis by infective trypanosomes may exacerbate the changes in testicular steroidogenesis frequently observed in trypanosomiasis.