This study examined changes in
choline acetyltransferase and
calcitonin gene-related peptide immunoreactivity in hypoglossal motoneurons of rats at 1, 3, 7, 20 and 50 days after three types of nerve
injury: crush, transection and resection. Peripheral reinnervation was assayed by retrograde labelling of the motoneurons after
injections of the exogenous
protein,
horseradish peroxidase, into the tongue. Maximal reduction in
choline acetyltransferase immunostaining occurred at seven days after nerve damage and the amount of the decrease was related to the nature of the injury. The recovery of
choline acetyltransferase to normal levels was related to the timing of reinnervation after
nerve crush, but not after transection or resection
injuries. In contrast to these findings, a rapid increase in
calcitonin gene-related peptide immunoreactivity preceded the decrease in
choline acetyltransferase levels. A striking increase in
calcitonin gene-related peptide immunoreactivity was observed at one day postoperative and was maximal at three days postoperatively for all
injuries. Later changes in
calcitonin gene-related peptide levels were dependent on the type injury. Increased
calcitonin gene-related peptide staining persisted to 20 days after
nerve crush. After nerve transection or resection,
calcitonin gene-related peptide immunoreactivity decreased to basal levels at seven days postoperatively. This declination was followed by a second rise in
calcitonin gene-related peptide immunolabeling at 20 days for nerve transection or 50 days after resection. Nearly complete reinnervation was established by 20 days after
nerve crush. At 50 days after transection, less than half the number of normally-labelled neurons contained
horseradish peroxidase. At this time only 1% of those whose axons had been resected were labelled. These observations suggest that different mechanisms regulate the responses of
choline acetyltransferase and
calcitonin gene-related peptide to nerve injury. The present results indicate that
choline acetyltransferase levels in motoneurons can not be used to predict either the likelihood of or the timing of reinnervation after nerve transection or resection. However, our results strengthen the premise that an increased of
calcitonin gene-related peptide immunoreactivity serves as a reliable index for predicting nerve regeneration/reinnervation after
cranial nerve injury.