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Serotonin induces central fatigue by inhibiting action potential initiation in motoneurons.

Florence Cotel1, 2*, Richard Exley3, Stephanie Cragg3 and Jean-François M. Perrier2
  • 1 Queensland Brain Institute, Australia
  • 2 University of Copenhagen, Department of Neuroscience and Pharmacology, Denmark
  • 3 University of Oxford, Department of Physiology, Anatomy and Genetics, United Kingdom

Neuronal output classically refers to the creation of action potentials at the axon initial segment that propagate along the axon to induce neurotransmitter release at the terminals, and, thereby, mediate a communication with postsynaptic partners. Hence information transfer critically relies on the initiation of spikes. In a slice preparation from the spinal cord of the adult turtle, we found that activation of serotonergic receptors 5-HT1A (5-HT1AR), specifically located at the axon initial segment (AIS), dampened spike initiation in motoneurons by inhibiting the Na+ channels responsible for action potential genesis [1-3]. This induced an increase of the action potential threshold and in some instances a failure of its propagation. We then tested if synaptic release of serotonin (5-HT) could activate the mechanism we uncovered. Immunohistochemical staining indicated that the somatodendritic membrane of motoneurons is densely innervated by serotonergic synaptic boutons. By contrast, the AIS is devoid of 5-HT terminals, suggesting that extrasynaptic 5-HT1AR in this compartment are activated by spillover of 5-HT. Indeed prolonged stimulations of the serotonergic raphe-spinal pathway increased extracellular 5-HT in the vicinity of the AIS to a concentration sufficient to activate 5-HT1AR and decreased the excitability of motoneurons. Raphe neurons activity level correlates with motor exercise. We showed that increased release of serotonin, as what occurs during intense motor activity, downregulated motoneuron excitability. This represents the first cellular mechanism for central fatigue and confirms many indirect evidence that serotonin is involved in the phenomenon. In conclusion the experimental evidence of a serotonergic modulation of action potential genesis at the AIS of motoneurons, that mediates central fatigue, highlights the functional importance of the regulation of spike initiation.

Acknowledgements

The project was funded by Danish Medical Research council Grant 09-123456, Lundbeckfonden, Owensenske Fond, Agnes and Poul Friis Fond, Association Française contre les Myopathies and Antidoping Danmark. The participation to the workshop was supported by a CNS 2015 travel award.

References

[1] F. Cotel, R. Exley, S.J. Cragg, and J.F. Perrier, Serotonin spillover onto the axon initial segment of motoneurons induces central fatigue by inhibiting action potential initiation. Proc Natl Acad Sci U S A 110 (2013) 4774-9.
[2] J.F. Perrier, and F. Cotel, Serotonin differentially modulates the intrinsic properties of spinal motoneurons from the adult turtle. J Physiol 586 (2008) 1233-8.
[3] J.F. Perrier, and F. Cotel, Serotonergic modulation of spinal motor control. Curr Opin Neurobiol 33C (2014) 1-7.

Keywords: spike initiation, motoneurons, central fatigue, Serotonin, spillover

Conference: Spike Initiation: Models & Experiments, Prague, Czechia, 22 Jul - 22 Jul, 2015.

Presentation Type: Oral Presentation

Topic: Neuroscience

Citation: Cotel F, Exley R, Cragg S and Perrier JM (2015). Serotonin induces central fatigue by inhibiting action potential initiation in motoneurons.. Front. Neurosci. Conference Abstract: Spike Initiation: Models & Experiments. doi: 10.3389/conf.fnins.2015.90.00004

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Received: 06 Jul 2015; Published Online: 06 Jul 2015.

* Correspondence: Dr. Florence Cotel, Queensland Brain Institute, St Lucia, Queensland, 4072, Australia, f.cotel@uq.edu.au