The Distribution of Serotoninergic and Noradrenergic Synapses on the Dendritic Trees of Spinal Motoneurons

Abstract

The currents generated by excitatory and inhibitory synapses on motoneurons can be amplified by noradrenalin and serotonin. Both of these neurotransmitters act, and interact, via the same Gq-protein second-messenger system to modulate L-type Ca++, persistent-Na+, and leak K+ channels on motoneuron dendrites. However, noradrenergic and serotonergic synapses only modulate nearby excitatory and inhibitory synapses, so their relative distributions play a major role in the regulation of the overall output of the motoneuron. Moreover, the relative proximity between noradrenergic and serotonergic synapses may allow their individual effects to combine nonlinearly when co-activated, thereby regulating the magnitude of the amplification. The goal of the present study is to determine whether the distributions of noradrenergic and serotonergic synapses are biased along motoneuron dendritic trees.

The dendritic trees of five intracellularly stained feline splenius motoneurons were reconstructed. On them were plotted the locations of noradrenergic and serotonergic contacts, as determined by immunohistochemistry. The distribution of noradrenergic contacts was moderately biased both dorsally and distally in all five cells. Serotonergic contacts on the same neurons showed a moderate ventral bias. These findings suggest that excitatory and inhibitory inputs located dorsally and/or distally are preferentially amplified by noradrenergic synapses. Also, those synapses which are located ventrally are favorably amplified by serotonergic synapses. Both serotonergic and noradrenergic contacts are strongly biased towards innervation along small diameter (<2μm) dendrites.

The relative distributions between serotonergic and noradrenergic contacts have also been analyzed for all five cells. There was a bias towards minimizing the distance between like contacts (NE to NE and 5-HT to 5-HT). This increases the likelihood of interaction within populations when contacts are co-activated. Conversely, the distances between neighbouring noradrenergic and serotonergic contacts (NE to 5-HT and 5-HT to NE) were biased towards greater separation. This decreases the likelihood of interaction between populations when contacts are co-activated.

In summary, these findings suggest that noradrenalin and serotonin, having different location biases along the dendritic tree, will amplify some synapses in a biased manner. Additionally, like synapses may work in a coordinated manner with respect to their relative proximity. Coordination between noradrenergic and serotonergic synapses is less likely.