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Pain Research and Treatment
Volume 2013, Article ID 407504, 6 pages
http://dx.doi.org/10.1155/2013/407504
Research Article

Chronic Opioid Therapy and Opioid Tolerance: A New Hypothesis

Anesthesiology Service, Durham Veterans Affairs Medical Center and Department of Anesthesiology, Duke University School of Medicine, 508 Fulton Street, Durham, NC 27705, USA

Received 3 November 2012; Revised 23 December 2012; Accepted 27 December 2012

Academic Editor: Michael G. Irwin

Copyright © 2013 Joel S. Goldberg. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Opioids are efficacious and cost-effective analgesics, but tolerance limits their effectiveness. This paper does not present any new clinical or experimental data but demonstrates that there exist ascending sensory pathways that contain few opioid receptors. These pathways are located by brain PET scans and spinal cord autoradiography. These nonopioid ascending pathways include portions of the ventral spinal thalamic tract originating in Rexed layers VI–VIII, thalamocortical fibers that project to the primary somatosensory cortex (S1), and possibly a midline dorsal column visceral pathway. One hypothesis is that opioid tolerance and opioid-induced hyperalgesia may be caused by homeostatic upregulation during opioid exposure of nonopioid-dependent ascending pain pathways. Upregulation of sensory pathways is not a new concept and has been demonstrated in individuals impaired with deafness or blindness. A second hypothesis is that adjuvant nonopioid therapies may inhibit ascending nonopioid-dependent pathways and support the clinical observations that monotherapy with opioids usually fails. The uniqueness of opioid tolerance compared to tolerance associated with other central nervous system medications and lack of tolerance from excess hormone production is discussed. Experimental work that could prove or disprove the concepts as well as flaws in the concepts is discussed.