Figure 6.

Transport into the nucleus plays a role in the induction of hyperalgesic priming. Rats received intraganglion (i.gl.) injection of vehicle (white bars) or ivermectin (10 μg, black bars), an inhibitor of the nuclear transporter importin. Thirty minutes later, MCP-1 [20 ng/μl, 20 μl, injected intrathecally (i.t.) into the spinal cord, A] or 8-bromo cAMP [10 μg, injected into the DRG, B] was administered. No effect of ivermectin on MCP-1- or 8-bromo cAMP-induced hyperalgesia was observed (data not shown). The injection of vehicle or ivermectin continued daily for 3 d. On the fourth day, PGE₂ (100 ng) was injected intradermally into the dorsum of the hindpaw. A, Average paw-withdrawal thresholds before injection of vehicle/MCP-1 and immediately before the injection of PGE₂ (4 d later) were 121.3 ± 2.1 and 118.6 ± 1.6 g, respectively (t₍₅₎ = 0.9325; p = 0.3939, NS); for the groups that received ivermectin/MCP-1, they were 123.0 ± 2.5 and 123.5 ± 1.6 g, respectively (t₍₅₎ = 0.0000; p = 1.0000, NS); no significant (NS) difference between these two values was observed (paired Student's t test). B, Average paw-withdrawal thresholds before injection of vehicle/8-bromo cAMP and immediately before the injection of PGE₂ (4 d later) were 117.3 ± 2.1 and 114.6 ± 1.8 g, respectively (t₍₅₎ = 0.9035; p = 0.4077, NS); for the groups that received ivermectin/8-bromo cAMP, they were 122.3 ± 1.6 and 122. 5 ± 2.6 g, respectively (t₍₅₎ = 0.0000; p = 1.0000, NS); no significant (NS) difference between these two values was observed (paired Student's t test). Mechanical hyperalgesia was then evaluated 30 min and 4 h after PGE₂ injection using the Randall–Sellitto paw-withdrawal test. In both A and B, in the paws ipsilateral to the DRGs that received vehicle, the magnitude of hyperalgesia was still significant at the fourth hour after PGE₂ injection. However, in the paws ipsilateral to the DRGs that received ivermectin, the prolongation of PGE₂-induced hyperalgesia was significantly attenuated (***p < 0.001, A and B, when vehicle and ivermectin groups are compared at the fourth hour; two-way repeated-measures ANOVA followed by Bonferroni post-test), indicating that the induction of priming by i.t. MCP-1 or i.gl. 8-bromo cAMP requires transport into the nucleus. When tested again with PGE₂ 1 week later; attenuation of the hyperalgesia induced by PGE₂ at the fourth hour was still observed (***p < 0.001, A and B, when vehicle and ivermectin groups are compared at the fourth hour), indicating that ivermectin prevented the development of hyperalgesic priming (N = 6 paws per group).