Activin injection induces thermal hyperalgesia in wild-type but not TRPV1-null mice. A, Activin (250 ng/5 μl) injection induces thermal hyperalgesia in wild-type mice (gray bars, n = 19) but not TRPV1-null mice (black bars, n = 16). Thermal hyperalgesia was measured by paw withdrawal latency (in seconds) with plantar heating at 1–48 h after injection. Data groupings represent assessments done at the indicated times after activin injection. Wild-type mice show 4–5 s withdrawal latency in the absence of activin (white bars), whereas TRPV1-null mice exhibit longer latencies (hatched bar, n = 8). *p < 0.05 relative to wild-type responses in activin. B, Activin (250 ng/5 μl) injection induces mild tactile allodynia at 1 h in wild-type mice (gray bar, n = 8) but not TRPV1-null mice (black bar, n = 8). Wild-type saline injected mice show 1 g withdrawal threshold (white bar, n = 4). *p < 0.05 denotes the significance level.