Prolactin in the Cat: II. Diurnal Patterns and Photoperiod Effects

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Abstract
Photoperiod effects on prolactin (Prl) secretory patterns were investigated in the domestic cat. Circadian patterns of Prl release were studied in the cat in blood samples obtained every 4 h. Plasma concentrations were significantly elevated during the dark period (.hivin.X = 31.7 ng/ml) compared to the light period (.hivin.X = 5.5 ng/ml) when blood samples were obtained under red light during the dark period. If a short period of exposure to white light occurred during the dark sampling time, significant differences were not observed. In a long-term study of the effect of photoperiod, animals subjected to a decreasing photoperiod (4 discrete changes from 14 to 7 h light per day occurring over an 8-wk period) showed no significant difference in morning Prl concentrations during the 7-wk period of 7 h light per day (.hivin.X = 12.6 .+-. 0.3 ng/ml) compared to a group of cats maintained under 14 h light per day throughout (.hivin.X = 17.2 .+-. 4.9 ng/ml). Three of 4 animals in decreased light did not have another estrous period following the initial light change (14 to 12 h light); 1 animal had 1 estrous period following the photoperiod change. When the light was increased to 10 h/day, a return to estrus occurred within a period of 2-6 wk. During the period of increased light, there was no change in Prl concentrations as compared to the decreased light period. Major shifts in Prl secretion apparently do not accompany the seasonal anestrous period of the cat. The possibility does remain (based on the circadian data) that Prl secretion may be modified on a daily basis in response to photoperiod.

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