Pharmacological Blockade of PPARα Exacerbates Inflammatory Pain-Related Impairment of Spatial Memory in Rats

Abstract

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors that exist in three isoforms: PPARα, PPARβ/δ and PPARγ. Studies suggest that the PPAR signalling system may modulate pain, anxiety and cognition. The aim of the present study was to investigate whether endogenous signalling via PPARs differentially modulates innate anxiety responses and mnemonic function in the presence and absence of inflammatory pain. We examined the effects of intraperitoneal administration of GW6471 (PPARα antagonist), GSK0660 (PPARβ/δ antagonist), GW9662 (PPARγ antagonist), and N-palmitoylethanolamide (PEA) on rat behaviour in the elevated plus maze (EPM), open field (OF), light-dark box (LDB), and novel object recognition (NOR) tests in the presence or absence of chronic inflammatory pain. Complete Freund's Adjuvant (CFA)-injected rats exhibited impaired recognition and spatial mnemonic performance in the NOR test and pharmacological blockade of PPARα further impaired spatial memory in CFA-treated rats. N-oleoylethanolamide (OEA) levels were higher in the dorsal hippocampus in CFA-injected animals compared to their counterparts. The results suggest a modulatory effect of CFA-induced chronic inflammatory pain on cognitive processing, but not on innate anxiety-related responses. Increased OEA-PPARα signalling may act as a compensatory mechanism to preserve spatial memory function following CFA injection.

Keywords: OEA; PEA; anxiety; cognition; complete Freund adjuvant; dorsal hippocampus; peroxisome-proliferator activated receptor; spatial memory.

Figure 1

Graphical representation of the experimental design.

Figure 2

Graphical representation of the NOR arena. The left-top image represents the organisation of the object on the sample (familiarisation) day. The left-bottom image represents the organisation of the objects on the test day (novel object in the position of object 3). The right image shows the equal distances of the object to the wall of the circular arena.

Figure 3

Effects of systemic administration of vehicle, selective PPARα (GW6471), PPARβ/δ (GSK0660) and PPARγ (GW9662) antagonists, and PEA on mechanical hypersensitivity in CFA-injected (CFA) and control (No-CFA) rats. Post hoc testing indicated significantly lower paw withdrawal thresholds in CFA vehicle-treated animals on days 1, 7, 21, and 28 (* p < 0.05 and *** p < 0.001, vs. No-CFA Vehicle) in the ipsilateral paw (B). The test also indicated lower paw withdrawal thresholds in CFA GW9662 and PEA-treated animals compared to their No-CFA counterparts (# p < 0.05 vs. No- CFA GW9662, days 7 and 28; # p < 0.05 vs. No-CFA PEA, days 7 and 21). The von Frey data, which were non-parametric, are presented in timeline graphs as means ± S.E.M. for presentation/readability purposes (n = 7–8 rats per group).

Figure 4

Effects of systemic administration of vehicle, selective PPARα (GW6471), PPARβ/δ (GSK0660) and PPARγ (GW9662) antagonists, and PEA on anxiety-related behaviours in the EPM test in CFA-injected (CFA) and control (No-CFA) rats. Data are expressed as means ± SEM (n = 8 rats per group). The symbols represent each individual data point.

Figure 5

Effects of systemic administration of vehicle, selective PPARα (GW6471), PPARβ/δ (GSK0660) and PPARγ (GW9662) antagonists, and PEA on exploratory and anxiety-related behaviours in the OF test in CFA-injected (CFA) and control (No-CFA) rats. Data are expressed as means ± SEM (n = 8 rats per group). The symbols represent each individual data point.

Figure 6

Effects of systemic administration of vehicle, selective PPARα (GW6471), PPARβ/δ (GSK0660) and PPARγ (GW9662) antagonists, and PEA on anxiety-related behaviours in the LDB test in CFA-injected (CFA) and control (No-CFA) rats. Data are expressed as median with interquartile range and min/max (A–E) or means ± SEM (F–I) (n = 7–8 rats per group). In the CFA-injected groups, the area under the curve (AUC) analysis indicated a trend for a decrease in time spent in the light side in GW6471-treated (p = 0.075; F) and GSK0660-treated (p = 0.07; G) rats compared to vehicle counterparts. The AUC analysis was done on all groups together but are presented separately for presentation/readability purposes. Data are presented as means ± S.E.M. (n = 7–8 rats per group).

Figure 7

Effects of systemic administration of vehicle, selective PPARα (GW6471), PPARβ/δ (GSK0660) and PPARγ (GW9662) antagonists, and PEA on behaviour in the NOR test in CFA-injected (CFA) and control (No-CFA) rats. Two-way ANOVA revealed an effect of day (^a p < 0.001) on the percentage of time spent exploring the novel object compared to object 3 (i.e., preference index = TObject3 or Novel Object/(TObject 1 + TObject 2/2) + TObject3 or Novel Object multiplied by 100; (A). CFA injection (^a p < 0.05; (B,C)) was also shown to have an effect on the discrimination index (Discrimination index = TNovel object/(TObject1 + TObject2/2) + TN multiplied by 100) and on the spatial discrimination index (Spatial discrimination index = TNovel object-TObject3/TNovel object + TObject3 multiplied by 100). GW6471 significantly reduced the spatial discrimination index compared to vehicle-treated CFA rats (* p < 0.05, vs. CFA-Vehicle; (C)). Data are expressed as means ± SEM (n = 7–8 rats per group). The symbols represent each individual data point.

Figure 8

Effects of intraplantar injections of CFA in vehicle-treated animals on the levels of OEA and PEA in the dorsal hippocampus (A,B) and entorhinal cortex (C,D). Two-way ANOVA has shown a significant overall effect of CFA on the levels of OEA in the dorsal hippocampus (B). Post hoc analysis with Student Newman-Keuls test indicated that OEA levels were significantly higher in the ipsilateral dorsal hippocampus of the CFA-vehicle group compared to No-CFA counterparts (B; * p < 0.05, vs. no-CFA; d = −0.390). Data are expressed as means ± SEM (n = 7–8 rats per group).