Circadian variations of "synaptic" bodies in the pineal glands of Brattleboro rats

The function of the mammalian pineal gland is regulated primarily by the sympathetic system. Arginine-vasopressin (AVP) may also be involved in the regulation of pineal melatonin synthesis under experimental conditions. The present study was conducted in the AVP-deficient rat strain, the Brattleboro rat, to investigate whether the numbers and rhythms of pineal "synaptic" bodies in this strain are different from those found in intact rats. AVP or its non-vasoconstrictive analog, deamino-D-AVP, was also injected intra-arterially in Brattleboro or Sprague-Dawley rats to test whether this procedure influences "synaptic" body numbers. Brattleboro rats were killed at different time-points throughout the 24 h-cycle in March, June and September. "Synaptic" ribbons, spherules and intermediate structures were quantified and examined with regard to their intracellular location, with or without nocturnal AVP or D-AVP treatment. Numbers of ribbons were low during the day and high during the night (as in genetically intact rats), whereas spherules and intermediate structures numbers exhibited inconstant daily patterns. Night levels of "synaptic" ribbons were highest in June, lowest in March, whereas day levels did not differ significantly. No significant alterations in pineal "synaptic" body numbers were found following administration of AVP or D-AVP. Our results therefore indicate that AVP does not play a crucial role in the regulation of pineal "synaptic" body numbers in rats.