A, Schematic of the protocol for behavioral testing. B,C, Histograms showing formalin-induced CPA. Mean ± SEM time spent in the treatment-paired compartment on pre-and post-conditioning day (B). Mean ± SEM F-CPA scores, i.e. the time spent in the treatment-paired compartment on the pre-conditioning day minus that on the post-conditioning day (C). D, Schematic of the behavioral procedure used in subsequent studies. E,F, Histograms showing effects of microinjection of NR2B antagonist Ifenprodil (0.2 μg/μl, 0.6 μl per side), Ro 25-6981 (2 μg/μl, 0.6 μl per side), or NR2A antisense (AS) ODNs (2 nmol/0.6 μl per side) into the bilateral rACC on F-CPA. Values are mean ± SEM time in the formalin-paired compartment on pre-conditioning and post-conditioning days (E), and F-CPA scores (F). G,H, Western blot analysis showing that formalin-induced increase in level of NR2A receptor is blocked by intra-rACC preadministration of NR2A AS but not NR2A MS ODNs. Tubulin serves as loading control. Both ipsilateral and contralateral sides of the rACC were pooled together in this experiment. I,J, Intra-rACC administration of NR2B antagonist Ifenprodil and NS (I), or NR2A AS and missense (MS) ODNs (J) has no effect on formalin-induced biphasic nociceptive responses. *p<0.05, **p<0.01 vs. preconditioning day. + p<0.05, ++p<0.01 vs. NS or MS ODNs control.