Distribution and phenotype of TrkB oligodendrocyte lineage cells in the adult rat spinal cord

• A majority of mature oligodendrocyte (OLs) in spinal cord expressed TrkB.
• Few oligodendrocyte precursor cells (OPCs) expressed detectable levels of TrkB.
• TrkB cells resided in gray and white matter of the spinal cord in similar abundance.
• A population of TrkB nonneuronal cells did not express markers for OLs or OPCs.
• TrkB ‘perineuronal’ cells nestling with neuronal cell bodies typically were mature OLs.

The distribution and phenotype of a previously undescribed population of nonneuronal cells in the intact spinal cord that expresses TrkB, the cognate receptor for brain derived neurotrophic factor (BDNF) and neurotrophin 4 (NT-4), were characterized by examining the extent of co-localization of TrkB with NG2, which identifies oligodendrocyte progenitors (OPCs) or CC1, a marker for mature oligodendrocytes (OLs). All TrkB nonneuronal cells expressed Olig2, confirming their role in the OL lineage. Similar to OPCs and OLs, TrkB cells resided in gray and white matter of the spinal cord in similar abundance. Less than 2% of TrkB cells expressed NG2, while over 80% of TrkB cells in the adult spinal cord co-expressed CC1. Most OPCs did not express detectable levels of TrkB, however a small OPC pool (~5%) showed TrkB immunoreactivity. The majority of mature OLs (~65%) expressed TrkB, but a population of mature OLs (~36%) did not express TrkB at detectable levels, and 17% of TrkB nonneuronal cells did not express NG2 or CC1. Approximately 20% of the TrkB nonneuronal population in the ventral horn resided in close proximity to motor neurons and were categorized as perineuronal. We conclude that TrkB is expressed by several pools of OL lineage cells in the adult spinal cord. These findings are important in understanding the neurotrophin regulation of OL lineage cells in the adult spinal cord.