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.

An experiment was conducted to determine whether active immunization against melatonin could modify the perception of abrupt photoperiodic changes in ewes. Two groups each containing six intact Ile-de-France ewes were submitted to alternate periods of short days for 2.5 months and long days for 2.5 days for about 70 weeks. Three series of active immunizations against a melatonin conjugate were carried out during the first of the three long-day periods. Control ewes were actively immunized at the same time against human serum albumin. Blood samples were taken once a week throughout the experiment to measure antibody titre and affinity, and prolactin and progesterone concentrations. Sera of all treated ewes demonstrated higher antibody titres than those of control ewes. Antisera were highly specific, as evidenced by the absence of displacement of iodinated melatonin in the presence of ten melatonin metabolites. Significant effects of photoperiod and of the interaction between treatment and photoperiod on prolactin concentration were detected. Prolactin concentrations in plasma of the control ewes were high during long days and low during short days. However, in the treated ewes, with the exception of the first period of long days, prolactin concentrations were not influenced by photoperiodic changes. Ovulatory activity of control ewes, as demonstrated by progesterone measurements, was stimulated by short days and inhibited by long days. In contrast, ovulatory activity of treated ewes, after a response identical to that of control ewes after the first photoperiodic shift from long to short days, showed a complete desynchronization of ovulatory activity relative to photoperiodic changes. Immunization against melatonin can therefore modify the influence of artificial photoperiodic changes on prolactin secretion and ovulatory activity in ewes.

Summary. In anoestrous ewes, male chemosignals elicit rapid increases in luteinizing hormone (LH) secretion that can ultimately lead to ovulation. To assess the possible involvement of the accessory (vomeronasal) olfactory system in the mediation of those chemical cues, we destroyed this pathway by vomeronasal organ electrocauterization (Exp. I) and vomeronasal nerve section (Exp. II). Neither of these lesions inhibited the LH response of ewes to the odour of the male. These results suggest that the vomeronasal system is not necessary to mediate the neuroendocrine response of the ewe to the male odour. As both surgical methods spared the main olfactory system but destroyed the vomeronasal system, it is likely that the main olfactory system is involved in the LH response to chemical stimulation in sexually experienced ewes.

Keywords: sheep; male effect; chemosignals; vomeronasal system