Pineal melatonin: Circadian rhythm and variations during the hibernation cycle in the ground squirrel, Spermophilus lateralis

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Abstract
Variations in pineal melatonin content throughout a 24‐hour period and during different phases of the hibernation bout cycle were studied in the golden‐mantled ground squirrel (Spermophilus lateralis). In addition to pineal melatonin, the circadian variation in the activities of pineal N‐acetyltransferase (NAT) and hydroxyindole‐O‐methyltransferase (HIOMT) were also investigated in summer animals maintained at 22 ± 2°C, on a light:dark (L:D) schedule of 12:12 hr for 1 month (lights on at 08.00 hr). Pineal glands were collected from six animals in each group at 1200, 1600, 2000, 2400, 0200, 0400, and 0800 hr. Changes in pineal melatonin content during the hiberation bout cycle were investigated in ground squirrels housed at 4 ± .05°C in relative darkness (1.9–3.4 lux; 10:14 LD). Pineal glands were obtained between 12:00 and 18:00 hr from 30 animals during one of three phases of the cycle (deep hibernation, euthermic interbout, and entrance into hibernation). Pineal melatonin was also measured for comparison in six winter euthermic animals that were housed at 22 ± 2°C, on a L:D schedule of 10:14 hr. Melatonin was measured in individual pineal glands by radioimmunoassay. The daily melatonin rhythm in S. lateralis was characterized by a marked increase in pineal melatonin during the dark phase, in which peak nighttime values were nearly 20‐fold greater than daytime basal levels. The daily rhythm for NAT activity paralleled the changes in melatonin, showing a peak activity at 0200 hr that was 45 times greater than mean daytime values. No significant variation was observed in HIOMT activity. Pineal melatonin levels in winter animals were lowest during the state of deep hibernation. Melatonin content was increased significantly in the interbout phase within 1 hr postarousal (p < .01). A significant decline was evident in interbout animals that were sacrificed 6–8 hr postarousal and in animals that were in the entrance phase of hibernation (p < .01), although these values remained significantly higher compared to hibernation.