Endogenous TrkB Ligands Suppress Functional Mechanosensory Plasticity in the Deafferented Spinal Cord

DRI induces BDNF and NT-3 expression in the dorsal horn. a, In the intact spinal cord, BDNF immunoreactivity was restricted to the terminals of primary afferent axons. Higher-power images from the same sections are shown on the right (boxes in lower-power images indicate their approximate origins). b–d, BDNF was upregulated in glial cells by 3 d after C7/8 DRI and remained elevated for at least 20 d. e, Ox-42 immunohistochemistry revealed BDNF expression by microglia in early (arrowhead) and intermediate (arrow) stages of reactivity. f–h, NT-3 immunoreactivity was intense in cerebellar Purkinje cells (inset), absent from the uninjured dorsal horn, but present in blood vessel-associated (asterisk) cellular processes as early as 3 d after DRI (g, h, arrows). h, i, DRI-induced NT-3 expression persisted for at least 20 d. j, NT-3 was expressed in glial fibrillary acidic protein (GFAP)-positive astrocytes around rat endothelial cell antigen 1 (RECA-1)-positive vasculature (a white-matter astrocyte from the degenerating dorsal columns is shown). Scale bars: a, 100 μm; e, i, 25 μm.

Sequestration of injury-induced BDNF promotes complete mechanosensory recovery after C7/8 DRI. a, Fc immunohistochemistry demonstrating spinal penetration of intrathecally delivered proteins. Scale bar, 100 μm. b, Only continuous TrkB-Fc treatment improved mechanosensation. c, Mechanical allodynia did not emerge in IgG- or TrkB-Fc-treated rats, and withdrawal thresholds were increased with TrkC-Fc treatment. Asterisks in b and c indicate significant ipsilateral (ipsi)–contralateral (contra) differences.

Acute BDNF antagonism does not affect mechanosensation 10 d after rhizotomy. a, Time course of mechanosensory recovery over the first 10 d after DRI. b, Time course of forepaw cold-pain development (increased response duration to an acetone squirt) in the same animals. Arrows indicate days on which intrathecal boli of IgG or TrkB-Fc were administered. c, Intrathecal boli of TrkB-Fc did not alter mechanosensory response latencies 30–90 min after administration. d, Acute BDNF antagonism reduced response durations to acetone in the same animals. Asterisks in a and b indicate significant ipsilateral (ipsi)–contralateral (contra) differences. Asterisks in c and d indicate significant differences between IgG andTrkB-Fc treatments.

TrkB-Fc promotes mechanosensory axon sprouting after C7/8 DRI. a, b, VGLUT1-positive (a) and CTB-positive (b) terminal density in superficial (II) and deep (III–IV) spinal laminas in uninjured animals. DRI reduced density in deeper laminas, which was partially reversed by intrathecal TrkB-Fc. Asterisks indicate significant differences between IgG- and Trk-Fc-treated rats. c, Low-power micrographs of VGLUT1 immunoreactivity (lamina II indicated). d, e, Higher-power micrographs of VGLUT1 (d) and CTB (e) staining from regions similar to that indicated in c (arrow indicates spared C6/T1 axons). f, The size distribution of VGLUT1-positive terminals was significantly right-shifted in TrkB-Fc-treated rats. g, Large VGLUT1-positive terminals were also CTB filled (arrowheads). Clusters of small VGLUT1-positive terminals are indicated by arrows. Scale bars: c, 100 μm; g, 20 μm.

Endogenous TrkB ligands stimulate serotonergic sprouting after C7/8 DRI. a, Serotonergic axon density measurements from superficial (IIo and IIi) and deeper (III/IV) spinal laminas: TrkB-Fc treatment prevented DRI-induced sprouting. Asterisks indicate significant differences from intact rats. b, Low-power micrographs of serotonin transporter (SERT)-expressing axons in the spinal dorsal horn. c, Higher-power micrographs of SERT immunohistochemistry from regions similar to that indicated in b. Scale bar, 100 μm.