HYPOTHYROIDISM IN THE RAT RESULTS IN DECREASED SOLEUS MOTONEURON SOMASIZE

Adult female rats were thyroidectomized. After an average of 17 weeks, horseradish peroxidase (HRP) was injected into the right side soleus muscle. Two days later, left side soleus muscle properties were record ed and muscles and spinal cord were removed for further histological m easurements. Soleus muscles from hypothyroid rats no longer contained type IIA fibers. Contraction and half-relaxation times of twitches had increased significantly compared to control rats. The average cross-s ectional surface areas of HRP-labeled soleus motoneurones from hypothy roid rats were slightly but significantly smaller than those of contro l rats. A similar decrease in size was found for other (presumed moto- ) neurones lying ventrolaterally to the soleus motor nucleus. It is co ncluded that changes in the soleus muscle fiber composition, as caused by lowered levels of thyroid hormone, are paralleled by corresponding changes in the size oi its motoneurones and also of other spinal (mot o)neurones. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved .