Elsevier

Neuroscience

Volume 134, Issue 3, 2005, Pages 889-899
Neuroscience

Molecular neuroscience
Early and transient increase in spontaneous synaptic inputs to the rat facial motoneurons after axotomy in isolated brainstem slices of rats

https://doi.org/10.1016/j.neuroscience.2005.05.002Get rights and content

Abstract

Section of motor nerve fibers (axotomy) elicits a variety of morphofunctional responses in the motoneurons in the motor nuclei. Later than the fifth post-operational day after section of the facial nerve, synapse elimination occurs in the facial motoneuron pool, leading to gradual abolishment of synaptic input-driven activities of the axotomized motoneurons. However, it remains unknown how the amount of synaptic input changes during this period between the axotomy and the synaptic elimination. Here we examined a hypothesis that axotomy of the motoneurons itself modifies the synaptic inputs to the motoneurons. One day after axotomy, the postsynaptic currents, mostly mediated by non-N-methyl-D-aspartic acid (non-NMDA) receptors, recorded from the axotomized facial motoneurons in the acute slice preparations of the rats were of higher frequency and larger amplitude than those in the intact motoneurons. This difference was not observed after the third post-operational day and appeared earlier than the changes in the electrophysiological properties and increase in the number of dead neurons in the axotomized motor nucleus. The larger postsynaptic current frequency of the axotomized motoneurons was observed both in the absence and in the presence of tetrodotoxin citrate, suggesting that increased excitability and facilitated release underlie the postsynaptic current frequency increase. These results suggest that synaptic re-organization occurs in the synapses of motoneurons at an early stage following axotomy.

Section snippets

Unilateral facial nerve axotomy

Our manipulation of animals conformed to the Guiding Principles for the Care and Use of Animals in the Field of Physiological Sciences of the Physiological Society of Japan (1988) and International guiding principles for biomedical research involving animals by the Council for International Organizations of Medical Sciences (1984). Experimental protocol was approved by the Animal Experiments Committee of the Jikei University School of Medicine. All possible efforts were made to minimize the

Identification of facial motoneurons after axotomy in the brain slice

We made unilateral axotomy in 50 rats (P1–P6, n=17; P7–P14, n=33). The brainstem slices were prepared on D1 (68 slices from 36 rats; P1–P6, n=10; P7–P14, n=26), D3 (10 slices from six rats; P1–P6, n=2; P7–P14, n=4), D5 (eight slices from five rats; P1–P6, n=3; P7–P14, n=2) and D7 (three slices from three rats; P1–P6, n=2; P7–P14, n=1). We first confirmed that surviving motoneurons after axotomy could be successfully visualized with IR-DIC optics and were healthy enough that they could undergo

Discussion

The axonal injury of the motoneurons leads to a variety of post-injury morphofunctional events. This is the first study that has analyzed the changes in the synaptic inputs of the motoneurons following section of their axons. The main novel finding of this study is that the amount of excitatory synaptic inputs to the surviving axotomized motoneurons is increased to ∼three-fold of that of the non-axotomized motoneurons at the first day following axotomy, whereas after the fifth day post-injury,

Conclusion

Axonal injury of the motoneurons is likely to result in the synaptic re-organization of the premotor-to-motoneuron synapses. This re-organization might modify the fate of the axotomized motoneurons through modifying excitability of these surviving motoneurons.

Acknowledgments

This work was supported in part by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Nos. 13680902, 15650071, 17023042, 17650116 and 17300123) and Grants for the Research on Health Sciences focusing on Drug Innovation from the Japan Health Sciences Foundation (KH21014) to F.K. The authors thank Dr. Frank Scott Howell (the Sophia University) for scientific and linguistic comments. The expert assistance in the experiments and during preparation of the

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