Figure 1.

Induction of hyperalgesic priming by intrathecal (i.t.) injection of MCP-1 is dependent on gene transcription. Rats received three daily intraganglion (i.gl.) injections of vehicle (white bars) or the protein transcription inhibitor actinomycin D (10 μg, black bars). Thirty minutes after the first injection of actinomycin D, the central terminal of these nociceptors was stimulated with MCP-1 (20 ng/μl, 20 μl), injected intrathecally. No effect of actinomycin D on MCP-1-induced hyperalgesia was observed (data not shown). The injections of vehicle or actinomycin D continued, once a day, for 3 d. On the fourth day, PGE₂ (100 ng) was injected intradermally into the dorsum of the hindpaw at the site of nociceptive testing to evaluate for the presence of priming. The average paw-withdrawal thresholds before injections of vehicle/MCP-1 and immediately before the injection of PGE₂ (4 d later) were 110.0 ± 3.6 and 110.0 ± 2.7 g, respectively; for the groups that received actinomycin D/MCP-1, 113.3 ± 3.3 and 113.5 ± 2.2 g, respectively (t₍₅₎ = 0.0000; p = 1.0000, NS in both cases; paired Student's t test). Mechanical hyperalgesia was then evaluated by the Randall–Sellitto paw-withdrawal test 30 min and 4 h after PGE₂ injection. We observed that, in the paws ipsilateral to the DRGs that received vehicle, the magnitude of hyperalgesia was still significant at the fourth hour after PGE₂ injection, indicating the presence of hyperalgesic priming, while in the paws ipsilateral to the DRGs that received actinomycin D, the PGE₂-induced hyperalgesia was significantly attenuated at this time point, which is compatible with the prevention of priming (***p < 0.001, when both groups are compared at the fourth hour; two-way repeated-measures ANOVA followed by Bonferroni post-test, N = 6 paws per group). The results of this experiment support the suggestion that the inhibition of transcription in the cell body prevented the induction of priming by intrathecal MCP-1.