Distribution of glutamic acid decarboxylase mRNA-containing neurons in ratmedulla projecting to thoracic spinal cord in relation to monoaminergic brainstem neurons

Recent neurophysiological work has suggested the existence of monosynaptic gamma-aminobutyric acidergic (GABAergic) projections from the medulla oblon gata to sympathetic preganglionic neurons. The purpose of the present study was to identify the possible anatomical location of these neurons. The loc ation of GABAergic neurons with projection to the thoracic spinal cord was studied by using in situ hybridization for both 65-kD and 67-kD isoforms of glutamic acid decarboxylase (GAD) mRNA (GAD-65 and GAD-67, respectively) c ombined with midthoracic spinal cord injections of the tracer Fast Blue. Ty rosine hydroxylase (TH) or tryptophan hydroxylase immunohistochemistry was combined with GAD mRNA detection and Fast Blue to determine whether any bul bospinal catecholaminergic or serotonergic cell groups in the medulla also are GABAergic. GAD-67 and GAD-65 mRNA-containing neurons had similar distri bution patterns in the medulla oblongata, with some areas exhibiting lighte r labeling for GAD-65 mRNA. GABAergic bulbospinal neurons were located in t he caudal part of the solitary nucleus, the parasolitary nucleus, the vesti bular nuclei, the ventral medial medulla, the raphe nuclei, and parapyramid al areas. TH-immunoreactive neurons in the A(1), A(2), C-1, and C-2 areas o r the area postrema did not contain either GAD-67 or GAD-65 mRNA. GAD mRNA- positive bulbospinal cells were present medial to the A(1) and C-1 catechol aminergic cell groups, with little or no overlap. Serotonergic neurons posi tive for GAD mRNA were found in the parapyramidal area and just dorsal to t he pyramidal tract in the raphe magnus. This population included bulbospina l neurons. In conclusion, GABAergic neurons with projections to the thoraci c spinal cord exist in a restricted number of medullary nuclei from which i nhibitory sympathetic control may originate. J. Comp. Neurol. 407:367-380, 1999. (C) 1999 Wiley-Liss, Inc.