The effect of a peripheral nerve lesion on calbindin D28k immunoreactivity in the cervical ventral horn of developing and adult rats.

ABSTRACT

Expression of calbindin D28k (CB) immunoreactivity by putative Renshaw cells is substantially downregulated by sciatic motoneuron axotomy in the adult rat. The present study investigated the effect of median and ulnar nerve lesion at different ages on ventral horn CB immunoreactivity 7 days after the injury to see whether similar results were obtained in the cervical cord and during development. Two major differences were observed. First, axotomy induced CB immunoreactivity in some motoneurons, confirmed by retrograde labeling of the injured neurons with fast blue (FB). Observation of fluorescent phagocytic microglia revealed that some motoneuron death occurred following lesions at postnatal day 2 (P2) and P7, but not at P21 or P63. A significantly higher proportion of remaining FB labeled motoneurons expressed CB following lesion at P2 (mean 33% +/- 7.6 SD) and P7 (30.6% +/- 5.2) than at P28 (14.0% +/- 1.9). Second, CB expression by putative Renshaw cells was not significantly downregulated ipsilateral to the lesion. CB immunofluorescent putative Renshaw cells were counted in sections containing FB labeled motoneurons. No consistent differences in the numbers of Renshaw cells ipsilateral and contralateral to the lesion were found at any age. To confirm that these neurons really were Renshaw cells, the mediators of recurrent inhibition to cholinergic motoneurons, we employed double-immunofluorescence labeling with confocal microscopy. The group of CB immunopositive neurons located among the converging ventral roots in the cervical cord were closely apposed by many axon terminals immunoreactive for (i) vesicular acetylcholine transporter and (ii) cholera toxin B localized to motor axon collaterals by injection of this tracer into a distal forelimb muscle. We conclude that motoneuron axotomy need not always downregulate CB expression in associated Renshaw cells. In addition, some brachial motoneurons respond to axotomy by expressing CB.