The distribution of cytoplasmic and membrane-associated tropomyosinrelated kinase b (Trkb) receptor in the dendritic tree of adult spinal motoneurons

Although neurotrophins are conventionally associated with the proper growth and survival of developing neurons, there is increasing evidence that they play an equally significant role in the functions of adult neurons. Specifically, brain derived neurotrophic factor activation of its preferred receptor TrkB is essential in the regulation of motoneuronal activity. Neurotrophin-dependent and independent activation of TrkB regulates the motoneuronal dendritic integrity, and maintains unique classes of synapses. In addition, it regulates the expression and function of ion channels as well as the strength of inhibitory and excitatory synapses via different intracellular pathways. The recent physiological findings in the regulatory roles of TrkB are implicative of its presence on motoneuronal dendrites. Although, the expression of TrkB in the soma has long been confirmed, its distribution on the dendritic tree of motoneurons remains unknown. We aimed to examine the distribution of TrkB in the cytoplasm and membrane-associated regions of the dendritic tree of adult neck motoneurons.;We have determined, via confocal microscopy, that TrkB is present and abundant throughout the cytoplasm and the membrane-associated regions of motoneuronal dendrites as well as the soma. TrkB is organized in clusters and its distribution is best described as punctated. We then developed a technique to separately extract and quantify the TrkB immunoreactivity associated with the membrane and the cytoplasm as function of distance from the soma and dendritic tree. We have demonstrated that there is no bias in TrkB immunoreactivity to a specific region of the dendritic tree in five trapezius motoneurons. These observations were confirmed for both cytoplasmic and membrane-associated TrkB.;There is compelling evidence that both mature full-length and immature hypoglycosylated TrkB isoforms are active in strengthening the response to excitatory synapses in motoneurons. We identified the full length TrkB as well as 3 hypoglycosylated isoforms in cervical spinal segments that contain trapezius motoneurons and phrenic motoneurons.;Taken together, these data indicate that TrkB is likely involved in regulating and maintaining different classes of ion channels and synapses on the dendrites as well as the soma. Various isoforms of TrkB may also be involved in regulating motoneuronal activity.