Figure 3.

Role of IB4+ and IB4– nociceptors in priming induced at the peripheral and central nociceptor terminal by systemic fentanyl. Rats received an intrathecal injection of control saporin (3.2 µg/20 µl), SSP-saporin (100 ng/20 µl), or IB4-saporin (3.2 µg/20 µl). Fourteen days later, at which time the mechanical nociceptive threshold was not different from pre-saporin baselines [t₍₅₎ = 0.59; p = 0.58, for the saporin (control)-treated group, t₍₅₎ = 1.3; p = 0.24, for the SSP-saporin-treated group, and t₍₅₎ = 0.47; p = 0.66, for the IB4-saporin-treated group, when the mechanical nociceptive threshold is compared before and 14 d after treatments; paired Student's t test], fentanyl (30 µg/kg) was injected subcutaneously. A, Four days later, at which time mechanical nociceptive threshold was not different from pre-fentanyl baseline [t₍₅₎ = 0.25; p = 0.81, for the saporin (control)-treated group, t₍₅₎ = 0.52; p = 0.62, for the SSP-saporin-treated group, and t₍₅₎ = 0.061; p = 0.95, for the IB4-saporin-treated group, when the mechanical nociceptive threshold was compared before and 4 d after systemic fentanyl; paired Student's t test] PGE₂ (100 ng/5 µl) was injected intradermally and mechanical nociceptive threshold evaluated 30 min and 4 h later. Hyperalgesia was present in all saporin-treated groups measured 30 min after its injection. Also, prolonged hyperalgesia induced by intradermal PGE₂ was present in all groups treated with saporins (F_(2,15) = 0.69, p = 0.52, when all saporin-treated groups are compared at the fourth hour after intradermal PGE₂; two-way repeated-measures ANOVA followed by Bonferroni post hoc test), indicating that in the peripheral terminal of the nociceptor systemic fentanyl does not require IB4+ and SP peptidergic nociceptors to develop priming. B, Seven days after systemic fentanyl and 3 d after intradermal PGE₂, rats were tested for priming in the central terminal of the nociceptor. PGE₂ (400 ng/20 µl) was injected intrathecally and mechanical nociceptive threshold evaluated 30 min and 4 h later. Intrathecal PGE₂-induced hyperalgesia in all saporin-treated groups, measured 30 min after its administration. However, the prolongation of PGE₂-induced hyperalgesia at the fourth hour was markedly attenuated in the group treated with SSP-saporin, but not in the saporin-control-treated and IB4-saporin-treated groups [F_(2,15) =16.0, ***p = 0.0002, when the SSP-saporin-treated group is compared with the saporin (control)-treated and the IB4-saporin-treated groups at the fourth hour after intrathecal PGE₂; two-way repeated-measures ANOVA followed by Bonferroni post hoc test]. These findings support the suggestion that at the central terminal, systemic fentanyl requires IB4– peptidergic nociceptors to induce priming (n = 6 paws/6 rats per group).