Contact: Jennifer O'Brien
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University of California, San Francisco
UCSF finding could lead to a new class of pain relieving drugs
UC San Francisco researchers have identified a new molecular pathway
through which chemical signals alert the body to pain, and inhibiting
the key protein in this pathway could bring relief in a broad spectrum
of pain syndromes, they say.
The finding, drawn from a study in mice and rats, applies to
inflammatory pain associated with such conditions as arthritis and
colitis, torn ligaments and sprained ankles, and post-operative pain.
However, the researchers expect the finding will apply even more
broadly.
"This discovery is extremely important," said the director of the
National Institutes of Health Pain Center at UCSF, Jon Levine, PhD, a
professor of oral and maxillofacial surgery and medicine and a senior
author of the paper. "I think this signaling pathway will be shown to
play a role in many kinds of pain."
The study, published in the Sept. 24 issue of Neuron, was funded by the
Ernest Gallo Clinical and Research Center at UCSF and the National
Institutes of Health.
The body's immune system responds to many forms of tissue injury by
producing an inflammatory response, which includes the release of
chemical signals into injured tissue, where they sensitize pain-sensing
neurons. As a result, stimuli that normally would not cause pain, such
as the brush of a shirt being drawn onto the body, become painful when
the skin is sunburned; likewise, the movement of a joint, normally
unnoticed, would cause pain in the presence of arthritis.
Chemical signals act on pain-sensing neurons by latching on to specific
cell-surface receptors that convey the signals into the cell. Once
inside, the chemical signal initiates a cascade of molecular events that
culminates with the neurons transmitting pain signals out of the cell
body and into the central nervous system, where pain is felt.
Current inflammatory-pain drugs -- the nonsteroidal anti-inflammatory
drugs, or NSAIDS, including the new COX-2 inhibitors -- act by blocking
the production of some of these chemical signals, or inflammatory
"mediators." However, because these drugs block only a small percentage
of these messages, their effectiveness is limited.
The significance of the UCSF finding is that the researchers have
identified a protein enzyme inside pain-sensing neurons through which
they believe many of these inflammatory mediators - including those
targeted by NSAIDS - act, suggesting a possible target for broad-based
pain therapy.
"Identifying the common signaling pathways inside these pain-sensing
cells would prevent us from having to identify blocks for every
inflammatory mediator," said Levine. "I think this enzyme will prove to
be the central signaling pathway by which most chemical mediators act on
pain-sensing neurons."
For more than a decade, researchers have thought that the protein kinase
C (PKC) enzyme played a role in the pain-sensing neurons' activity, but
they have not known which of the ten known forms of the enzyme might be
involved. In the current study, the researchers discovered the role
played by protein kinase C epsilon (PKC).
The researchers discovered the PKC signaling pathway by conducting
studies in mice that lacked the enzyme and in rats in which the enzyme
was inhibited by a drug.
In one study, they compared the responses of normal mice, and mice
lacking the PKC enzyme, to painful stimuli, and determined that the mice
responded equally to stimulation.
However, when they added epinephrine, an inflammatory mediator that
heightens the sensitivity of pain sensing neurons, those without the
enzyme exhibited a "significantly reduced" reaction to stimulation.
In a second study, the researchers applied the chemical irritant acetic
acid. The response to the painful stimulus, which causes inflammation,
was "almost completely blocked" in the mice lacking PKC, they said. In a
third study, the researchers examined rats in which PKC was inhibited.
Predictably, both these animals and control animals responded to
stimulation. However, when epinephrine was added to increase pain
sensitivity, the animals with the inhibited enzyme became markedly less
sensitive to the pain.
Epinephrine acts on pain-sensing neurons, or nociceptors, by enhancing
an ion channel known as TTX- RINA, which sensitizes the pain-sensing
neurons to previously innocuous stimuli. As a check on the animal study
results, the researchers examined whether inhibiting PKC would blunt
epinephrine's action in pain-sensory neurons in laboratory cultures. It
did. In cultured cells in which the enzyme was inhibited, epinephrine's
effect was decreased by half, demonstrating that epinephrine depends on
PKC to prompt a full effect on the TTX-RI NA channel in a critical group
of pain-sensing neurons, the researchers said.
The researchers further demonstrated PKC's role by examining the
response of rats to a potent irritant known as carrageenan. When they
applied the seaweed compound in rats exposed to stimulation, the animals
exhibited pain. But when the animals were pretreated with the chemical
that inhibits the PKC enzyme, the painful response was "almost
completely reversed," the researchers report. Carrageenan is commonly
used by the pharmaceutical industry as a model to screen for
pain-reducing drugs.
Finally, the researchers showed that PKC modulates the pain response
induced by the chemical known as nerve growth factor. When the factor
was injected into normal rats exposed to stimulation, the animals
experienced heightened pain. But when the factor was injected in animals
in which PKC was inhibited, their pain threshold was higher.
"These results suggest that PKC plays a key role in regulating pain
sensitivity," said a senior author of the UCSF paper, Robert Messing,
MD, an associate professor of neurology. "The fact that inhibiting PKC
reduced pain in response to several different sensitizing agents is
significant."
Since absence or inhibition of PKC does not disturb basic pain-sensory
thresholds, needed to help alert the body to possible danger, and the
mice in which the enzyme was missing appeared normal, it may be
possible, the researcher said, to develop PKC inhibitors that reduce
pathologic pain without producing serious systemic side effects or
interfering with normal pain responses.
Co-authors of the UCSF study were Sachia G. Khasar, PhD, an assistant
research pharmacologist, K.O. Aley, PhD, an assistant research
pharmacologist, William Isenberg, MD, PhD, an assistant reseach
endocrinologist, Gordon McCarter, PhD, a post-doctoral fellow, Paul G.
Green, PhD, an assistant professor, all in the Department of Internal
Medicine and Oral Surgery and NIH/UCSF Pain Center; and Yu-Huei Lin,
PhD, at the time a postdoctoral fellow, Annick Martin, PhD, a
post-graduate research fellow, Jahan Dadgar, BS, a staff research
associate, Thomas McMahon, BS, a staff research associate, Dan Wang, MS,
BS, a a staff research associate, Bhupinder Hundle, PhD, at the time a
postdoctoral fellow, and Clyde Hodge, PhD, an assistant adjunct
professor in the Department of Neurology, Ernest Gallo Clinical and
Research Center at UCSF.