Calcitonin gene-related peptide-like immunoreactivity in motoneuron pools innervating different hind limb muscles in the rat

Abstract

The content of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) in motoneurons was studied in four motor pools supplying muscles in the rat hind limb subserving different types of motor activity. The motor pools were identified by retrograde labeling with horseradish peroxidase or fluorophore-conjugated dextran amines, which were injected into the soleus, tibialis anterior, lateral gastrocnemius, or abductor digiti minimi muscles. After processing for immunohistochemistry, a semiquantitative evaluation was carried out to estimate the proportion of strongly, intermediately, and weakly labeled motoneurons, as well as motoneurons totally lacking CGRP staining. This revealed a considerable diversity in the intensity of CGRP labeling even for motoneurons in the same motoneuron pool. Thus, strongly labeled cells, as well as cells devoid of CGRP label, were found in all four motoneuron pools. However, a difference was found in the distribution of motoneurons innervating muscles with a dominant composition of fast and slow motor units, respectively, in that a larger fraction of the latter type lacked CGRP-LI. Moreover, generally motoneurons in the small motor units of the abductor digiti minimi muscle displayed weaker staining, and a larger proportion of cells was totally devoid of CGRP-LI (16%) compared with larger motor units of the other three muscles (1-10%). Small-sized cells within the gamma-motoneuron size range were weakly stained or, more frequently, totally devoid of CGRP label (50%) as compared to larger cells, presumably representing alpha-motoneurons (1-16%). Five days after axotomy all four studied motoneuron pools displayed stronger CGRP labeling than corresponding unlesioned pools. However, a considerable variation in CGRP labeling persisted also among axotomized motoneurons. These results indicate that motoneurons normally display a great variation in CGRP-LI levels, but that motoneurons of small and slow-twitch motor units in general have lower levels than motoneurons of large and fast-twitch motor units, respectively. After axotomy, CGRP-LI increases in lesioned motoneuron pools compared with normal, but in a fraction of the axotomized motoneurons the increase seems to be discrete or even absent. The possible physiological implications of these findings are discussed.