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

Groups of 15 adult, castrated, male Soay sheep were housed under natural daylength conditions at 57 degrees N and fed a complete diet ad libitum (AL) or at a restricted rate (R) of 35 g dry matter (DM)/kg(0.75) initial liveweight per day. The diet was based on barley and dried grass pellets and contained an estimated 11.6 MJ of metabolisable energy, 83% DM and 140 g crude protein/kg DM. In the AL animals, higher levels of feed intake during the periods of long daylength were associated with shorter inter-meal intervals (p<0. 001), a greater meal frequency (p<0.001), and a greater proportion of time 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 than in the autumn. Mean plasma GH concentrations did not differ with month. Compared with R animals, AL animals had higher mean plasma concentrations of insulin (p<0.001), prolactin (p<0.01), T(3) (p<0.01), and T(4) (p<0.01). Plasma GH and IGF-1 concentrations did not differ significantly with treatment. There was a greater increase in plasma insulin concentrations following feeding in R than AL animals (p<0.001) owing to higher pre-feeding concentrations in AL animals and the ingestion of larger amounts of feed by R than AL animals in the period after fresh feed was introduced. There were significant differences between months in this response, in R animals (p<0.01). Mean CSF insulin concentrations were significantly higher in AL than R animals (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 plasma insulin concentrations to feeding at different times of year, which were detected in R animals, were attributable, primarily, to differences in the vagally-induced insulin response to feeding and that these differences may provide important feedback signals to the appetite centre.