Why does injury or surgery in some people lead to lasting pain, while in others the wound heals and the pain resolves along with it? A new study takes on that question by comparing rats that develop chronic pain after nerve injury with identically injured animals that do not. Researchers directed by Frank Porreca and Theodore Price at the University of Arizona, Tucson, found that differences in pain modulation by the central nervous system underlie the rats’ opposite fates. The data suggest that descending inhibitory mechanisms protect some rats by continuously blocking pain. The study, led by first authors Milena De Felice, Raul Sanoja, and Ruizhong Wang, was published online July 9 in Pain.
Crucial to the study was identifying animals that do not develop neuropathic pain after injury. In humans, most surgeries do not generate ongoing pain, but in animals, models are optimized so that they almost always produce chronic pain, Porreca told PRF. In practice, he said, nine out of 10 animals that undergo procedures such as spinal nerve ligation (SNL), a model for neuropathic pain, develop persistent pain. The one that does not is written off to experimental error or other causes. That kind of thinking may be wrong, Porreca says—the wayward rat may, in fact, reveal something important. Rather than asking why neuropathic pain develops, he and his colleagues set out to investigate why it sometimes does not.
Porreca and his group knew that rat strains show different susceptibilities to neuropathic pain, as measured by the development of allodynia, or hypersensitivity to tactile stimuli, after SNL (Yoon et al., 1999). In two closely related strains, Sprague Dawley (SD) and Holtzman (HZ), they found that before surgery, the rats displayed similar responses to tactile stimuli (probing the hind paw with von Frey filaments). After SNL, however, the strains behaved very differently: In both strains, some rats showed allodynia, while others responded as if they had no nerve injury. What differed between the strains was the number of individuals in each group: Among 150 male SD rats (plus historical data from a whopping 633 more), approximately 90 percent became allodynic after SNL. In contrast, of 185 HZ rats, half developed allodynia and half did not. That provided the researchers with large cohorts of rats that developed neuropathic pain behaviors after nerve injury (“allodynic” rats), and those that did not (“nonallodynic”).
The researchers wondered whether the distinct behaviors might involve differences in descending pain-modulating systems. The rostral ventromedial medulla (RVM) is thought to play an important role in descending pain modulation, receiving signals from the brain and conveying them down to the spinal dorsal horn. Within the RVM, some cells (ON cells) enhance nociceptive traffic, while others (OFF cells) inhibit it. (For a review, see Ossipov et al., 2010).
To investigate the contribution of descending modulation to pain susceptibility, the researchers deactivated the RVM by a local injection of lidocaine. RVM lidocaine blocked the hypersensitivity to tactile stimuli in allodynic rats, presumably by blocking pain-facilitating systems (as previously reported in Burgess et al., 2002). In nonallodynic rats, however, RVM lidocaine precipitated allodynia.
The researchers found similar differences when it came to spontaneous (non-evoked) pain—the primary complaint of patients with neuropathic pain. To measure spontaneous pain, they used a conditioned place preference test in which rats are trained to associate one of two chambers with administration of RVM lidocaine. When unmedicated animals are allowed to move freely between the two chambers, a display of preference for the lidocaine-associated chamber is thought to reflect that they are experiencing spontaneous pain. (For recent work from the Porreca lab on how pain drives conditioned place preference, see Qu et al., 2011) In this study, allodynic rats preferred the lidocaine-associated chamber, indicating that they had spontaneous pain. Nonallodynic rats, on the other hand, avoided the lidocaine chamber, signaling that for them, RVM lidocaine unleashed spontaneous pain.
The researchers suspected that RVM lidocaine precipitated pain by shutting down inhibitory circuits. To probe that idea, they used a kappa opioid agonist that specifically blocks RVM OFF cells. Administering this compound to nonallodynic HZ rats caused allodynia. This and other pharmacological experiments supported the idea that many HZ rats are able to avoid neuropathic pain because they engage descending pain-inhibiting systems. “Whatever was driving the pain was there, but it was regulated by this descending inhibitory mechanism,” Porreca said.
Finally, the team performed recordings of RVM neurons in live rats to directly measure the activity of ON and OFF cells. To drive nociceptor activity, they injected formalin into the hind paw, which caused heightened persistent pain behaviors in the HZ rats compared to SD. After the injection, OFF cells fired more in HZ rats, showing fewer and shorter pauses than in SD rats. Thus, in the HZ animals, the descending inhibition system apparently turned on and stayed on, whereas in the SD rats, it shut off frequently. In addition, ON cells fired less in the HZ rats.
“That was nice support for the hypothesis that these animals failed to develop chronic neuropathic pain because they are able to maintain descending inhibition in the face of ongoing nociceptive activity,” Price said.
Porreca and Price note that the mechanism they have identified for how certain rats escape neuropathic pain lines up well with the routes by which known therapies alleviate pain in people. Opioids and other drugs that are effective against neuropathic pain either mimic, or actively engage, descending inhibition.
Now the investigators want to use the pain-prone and pain-protected rats to find out what turns on inhibitory systems, and why they sometimes fail to operate. If the same mechanisms operate in humans, Price hopes that for people in whom descending inhibition has failed, he and his colleagues can one day find a pharmacological solution to flip the system back on.