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Effects of Trypanosoma congolense infection on the pituitary gland of Baoulé bulls: immunohistochemistry of LH- and FSH-secreting cells and response of plasma LH and testosterone to combined dexamethasone and GnRH treatment

The effects of Trypanosoma congolense infection were investigated at the pituitary level on trypanosome resistant Baoulé bulls (aged 3–6 years), using immunohistochemistry of LH-and FSH-secreting cells and a combined dexamethasone and GnRH challenge. The pituitaries of two control and five naturally infected Baoulé bulls were removed after slaughter and the LH- and FSH-secreting cells were examined immunohistochemically, using specific polyclonal antibodies against βLH and βFSH. No significant impairment of the labelling and distribution of LH- and FSH-secreting cells was seen in infected bulls when compared with control animals. No parasites were found in the pituitary glands. Plasma LH and testosterone concentrations were determined in eight control and eight infected bulls by enzymeimmunoassay and radioimmunoassay techniques, respectively. Blood samples were collected at intervals of 30 min two times before and nine times after dexamethasone treatment (20 mg i.m.). GnRH (Busereline: 20 µg, i.m.) was injected 4.5 h later and samples were collected every 15 min for 180 min. After dexamethasone treatment, LH and testosterone concentrations declined dramatically in both groups. Four hours after treatment, the mean testosterone concentration for both groups was 0.44 ng ml^–1. After GnRH injection, LH concentrations in the infected group increased rapidly to a mean maximum value of 30 ng ml^–1 by 165 min. In contrast, the increase in LH concentration in non-infected bulls was more gradual and the mean maximum value, reached at the same time, was only 20 ng ml^–1. Testosterone concentration increased rapidly and in a similar manner in both groups for the first 90 min (0.08 ± 0.04 ng ml^–1). There was almost no further increase in testosterone concentration in the infected group (different from controls; P < 0.05) although LH concentrations continued to rise. The testosterone concentration of the non-infected group increased steadily, up to the end of the sampling period. It is concluded from the immunohistochemical study and from the pituitary response to GnRH that the parasites do not alter pituitary function but that they do affect testicular function.