Pain is more than a physical sensation. For many people, it extends beyond the sensory “ouch” and is associated with negative affective states, including anxiety, depression, and reduced motivation. However, these aspects of pain conditions are largely undertreated, and their mechanisms are poorly understood.
Now, new research in rodents sheds light on this phenomenon by identifying kappa opioid receptors (KORs) within specific areas of the nucleus accumbens (NAc) as drivers of pain-induced negative affect.
The study, led by Jose Morón-Concepción, Washington University Pain Center, St. Louis, US, demonstrates that KOR activation in discrete subregions of the NAc in both mice and rats spurs inflammatory pain-induced negative affect, which was attenuated by KOR blockade. In addition, inflammatory pain enhanced recruitment of neurons containing dynorphin, an opioid peptide that activates KORs, in the same NAc subregions, and activation of those neurons was sufficient to produce negative affect.
“This study is a very comprehensive analysis of the dynorphin and kappa opioid receptor systems, and how they are upregulated in pain negative affect,” said Thomas Martin, Wake Forest University, Winston-Salem, US, who was not involved in the study.
The findings were published online March 13 in Neuron.
Kappa opioid receptors and motivation
Morón-Concepción’s work has long focused on opioids and pain, including their effects on motivational behavior. “We had demonstrated before that pain decreases a rat’s motivation to obtain a reward and studied the role of the mu opioid receptor to that effect. However, as the kappa opioid receptor is more involved in negative affect, we decided to investigate if this receptor was driving the negative emotional component of pain,” he told PRF.
The researchers decided to focus on a specific brain area that had been implicated in motivation and reward. “Coming from the addiction field, I had done a lot of work previously with the nucleus accumbens,” Morón-Concepción said. “As the focus of this new study was primarily on how pain was affecting motivation and reward rather than the neurobiology of pain itself, and when you consider that reward triggers dopamine release into the nucleus accumbens, it made sense to focus on this region initially rather than other regions more traditionally associated with the emotional component of pain, such as the amygdala,” according to Morón-Concepción.
A decreased motivation to obtain goals is a characteristic feature of pain-induced negative affective states. The researchers took advantage of this knowledge to investigate the role that the KOR may play in the motivation to seek rewards in rodents that had received complete Freund’s adjuvant (CFA), a model of inflammatory pain. Using a sucrose self-administration assay in which the animals are trained to press a lever in order to obtain a sucrose pellet, the researchers found that the motivation to obtain the pellet decreased as the number of lever presses required to get it increased.
“Uninjured animals will work a lot in order to receive the reward, pressing the lever up to 100 times to receive a single reward, so they clearly have a strong motivation to obtain that reward,” explained first author Nicolas Massaly. But “when you induce pain, you decrease this motivation for the reward and therefore the number of lever presses.”
Injection of the KOR antagonist norbinaltorphimine (norBNI) directly into a subregion of the NAc known as the NAc shell cold spot (NAcShCS) prevented the pain-induced decrease in motivation seen in the rats. Importantly, this was not due to an intrinsic effect of norBNI as this agent had no effect of the number of lever presses in sham pain conditions. Furthermore, blocking KOR did not affect the thermal hyperalgesia caused by CFA, demonstrating that the KOR is a key mediator of the negative affective component of pain but not the sensory aspect, in this case heat pain.
Conversely, activating KOR in the NAcShCS with the KOR agonist U50,488 in naive rats reduced the motivation for sucrose self-administration. This suggested that KORs are sufficient to decrease such behavior.
The decreased motivation resulting from KOR activation appears to occur across species, as Massaly and colleagues demonstrated similar pain-induced decreases in reward-seeking behavior in male and female CFA mice. This was reversed with local norBNI treatment in the ventromedial NAc shell (vNAcSh).
A role for dynorphin neurons
The vNAcSh contains a large population of neurons that express dynorphin, an endogenous opioid that binds to the KOR and controls the release of various neurotransmitters. Using optogenetic techniques, the researchers directly targeted those dynorphin-containing neurons in the vNAcSh and discovered that, similar to the KOR, activating them was also sufficient to decrease sucrose self-administration in sham mice.
Further, previous work had shown that activating dynorphin-containing neurons in the vNAcSh produced aversive behavior in mice in a real-time place aversion assay (RTPA; Al-Hasani et al., 2015). In the current study, stimulating the dynorphin-containing neurons via optogenetics in CFA mice produced aversive behavior in the RTPA. Local pretreatment with norBNI in the NAc abolished this effect, providing evidence of the role of KORs in this brain region during aversive behaviors.
