Source: Science, 1999-11-19 (see below)
Researchers at the University of Minneapolis have devised a way of destroying particular nerve cells that pass on pain messages. The nerve cells that are destroyed are ones that express Substance P receptor. A poison (Substance P - Saporin, or SP-SAP) is injected into the spine, and kills these cells. Tests on rats have shown that they were less sensitive to chronic pain after this treatment. However, the treatment did not appear to affect normal sensation, and morphine was still an effective painkiller for mild pain.
The researchers suggest that because this treatment destroys cells, it may only ever be suitable for terminally ill patients. They hope to develop ways of making these cells less sensitive without destroying them, for non-fatal conditions.
This research is in the early stages, and if it proves effective and safe in humans it is likely to be years before it is widely available.
Science 1999 November 19; 286: 1558-1561
Transmission of Chronic Nociception by Spinal Neurons Expressing the Substance P Receptor
Michael L. Nichols, Brian J. Allen, Scott D. Rogers, Joseph R. Ghilardi, Prisca Honore, Nancy M. Luger, Matthew P. Finke, Jun Li, Douglas A. Lappi, Donald A. Simone, and Patrick W. Mantyh