Distribution and density of contacts from noradrenergic and serotonergic boutons on the dendrites of neck flexor motoneurons in the adult cat

ABSTRACT Serotonergic (5‐HT) and noradrenergic (NA) input to spinal motoneurons is essential for generating plateau potentials and self‐sustained discharges. Extensor motoneurons are densely innervated by 5‐HT and NA synapses and have robust plateau potentials and self‐sustained discharges. Conversely, plateau potentials and self‐sustained discharges are very rare in flexor motoneurons. The most likely reasons for this difference are that flexor motoneurons have few 5‐HT and NA synapses and/or they are distributed distant to the channels responsible for plateau potentials and self‐sustained discharges. However, the distribution of 5‐HT and NA synapses on flexor motoneurons is unknown. Here we describe the distribution and density of 5‐HT and NA synapses on motoneurons that innervate the flexor neck muscle, rectus capitis anterior (RCA), in the adult cat. Using a combination of intracellular staining, fluorescent immunohistochemistry, and 3D reconstruction techniques, we found that 5‐HT and NA synapses are widely distributed throughout the dendritic trees of RCA motoneurons, albeit with a strong bias to small‐diameter dendrites and to medial dendrites in the case of NA contacts. The number of 5‐HT and NA contacts per motoneuron ranged, respectively, from 381 to 1,430 and from 642 to 1,382, which is 2.3‐ and 1.4‐fold less than neck extensor motoneurons (Montague et al., J Comp Neurol 2013;521:638–656). These results suggest that 5‐HT and NA synapses on flexor motoneurons may provide a powerful means of amplifying synaptic currents without incurring plateau potentials or self‐sustained discharges. This feature is well suited to meet the biomechanical demands imposed on flexor muscles during different motor tasks. J. Comp. Neurol. 523:1701–1716, 2015. © 2015 Wiley Periodicals, Inc. Quantitative maps of the locations of serotonergic and noradrenergic synapses on the dendrites of flexor motoneurons reveal that the distributions of monoamine synapses on flexor and extensor motoneurons are similar. However, the density of synapses is 45–65% less, suggesting that monoamine modulation of flexors motoneurons is equally specific, but weaker.