Mechanisms of intake depression in parasitised lambs

Abstract

Two experiments each with a similar design were conducted to determine a mechanism by which parasites of the gastrointestinal tract depress food intake in infected lambs. The first experiment involved blocking the activity of the ventromedial hypothalamus (VMH) using mederantil, a compound with the active ingredient brotizolam, which blocks the diazapem receptor in the VMH. The second eXI)eriment utilised loxiglumide (CR1505) which antagonises the peripheral receptor of cholecystokinin (CCK). For each experiment a group of eight Dorset Down cross Coopworth ewe lambs were individually penned indoors. Half of the animals were dosed three times each week for the duration of the trial, at the daily level of 4000 Trichostrongylus collibrijolllis per head and allocated to treatments in a 4*4 latin square design. Control lambs were pair fed a complete pelleted ration ad libitium to the intake of the parasitised lambs. Intake depression occurred after nine weeks of continuous larval intakes. Parasitised lambs ate less on a daily basis compared to the control animals (90 vs 75g/MLWT/D control vs parasitised animals). The daily cumulative feed intake pattern was also similar, but the cumulative amount consumed differed between the parasite infected and control animals. This occurred in both experiments. In the brotizolam experiment, dose rates of 0, 0.2, 0.4 and 0.8 ml brotizolam were administrated at two time intervals on two separate occasions (prior to feeding and 45 minutes after feed was offered). In both parasitised and control groups of animals a response in feeding at the 50 minute time interval was observed for both administeration times with increased dose rates of brotizolam (mean intakes 14, 24, 26, 29 g/MLWT/D for 0, 0.2, 0.4, 0.8 mls brotizolam respectively). There was no difference in total daily food consumption in both groups with increasing dose rate when brotizolam was administered prior to feeding, but both groups increased daily intake when hrolizolarn was given at 45 minutes post feed offer (71 vs 76 g/MLWT/d parasitisecl lambs and 55 vs 67g/MLWT/cI control lambs; control vs brotizolam respectively. P< 0.00 I). In the second experiment a potent peripheral CCK blocker, CR1505 was infused at O. 5. 10. 20 mg/BW. This had no effect on feed intake. It was concluded that the VMH plays a role in mediating satiety as brotizolam successfully stimulated appetite. The results from the CR 1505 experiment suggest that either peripheral CCK levels were not elevated under parasitism or peripheral CCK is not a factor involved in intake depression of parasitised lambs