Effect of long-term feed restriction on seasonal endocrine changes in Soaysheep

Groups of 15 adult, castrated, male Soay sheep were housed under natural da ylength conditions at 57 degreesN and fed a complete diet ad libitum (AL) o r at a restricted rate (R) of 35 g dry matter (DM)/kg(0.75) initial livewei ght per day. The diet was based on barley and dried grass pellets and conta ined an estimated 11.6 MJ of metabolisable energy, 83% DM and 140 g crude p rotein/kg DM. In the AL animals, higher levels of feed intake during the pe riods of long daylength were associated with shorter intermeal intervals (p <0.001), a greater meal frequency (p<0.001), and a greater proportion of ti me spent eating (p<0.001) together with a greater rate of feed ingestion (p <0.001) and an increased meal size (p<0.001). Mean plasma concentrations of insulin, prolactin, insulin-like growth factor 1 (IGF-1), triiodothyronine (T-3), and thyroxine (T-4) were higher(p<0.001) in the spring or summer th an in the autumn. Mean plasma GH concentrations did not differ with month. Compared with R animals, AL animals had higher mean plasma concentrations o f insulin (p<0.001). prolactin (p<0.01), T-3, (p<0.01), and T-4 (p<0.01). P lasma GH and IGF-I concentrations did not differ significantly with treatme nt. There: was a greater increase in plasma insulin concentrations followin g feeding in R than AL animals (p<0.001) owing to higher pre-feeding concen trations in AL animals and the ingestion of larger amounts of feed by R tha n AL animals in the period after fresh feed was introduced. There were sign ificant differences between months in this response. in R animals(p<0.01). Mean CSF insulin concentrations were significantly higher in AL than R anim als (p<0.05) but were not affected by month. Neither was there a difference between pre-feeding concentrations and concentrations at approximately 12 h after feeding. It is concluded that the differences in the response of pl asma insulin concentrations to feeding at different times of year, which we re detected in R animals, were attributable. primarily, to differences in t he vagally-induced insulin response to feeding and that these differences m ay provide important feedback signals to the appetite centre. (C) 2000 Else vier Science Inc. All rights reserved.