CALCITONIN-GENE-RELATED PEPTIDE EXPRESSION AT END-PLATES OF DIFFERENT FIBER TYPES IN MUSCLES IN RAT HIND LIMBS

Calcitonin gene-related peptide (CGRP) occurs only in some motoneurons. In this study, the presence of CGRP in motor endplates in relation to muscle fibre types was examined in slow (soleus muscle) and fast [tibialis anterior (TA) and extensor digitorum longus (EDL)] leg muscles of the rat. CGRP was detected by use of immunohistochemical methods, and staining for the mitochondrial-bound enzyme NADH-TR was used for demonstration of fibre types. The fibres showing low NADH-TR activity were interpreted as representing IIB fibres. All such fibres located in the superficial portion of TA were innervated by endplates displaying CGRP-like immunoreactivity (LI), whereas in the deep portion of TA some of these fibres lacked CGRP-LI at their endplates. Thirty per cent of the IIB fibres in EDL showed CGRP-LI at the endplates. All fibres in TA and EDL displaying high NADH-TR activity and interpreted as type-IIA fibres, lacked CGRP-LI in their motor innervation. One third of the fibres with intermediate NADH-TR activity in TA exhibited CGRP-LI at their endplates, whereas in EDL only few such fibres displayed CGRP-LI in the endplate formation. These fibres are likely to belong to type-IIX or type-I motor units. CGRP-LI was very rarely detected at the endplates in the soleus muscle. These observations show that distinct differences exist between the slow muscle, soleus, and the fast muscles, TA and EDL, but that there are also differences between the different types of fibres in TA and EDL with respect to presence of CGRP-LI at the endplates. As CGRP-LI was frequently detected at endplates of IIB fibres, it is likely that CGRP has a particular role related to the differentiation and maintenance of these fibres.