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Coupling between serotoninergic and noradrenergic neurones and -motoneurones in the cat

M. H. Gladden, D. J. Maxwell, A. Sahal and E. Jankowska*

1. Noradrenaline is known to suppress transmission from group II muscle afferents when locally applied to -motoneurones, and serotonin (5-HT) facilitates the transmission. The purpose of this investigation was to search for evidence of monoaminergic innervation of -motoneurones.

2. Eight -motoneurones were labelled with rhodamine-dextran, and 50 µm thick sagittal sections of the spinal cord containing them were exposed to antibodies against dopamine -hydroxylase (DBH) and 5-HT. All the cells were directly and/or indirectly excited by muscle group II afferents from the muscle they innervated and/or other muscles.

3. Appositions between monoaminergic fibres and the labelled somata and dendrites were located with three-colour confocal laser scanning microscopy by examining series of optical sections at 1 or 0·5 µm intervals.

4. DBH and 5-HT varicosities formed appositions with the somata and dendrites of all the -motoneurones. The mean packing densities for 5-HT (1·12 ± 0·11 appositions per 100 µm² for somata and 0·91 ± 0·07 per 100 µm² for dendrites) were similar to the densities of contacts reported for -motoneurones. Monoaminergic varicosities in apposition to dendrites greatly outnumbered those on the somata.

5. The density of DBH appositions was consistently lower - corresponding means were 53 % and 62 % of those for 5-HT on the somata and dendrites, respectively.

6. It is concluded from an analysis of the distribution and density of varicosities in apposition to the -motoneurones compared with the density in the immediate surround of the dendrites that there is indeed both a serotoninergic and noradrenergic innervation of -motoneurones.

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