How does chronic pain start? Many researchers now believe the bulk of the blame falls on the brain. But how can we make the pain stop? When it comes to addressing that problem, most studies focus on targets in the spinal cord and peripheral nerves, and steer clear of the cerebral cortex.
A pair of recent papers may help open up the brain as a target for pain therapy. The studies, from the laboratory of Min Zhuo at the University of Toronto in Canada, take aim at pain-related synaptic plasticity in the anterior cingulate cortex (ACC), a brain region that connects the frontal cortex, amygdala, and hypothalamus, bringing together autonomic functions—including sensation—with emotion and cognition.
Zhou’s group previously showed that calcium/calmodulin-stimulated adenylyl cyclase type 1 (AC1) was required for synaptic changes in the ACC after nerve injury in mice. In a new study, they identify a small molecule inhibitor of the enzyme, and show it has analgesic effects in animals. In a separate report, they use an inhibitor of protein kinase M zeta (PKMζ), another enzyme involved in synaptic plasticity in the ACC and elsewhere, to alleviate pain hypersensitivity in a mouse model of neuropathic pain.
“This isn’t a magic bullet for all chronic pain,” Zhuo told PRF. “But for severe neuropathic pain, such as long-lasting spinal cord injury pain or cancer pain, if we can reduce the sensitization in the brain, I think we’ll be able to make patients feel better.”
The ACC is a well known participant in pain. For decades, clinicians have observed that lesions in that region, either from strokes or surgery, can selectively reduce chronic pain. More recently, functional imaging studies in humans have revealed that pain elicits a rash of activity in the ACC.
Zhuo and his colleagues previously found that peripheral nerve injury induces long-term potentiation (LTP) of ACC synapses (Wei and Zhuo, 2001) that is similar to the central sensitization that occurs in the dorsal horn of the spinal cord. That indicated that blocking LTP in the ACC might be a good target for treating persistent pain.
Unfortunately, the mechanism of pain-related LTP in the ACC turns out to be similar to LTP in the hippocampus, which underlies learning and memory. Both use NMDA receptor pathways, and as a result, targeting LTP as a pain treatment strategy runs the risk of producing cognitive problems. NMDA receptor antagonists, for example, are used as analgesics, but they are limited by cognitive and motor side effects (Chizh et al., 2001).
Zhuo’s group has been hunting for proteins downstream of the NMDA receptor that might make better targets for treating pain. Earlier, they showed that AC1 knockout mice experienced a dramatic reduction in inflammatory and neuropathic pain, but no effects on acute pain (Wei et al., 2002, Xu et al., 2008).
The group set out to find a drug with a similar effect, and their results are reported in the January 12 issue of Science Translational Medicine. By screening a small collection of rationally designed compounds, they identified NB001 as the most potent inhibitor of AC1 in cultured human cells and mouse brain slices. In mouse and rat models of neuropathic and inflammatory pain, systemic administration of the compound reduced the animals’ pain hypersensitivity. NB001 appeared to act in the brain, because it was effective when injected into the ACC, but not when applied intrathecally or subcutaneously. In normal mice, responses to noxious heat and punctate stimuli were untouched, indicating that the molecule does not meddle with acute pain responses. Treated animals also sailed though learning and memory tests, as well as tests for anxiety and motor function.
These results hint that it may be possible to target AC1 without the serious side effects that typically plague attempts to take pain treatment into the brain. Now, further studies are needed to make sure that memory processing in the hippocampus is really spared, say Reza Sharif-Naeini and Allan Basbaum of the University of California, San Francisco. Writing in an accompanying perspective, they point out that in previous studies, AC1 mutant mice showed depressed hippocampal LTP and had problems with a water maze memory test (Wu et al., 1995). “It will be essential,” they write, “to conduct a thorough examination of the effects of NB001 on hippocampal function in particular and on memory processing in general.”
In the other paper, published at the end of last year in Science, Zhuo and his collaborators reported results on another protein involved in ACC plasticity, protein kinase M zeta (PKMζ). They found that PKMζ, acting downstream of AC1, maintains the injury-induced synaptic changes that AC1 helps to create. When the researchers inhibited PKMζ with a peptide in mice, they were able to alleviate neuropathic pain. However, PKMζ is also crucial for long-term memory storage in the hippocampus and other brain regions (Sacktor, 2011). Inhibiting PKMζ to treat pain “is going to give you some side effects,” says Zhuo.
