3.4. Kappa opioid receptor blockade rescues inflammatory pain-induced anxiety-like behaviours in female rats.
Previous data have shown that the blockade of the KOR/DYN signalling in the NAc shell was an appropriate strategy to impair the development of negative affect and alcohol-relapse-like behaviour induced by pain (Massaly 2019, Lorente 2021). Thus, we found interesting to pharmacologically analyse the role of NAc KOR in the inflammatory pain-induced negative affective states reported in female rats in the present study (figure 1). The blockade of the KOR locally in the nucleus accumbens shell of female rats (figure 5A and S3A) impacted the behaviour in the LDB without altering the SPT. In fact, in the case of the SPT 48h, the ANOVA only detected differences for the time variable (F(2,130)= 15.259; p= 0.0001; η2= 0.19) and the interaction time x pain (F(2,130)= 11.576; p= 0.0001; η2= 0.151). Further post-hoc analyses revealed that CFA groups decreased their sucrose consumption compared with saline groups at days 1-2 (p= 0.010), while no differences were found for the other variables and interactions (Figure 4B) including the treatment variable and its interactions (see table 1). The analysis of the SPT of 2h, revealed that the consumption of sucrose increases over the time and the animals whose received NorBNI infusion showed a high sucrose consumption than aCSF groups. Indeed, the ANOVA showed differences for the time and treatment variables (F(2,130)= 51.364; p=0.0001; η2= 0.597; and F(1,65)= 5.054; p= 0.028; η2= 0.072). However, we did not detect differences for pain variable and the interaction between variables (Figure 4C).
Finally, when analysing the LDB test we observed that NorBNI reverse the decrease in the time spent in light box of CFA female rats without altering the transition between boxes. For the time in light, we detected differences in the time variable (F(1,65)= 12.969, p= 0.001; η2= 0.166), showing that animals spent more time in light during the second week. Furthermore, we detected differences in the pain variable (F(1,65)= 11.470, p= 0.001; η2= 0.15) and in the interaction between pain x treatment (F(1,65)= 3.954, p= 0.05; η2= 0.057). The treatment variable and the other interactions showed no significative differences, and their F values are showed in table 1. Post-hoc analyses of the significant interaction and variables revealed that pain-suffering female rats that received aCSF spent a shorter time in light box, compared to the pain-free animals at 7 and 14 days (p7= 0.001, p14= 0.012) while the pain-suffering female rats receiving NorBNI spent in the light compartment the same time as saline groups (Figure 4D, p7= 0.181; p14= 0.974). Thus, the administration of NorBNI counteracted the anxiety-like behaviour induced by CFA injection in female rats. The analysis of the transitions between boxes detected a significant effect for the time (F(1,65)= 45.153; p= 0.0001) but not for the other variables and their interactions (see table1). Thus, the transitions at 7 days were lower than the transitions at 14 days either after saline or after CFA injection.
Finally, we analysed the mechanical sensitivity thresholds with the VFT to confirm that all CFA-treated rats maintained values until the end of the protocol. We detected differences in pain (F(1,65)= 69.774; p= 0.0001; η2= 0.518) and time (F(1,65)= 55.861; p= 0.0001; η2= 0.521) variables and in the interaction between time x pain (F(1,65)= 36.695); p= 0.0001; η2= 0.41), while no differences were detected for treatment variables and for their interactions (see table 1). Post-hoc analyses revealed that female rats showed lower paw withdrawal thresholds after 7 and 14 days following CFA injection compared with their own basal values (before the injection, p7= 0.0001); p14= 0.0001) and with their saline counterparts (p7= 0.0001; p14= 0.0001).