ALPHA-CGRP MESSENGER-RNA LEVELS IN MOTONEURONS INNERVATING SPECIFIC RAT MUSCLES

Along with acetylcholine, motoneurons express several neuromodulatory peptides. The most extensively studied of these peptides is calcitonin gene-related peptide (CGRP). CGRP modulates the biochemical, physiolo gical and metabolic properties of skeletal muscle primarily through ac tivation of membrane receptors. Virtually all motor pool contain moton eurons that are immunoreactive for CGRP. The purpose of this study was to determine the proportions of motoneurons that express alpha-CGRP i n motor pools innervating muscles with different motor unit compositio ns. These include the soleus, extensor digitorum longus, tensor fascia latae and the diaphragm muscles as well as the spinal nucleus of the bulbocavernosus. The spinal nucleus of the bulbocavernosus provides in nervation to the bulbocavernosus/levator ani muscle complex and the ex ternal anal sphincter muscle. The spinal nucleus of the bulbocavernosu s contained the greatest proportion of alpha-CGRP mRNA-positive motone urons, followed in descending rank order by the tensor fascia latae, t he extensor digitorum longus, the soleus and the diaphragm motor pools . In addition, significant differences between motor pools were observ ed in the mean relative alpha-CGRP mRNA level among those motoneurons expressing alpha-CGRP. The highest mean relative alpha-CGRP mRNA level was observed in soleus and the extensor digitorum longus motor pools; followed in descending rank order by the tensor fascia latae, the dia phragm and the spinal nucleus of the bulbocavernosus. We have previous ly shown that muscle contractile inactivity increases the number of mo toneurons that express alpha-CGRP and in the relative mRNA levels. The results of the present study suggest that the proportion of motoneuro ns within a motor pool that express alpha-CGRP may be closely related to the contractile activity (i.e. activation history) of the target mu scle.