Effects of photoperiod on the cessation of growth during autumn in male red deer and growth hormone and insulin-like growth factor-I secretion.

Male red deer undergo seasonal cycles of food intake and growth rate, which are high during spring and low during winter, despite high quality food ad libitum. Hormonal profiles during the cessation of growth in autumn and the potential role of photoperiod in the timing of the observed changes have been investigated. Whether this seasonal decrease in growth affected the response of GH and IGF-I to fasting was also examined. Two groups of six male 1-year-old red deer were exposed to different photoperiods after the summer solstice. One group (C) was given a simulated natural photoperiod while the other group (SS) was maintained on a summer solstice photoperiod (16L:8D). GH was measured in blood collected continuously and divided into pools every 5 min for 24 h in the fed state and after a 48-h fast on two occasions; the first was in November before photoperiod manipulation began and the second was in April approximately 16 weeks after initiating treatments. IGF-I, prolactin, and testosterone were measured in weekly samples. Individual live weight and group food intake were also measured each week. The normal growth pattern seen in the C group was delayed in the SS group. Thus, from 7 March until the second GH sampling on 11 April the live weight of deer in group C fell; in contrast, deer in group SS continued to grow (-43 vs 186 g/day s.e.d. = 65.5, P < 0. 01). Food intake changes reflected the pattern of growth in both groups. Mean GH (P < 0.05), GH pulse amplitude (P < 0.01), and IGF-I (P < 0.001) declined in both groups from November to April. This decline was more marked in group C and in April these parameters were all lower in group C than in group SS (GH, P < 0.05; IGF-1, P < 0.01). Prolactin levels in April were also lower in group C than in group SS (P < 0.01); testosterone was not affected by treatment. Fasting increased mean GH and GH pulse amplitude in both groups in November (P < 0.05). In April, the fasting response differed between the groups. In group C, mean GH, pulse amplitude, and pulse frequency were all greater in the fasted state than in the fed state (P < 0.05), while in group SS there were no significant differences (P > 0.05). IGF-I was lower in the fasted state than in the fed state at both sampling dates (P < 0.001). The seasonal decline in food intake and growth is associated with decreased GH, IGF-I, and prolactin concentrations, and increased testosterone and the GH response associated with fasting. All these changes except those of testosterone were delayed or reduced by continued exposure to a summer solstice photoperiod in autumn. The decreased photoperiod in autumn may thus influence the normal timing of the seasonal growth cycle.