Studying mice, pain researchers at Washington University School of Medicine in St. Louis have identified two key components in the pain cascade that may provide targets for more effective analgesic drugs with potentially fewer side effects.
A team led by Robert W. Gereau IV, Ph.D., associate professor of anesthesiology, reports in the April 6 issue of the journal Neuron the identification of a potassium channel that plays a crucial role in what scientists call pain plasticity, the ability of molecules in the spinal cord to amplify or diminish the response to a painful stimulus.
Electrical activity in neurons is produced by subtle changes in the cell's potassium concentration. To maintain correct amounts of potassium, cells are equipped with proteins that poke through the cell membrane like small pores. The proteins are called ion channels or potassium channels, and they create tiny sieves through which potassium can flow from the inside to the outside of the cell.
"The potassium channel we are studying is called Kv4.2," says Gereau, who also is chief of the basic research division of the Washington University Pain Center. "Through a series of experiments, we've been able to determine that Kv4.2 decreases transmission through the pain pathway. It helps regulate the ability of pain-transmitting neurons to transmit their signals to the brain."
We sense pain through primary sensory neurons with nerve endings in the skin, the joints, internal organs or muscles. Those nerve cells interpret signals indicating tissue injury or potential injury and transmit these signals to a part of the spinal cord called the dorsal horn. Pain-transmission neurons in the dorsal horn receive those messages and transmit their own pain signals to the brain.
The signals from neurons in the dorsal horn can be either damped down or enhanced, depending upon many factors, according to Gereau. That's the plasticity that makes some things hurt more than others, even though the painful stimulus itself might not change.
"Say you pinch your finger," Gereau says. "It might not feel painful because even though you activate some of these pain-sensing neurons, that pain signal doesn't just pass through the spinal cord to the brain. Active potassium channels in the spinal cord neurons may inhibit their firing."
On the other hand if a person has a sunburn, a light touch that would not normally cause pain may suddenly hurt a great deal. In that case, the potassium channels in dorsal horn neurons are less active, and they can't interfere with the transmission of pain signals to the brain.
The researchers tested the role of Kv4.2 in damping down the pain response by studying knockout mice that had no Kv4.2 gene. The mice were bred so that some pups in a litter were knockout mice while others were normal, wild-type mice with the gene. Knockout mice withdrew their paws from a heat source or mechanical stimulus more quickly than their wild-type siblings.
The scientists also looked at dorsal horn neurons in culture from both wild-type and knockout mice and found that the neurons from the knockout mice fired more readily than neurons from wild-type mice.
"That's because the inhibitory Kv4.2 channel was gone in the knockout mice," Gereau says. "It's hard to say that these mice somehow sense pain more intensely, but their thresholds for withdrawal from heat and touch are much lower than their brothers and sisters that are genetically normal."
Potassium channels in dorsal horn neurons are regulated by a molecule called extracellular signal-related kinase (ERK). Past research has demonstrated that if ERK activity is inhibited, much of the spinal cord's sensitivity to pain can be diminished. But scientists haven't really known what ERK was doing.
In this study, the research team looked for targets that might interact with ERK, and the potassium channel Kv4.2 happened to be one of those potential targets. They studied dorsal horn neurons from mice to clarify the relationship between ERK and Kv4.2.
"When an injury occurs, there is a massive barrage of activity in pain-sensing neurons, and as those neurons fire, that causes neurochemical changes in dorsal horn neurons," Gereau explains. "Those neurochemical changes activate the ERK pathway. One of the things ERK does is modify Kv4.2, so it can't inhibit the firing of dorsal horn neurons as efficiently as it normally does. Because Kv4.2 can't do that, more pain signals get sent to the brain."
In tests of neural response to inflammation, the researchers found that although Kv4.2 knockout mice normally were more sensitive to heat and touch, after injections with a chemical that causes inflammation, the knockout mice became less sensitive to heat and touch than their wild-type siblings. But it's not because the inflammation made the knockout mice less sensitive. It's because the wild-type mice became more sensitive.
"After inflammation, normal mice have a dramatically reduced threshold for withdrawal from these mechanical and thermal stimuli," Gereau says. "The knockout mice don't develop this hypersensitivity after inflammation because they lack the Kv4.2 potassium channel that is affected by ERK. But in normal mice, ERK inhibits Kv4.2, so they become more sensitive after inflammation."
Gereau says the experiments demonstrate that Kv4.2 is a primary target for ERK, and he says both molecules are potential targets for drugs to control or eliminate pain.
"Any intervention that would decrease the activity of ERK or increase the activity of this potassium channel should have an analgesic effect in animals and in people," he says.
In fact, many currently prescribed anti-inflammatory drugs and opioids are known to decrease ERK activity in the spinal cord. It's not clear whether those drugs also affect the Kv4.2 potassium channel, but because these experiments have demonstrated that Kv4.2 is mediated by ERK, Gereau hypothesizes that the drugs affect both.
Although they inhibit ERK activity in the spinal cord, Gereau says many current drugs have unwanted side effects and potential addiction liabilities. His team is working to identify all of the molecular pathways responsible for making neurons more sensitive to pain signals. They hope to develop more specific targets for analgesic drugs that might work differently than opioids, which often require higher doses for subsequent treatments to produce the same effect and can interfere with respiratory function.
Gereau says there also have been problems associated with anti-inflammatory cox-2 inhibitors, such as Vioxx, which was found to increase the risk of serious cardiovascular events, including heart attacks and strokes. He is searching for approaches to pain relief that rely on different mechanisms like ERK and Kv4.2.
"We've been in contact with various companies with drug discovery programs in place," he says. "We're interested in pursuing the idea that inhibitors of the ERK pathway or any compounds that would up-regulate the activity of these potassium channels might be effective treatments for pain."
If such drugs are shown to be safe for use in humans, Gereau says they quickly could be tested in small-scale trials conducted through the Washington University Pain Management Center. In addition, Gereau's laboratory is working to identify the longer-term effects of ERK. Because the molecule is known to cause gene transcription changes, Gereau believes it might have a role in chronic pain by changing the proteins present in the spinal cord and influencing neurons to report pain even in the absence of a painful stimulus.