Sham surgeries for central and peripheral neural injuries persistently enhance pain-avoidance behavior as revealed by an operant conflict test

Abstract

Studies using rodent models of neuropathic pain use sham surgery control procedures that cause deep tissue damage. Sham surgeries would thus be expected to induce potentially long-lasting postsurgical pain, but little evidence for such pain has been reported. Operant tests of voluntary behavior can reveal negative motivational and cognitive aspects of pain that may provide sensitive tools for detecting pain-related alterations. In a previously described operant mechanical conflict test involving lengthy familiarization and training, rodents freely choose to either escape from a brightly lit chamber by crossing sharp probes or refuse to cross. Here, we describe a brief (2-day) mechanical conflict protocol that exploits rats' innate exploratory response to a novel environment to detect persistently enhanced pain-avoidance behavior after sham surgeries for 2 neural injury models: thoracic spinal cord injury and chronic constriction injury of the sciatic nerve. Pitting the combined motivations to avoid the bright light and to explore the novel device against pain from crossing noxious probes disclosed a conflicting, hyperalgesia-related reluctance to repeatedly cross the probes after injury. Rats receiving standard sham surgeries demonstrated enhanced pain-like avoidance behavior compared with naive controls, and this behavior was similar to that of corresponding chronic constriction injury or spinal cord injury rats weeks or months after injury. In the case of sham surgery for spinal cord injury, video analysis of voluntary exploratory behavior directed at the probes revealed enhanced forepaw withdrawal responses. These findings have important implications for preclinical investigations into behavioral alterations and physiological mechanisms associated with postsurgical and neuropathic pain.

Transcriptional Profiling of Non-injured Nociceptors After Spinal Cord Injury Reveals Diverse Molecular Changes

“…For example, removal of bone and muscle alone could trigger chronic pain-like symptoms, analogous to post-surgical pain reported in humans (Woolf, 2011). Sham surgery for SCI has also recently been reported to produce significant increases in pain-avoidance behavior in rats, which is indicative of persistent post-surgical pain (Odem et al, 2019). Surprisingly, there were no significant differences between naïve and sham mice at any time point (Figures 1A,B), suggesting that the laminectomy did not produce any locomotor differences or behavioral hypersensitivity 1-7 days post-surgery in mice.…”

EPAC1 and EPAC2 promote nociceptor hyperactivity associated with chronic pain after spinal cord injury

“…In our preliminary testing we observed that repeated trials significantly decreased the number of crossings and increased the latency to cross. Similar studies with rats suggested that a large component of the motivation to cross the aversive probes is the strong drive of rodents to explore an unfamiliar context (Odem et al, 2019). To reduce possible habituation to the MC device, we modified our procedure to allow the mice to experience the device only twice, with a period of 3–4 weeks separating each exposure.…”

“…The operant MC test, which reflects affective-motivational and cognitive-evaluative dimensions of pain (Harte et al, 2016, Odem et al, 2019, Pahng et al, 2017) revealed enhanced pain-avoidance behavior after SCI and no effects of deleting EPAC2 or EPAC1. SCI wild-type and EPAC2 −/− or EPAC1 −/− mice crossed the probes less often than sham-operated animals (Fig.…”

“…Reliance on an enhancement of brisk withdrawal reflexes as a measure of increased pain sensitivity is problematic, especially after SCI, as sensitization of hindlimb reflexes can be part of spastic syndromes, and reflexive measures may not capture the motivational/affective dimensions of pain (Baastrup et al, 2010, Odem et al, 2019, Yezierski and Vierck, 2010). In contrast, an operant mechanical conflict (MC) test is a paradigm in which voluntary behavior reveals the aversiveness of a rough substrate that animals must cross to explore an unfamiliar setting and to escape from an aversive bright light (Harte et al, 2016, Odem et al, 2019, Pahng et al, 2017). When tested in the MC device, naïve wild type and EPAC2 −/− mice crossed the aversive probes a similar number of times, and they spent about the same amount of time on the probes (Table 1).…”

“…The MC test (Harte et al, 2016, Odem et al, 2019) was used to assess voluntary avoidance of noxious probes as an operant measure of altered pain sensitivity. The commercially available MC test device (Mechanical Conflict-Avoidance System, Coy Lab Products, Grass Lake, MI, USA) consists of two rectangular chambers (each 16.5 cm wide by 21.5 cm deep by 15.25 cm high) connected by a narrow 30.4 cm long tunnel containing a dense array of sharp probes on the floor (Harte et al, 2016).…”