Amitriptyline, minocycline and maropitant reduce the sevoflurane minimum alveolar concentration and potentiate remifentanil but do not prevent acute opioid tolerance and hyperalgesia in the rat: a randomised laboratory study.

BACKGROUND: The antidepressant amitriptyline, the inhibitor of microglia activation minocycline, and the neurokinin-1 antagonist maropitant have all been used to prevent or treat hyperalgesia and opioid tolerance.

OBJECTIVES: To determine the effect of amitriptyline, minocycline, maropitant, independently or with remifentanil, on the sevoflurane minimum alveolar concentration in rats and whether these drugs may block opioid-induced hyperalgesia and acute opioid tolerance under inhalational anaesthesia.

DESIGN: A randomised, laboratory study.

SETTING: Experimental Unit, La Paz University Hospital, Madrid, Spain.

ANIMALS: One hundred and fourteen adult male Wistar rats.

INTERVENTIONS: Intraperitoneal administration of amitriptyline (10 and 50  mg  kg-1), minocycline (30 and 100  mg  kg-1), maropitant (10 and 30 mg  kg-1) or isotonic saline, combined with a constant rate intravenous infusion of remifentanil (240 μg  kg-1  h-1) or saline.

MAIN OUTCOME MEASURES: Sevoflurane minimum alveolar concentration was determined before and after administration of the drugs; acute opioid tolerance was defined as a decreased ability of remifentanil to reduce the minimum alveolar concentration in the short term. In addition, mechanical nociceptive thresholds were determined before and after these treatments. Opioid-induced hyperalgesia was defined as an increase in mechanical nociceptive thresholds after opioid administration.

RESULTS: Amitriptyline, minocycline and maropitant reduced minimum alveolar concentration up to 24 (8)%, 23 (6)% and 15 (5)%, respectively (P <0.001). Remifentanil alone reduced minimum alveolar concentration by 36 (6)% (P <0.001), and in combination with amitriptyline, minocycline and maropitant, the reduction was 76 (9)%, 75 (16)% and 59 (5)%, respectively (P <0.001). An acute tolerance effect (P < 0.01) and a decrease in the mechanical nociceptive thresholds were observed with remifentanil in all groups.

CONCLUSION: Amitriptyline, minocycline and maropitant reduced the minimum alveolar concentration and potentiated the remifentanil minimum alveolar concentration reduction but failed to block opioid-induced hyperalgesia and acute opioid tolerance.