Distribution of glutamic acid decarboxylase mRNA-containing neurons in rat medulla 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 oblongata to sympathetic preganglionic neurons. The purpose of the present study was to identify the possible anatomical location of these neurons. The location 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) combined with midthoracic spinal cord injections of the tracer Fast Blue. Tyrosine hydroxylase (TH) or tryptophan hydroxylase immunohistochemistry was combined with GAD mRNA detection and Fast Blue to determine whether any bulbospinal catecholaminergic or serotonergic cell groups in the medulla also are GABAergic. GAD-67 and GAD-65 mRNA-containing neurons had similar distribution patterns in the medulla oblongata, with some areas exhibiting lighter labeling for GAD-65 mRNA. GABAergic bulbospinal neurons were located in the caudal part of the solitary nucleus, the parasolitary nucleus, the vestibular nuclei, the ventral medial medulla, the raphe nuclei, and parapyramidal areas. TH-immunoreactive neurons in the A1, A2, C1, and C2 areas or the area postrema did not contain either GAD-67 or GAD-65 mRNA. GAD mRNA-positive bulbospinal cells were present medial to theA1 and C1 catecholaminergic cell groups, with little or no overlap. Serotonergic neurons positive for GAD mRNAwere found in the parapyramidal area and just dorsal to the pyramidal tract in the raphe magnus. This population included bulbospinal neurons. In conclusion, GABAergic neurons with projections to the thoracic spinal cord exist in a restricted number of medullary nuclei from which inhibitory sympathetic control may originate.