Cortical Dendrites: A New Site for Chronic Pain Plasticity
Chronic pain leads to changes in how neurons in the pain pathway communicate with one another. So far, research on pain plasticity has focused mainly on synapses, the communication points between neurons (see PRF related news story). But a new study shows that changes in the properties of dendrites themselves, in neurons of the anterior cingulate cortex (ACC), contribute to nerve injury-induced neuropathic pain in a mouse model.
Thomas Nevian, University of Bern, Switzerland, and postdoctoral fellow Mirko Santello determined that downregulation of hyperpolarization-activated cyclic nucleotide-regulated (HCN) channels was responsible for elevated cortical dendritic excitability they saw in mice with nerve injury. Moreover, activation of 5-HT7 serotonergic receptors counteracted this change—and reduced mechanical hyperalgesia in the animals. These results provide the first hint of a cortical mechanism of action for antidepressant drugs targeting serotonin that are used to treat chronic pain. The research was published April 8 in Neuron.
Min Zhuo, University of Toronto, Canada, who studies synaptic plasticity in the ACC in chronic pain (see PRF related news stories here and here) but was not involved in the current investigation, called the new report “very important and very interesting work.” Quite different from studies of synaptic plasticity that focus on mechanisms involving, for instance, glutamate release, he added, “This study actually shows in a chronic pain condition that there can also be changes in neuronal properties related to action potential firing.” This work also takes the next step toward understanding cortical processes in pain, Zhuo said.
Defects in dendritic integration of incoming signals
“If you think about chronic pain as a memory trace, one mechanism could be that, rather than synapses changing, the electrical properties of the cell itself change,” Nevian told PRF. A lasting change in cells’ firing properties provides a potent way to store information, he added.
To begin to test that hypothesis, the authors first gave mice either a chronic constriction injury (CCI) or a sham surgery. In brain slices taken from both groups, the investigators performed dual electrophysiological recordings from the cell soma and the apical dendrite of pyramidal neurons in layer 5 of the ACC—the primary output neurons of the cortex. “We can look at the electrical communication between these two points on the neuron, and we can infer the electrical properties of the dendrite,” said Nevian. The challenging technique affords the best way to measure those properties independent of synaptic activity.
As has been observed in other cortical neurons, the dendrites sat at a slightly more depolarized voltage than the cell bodies, in both CCI and sham-operated mice. But, “this difference in the gradient of the resting membrane potential was smaller in CCI mice,” Nevian said.
Thomas Nevian (left) and Mirko Santello
The membrane potential gradient—becoming more depolarized from soma to dendrite—is thought to arise from tonic Ih current, which is a nonselective cation current that flows through HCN channels. The authors found that Ih current was significantly smaller in dendrites of CCI animals compared to sham-operated mice, “whereas at the soma, we did not see a difference in Ih current between groups,” Nevian said. The finding, he added, “showed that applying this technique [of recording directly from the dendrite] to the problem of neuropathic pain is fruitful. Without it, we would have missed this difference.” The researchers did not detect any other alterations in the active or passive electrical properties of dendrites of neurons from CCI mice, indicating that nerve injury did not significantly change expression of other ion channels.
Although Ih current is slightly depolarizing—or excitatory—the current paradoxically acts as a brake on neuronal excitability in dendrites of pyramidal neurons, serving as a shunt to defuse incoming synaptic stimuli. Consequently, the researchers next tested how the change in Ih in the CCI animals would affect how dendrites integrated incoming signals. “Independent of synapses, we injected into the dendrites currents representative of synaptic stimuli to see how they propagated to the soma,” Nevian explained. With reduced Ih, neurons from CCI mice displayed increased temporal summation of inputs, leading to increased neuronal excitability. The end result: “The cells need fewer synapses activated in order to fire action potentials,” Nevian said.
Seeking a mechanism
The researchers next wanted to determine the mechanism behind the change in the current carried by HCN channels in L5 neurons from the nerve-injured animals. “Plasticity of ion channels can appear in two ways. Either the channel subunit composition changes, affecting conductance properties, or channel number is altered,” said Nevian, but they saw no indication of the former. Instead, immunohistochemistry experiments suggested that fewer HCN proteins were present on the cell surface. “There seemed to be a retraction or removal of the channels from dendrites in the CCI condition,” he said.
Then, to understand how nerve injury might trigger the dendritic plasticity, the authors used an electrode to stimulate L5 neurons in sham-operated mice with a specific stimulation pattern to induce a reduction in HCN-mediated currents. Activation of metabotropic glutamate receptor type I (mGluR1) was required to see the change in Ih current; bath application of an mGluR1 antagonist prevented the plasticity. In CCI mice, however, stimulation did not change Ih, presumably because HCN channels were already downregulated by the barrage of input following nerve injury. The findings suggest that intensified inputs to the ACC, in the form of increased glutamate release, lead the L5 neurons to retract HCN channels, thereby changing their dendritic properties.
Confocal image of fluorescently labeled 5-HT7 receptors in ACC. The arrowheads indicate clearly labeled apical dendrites of L5 pyramidal neurons. Figure and legend reprinted from Santello and Nevian. Neuron. 2015 Apr 8; 86:1-15, with permission from Elsevier.
The authors also considered how they might reverse the dendritic change in Ih observed in CCI animals. “HCN channels are modulated by cyclic AMP [adenosine monophosphate],” Nevian said, “so we had the idea to look at metabotropic receptors coupled to adenylate cyclase,” which produces cyclic AMP. One candidate was the serotonergic receptor type 7 (5-HT7). When the group applied a 5-HT7 agonist to brain slices from CCI mice, “we found that it completely reversed the phenotype on the dendrites,” Nevian said—the Ih current was restored. Moreover, when they injected the agonist directly into the ACC of CCI animals, they saw a reduction in the animals’ mechanical pain sensitivity at the affected paw. “By modulating dendritic integration, we can change the behavioral response of the animal—that’s very exciting,” Nevian said. Over two hours, the animals’ pain sensitization returned. “We suspect that a higher dose of drug or repeated injections might prolong the analgesic effect,” Nevian said.
The serotonergic system is known to be involved in pain modulation; descending modulation from the midbrain to the spinal cord dampens incoming pain signals via serotonin, which is how tricyclic antidepressants and serotonin-reuptake inhibitors are thought to alleviate neuropathic pain. “This is the first report that shows serotonergic modulation of pain perception in the cortex, specifically in the ACC,” Nevian said.
The new findings clearly show a role for serotonin in the ACC in a chronic pain model, said Zhuo. He can see many directions for future investigations. “The other functions of the ACC, like encoding emotional aspects of pain, could use the same mechanism of plasticity,” Zhuo postulated. Testing this hypothesis will require use of behavioral tests that measure emotional and other facets of pain. Understanding the effects of activating other subtypes of serotonin receptors in the ACC will also be important, Zhuo said.
Nevian offered a caveat regarding the therapeutic implications of the work. “Just globally elevating serotonin might not be a good idea,” he said, considering how complex the neuromodulator’s effects are in the brain, but selectively activating 5-HT7 in the ACC might provide beneficial effects. Similarly, changing HCN channel function globally would not necessarily produce analgesia, Nevian said, because the role of these channels is highly variable, depending on their cellular location (e.g., see Emery et al., 2011). “It might be beneficial in cortex, but evidence indicates it might severely enhance discharge of nociceptors—and we can’t predict what that would mean for pain perception.”
Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California.
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