Intra-amygdala infusion of zeta inhibitory peptide attenuates neuropathic pain but not inflammatory pain in adult rats

Background: Protein kinase Mζ (PKMζ), a typical brain-specific PKC isoform, has been shown to be critical in the maintenance of long-term potentiation and memory storage. Zeta inhibitory peptide (ZIP), a peptide with selective inhibition of PKMζ, has been used in relieving experimental neuropathic pain and disrupting memory. The aim of this study was to investigate the effects of intra-amygdalar infusion of ZIP on neuropathic pain induced by chronic constriction injury (CCI), and inflammatory pain induced by complete Freund's adjuvant (CFA) in adult rats.

Methods: (I) ZIP was infused into the amygdala 30 minutes (min) before CCI was performed. Mechanical withdrawal threshold (MWT) was determined prior to the infusion of ZIP, and 30, 60, 90, 120, 150, 180 min after CCI (n=8 per group). (II) ZIP was infused into the amygdala 3, 7, 14 days after CCI (n=8 per group). MWT was measured 30 min before the infusion and 2, 12, 24 h after the infusion. (III) Three days after CCI, ZIP was infused into the amygdala repeatedly once a day for 2 days. MWT was measured before each infusion and 2 or 24 h after each infusion (n=8 per group). (IV) ZIP was infused into the amygdala 24 h after the establishment of inflammatory pain induced by complete Freund's adjuvant (CFA). MWT was determined 30 min before the infusion and 30, 60, 90, 120 and 150 min after the infusion (n=8 per group).

Results: As shown in figures, (I) amygdalar infusion of ZIP prior to the CCI produced no effect on CCI-induced hyperalgesia when compared to scr-ZIP or saline infusion [n=8, interaction effect, F (7.312, 76.776) =1.237, P>0.05; time as main factor, F (3.656, 76.776) =115.346, P<0.001; group as main factor, F (2, 21) =0.648, P>0.05]. (II) BLA infusion of ZIP significantly increased mechanical withdrawal threshold 7 days after CCI [n=8, interaction effect, F (5.476, 57.500) =15.279, P<0.001; time as main factor, F (2.738, 57.500) = 242.357, P<0.001; group as main factor, F (2,21) =4.786, P<0.05], just the same as 3 and 14 days after CCI (data not shown). (III) Bilateral injection of ZIP into the BLA significantly reduced mechanical hyperalgesia 2 h after the administration [n=8, paired t-test, t (0.05,7) =-5.561, P<0.05; n=8, paired t-test, t (0.05,7) =-4.745, P<0.05], and returned to baseline 24 h after the administration [n=8, paired t-test, t (0.05,7) =1.039, P>0.05; n=8, paired t-test, t (0.05,7) =-1.173, P>0.05]. (IV) ZIP produced no effect on mechanical withdrawal thresholds at all time points examined after the infusion, compared with scr-ZIP or saline control groups (n=8, interaction effect, F (6.135, 126) =1.724, P>0.05; group as main factor, F (2,21) =0.608, P>0.05).

Conclusions: Intra-amygdala infusion of ZIP attenuates mechanical hyperalgesia induced by CCI but has no effect on inflammatory pain induced by CFA in rats, suggesting that amygdala PKMζ may be a therapeutic target in the treatment of neuropathic pain.