The objective of the present study was to characterize the trkB receptor immunoreactive (-ir) cells in the intermediolateral cell column (IML) of the upper thoracic spinal cord. Small trkB-ir cells (area =56.1 +/− 4.4 µm 2 ) observed in the IML showed characteristics of oligodendrocytes and were frequently observed in close apposition to choline acetyltransferase (ChAT)-ir cell bodies. Large trkB-ir cells (area =209.3 +/− 25.2 µm 2 ) showed immunoreactivity for the neuronal marker NeuN, indicating their neuronal phenotype, as well as for ChAT, a marker for preganglionic neurons. TrkB and ChAT were co-localized in IML neurons primarily in cases that had received in vivo administration of nerve growth factor (NGF). These findings reveal two different cell types, oligodendrocytes and neurons, in the IML of the spinal cord that show trkB immunoreactivity, suggesting their regulation by BDNF and/or NT-4. In addition, there is evidence that NGF may play a role in the regulation of trkB-ir preganglionic neurons in the IML. Keywords preganglionic neurons; BDNF; NT-4; oligodendrocytes; sympatheticThe regulation of neurons and glial cells in the spinal cord is poorly characterized. TrkB expressing cells have been reported in the spinal cord, suggesting that they are regulated by either brain derived neurotrophic factor (BDNF) and/or neurotrophin -4 (NT-4). For example, in the lower intermediolateral cell column (IML; T7-T10), pre-ganglionic neurons have been shown to express trkB mRNA, are trkB immunoreactive (-ir) [17], and appear to be dependent on neurotrophin-4 (NT-4) derived from adrenal medullary chromaffin cells for survival [16]. However, retrograde transport of NT-4 from the target could not be demonstrated in these preganglionic neurons, and thus the exact mechanisms involved in the NT-4-mediated survival of the lower intermediolateral cell column are not known. Preganglionic neurons located in the IML of the upper thoracic cord may be regulated by brain-derived neurotrophic factor (BDNF) , though these phenomena have not been well documented.In addition to preganglionic neurons, trkB-ir glial cells have been reported in the motor areas of the lumbar spinal cord [19] and were shown to be altered by exercise [13]. The extent to which the trkB-ir glial cells were present in the upper thoracic cord has not been described.*Corresponding author, Lori G. Isaacson, Center for Neuroscience and Behavior, Department of Zoology, Miami University, 280 Pearson Hall, Oxford, OH 45056, Fax: 513-529-6900, isaacslg@muohio.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The go...