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.
Dorsal horn neurons (shown here in green) have different firing patterns in wild-type mice, which fire less frequently, from those bred without Kv4.2 potassium channels, which fire more often in response to a potentially painful stimulus. (Image courtesy of Washington University School of Medicine)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."....
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