Daily changes in concentrations of prolactin in serum of prepubertal bulls exposed to short- or long-day photoperiods.
Early temporal changes in concentrations of prolactin (PRL) in serum after a sudden change in photoperiod and daily responsiveness to PRL-releasing and inhibiting factors were investigated in prepubertal Holstein bull calves exposed to different photoperiods. In calves switched from 8-hr light: 16-hr dark to 16-hr light:8-hr dark, there was no observable change in the daily pattern of serum concentrations of PRL after 1, 2, or 4 days. On the other hand, in animals switched from 16-hr light:8-hr dark to 8-hr light:16-hr dark, there was a consistent increase in serum PRL from 33.4 ng/ml on Day 0 to maximum values of 57.3, 62.7, and 78.9 ng/ml between 14 and 18 hr after onset of light on Days 1, 2, and 4, respectively. Thus, absence of light allowed expression of a daily rhythm in serum concentrations of PRL that persisted for at least 4 days after the photoperiod switch. There were no differences in L-dopa inhibition of PRL release in animals exposed to 16-hr light:8-hr dark at 3 or 15 hr after onset of light. However, thyrotropin-releasing hormone-induced release of PRL was greater 3 hr after onset of light (11 hr after onset of dark) compared with release at 9, 15, and 21 hr after onset of light in animals exposed to 16-hr light:8-hr dark, but not in bulls exposed to 8-hr light:16-hr dark. The results provide evidence that the cue for the putative photosensitive period of PRL secretion in cattle may be more closely associated with onset of dark, not onset of light.
Petitclerc
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D
Department of Animal Science, Michigan State University, East Lansing 48824.
Chapin
L T
LT
Tucker
H A
HA
D000818:Animals; D002417:Cattle / Q000097:blood*; D007980:Levodopa / Q000494:pharmacology; D008027:Light; D008297:Male; D010507:Periodicity; D011388:Prolactin / Q000097:blood*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology
D007980:Levodopa; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin