Spiny interneurons identified in the normal mouse spinal cord show alterations in the Wobbler mouse: a model for inherited motoneuron disease.

Presumed interneurons are described in the Golgi-impregnated cervical spinal cord taken from normal phenotype and motoneuron-diseased mice of the Wobbler mouse strain (NFR/wr), as well as from the spinal cord of two related normal mouse strains (C57B1/6N and NFR/N). The interneurons, distributed throughout Rexed's laminae V-VIII, are characterized by numerous spines clustered along the distal dendrites. Quantitatively, the soma size (μm2) of the interneurons in the Wobbler specimens studied late in the motoneuron disease is smaller than that measured in the pair-matched (3-week-old) normal phenotype littermates. Early in the disease, the spine density (number of spines per 100 μm length dendrite) is greater compared with the normal phenotype littermates, perhaps implying that sprouting may occur. At a later stage in the disease process, the spine density does not differ significantly. However the increase in the spine density expected with advancing age is not observed for the Wobbler interneurons. It is proposed that perhaps the normal age-related proliferation of spines is impaired in the Wobbler mice. Since the measurements for spine length are lower in the Wobbler interneurons studied late in the motoneuron disease compared with the pair-matched (3-month-old) normal phenotype littermates, the normal age-related lengthening of the spines seems to be lacking. In addition, the spine length measured in the normal phenotype littermates is significantly greater compared with the normal mice (NFR/N, C57B1/6N). Thus the growth patterns of the spines may differ in the Wobbler mouse strain compared with the normal (C57B1/6N, NFR/N) mouse strains. It is proposed that the Wobbler motoneuron disease affects interneurons as well as motoneurons.