Ornidazole (400 mg kg⁻¹ day⁻¹) given by oral gavage rendered male rats infertile by 6.6 ± 0.7 days (mean ± sem, n = 9, range 3–10) after beginning the treatment and fertility returned within 5–10 days after treatment with ornidazole for 6–7 days. At 200 mg ornidazole kg⁻¹ day⁻¹, fertility was reduced but total infertility was not achieved. No differences were found in the percentage motility of spermatozoa recovered from any region of the epididymides of ornidazole-treated rats compared with controls. However, computer aided sperm analysis revealed significantly lower straight-line and average path velocities in ornidazole-treated animals (400 mg kg⁻¹ day⁻¹) for spermatozoa from the distal regions of the tract than for controls. Curvilinear velocity was significantly lower than that of controls in the distal corpus and cauda regions. The motility characteristics of spermatozoa from animals receiving 200 mg ornidazole kg⁻¹ day⁻¹ were lower than, but not significantly different from, motility in controls. There were no differences between the total protein, l-carnitine, glycerophosphocholine or total α-glucosidase content in epididymal homogenates from fertile control and infertile ornidazole-treated animals. Spermatozoa released from the cauda epididymidis of untreated rats into ornidazole solutions displayed no changes in the percentage motility up to 20 mmol l⁻¹ and were only depressed at 50 mmol l⁻¹. All velocities revealed a biphasic response with an initial increase in motility and then inhibition at higher concentrations, but a significant difference from velocities in the absence of ornidazole was evident only for straight line velocity (VSL) at 50 mmol l⁻¹. The rapidity of action in inducing infertility is compatible with post-testicular action and an action on the epididymis is suggested by the decline in motility parameters of luminal spermatozoa. The lack of effect on epididymal secretions in vivo and on sperm motility in vitro, except at very high doses, suggests that there may be a direct action of an ornidazole metabolite on epididymal spermatozoa.

Infertility is induced in male rats by oral administration of 400 mg ornidazole kg⁻¹ body mass day⁻¹ within 10 days, without drastic effect on the motility of epididymal spermatozoa obtained after 15 days of treatment. Spermatozoa were recovered from the female tract 9 h after mating with males treated with ornidazole for 10 days to identify the cause of infertility. The number and motility of spermatozoa in the uterus indicated normal seminal deposition. Spermatozoa could pass the utero–tubal junction but total numbers of spermatozoa and their velocities in the oviductal isthmus were significantly lower than those recovered from females mated to fertile, control males fed vehicle alone. However, the number of spermatozoa recovered from the ampulla was not different from control and the number of spermatozoa that had penetrated the cumulus mass was lower and none of the eggs was fertilized by spermatozoa from rats treated with ornidazole. The ability of cauda epididymal spermatozoa from ornidazole-fed rats to penetrate viscous media in vitro was lower than that of the controls. When incubated in media containing different concentrations of substrates and ions, a reduction in movement of spermatozoa was observed when glucose was the only exogenous substrate or after preincubation in substrate-free medium. These results indicate that the antifertility action of orally administered ornidazole occurs via damage to spermatozoa in the upper regions of the female tract, possibly reflecting a failure in capacitation as a result of reduced energy production.

Summary. A computer-aided sperm analysis system was optimized for objective assessment of the movement characteristics of mature and immature rat spermatozoa by testing different settings. Measurements of straight line velocity of individual motile cells were validated by manual tracking with a digitizer. Better agreement between the two methods and better performance in distinguishing between mature and immature spermatozoa was obtained by reducing the tracking rate to increase the time of analysis. However, numbers of motile and immotile cells could not be determined accurately. Manual counting of videotaped images revealed no significant differences in percentage motility of spermatozoa from five epididymal regions. Caput spermatozoa were characterized by low straight-line (VSL) and averaged-path (VAP) velocities and low path straightness (STR), whereas mature cells displayed high VSL, VAP and STR. An increase in curvilinear velocity on maturation was less obvious. Spermatozoa in the proximal corpus epididymidis were heterogeneous in their acquisition of motility maturation and the uniformity of movement pattern achieved in the distal corpus and proximal cauda regions tended to decrease again in the distal cauda epididymidis. Such objective measurements of motility patterns will facilitate studies on the regulation of motility development upon sperm maturation.

Keywords: epididymal spermatozoa; motility maturation; sperm analysis; computer-aid; rat

The chlortetracycline fluorescence assay was used to study the status of capacitation and the extent of induced acrosome reactions in cauda epididymidal spermatozoa from fertile and infertile rats fed, respectively, with vehicle or ornidazole (400 mg kg⁻¹ day⁻¹) for 10 days. Uniform bright fluorescence over the whole head was classified as the uncapacitated pattern, whereas a postacrosomal dark band, and a uniformly weaker fluorescence over the acrosome, reflected patterns intermediate between the uncapacitated and acrosome-reacted states. Acrosome-reacted spermatozoa displayed a dark head but always retained fluorescence at their tip. There was no difference between experimental and control groups of rats with regard to the development of the chlortetracycline fluorescence patterns during incubation. Under basal incubation conditions, the acrosome reaction was slightly delayed in spermatozoa from ornidazole-treated animals. In contrast, more spermatozoa were acrosome reacted in this group after incubation for 5 h when the concentration of BSA was increased from 4 to 20 mg ml⁻¹. The Ca²⁺-ionophore A23187 induced a similar stimulation of capacitation and acrosome reactions in spermatozoa from control and ornidazole-fed animals, but in the latter group A23187 caused strong immobilization of spermatozoa. In the capacitation medium containing 5 mmol lactate l⁻¹ and 5 mmol glucose l⁻¹, the straight-line velocity of spermatozoa from ornidazole-treated rats was reduced by 50% compared with controls, irrespective of the concentration of BSA. Two glycolytic enzymes, triose phosphate isomerase and glyceraldehyde 3-phosphate dehydrogenase, displayed reduced activity (48% and 68% of controls, respectively) in cauda epididymidal spermatozoa from ornidazole-fed rats, whereas the activities of hexokinase and lactate dehydrogenase remained unchanged. This finding suggests that the fertility-compromising action of ornidazole is due to a disturbed glycolytic pathway.