Immature corticospinal neurons respond to axotomy with changes in tubulin gene expression

We have examined the expression of two different tubulin mRNAs in hamster corticospinal neurons that were axotomized at three different developmental stages; postnatal day 8 (P8), P20, and adult. In situ hybridization of histological sections of the sensorimotor cortex was done with 35S-labeled cDNA probes specific to alpha 1-tubulin and beta III-tubulin mRNAs at 2-14 days following unilateral transection of the corticospinal tract in the caudal medulla. Both film and emulsion autoradiography were used to detect changes in tubulin mRNA levels. Qualitative assessment indicated substantial decreases in both alpha 1-tubulin and beta III-tubulin mRNA levels in layer V neurons of the sensorimotor cortex following axotomy. The changes were apparent as early as 2 days postinjury for P20 and adult operates, but not for P8 operates. However, by 14 days postinjury, decreases in alpha 1-tubulin and beta III-tubulin gene expression were apparent in animals operated at all three developmental stages. These findings indicate that both immature and adult corticospinal neurons respond to axonal injury in a manner that is distinctly different from the peripheral neuron response.