Distribution of cholinergic contacts on Renshaw cells in the rat spinal cord: a light microscopic study

1. Cholinergic terminals in the rat spinal cord were revealed by immunohist ochemical detection of the vesicular acetycholine transporter (VAChT). In o rder to determine the relationships of these terminals to Renshaw cells, we used dual immunolabelling with antibodies against gephyrin or calbindin D2 8k to provide immunohistochemical identification of Renshaw cells in lamina . VII of the ventral horn. 2. A total of 50 Renshaw cells were analysed quantitatively using a compute r-aided reconstruction system to provide accurate localization of contact s ites and determination of somatic and dendritic surface area. Dendrites cou ld be traced for up to 413 mu m from the soma in calbindin D28k-identified Renshaw cells and up to 184 mu m in gephyrin-identified cells. 3. A total of 3330 cholinergic terminals were observed on 50 Renshaw cells, with a range of 21-138 terminal appositions per cell (mean 66.6 +/- 25.56 contacts per cell). The vast majority (83.5%) of the terminals were apposed to dendrites rather than the soma. The overall density of cholinergic cont acts increased from a little above 1 per 100 mu m(2) on the soma and initia l 25 mu m of proximal dendrites to 4-5 per 100 mu m(2) on the surface of de ndritic segments located 50-250 mu m from the soma. Single presynaptic fibr es frequently formed multiple contacts with the soma and/or dendrites of in dividual Renshaw cells. 4. VAChT-immunoreactive terminals apposed to Renshaw cells varied in size f rom 0.6 to 6.9 mu m in diameter (mean 2.26 +/- 0 94; n. = 986) and were on average smaller than the cholinergic C-terminals apposed to motoneurones, b ut larger than VAChT-immunoreactive terminals contacting other ventral horn interneurones. 5. The high density and relatively large size of many cholinergic terminals on Renshaw cells presumably correlates with the strong synaptic connection between motoneurones and Renshaw cella. The fact, that the majority of con tacts are distributed over the dendrites makes the motoneurone axon collate ral input susceptible to inhibition by the prominent glycinergic inhibitory synapses located on the soma and proximal dendrites. The relative position s and structural features of the excitatory cholinergic and inhibitory glyc inergic synapses may explain why Renshaw cells, although capable of firing at very high frequency following motor axon stimulation, appear to fire at relatively low rates during locomotor activity.