CALCITONIN-GENE-RELATED PEPTIDE-LIKE IMMUNOREACTIVITY IN MOTONEURON POOLS INNERVATING DIFFERENT HIND-LIMB MUSCLES IN THE RAT

The content of calcitonin gene-related peptide-like immunoreactivity ( CGRP-LI) in motoneurons was studied in four motor pools supplying musc les in the rat hind limb subserving different types of motor activity. The motor pools were identified by retrograde labeling with horseradi sh peroxidase or fluorophore-conjugated dextran amines, which were inj ected into the soleus, tibialis anterior, lateral gastrocnemius, or ab ductor digiti minimi muscles. After processing for immunohistochemistr y, a semiquantitative evaluation was carried out to estimate the propo rtion of strongly, intermediately, and weakly labeled motoneurons, as well as motoneurons totally lacking CGRP staining. This revealed a con siderable diversity in the intensity of CGRP labeling even for motoneu rons in the same motoneuron pool. Thus, strongly labeled cells, as wel l as cells devoid of CGRP label, were found in all four motoneuron poo ls. However, a difference was found in the distribution of motoneurons innervating muscles with a dominant composition of fast and slow moto r units, respectively, in that a larger fraction of the latter type la cked 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%) compar ed with larger motor units of the other three muscles (1-10%). Small-s ized 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 strong er CGRP labeling than corresponding unlesioned pools. However, a consi derable variation in CGRP labeling persisted also among axotomized mot oneurons. These results indicate that motoneurons normally display a g reat variation in CGRP-LI levels, but that motoneurons of small and sl ow-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 dis crete or even absent. The possible physiological implications of these findings are discussed.