Modulation of Motoneuron Activity: Distribution of Glycine Receptors

Physiologically identified alpha- and gamma-motoneurons (MNs) were labeled by intracellular injection of neurobiotin (NB) or horseradish peroxidase (HRP). NB-labeled cells were revealed using 7-amine-4-methylcoumarin- 3-acetic acid and glycine receptors (Gly-Rs) on these cells were detected by immunofluorescence with an antibody against the 93 kDa Gly-R-associated protein, and fluorescein-labeled secondary antibodies. a-MN display Gly-Rs throughout their dendritic tree including dendrites in the white matter and in some dendritic spines. On soma and proximal dendrites, Gly-R labeling appeared in small clusters, whereas on distal dendrites the clusters were larger. In contrast, y-MNs had few Gly-R clusters, and they were usually found only on the distal dendrites. Standard HRP histochemistry combined with serotonin immunocytochemistry, using diaminobenzidine as chromogen, revealed that a-MNs receive contacts from l-2 x lo3 serotonergic boutons. The contacts were distributed over soma and dendrites but were more frequent within 400 pm of the soma. We conclude that both inhibitory and “gain setting” synaptic inputs are widely distributed on MN dendrites. Supported by NIH grant NS25547.