But when it comes to targeting AC1, he is very optimistic: “So far so good. We haven’t really seen anything negative.” In fact, Zhuo says he is now working to move NB001 into pre-clinical testing.
In the meantime, Zhou’s group will keep looking for other therapeutic targets in the LTP pathways of the cortex. “This is just the beginning of looking at cortical plasticity and chronic pain,” he said.
Comments
Min Zhuo, University of Toronto
In the commentary by Sharif-Naeini and Basbaum that accompanied our Science Translational Medicine paper, the authors suggest that LTP and memory processing are disturbed in AC1 knockout mice. On the contrary, learning related late-phase LTP (L-LTP) is normal in AC1 knockout mice, and AC1 knockout mice only showed a mild learning defect in the hidden platform test but not visual platform test.
Wu et al., 1995 was the first study of Dan Storm's AC1 knockout mice. In this early paper, they only recorded CA3 early LTP for 30 min. It is commonly accepted now that late-phase LTP is more important for long-term memory. Also, in this report, only field recording was used. Even for early LTP, they only saw the reduction, not a complete disruption. For learning phenotypes, they found that AC1 knockout mice performed normally in the visual platform Morris Water Task. The authors indicated that AC1 mutant mice were not generally deficient in learning ability. To push the task harder, they also used the hidden platform task. AC1 mutant mice also knew to use spatial cues to find the hidden platform, suggesting that they are still learning in this test. The only difference is that they spent less time in that hidden area than wild-type mice. It is important to point out that this test was done within the same day, thus it did not represent long-term memory. This defect can be affected by other factors. Thus, in general, even in the original paper, authors did not see robust memory defects in AC1 knockout mice.
In a following paper (Wong et al., 1999), the same authors reported that AC1 knockout did not affect L-LTP (3 hrs recording) in the CA1 region of the hippocampus. They also did not find any long-term memory defects in two different memory tests (Figs 5 and 6) in AC1 knockout mice. The general conclusion is that both AC1 and AC8 are essential for learning and memory. Inhibiting only one of the subtypes is not sufficient to interfere with cognitive learning and memory. This has been beautifully demonstrated in this paper.
More importantly, due to the lack of novel drug treatment of neuropathic pain (because of everyone is worried of side-effects in the brain), doctors have to use surgical lesions to the ACC/brain to relieve chronic pain in patients. Two recent reports (Yen et al., 2005 and Wilkinson et al., 1999) found that bilateral anterior cingulotomy was useful in alleviating chronic pain in patients. Only small or subtle cognitive impairments were noted.
I hope that these comments may help to show why I am favoring for AC1 as a target for chronic pain. It is also important to remember that all chronic pain drugs currently approved by FDA and used clinically such as opioids and gabapentin affect ACC functions in the brain, and they also have significant cognitive side effects, not to mention that they fail to stop chronic pain at very high doses.
References:
Wu ZL, Thomas SA, Villacres EC, Xia Z, Simmons ML, Chavkin C, Palmiter RD, Storm DR. Altered behavior and long-term potentiation in type I adenylyl cyclase mutant mice. Proc Natl Acad Sci U S A. 1995 Jan 3;92(1):220-4. PMID: 7816821.
Wong ST, Athos J, Figueroa XA, Pineda VV, Schaefer ML, Chavkin CC, Muglia LJ, Storm DR. Calcium-stimulated adenylyl cyclase activity is critical for hippocampus-dependent long-term memory and late phase LTP. Neuron. 1999
Aug;23(4):787-98. PubMed PMID: 10482244.
Yen CP, Kung SS, Su YF, Lin WC, Howng SL, Kwan AL. Stereotactic bilateral anterior cingulotomy for intractable pain. J Clin Neurosci. 2005 Nov;12(8):886-90. PubMed PMID: 16326270.
Wilkinson HA, Davidson KM, Davidson RI. Bilateral anterior cingulotomy for chronic noncancer pain. Neurosurgery. 1999 Nov;45(5):1129-34; discussion 1134-6. PubMed PMID: 10549929.