Coupling between serotoninergic and noradrenergic neurones and γ‐motoneurones in the cat

1 Noradrenaline is known to suppress transmission from group II muscle afferents when locally applied to γ‐motoneurones, and serotonin (5‐HT) facilitates the transmission. The purpose of this investigation was to search for evidence of monoaminergic innervation of γ‐motoneurones. 2 Eight γ‐motoneurones were labelled with rhodamine‐dextran, and 50 μm thick sagittal sections of the spinal cord containing them were exposed to antibodies against dopamine β‐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 γ‐motoneurones. The mean packing densities for 5‐HT (1.12 ± 0.11 appositions per 100 μm 2 for somata and 0.91 ± 0.07 per 100 μm 2 for dendrites) were similar to the densities of contacts reported for α‐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 γ‐motoneurones compared with the density in the immediate surround of the dendrites that there is indeed both a serotoninergic and noradrenergic innervation of γ‐motoneurones.