Effect of high trait anxiety on mechanical hypersensitivity in male rats.

Recently, we have shown that neuropathic pain is associated with anxiety-like behaviour in rats with sciatic nerve lesion. An enhanced pain perception has also been described in patients with anxiety disorders. However, there is only limited knowledge about the relationship between anxiety and pain in animals. To investigate whether trait anxiety influences nociception, we measured mechanical hypersensitivity following chronic constriction injury (CCI) in rats selectively bred for high (HAB) or low (LAB) anxiety-like behaviour. Pain sensitivity was assessed before surgery and at day 7, 14, 21, 36 and 57 after CCI by determination of withdrawal thresholds. Additionally, we examined pain-induced anxiety-like behaviour using the elevated plus-maze (EPM). HAB and LAB rats exhibited similar levels of mechanical hypersensitivity 7 days post-injury. However, at day 14 and 21 after surgery, mechanical-induced pain thresholds were significantly decreased in HAB rats (p<0.05) in comparison to LAB rats. From day 21 onward HAB rats displayed a faster return of paw withdrawal threshold to baseline level when compared to LAB rats (p<0.01). In the EPM anxiety-like behaviour was observed following CCI injury in HAB and LAB rats on top of low and high trait anxiety reflected in a reduced number of entries in open arms. These findings indicate that trait anxiety increases mechanical hypersensitivity in CCI rats during the chronic phase of pain, thereby suggesting that affective processes modulate even simple pain-related behaviour. Furthermore, we demonstrated that neuropathic pain in the CCI model increases anxiety-like behaviour even in LAB rats.