How Chronic Pain Changes Nerve Signals

By Pat Anson, Editor

Swedish researchers have developed a surprising new theory about what causes chronic nerve pain and why it is so difficult to treat.  

It has long been assumed that some sensory neurons only transmit pleasant tactile sensations, while others specialize in transmitting pain. But scientists at Karolinska Institutet have discovered that neurons that normally allow us to feel a caress or soft touch can switch roles and start signaling pain after nerve damage.

The researchers identified a small RNA molecule (microRNA) in neuron cells that regulates how touch is perceived. Levels of the molecule drop after neurons are damaged, which raises levels of a specific ion channel that makes the nerves sensitive to pain.

"Our study shows that touch-sensitive nerves switch function and start producing pain, which can explain how hypersensitivity arises," says Professor Patrik Ernfors at Karolinska Institutet's Department of Medical Biochemistry and Biophysics.

"What's interesting about our study is that we can show that the RNA molecule controls the regulation of 80 per cent of the genes that are known to be involved in nerve pain. My hope, therefore, is that microRNA-based drugs will one day be a possibility."

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The research was primarily conducted on mice but also verified in tests on human tissue, where low microRNA levels could be linked to high levels of the ion channel and vice versa, suggesting that the mechanism is the same in humans. Researchers believe the study findings, published in the journal Science, could lead to more effective pain treatments   

"It's vital that we understand the mechanisms that lead to chronic nerve pain so that we can discover new methods of treatment," says Ernfors. "The pharmaceutical companies have concentrated heavily on substances that target ion channels and receptors in pain neurons, but our results show that they might have been focusing on the wrong type of neuron."

Neuropathy and chronic nerve pain are common conditions, but the drugs available to treat them have limited efficacy. One widely used medication that blocks ion channels -- gabapentin (Neurontin) – is only effective in about half of the patients who take it, according to Ernfors.

Research Uncovers Why Some Pain Meds Don’t Work

By Pat Anson, Editor

An international team of researchers may have discovered why some pain medications are inneffective: they target receptors on the surface of nerve cells that have moved out of reach.

Their findings, published in the journal Science Translational Medicine, may lead to the development of a new class of pain medication that is more potent and less prone to side effects than opioids and non-steroidal anti-inflammatory drugs (NSAIDs).

"Opioids and NSAIDs do not work for everyone and have unacceptable side effects, particularly when used over a long period of time," said Nigel Bunnett, PhD, a professor of surgery and pharmacology at Columbia University Medical Center.

"However, previous efforts to develop more effective analgesics have been stalled by our limited understanding of the mechanisms that allow nerves to sense and transmit pain signals."

Many pain medications work by targeting protein receptors on the surface of nerve cells that transmit pain signals. One receptor – known as the neurokinin 1 receptor (NK1R) -- causes pain and inflammation when activated.

In a series of laboratory experiments on rodents, Bunnett and his colleagues discovered that NK1R, when stimulated by pain, quickly moves from the cell surface to inside the cell membrane, where it continues to function outside the reach of pain medication. Researchers found that when they added a lipid (fat molecule) to painkillers that can cross the cell membrane, they effectively blocked NK1R and provided potent and durable pain relief to the rodents.

"From these experiments, we have demonstrated that designing NK1R inhibitors that are capable of reaching the endosomal network within nerve cells may provide much longer-lasting pain relief than currently available analgesics," said Bunnett. "We think that modification of many existing compounds, as we did with NK1R inhibitors, may have the potential to enhance the effectiveness of many different classes of medications."

The next step for researchers is to see if the same results can be found in humans. If proven, it could mean that current pain medications could be redesigned to make them more effective.

"This is a proof-of-concept study that shows that we can re-engineer current pain drugs and make them more effective. The challenge is now to translate the technology into human clinical trials. This is a complex and challenging path – but the ultimate benefits to patients with nerve pain are potentially highly significant," said Dr. Meritxell Canals of Monash Institute of Pharmaceutical Sciences at Monash University in Australia.

The study was supported by grants from Australia’s National Health and Medical Research Council, the Australian Research Council, and Takeda Pharmaceuticals.