Glial reaction in the spinal cord of the degenerating muscle mouse (Scn8a (dmu)).

The glial reaction was investigated in the spinal cord of the degenerating muscle (dmu) mouse, which harbours a null mutation in the voltage-gated sodium channel gene Scn8a and does not produce functional Nav1.6 channel. Glial fibrillary acidic protein (GFAP)- and Iba1-immunoreactivity were detected in numerous cells throughout the spinal cord of wild type mice. These cells had small cell bodies and ramified processes. The dmu mutation increased the number of GFAP-immunoreactive (-IR) cells and the length of their processes in the ventral horn but not in the dorsal horn of the lumbar spinal cord. The number of Iba1-IR cells was similar in cervical and lumbar spinal cords of wild type and dmu mice. However, Iba1-IR processes and their branches became thinner and showed a fine varinose appearance in dmu mice. The length of Iba1-IR processes was significantly reduced in dorsal and ventral horns of dmu mice. Double immunofluorescence also demonstrated the relationship between glial cells and motor neurons containing calcitonin gene-related peptide (CGRP), a marker for their degeneration. The dmu mutation caused increase in the length of GFAP-IR processes surrounding CGRP-IR motor neurons in the ventral horn. However, the thickness and length of Iba1-IR processes around CGRP-IR motor neurons were reduced by the mutation. The present study suggests that the dmu mutation causes astrocytic activation and microglial inactivation in the spinal cord. These changes may be associated with degeneration and activity of motor and sensory neuron in dmu mice.

Increase of CGRP expression in motor endplates within fore and hind limb muscles of the degenerating muscle mouse (Scn8a(dmu)).

The distribution of calcitonin gene-related peptide (CGRP) was examined in skeletal muscles of fore and hind limb as well as in oral and cranio-facial regions of the degenerating muscle (dmu) mouse, which harbours a null mutation in the voltage-gated sodium channel gene Scn8a. In limb, oral and cranio-facial muscles of wild type mice, only a few motor endplates contained CGRP-immunoreactivity. However, many CGRP-immunoreactive motor endplates appeared in the triceps brachii muscle, the biceps brachii muscle, the brachialis muscle, and the gastrocnemius muscle of dmu mice. CGRP-immunoreactive density of motor endplates in the skeletal muscles was also elevated by the mutation. In these muscles, the atrophy of muscle fibers could be detected and the density of cell nuclei in the musculature increased. In the flexor digitorum profundus muscle, the flexor digitorum superficialis muscle, and the soleus muscle as well as in oral and craniofacial muscles, however, the distribution of CGRP-immunoreactivity was barely affected by the mutation. The morphology of muscle fibers and the distribution of cell nuclei within them were also similar in wild type and dmu mice. In the lumbar spinal cord of dmu mice, CGRP-immunoreactive density of spinal motoneurons increased. These findings suggest that the atrophic degeneration in some fore and hind limb muscles of dmu mice may increase CGRP expression in their motoneurons.

Increase of c-Fos and c-Jun expression in spinal and cranial motoneurons of the degenerating muscle mouse (Scn8a(dmu)).

The degenerating muscle (dmu) mouse harbors a loss-of-function mutation in the Scn8a gene, which encodes the alpha subunit of the voltage-gated sodium channel (VGSC) Na(V)1.6. The distribution of c-Fos and c-Jun was examined in spinal and cranial motoneurons of the dmu mouse. In the cervical spinal cord, trigeminal motor nucleus (Vm), facial nucleus (VII), dorsal motor nucleus of the vagus (X), and hypoglossal nucleus (XII) of wild-type mice, motoneurons expressed c-Fos and c-Jun-immunoreactivity. The immunoreactivity in wild-type mice was mostly weak and localized to the nucleus of these neurons whereas in the spinal cord and brain stem of dmu mice motoneurons showed intense c-Fos and c-Jun-immunoreactivity. The number of c-Fos-immunoreactive motoneurons was dramatically elevated in the cervical spinal cord (wild type, 4.8 +/- 1.0; dmu, 17.3 +/- 1.6), Vm (wild type, 76.2 +/- 21.6; dmu, 216.9 +/- 30.9), VII (wild type, 162.4 +/- 43.3; dmu, 533.3 +/- 41.2), and XII (wild type, 58.2 +/- 43.3; dmu, 150.9 +/- 25.7). The mutation also increased the number of c-Jun-immunoreactive motoneurons in the cervical spinal cord (wild type, 1.6 +/- 0.8; dmu, 12.1 +/- 2.1), Vm (wild type, 41.4 +/- 18.0; dmu, 123.1 +/- 11.7), and X (wild type, 39.1 +/- 10.7; dmu, 92.8 +/- 17.8). The increase of these transcription factors may be associated with the uncoordinated and excessive movement of forelimbs and degeneration of cardiac muscles in dmu mice.