Proceedings of The Physiological Society

Physiology 2014 (London, UK) (2014) Proc Physiol Soc 31, PCB062

Poster Communications

Mechanisms of hypereflexia following spinal cord injury: The role of the axon initial segment

A. Azam1, J. Wienecke2, L. Grøndahl1, C. F. Meehan1

1. Neuroscience & Pharmacology, Copenhagen University, Copenhagen, Denmark. 2. Department of Nutrition, Exercise and Sports, Copenhagen University, Copenhagen, Denmark.

When neurones below a spinal cord injury loose their descending input, over time, they become hyper-excitable, responding to remaining inputs (including sensory inputs) with an exaggerated response. This "hyper-reflexia" may cause a number of problems, in the case of motoneurones this could be spasticity.As action potentials are initiated in the axon initial segment (AIS), the excitability of a neurone can be partly determined by AIS structure, by its length and distance from the cell body (Kuba et al 2006). Recently, in the auditory system, it has been shown that AISs can respond homeostatically to changes in inputs (Kuba et al 2010). If similar changes occur in motoneurones below a spinal cord injury this could contribute to their exaggerated response to sensory inputs. To test this hypothesis, we performed a complete transection of the spinal cord at the sacral (S2) level, under isofluorane anaesthesia, in 13 adult rats (Wistar 150-200gm). Buprenorphine (Temgesic, 0.1mg/kg s.c.) was given (3/day, 48 hours). All rats developed a hyper-reflexia, measured by clinical testing of the tail, stretch rub manoeuvre, and seen as spasms of the tail. At either 6-7 weeks (6 rats) or 14-16 weeks (7 rats) post-transection, immunohistochemistry was used to label Ankyrin G as a marker of AISs and Choline Acetyltransferase as a marker of motoneurones. 3-dimensional images were captured using confocal microscopy (Zeiss LSM 700) and AISs were measured in 3-dimensions and compared to controls (6 rats).Kruskall Wallis tests revealed a significant effect on length (P<0.0001). Means (+SD): control, 19.37 μm (4.88) n=349 cells, 6-7 week post lesion, 22.31 μm (6.452) n=288 cells, 14-16 weeks post lesion, 23.28 μm (6.067) n=335 cells. Post hoc tests (Dunn's Multiple Comparisons) revealed this to be due to a significant increase in length in both lesion groups compared to controls (P<0.0001) and a further significant increase in length at 14-16 weeks post-lesion compared to 6-7 weeks (P<0.05). Kruskall Wallis tests also revealed a significant effect on the distance from the cell body (to start of AIS) (P<0.013). Means (+SD): control, 12.85 μm (6.671) n=138 cells, 6-7 week post lesion 10.6 μm (5.119) n=158 cells, 14-16 weeks post lesion, 11.09 μm (5.777) n=188 cells. Post hoc tests (Dunn's Multiple Comparisons) revealed this effect to be due to AISs being significantly closer to the cell body at 6-7 weeks post-injury (P<0.01). This distance did not change significantly between 6-7 and 14-16 weeks post-injury. Conclusions and implications: A complete transection of the spinal cord results in a compensatory increase in the length of AISs on spinal motoneurones and a proximal shift closer to the cell body. Both changes would contribute to an increased response of the neurone to sensory input which would contribute to a hyperflexia.

Where applicable, experiments conform with Society ethical requirements