Document Detail

Some new insights into the effects of opioids in phasic and tonic nociceptive tests.
MedLine Citation:
PMID:  9839818     Owner:  NLM     Status:  MEDLINE    
In this review we critically appraise the value of some phasic and tonic nociceptive tests as models for differentiating the antinociceptive effects of opioid agonists. Using heat-evoked withdrawal of the hind paw or tail of a rodent, several early studies have assessed the effects of stimulus intensity upon antinociceptive potency of opioid agonists. After intrathecal (i.t.) administration of either morphine or sufentanil, for example, for any incremental change in stimulus intensity, the degree of right shift in the dose-response relationship was greater for morphine than for sufentanil. At first glance, such data appear to provide robust support for the pharmacological model of fractional receptor occupancy (FRO), which, according to the historical tenets of classical receptor theory, describes the relationship between intrinsic efficacy and the total receptor concentration. However, new data which elegantly characterize the relative contribution of small calibre unmyelinated and myelinated nociceptive afferents in mediating thermal-evoked responses challenge the exclusivity of such explanations with origins in classical theory. Within our review we report the results of experiments which provide direct electrophysiological evidence that noxious skin heating at a low rate activates C-polymodal nociceptors, but does not effectively activate A-delta mechanothermal nociceptors. In contrast, a high rate of skin heating activates both nociceptor classes, but produces a more intense activation of A-delta nociceptors that occurs after a shorter onset latency compared with the activation of C-fibre nociceptors. Thus, in direct challenge to the traditional model of FRO, a shift in the dose-response relationship of morphine to the right with a reduction in efficacy, may reflect the limited effectiveness of morphine to attenuate the A-delta-mediated component which assumes increasing dominance at high intensity heating. In our appraisal of other nociceptive models we provide an in-depth characterization of afferent processing in the early neonate rat, in which opioids have been tested in both phasic (tail flick and hot-plate) and tonic (formalin) tests. Afferent processing in this model is typified by several behavioural, anatomical and functional features which, although not pathological, are characteristic of those observed in models of nerve injury using the adult rat. Notably, these features include a lack of segmental inhibition ('disinhibition') and afferent input in large diameter myelinated fibres which make synaptic contacts within superficial laminae of the dorsal horn that in the adult are predominantly nociceptive. Paradoxically, because this paradigm demonstrates increased sensitivity to the antinociceptive effects of opioids it may have special merit as a model of tonic pain. It was recently announced that the i.t. administration of pertussis toxin (PTX) caused hyperalgesia and allodynia that appears similar to the symptoms reported by patients suffering from neuropathic pain. Unlike the effects of other opioids so far tested, buprenorphine-induced antinociception is not blocked in this model. This is an exciting finding and provides new optimism that some opioids, notably buprenorphine, may have a special role in managing some types of neuropathic pain.
K McCormack; P Prather; C Chapleo
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Publication Detail:
Type:  Journal Article; Review    
Journal Detail:
Title:  Pain     Volume:  78     ISSN:  0304-3959     ISO Abbreviation:  Pain     Publication Date:  1998 Nov 
Date Detail:
Created Date:  1999-03-09     Completed Date:  1999-03-09     Revised Date:  2005-11-16    
Medline Journal Info:
Nlm Unique ID:  7508686     Medline TA:  Pain     Country:  NETHERLANDS    
Other Details:
Languages:  eng     Pagination:  79-98     Citation Subset:  IM    
Drug Research Group, McCormack Limited, Leighton Buzzard, Bedfordshire, UK.
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MeSH Terms
Animals, Newborn / physiology
Formaldehyde / pharmacology
Injections, Spinal
Narcotics / classification,  pharmacology*
Nervous System / drug effects
Nociceptors / drug effects*
Pain / chemically induced
Pertussis Toxin
Virulence Factors, Bordetella / administration & dosage
Reg. No./Substance:
0/Narcotics; 0/Virulence Factors, Bordetella; 50-00-0/Formaldehyde; EC Toxin

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