A photophase and seasonal study of the pineal parenchyma and melatonin serum levels in the albino rat subjected to immobilization stress.

Sixty-four male Wistar rats (mean weight 215 +/- g) were divided into two groups of 32 animals each, with four additional subgroups (n = 8). The experimental group was subjected to immobilization stress for 24 hours and sacrificed during the photo - (10:00-14:00 hours) and scotophases (00:00-02:00 hours) of spring and winter. Photophase and seasonal differences were observed (p < 0.0001) between the corticomedullary karyometric values obtained in the control group. Similar results were obtained in the experimental group, though the seasonal differences were greater and favored spring (p < 0.0001), while the photophase differences were less important (p < 0.03). The number of synaptic ribbons showed a significant inversion of the physiological day/night ratio in both seasons. Similar results were obtained for the melatonin serum levels. The results suggest that photophase and season may be conditioning factors in the albino rat regarding pineal response to stress immobilization.