Serotonin and seasonal variation in the pancreatic structure of bats: Possible presence of serotonergic axons in the gland

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Abstract
The serotonin concentration in the bat pancreas was determined at nine times during the year. Pancreatic ultrastructure was also examined. The pancreatic level of serotonin was uniform throughout the year except in late May, when it fell dramatically from a normal level of 3.14 ±.19 μgm/gm to 0.40 ± .05 μgm/gm. No seasonal changes in islet fine structure were found; however, the ultrastructure of exocrine cells was seen to be changed radically only in bats captured in late May. At this time, bats develop unusual cytoplasmic structures in the exocrine cells. These structures consist of membrane-limited bodies containing long, straight, and parallel paracrystalline arrays of stacked membranes and round or pleomorphic osmiophilic granules. Depletion of serotonin with reserpine or parachlorophenylalanine, or incubation of pancreatic tissue with different concentrations of serotonin, failed to provoke the formation of these structures in exocrine cells of active bats. Administration of 3H-5-hydroxytryptophan to bats, or incubation of bat pancreas with 3H-serotonin, failed to demonstrate a labeling of pancreatic exocrine cells. Incubation of the gland with 3H-serotonin, however, led to intense labeling of a subset of pancreatic terminal axons. It is concluded that the natural fall in the concentration of serotonin in late May is not the cause of the formation of the paracrystalline structures in pancreatic exocrine cells. The specific pancreatic uptake mechanism for serotonin found by Kovary et al. ('80) is probably a property of pancreatic neurites; these neurites might be serotonergic.