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G M Webster

Publications and source records attributed to G M Webster.

5 recordsLinked to original sources
PubMed1983

Seasonal changes in LH and prolactin concentrations in ewes of two breeds.

The current experiment was designed to investigate whether changes in prolactin concentrations might be involved in the seasonal change in responsiveness of the hypothalamic-pituitary axis to the negative feedback effects of oestradiol in the ewe. Twelve Dorset Horn ewes (long breeding season) and 12 Welsh Mountain ewes (short breeding season) were ovariectomized after insertion of empty subcutaneous implants (N = 2 per breed), or ones containing oestradiol-17 beta (N = 10 per breed). Intact ewes, 10 of each breed, were used to monitor breeding activity. Mean weekly plasma LH concentrations in oestrogen-treated ovariectomized ewes of both breeds were elevated over the period corresponding to their respective breeding seasons, but were basal during anoestrus. The changes in prolactin concentrations, however, followed the same temporal pattern in both breeds and were approximately parallel to changes in daylength. These data support the suggestion of a seasonal change in negative feedback responses to oestradiol which corresponds to changes in breeding activity, but any involvement of prolactin in this response remains questionable.

Animals↗
PubMed1980

The effect of rearing under a long or short photoperiod on testis growth, plasma testosterone and prolactin concentrations, and the development of sexual behaviour in rams.

Two groups of 6 rams were maintained under constant photoperiodic conditions consisting of short days (8 h light: 16 h dark; Group S) and long days (16 h light; 8 h dark; Group L) from 4 to 20 months of age. Five other rams were reared under a photoperiod representative of that occurring naturally (Group N). Testis size, plasma testosterone and prolactin concentrations were monitored weekly and sexual behaviour tests were carried out at regular intervals. Over the treatment period Groups S and L did not differ in terms of testis growth or plasma testosterone. Both groups had a phase of testis growth and increased testosterone followed by a decline and the temporal patterns for the two groups were equivalent. Sexual behaviour was slower to develop in Group L than in Group S, indicating that photoperiod can affect the development of sexual behaviour irrespective of peripheral plasma testosterone concentrations. Plasma prolactin levels showed a cyclic variation in Group L and were significantly higher overall than in Group S rams. This, together with a trend towards negative correlations between prolactin concentrations and sexual behaviour in Group L, indicates that prolactin may be involved in the effect of photoperiod on sexual behaviour. The presence of a cycle of testicular growth and of hormone concentrations in young animals under constant photoperiod tentatively suggests that these cycles are endogenous. The constant photoperiod did, however, affect the animals because the cycles which occurred in Groups S and L were out of phase with those of Group N by about 4 months.

Animals↗