The Effect of Pinealectomy on Seasonal Changes in Prolactin Secretion in the White-Tailed Deer (Odocoileus virginianus borealis)*

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Abstract
Surgery was performed on four buck and four doe 9-month-old white-tailed deer in March of 1978. Pinealectomy was performed on two deer of each sex, and the remaining animals received sham operations. At monthly intervals over the following year, baseline and TRH-stimulated (200 μg/deer; iv bolus) serum PRL was measured over a 3-h period by RIA. Baseline PRL levels in sham-operated animals followed a circannual pattern, with peak levels occurring in June (74–237 ng/ml for does; 34–193 ng/ml for bucks) and lowest levels occurring in midwinter (0.41–0.44 ng/ml for does; 0.10–0.13 ng/ml for bucks). Pituitary responsivity to TRH followed the same pattern as that seen for basal PRL levels in sham-operated deer, with the highest peak serum PRL responses in June (198–568 ng/ml for does; 190–395 ng/ml for bucks) and the lowest peaks seen in midwinter (0.27–0.80 ng/ml for does; 0.29–2.62 ng/ml for bucks) Pinealectomy appeared to abolish the circannual basal serum PRL rhythms in bucks, while this rhythm was maintained in does. Basal PRL levels in pinealectomized bucks ranged from 0.36–10.5 ng/ml, and basal levels in pinealectomized does ranged from 0.10–29.4 ng/ml. The greatest peak PRL response to TRH in pinealectomized deer was seen in August (41.2–93.4 ng/ml for does; 32.0–40.5 ng/ml for bucks), while the lowest peak response occurred in January (0.33–11.0 ng/ml for does; 0.50–17.0 ng/ml for bucks). Both sexes retained a degree of seasonally in their pituitary responsiveness to TRH, but the magnitude of the response in pinealectomized deer was greatly diminished in the summer months and increased in the winter months. Our results show that pinealectomy alters the naturally occurring photoperiod-linked seasonal profile of serum PRL in white-tailed deer and the associated pituitary responsiveness to TRH.