We have sought to determine (1) if thalamic neurons upregulate the growth associated protein GAP-43 as a response to injury, or if a peripheral nerve graft is required to induce, enhance or sustain such a response, and (2) if thalamic neurons with different regenerative potentials also display different GAP-43 responses. Levels of GAP-43 protein (detected by LM and EM immunohistochemistry) and of GAP-43 mRNA (detected by in situ hybridization) were compared in the thalamus of adult rats between 1 d and 180 d after making a stab lesion or after implanting a peripheral nerve autograft. Stab injury is a sufficient stimulus to cause a transient upregulation in GAP-43 expression by neurons in the thalamus (both around the graft tip and in particular in the thalamic reticular nucleus) in the first week after injury but this response is both prolonged, and enhanced in the presence of a peripheral nerve graft. In addition, we demonstrate directly, by double labelling, that neurons of the thalamic reticular nucleus displaying high levels of the mRNA for GAP-43, have axons regenerating in the distal portion of the graft. These findings lend direct support to the hypothesis that upregulation of the GAP-43 gene is essential for prolonged regenerative axonal growth. We also demonstrate GAP-43 protein in graft Schwann cells and in brain astrocytes close to the site of graft implantation.

A segment of tibial nerve was autografted to the right corpus striatum of deeply anesthetized adult rats; the distal graft was left beneath the scalp. Horseradish peroxidase (HRP) conjugates were injected into the distal graft after 2-30 weeks, and the animals were killed 2-3 days later. Small numbers of neostriatal perikarya were HRP labeled at all survival times; most were large (ca. 20 microm in diameter), and many contained acetycholine esterase (AChE). Many more neurons were labelled in the substantia nigra pars compacta (SNpc) 4 weeks or more after grafting. When the graft encroached on the globus pallidus, numerous pallidal neurons, most of them AChE positive, were also labeled. Nigrostriatal neurons, a population of pallidal cholinergic neurons, and a subclass (or classes) of neostriatal neurons, including cholinergic interneurons, thus can be classified as central nervous system (CNS) neurons with a relatively strong regenerative response. In a second experimental series, animals were killed 1-4 weeks after grafting, and sections were probed for the expression of mRNAs encoding growth-associated protein 43 (GAP-43) and the cell adhesion molecules N-CAM and L1. Subpopulations of mostly large neurons scattered throughout the neostriatum gave moderate signals for GAP-43 and N-CAM mRNAs and a stronger signal for L1 mRNAs. Most SNpc neurons were strongly labeled with all three probes. Neostriatal grafts had no apparent effect on the expression of any of the mRNAs in the SNpc or on L1 and N-CAM mRNAs in the striatum. However, GAP-43 mRNA levels were increased in a few, mainly large neostriatal neurons around the graft tip, resembling the HRP-labeled cells. In contrast, previous work has shown upregulation (from an undetectable level) of GAP-43 and L1 mRNAs in neurons regenerating axons into grafts placed in the thalamus and cerebellum. Thus, GAP-43 and L1 mRNA expression, but not necessarily marked upregulation, may correlate with, and be intrinsic determinants of, the ability of CNS neurons to regenerate their axons.