The Distribution of Serotonergic, Noradrenergic and Dopaminergic Synapses on Flexor Motoneurons
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Serotonin (5-HT) and noradrenalin (NA) increase persistent inward currents mediated by sodium and calcium channels on the dendrites of motoneurons. The ability of 5-HT and NA to modulate these channels depends on the distributions of 5-HT and NA synapses. Recent studies of the distribution of 5-HT and NA synapses on motoneurons innervating the neck muscle splenius reported that these synapses are rare on the somata and have a strong bias to dendrites with small diameters. It is unknown whether this distribution pattern represents a general principle of organization (1) for all motoneuron groups or (2) for all types of modulators. To address the first question, we have examined the distribution of 5-HT and NA synapses on flexor motoneurons, which unlike extensor motoneurons, are not able to generate self-sustained discharges known to involve the activation of persistent inward currents. To answer the second question, we have mapped the distribution dopamine (DA) synapses. The dendrites of motoneurons that innervate the neck flexor rectus capitis anterior (RCA) were stained. Synapses containing 5-HT, NA and DA were identified using immunohistochemical techniques. Observations based on five RCA motoneurons indicate that the average densities of 5-HT and NA contacts are 2.3 and 1.4 times less dense than the average densities of 5-HT and NA contacts on splenius motoneurons, respectively. Moreover, pairs of 5-HT contacts and pairs of NA contacts were found to be 3.0 and 1.8 times closer together on splenius compared to RCA motoneurons, respectively. These observations may reflect the inability of flexor motoneurons to generate self sustained discharges. Similar to splenius motoneurons, 5-HT and NA synapses were found to preferentially innervate dendrites with diameters less than 2 µm. Thus, 5-HT and NA synapses facilitate channels in regions where excitatory or inhibitory signals undergo the largest attenuations. DA synapses on the dendritic tree were sparse (0.2 and 0.1 contacts per 1000 µm2), suggesting that the actions of DA synapses are confined to local regions on the dendritic tree. These results highlight that motoneurons do not all share the same intrinsic properties, and the distribution of modulatory synapses have a crucial role in determining these properties.