While still held up as the “gold standard” for treating chronic and post-operative pain, morphine and other opiate drugs come with serious side effects. Tolerance and drug-induced hyperalgesia top the list, and until now were thought to share a common mechanism. But new work from Yves De Koninck at Institut Universitaire en Santé Mentale de Québec, Canada, shows that they are, in fact, distinct processes.
The work “really puts another nail in the old hypothetical coffin that you can’t separate these effects of tolerance and hyperalgesia,” says Howard Gutstein, a researcher and clinician at MD Anderson Cancer Center, Houston, US, who was not involved in the current work.
The separation of hyperalgesia from tolerance, says De Koninck, means that the insidious side effect can also be separated from the drugs’ pain-killing power. “Hyperalgesia is really a separate reaction of the CNS. This opens an avenue to prevent that side effect without losing the analgesia of morphine.”
The new findings, from De Koninck and co-first authors Francesco Ferrini (also in Québec) and Tuan Trang, from Michael Salter’s lab at the Hospital for Sick Children in Toronto, Canada, appear online in the January 6 Nature Neuroscience.
In defiance of Occam’s razor—a principle dictating that among competing hypotheses, the simplest is preferable to the more complex—De Koninck’s team uncovered an intricate signaling pathway that leads to aberrant pain signaling. For reasons unknown, microglia—the nervous system’s sentinel immune cells—within the dorsal horn of the spinal cord express μ (mu)-opioid receptors. After morphine binds to those receptors, microglia increase their expression of ATP-binding P2X4 receptors, which in turn leads to microglial release of brain-derived neurotrophic factor (BDNF). BDNF then acts on lamina I projection neurons, decreasing their expression of KCC2, a transporter that maintains chloride homeostasis, which is key to inhibitory neurotransmission.
The researchers showed that interrupting any step of the pathway—depletion of spinal microglia, deletion of microglial BDNF, or inhibition of the P2X4 receptor—prevented the development of hyperalgesia in mice. However, all three manipulations left analgesia and tolerance intact. Gutstein describes the experiments as a “tour de force dissecting the signaling pathway.”
The results on tolerance stand in contrast to previous work showing a role for microglia and neuroinflammation in both opioid-induced hyperalgesia and tolerance (see PRF news story and comment below from Mark Hutchinson).
The team demonstrated the drug-induced collapse of the chloride gradient in neurons in an elegant series of electrophysiological experiments. The ion balance disruption directly affected the directional flow of ions through open GABA-receptor channels. Whereas the inhibitory neurotransmitter usually evokes a hyperpolarizing current, the chloride disruption perturbed the reliability of that signal. The end result is that morphine appears to release inhibition of spinal cord projection neurons by handicapping chloride extrusion, affecting GABA signaling as well as the membrane's overall excitability.
In previous work, the authors described the same basic signaling mechanism as underlying the pain following nerve injury (Coull et al., 2005). Ironically, says De Koninck, the new results suggest that “morphine tends to recapitulate what happens in neuropathic pain.” Why this inflammatory cascade—initiated in microglia and culminating in neuronal chloride balance disruption—follows chronic morphine exposure presents an interesting puzzle. De Koninck asks, “Is it a typical homeostatic response of the body to drugs? Or is it just bad luck?”
In either case, the upshot is that normally painless sensory events become painful, but how? The majority of projection neurons in lamina I of the spinal cord normally respond only to noxious input, dependably signaling the brain that a potentially damaging event is happening out in the periphery. But after nerve injury—and presumably after hyperalgesia brought on by morphine—over 80 percent of the projection neurons begin to respond to painless touch. While the disruption in chloride concentration may be localized to excitatory projection neurons, De Koninck explains, those neurons are part of a larger pain-signaling network that encompasses both nociceptive and non-nociceptive sensory neurons. The connections with non-nociceptive inputs are likely indirect and normally suppressed. “This [hyperalgesia] pathway basically unmasks this existing interconnection, causing a noxious response to innocuous input.”
In terms of a therapeutic “avenue,” the new results reveal targets to combat hyperalgesia, including P2X4 receptors, BDNF signaling, and GABA receptors. Despite significant efforts, specific antagonists for the P2X4 receptor have so far eluded chemists. And while BDNF is a more tractable target, most neuroscientists would agree that shutting down that molecule might do more harm than good, because it provides neurons with critical support. Enhancing GABA transmission might also be folly, says De Koninck, because inhibitory currents in this scenario might either collapse, “or worse, switch to an excitatory signal,” he says. Instead, the group has chosen to focus on KCC2 itself with a double-pronged strategy: to stabilize the protein at the membrane, and to enhance its activity.
The corollary to the current work came from Gutstein, who last year described a mechanism for opioid tolerance involving a growth factor receptor kinase (see PRF related news story). While the side effects of morphine present significant limitations, says Gutstein, “opiates are all we’ve got” to battle some types of pain. But now, he says, it appears that we might develop ways to contend with their negative consequences, “one side effect at a time.”
Stephani Sutherland, PhD, is a freelance neuroscience writer based in Southern California.
Image Credit: © Deeboldrick | Dreamstime.com