Central GABAergic innervation of the mammalian pineal gland: A light and electron microscopic immunocytochemical investigation in rodent and nonrodent species
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Light and electron microscopic immunocytochemical observations were made to demonstrate central pinealopetal fibers immunoreactive for gamma-aminobutyric acid (GABA) and synapses between their terminals and pinealocytes in the pineal gland of four rodent (Wistar-King rat; mouse; Syrian hamster, Mesocricetus auratus; Hartley strain guinea pig) and one nonrodent (tree shrew, Tupaia glis) species. GABA-immunoreactive myelinated and unmyelinated fibers and endings were found in the parenchyma of the pineal gland of all the animals examined. In the rodent species, GABAergic fibers were mainly found in the intermediate and proximal portions of the pineal gland and were nearly or entirely absent in the distal portion of the gland. Abundant GABAergic fibers were evenly distributed throughout the gland of the tree shrew. In all the animals, the habenular and posterior commissures contained abundant GABA-positive fibers, and some of them were followed to the pineal gland. GABA-positive endings made synaptic contact with pinealocytes, occasionally in mice and guinea pigs, and frequently in tree shrews; no synapses were observed in Syrian hamsters and rats. In the pineal gland of all the animals, GABA-immunoreactive cell bodies were not detected, and sympathetic fibers were not immunoreactive for GABA. These data indicate that GABAergic fibers are main pinealopetal projections from the brain. In view of the difference in the distribution of these fibers, central GABAergic innervation may play a more significant role in nonrodents than in rodents. The frequent occurrence of GABAergic synapses on pinealocytes in the tree shrew suggests that GABA released at these synapses directly controls activity of pinealocytes of this animal.
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