Say you stub your toe. Five or six feet north of thebruised digit, your brain feels the pain. Perhaps itcommands your mouth to utter "Ouch!" or some otherappropriate expletive. What's going on?
First, that insult to the toe contuses or breaks its skin.Tissue damage releases ATP _ adenosine triphosphate_ from those bruised skin cells to activate the nervesnearest the injury. ATP is ubiquitous in mammalian cells,to which it supplies energy. And recently neurologistshave recognized ATP as one of the brain'sneurotransmitters.
Next, the sensory neuronal axons transmit a pain signalfrom that toe to their nerve terminals in the dorsalganglion of the spinal cord. This switching centerbetween peripheral and central nervous system networksrelays the painful news to regions of the brain, which feelthe hurt, and react to it.
That's the brief, broad-brush picture. Getting down tobasics, ATP's activation of those toe-adjacent sensoryneurons involves a number of chemical mediators, whichinduce an electrical signal in the nerve cells.
This sets sodium ions pouring into the cell, through ionchannels traversing its surface membrane. These tunnelsopen and shut in response to specialized proteins that actas ATP nociceptors _ pain-signaling receptors.
"It's been known for 30 years," observes molecularneurobiologist John Wood, "that ATP can induce thesensation of pain in humans. So the fact that ATP ispresent in all cells as a component of the energytransduction system means that it's a wonderful signalingmolecule."
Wood, a senior lecturer at University College in London,is first author of a paper in today's Nature titled "A P2Xpurinoceptor expressed by a subset of sensory neurons."
This ATP receptor, known as P2X, had only two knownsub-subsets, P2X1 and P2X2, until Wood's article inNature introduced a third, P2X3. "What's novel in ourpaper," he said, "is the cloning and characterization of anew member of that protein family. The gene thatencodes it has never before been discovered."
Dual Papers Were `Pure Coincidence'
He and his co-authors "found a variety of receptors,which are only expressed on neurons that respond totissue damage." Wood added, "These are potential newtargets for analgesic drug development, and should bepotentially interesting to the pharmaceutical industry."
The leading exponent of that industry is Glaxo Holdingsplc, of London. "By pure coincidence," Wood said, theGlaxo Institute for Molecular Biology in Geneva has apaper in the same issue of Nature. Its title: "Co-expression of P2X2 receptor subunits can account forATP-gated currents in sensory neurons." It too reportscloning of P2X3 cDNA from rat dorsal root ganglia.
Alan North, a co-author of that paper, and an ion-channelresearcher at the Glaxo institute, told BioWorld Todaythat, "We and Wood cloned the same receptor, P2X3."The Glaxo group, he added, "showed that when P2X2 isco-expressed with subunit, P2X3 the two assemble into asingle protein," which passes the message of ATP-transmitted pain sensation. He suggested that "analgesiccompounds might be developed to block that assembly."
Wood observed that his subunit three "was actuallyidentified by a piece of virtuoso molecular genetics." Histeam, like Glaxo's, made cDNA libraries from sensoryneuron channel transcripts derived from the dorsalganglia of rats. Then they subtracted RNA sequenceswhich are expressed throughout the central nervoussystem and viscera.
"We ended up," he recalled, "with just 46 specific geneproducts that seem to be expressed only by peripheralsensory neurons. By sequencing these particular geneproducts, and comparing them with known ATP-gatedreceptors, we realized that we'd found a new P2Xreceptor specifically expressed only by sensory neuronsthat respond only to tissue damage, and to no other cells."Wood observed that "P2X3 is the only ligand-gatedchannel known to be expressed exclusively by a subset ofsensory neurons."
Given this "very fine specificity pattern of expression,"he observed, "one might be able to selectively target thisreceptor. But rather than focusing on specificallyblocking it to create an analgesic drug," he continued,"what we are now doing is trying to define in humantissues the regulatory elements that control its expressionin those particular cells that seem important in painperception."
Mute, Not Abolish, Chronic Pain
Wood aims not merely at making more finely tunedtopical pain-killers but "down-regulating the proteins'expression, as opposed to blocking their action." By thisrationale, he explained "we might be able to mute thesensitivity to pain, which causes so much unhappiness invarious chronic-pain states, without creating a state ofcomplete analgesia, which would be a disaster."
He went on: "If we can map the regulatory elements, it'snot unreasonable or far-fetched to set up some kind ofdrug screen, where we may come across something thatcould exert analgesic action in that way."
He allowed that "the human gene product wouldobviously be of potential commercial interest in a drug-screening program. We haven't actually gotten the humanreceptor yet, so we haven't been able to patent that.We're funded by the Wellcome Trust in England, sowe're completely non-commercial.
"I'm sure," he added, "that our colleagues at Glaxo havepatented that application." North told BioWorld Todaythat in May 1994, Glaxo had applied to patent subunitsone, three and four.
Pharmacologist Charles Kennedy, a lecturer at theUniversity of Strathclyde in Glasgow, wrote an editorialaccompanying the two Nature papers, titled "Painfulconnection for ATP" .
He told BioWorld Today that "new drugs selective for thenew receptor would have fewer side effects" thancomparable existing compounds. He pictures suchinnovative pharmaceuticals as being "neurally availablein the form of creams or sprays."
Kennedy makes the "important point that this is apossible opportunity to create non-addictive analgesics,as an alternative to opioids, which are the basis of mostanalgesics today." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.