Chronic inflammatory pain does not attenuate the development of tolerance to chronic morphine in adult male rats.
Pharmacol Biochem Behav
; 98(2): 325-30, 2011 Apr.
Article
in En
| MEDLINE
| ID: mdl-21277888
ABSTRACT
The overall impact of chronic pain on the response to opioids is ambiguous in the literature, and comparisons between human and animal studies are complicated by vast differences between the manner and dosage of opioids given to humans treated for pain in comparison to rodents as well as a lack of healthy participant studies examining the impact of chronic opioids. The purpose of this study was to evaluate the impact of chronic pain on the development of tolerance to morphine and to assess how the concentration of drug affects this process. Twenty-four hours after the injection of CFA or normal saline in the left hind paw, the level of mechanical hypersensitivity was assessed and animals were randomly assigned to a morphine dose (1, 3 or 8 mg/kg or saline). Morphine was administered by subcutaneous injection twice a day for 5 days. On Day 6, animals were challenged with a single dose of 3 mg/kg morphine prior to formalin testing. Evidence of tolerance was mixed, and the results varied widely among the conditions. Analysis of mean paw withdrawal thresholds indicated that the analgesic efficacy of subcutaneous morphine diminished following repeated dosing. The presence of the chronic inflammatory pain condition during the morphine dosing period produced an increase in formalin pain behaviors compared to saline controls, such that animals given any dose of morphine during the 5-day dosing period showed higher responding to formalin following the 3 mg/kg dose than animals that had received saline injections. These results indicate that chronic pain does influence the development of opioid tolerance, but it does not prevent this phenomenon from occurring as suggested by some researchers.
Full text:
1
Collection:
01-internacional
Database:
MEDLINE
Main subject:
Pain
/
Drug Tolerance
/
Morphine Dependence
Limits:
Animals
/
Humans
/
Male
Language:
En
Journal:
Pharmacol Biochem Behav
Year:
2011
Document type:
Article
Affiliation country:
United States
Publication country:
EEUU
/
ESTADOS UNIDOS
/
ESTADOS UNIDOS DA AMERICA
/
EUA
/
UNITED STATES
/
UNITED STATES OF AMERICA
/
US
/
USA