Interestingly, by examining the time spent in the center zone of an open field arena, a surrogate measure of anxiety in rodents, Massaly and colleagues demonstrated that blocking KORs in the vNAcSh in mice did not impact inflammatory pain-induced anxiety behavior. This is in agreement with previous studies suggesting a role for KORs during anxiety in a different brain region, in particular, the amygdala.
To better understand how inflammatory pain engages the KOR system in the NAc, the team examined the KOR, which is a G protein-coupled receptor (GPCR), in an assay for G protein binding activity. When a GPCR like KOR is bound by an agonist, this activates the G protein signaling pathway, which researchers believe is responsible for analgesic effects. However, GPCR activation can also recruit a different pathway, the beta-arrestin pathway, which is thought to underlie the adverse effects associated with opioid drugs. Using this binding assay, the researchers demonstrated that following stimulation with dynorphin A (a form of dynorphin), GPCR binding was elevated in the NAc of CFA-treated rats compared to controls, indicating increased KOR-induced G protein activity.
However, as Martin noted, “The majority of evidence concerning KOR-mediated negative affect points to non-G protein-mediated signaling events, namely beta-arrestin-mediated signaling, and this pathway is thought to underlie the mechanism by which KOR produces negative affect. It would be nice to know if there is also an upregulation of beta-arrestin signaling in this model,” in response to KOR activation.
Next, rats that received CFA also displayed an increase in dynorphin A peptide within the NAcShCS. This increased dynorphin expression was coupled to increased excitability of dynorphin-containing neurons. According to electrophysiological recordings, this change in excitability resulted from disinhibition of dynorphin-containing neurons because of a decrease in spontaneous inhibitory postsynaptic currents in these neurons.
Finally, the researchers extended their investigation of the role of dynorphin-containing neurons in pain-induced negative affect by employing a chemogenetic approach to selectively target these neurons within the NAcShCS of rats. Silencing these cells in animals with CFA-induced inflammatory pain blocked the decreased motivation to obtain a sucrose pellet, compared to controls.
So is it the KOR or the dynorphin-containing neurons within the NAc that are critical for pain-induced negative affect in the rodents? There is evidence to make a case for either, but Massaly doesn’t see it necessarily as an either/or phenomenon.
“These findings show that the KORs in the NAc are necessary and sufficient to drive this effect, and that the dynorphin-containing neurons are also necessary and sufficient. But it doesn’t exclude the possibility, for example, that the dynorphin-containing neurons are only stimulating the KORs in other brain regions.”
In that regard, of relevance is a previous study that identified a role for dynorphin/KOR signaling in the aversive component of neuropathic pain, but not the tactile sensory component, in the amygdala (Navratilova et al., 2019).
In the pipeline
One of the intriguing possibilities from the new work is the potential of blocking the KOR to treat the negative affective component of pain. In recent years, interest has been growing in KOR antagonists for the treatment of pain (see recent PRF related webinar presented by Frank Porreca, University of Arizona, Tucson, US), and Morón-Concepción believes that the KOR may well be a viable target, albeit with some significant hurdles to overcome.
“An antagonist specific for the KOR that can penetrate the blood-brain barrier and act centrally, and also be short acting, without any of the serious opioid-related adverse effects would be very beneficial,” he said. “We are in the process of testing such compounds in our animal models, with the aim to bring them forward for clinical testing.” One such compound from another group is currently undergoing testing in people for treatment of neuropsychiatric disorders (Guerrero et al., 2019).
Martin also sees the potential of this approach. “From the receptor point of view, it’s certainly intriguing to consider using KOR antagonists to elevate mood in the presence of pain, which is clearly a significant problem in chronic pain. If we truly want to find alternatives to mu opioid receptor agonists, it’s critical to examine the negative affect of pain because that’s what the opioids, at least in the beginning, are very effective at treating. One of the really nice things about this study is that they are investigating the mechanism of negative affect, as any new pain drug will have to be effective against that at some level.”
Dara Bree is a postdoctoral fellow at Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, US.
Featured image credit: Rob Hurt/Wikimedia Commons. Creative Commons Attribution-Share Alike 4.0 International license.