Effects of shortened daylength and melatonin treatment on plasma prolactin and melatonin levels in pinealectomised and sham-operated ewes

Eight red deer stags, 2 control, 3 control-immunized (i.e. a low titre of LHRH antibodies after active immunization 2 years earlier) and 3 superior cervical ganglionectomized, were given a s.c. implant of melatonin in May at the nadir of the sexual cycle; 5 other stags remained untreated. All the melatonin-treated animals shed the velvet-like skin from the antlers in June or July at least 1 month before the untreated controls, and had an early increase in blood plasma testosterone concentrations. The treated stags were also precocious in the development of rutting behaviour, although this inductive effect was blocked dramatically in the control-immunized stags after a booster immunization against LHRH; these animals failed to show any further reproductive development and cast their antlers. It is concluded that continuous exposure to melatonin in early summer will induce premature seasonal testicular development, an effect dependent on the secretion of LHRH, and similar to that produced by exposure to short daylengths.

Section: Discussionmentioning

confidence: 92%

Induction of early rutting in male red deer (Cervus elaphus) by melatonin and its dependence on LHRH

Lincoln¹,

Fraser²,

Fletcher³

1984

“…This is further confirmation that the long daylength prolactin surge is not mediated by pineal influences. We have previously reported the occurrence of the dusk surge in prolactin in pinealectomized adult ewes in spring and its disappearance after an abrupt shift to short daylength (Kennaway, Dunstan, Gilmore & Seamark, 1982). Rodway, Brinklow, Crump, Oliver & Owen (1983) reported that a 'short day' melatonin infusion into ewes maintained in long daylength abolished the dusk surge of prolactin.…”

Section: Discussionmentioning

confidence: 97%

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Kennaway¹,

Peek²,

Gilmore³

et al. 1986

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.

“…In adult ewes melatonin implants simulate short daylength, decreasing prolactin levels (Kennaway et al, 1982b(Kennaway et al, , 1983a (Kennaway, Sanford, Godfrey & Friesen, 1983b). Future experiments with sheep maintained in controlled environments and utilizing multiple blood sampling may clarify the question of prolactin suppression by melatonin in young lambs.…”

Section: Discussionmentioning

confidence: 99%

Effects of melatonin implants in ewe lambs

Kennaway¹,

Gilmore²

1984