Expression of beta-calcitonin gene-related peptide in axotomized rubrospinal neurons and the effect of brain derived neurotrophic factor.

The mRNA levels for alpha- and beta-calcitonin gene-related peptide (CGRP) in rat rubrospinal neurons were studied by in situ hybridization 3, 7, 14, 28 and 56 days following cervical spinal hemisection. CGRP-like immunoreactivity (LI) in the rubrospinal neurons and the rubrospinal tract in cervical spinal cords were examined using immunohistochemistry. There was almost no signal for alpha- and beta-CGRP mRNAs and undetectable level of CGRP-LI in the rubrospinal neurons ipsilateral to cervical spinal hemisection (control side). Fourteen days after spinal hemisection, the rubrospinal neurons contralateral to cervical hemisection (axotomized side) showed CGRP-LI in their cell bodies, and CGRP containing fibers were observed in the lateral funiculi just proximal, but not distal, to the injury sites. In situ hybridization showed upregulation of beta-CGRP mRNA in a subpopulation of the rubrospinal neurons on the axotomized side. The proportion of beta-CGRP mRNA-expressing neurons reached its maximum (approximately 19%) 4 days following axotomy and slowly decreased to about 5% 56 days after axotomy. The percentage of alpha-CGRP mRNA-expressing neurons was much lower than that of beta-CGRP mRNA (maximum about 2.6% 4 days after axotomy) and not significantly different from the control side throughout the time period studied. These data indicate that axotomy induces de novo synthesis of the CGRP beta-subtype in rubrospinal neurons and that the beta-CGRP is transported to the injury site through the rubrospinal tract. In addition, we studied the effect of the intracerebral injections of brain derived neurotrophic factor (BDNF). BDNF treatment fully reversed the severe cell atrophy that followed axotomy and increased the number of neurons labeled for beta-CGRP mRNA, but did not increase the percentage of rubrospinal neurons expressing beta-CGRP mRNA. Thus, topical application of BDNF does not have direct modulatory effect on CGRP induction in axotomized neurons in the red nucleus.