Zhan, When Zhi

Spinal cord hemisection at C2 (SH) disrupts descending drive to ipsilateral phrenic motoneurons (PMNs) causing unilateral diaphragm muscle (DIAm) paralysis. Previously, we found a decrease in motoneuron size at 2 weeks post‐SH. Recently, we found neuregulin (NRG) in synaptic boutons surrounding PMNs. NRG is a trophic factor known to regulate development of the neuromuscular junction, but the effect of NRG on motoneurons is unknown. We hypothesized that 1) NRG input to PMNs is supraspinal in origin, and 2) SH disrupts NRG input to PMNs. Adult male Sprague‐Dawley rats underwent SH or sham‐surgery. PMNs were retrogradely labeled by intramuscular injection of Alexa488‐conjugated cholera toxin B. Using laser capture microdissection, we found that PMNs express mRNA for the NRG receptors ErbB2 and ErbB3, providing a functional substrate for a NRG effect. Two weeks post‐SH, labeled PMNs and NRG boutons were imaged using confocal microscopy. PMN morphology and the number of NRG boutons in the vicinity of PMNs were determined using a computerized, semi‐automated reconstruction algorithm. NRG boutons were fairly homogeneously distributed on motoneurons independent of size. Following SH, few NRG boutons were observed. We conclude that 1) NRG input to PMNs derives from a supraspinal source; and 2) SH removes NRG input to PMNs. It is possible that NRG input exerts a trophic influence on PMNs. Supported by NIH grants HL37680, AR51173 and Mayo Clinic

Neuregulin (NRG) is a nerve derived trophic factor that regulates acetylcholine receptor expression during neuromuscular junction development. NRG also has important trophic effects in the adult central nervous system. We hypothesized that NRG is a trophic factor for phrenic motoneurons. In adult male Sprague‐Dawley rats, we examined NRG‐1 immunoreactivity in the vicinity of phrenic motoneurons. Retrogradely‐labeled phrenic motoneurons were imaged and reconstructed in 3D. NRG‐1 boutons were found mainly around the soma and primary dendrites. The co‐localization of NRG boutons and synaptic markers was determined using multicolor confocal microscopy and a custom‐developed application (Metamorph; Universal Imaging). We found that 94 ± 3% of NRG‐containing boutons resided on synapses, as determined by co‐localization with synaptophysin. In addition, 6 ± 3% of synapses on phrenic motoneurons (within 1 micrometer of soma and primary dendrite surface) expressed NRG. Overall, 13 ± 5 NRG boutons were present at the soma vs. 6 ± 3 at each primary dendrite of a motoneuron.Thus, 57% of NRG‐containing boutons were surrounding the motoneuron soma. In summary, NRG‐1 synaptic boutons are present at phrenic motoneurons in the adult, although its role is presently unknown. Supported by NIH grant AR51173.