Central GABAergic innervation of the mammalian pineal gland: A light and electron microscopic immunocytochemical investigation in rodent and nonrodent species

Light and electron microscopic immunocytochemical observations were made to demonstrate central pinealopetal fibers immunoreactive for gamma -aminobut yric acid (GABA) and synapses between their terminals and pinealocytes in t he pineal gland of four rodent (Wistar-King rat; mouse; Syrian hamster, Mes ocricetus auratus; Hartley strain guinea pig) and one nonrodent (tree shrew , Tupaia glis) species. GABA-immunoreactive myelinated and unmyelinated fib ers and endings were found in the parenchyma of the pineal gland of all the animals examined. In the rodent species, GABAergic fibers were mainly foun d in the intermediate and proximal portions of the pineal gland and were ne arly or entirely absent in the distal portion of the gland. Abundant GABAer gic fibers were evenly distributed throughout the gland of the tree shrew. In all the animals, the habenular and posterior commissures contained abund ant GABA-positive fibers, and some of them were followed to the pineal glan d. GABA-positive endings made synaptic contact with pinealocytes, occasiona lly in mice and guinea pigs, and frequently in tree shrews; no synapses wer e observed in Syrian hamsters and rats. In the pineal gland of all the anim als, GABA-immunoreactive cell bodies were not detected, and sympathetic fib ers were not immunoreactive for GABA. These data indicate that GABAergic fi bers are main pinealopetal projections from the brain. Ln view of the diffe rence in the distribution of these fibers, central GABAergic innervation ma y 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 pinealo cytes of this animal. J. Comp. Neurol. 430:72-84, 2001. (C) 2001 Wiley-Liss , Inc.