The enigma of morphine tolerance: recent insights

Acquired pharmaco-dynamic opioid tolerance: a concept analysis

AIM

To report an analysis of the concept of acquired pharmaco-dynamic opioid tolerance.

BACKGROUND

Acquired pharmaco-dynamic opioid tolerance is a complex and poorly understood phenomenon associated with strong opioid therapy for managing pain. Critical review of the concept provides greater clarification of the attributes, assisting healthcare professionals in addressing pain and functional management of patients, particularly those with non-malignant pain.

DESIGN

Concept analysis.

DATA SOURCES

A systematic literature search was undertaken using electronic data bases: CINAHL, British Nursing Index, EMBase, Medline, Pubmed and AMED. All literature reviewed was in English and published between 1976 and 2012. The key search terms were 'chronic non-malignant pain', 'strong opioid therapy' and 'development of acquired pharmaco-dynamic opioid tolerance'; all possible variant terms were also searched.

METHOD

The Walker and Avant approach was used to guide the concept analysis.

RESULTS

The concept analysis revealed four empirical referents: plasticity, drug administration, reduced analgesic efficacy and increased drug dosing. Tachyphylexia was identified as a borderline case, opioid induced hyperalgesia as a related case and pseudo-tolerance as a contrary case. The antecedent is administration of an opioid analgesic drug and the consequences, increasing opioid drug dose to maintain analgesic effect.

CONCLUSION

Untangling the antecedents, empirical referents and consequences of tolerance help healthcare professionals understand its complexities. Improved knowledge may ultimately influence patient outcomes through the construction of better monitoring systems. This concept analysis may also provide insights for policy change and give empirical direction for future research.

Enhanced analgesic effect of morphine-nimodipine combination after intraspinal administration as compared to systemic administration in mice

Calcium plays an important role in the pathophysiology of pain. A number of studies have investigated the effect of L-type calcium channel blockers on the analgesic response of morphine. However, the results are conflicting. In the present study, the antinociceptive effect of morphine (2.5 microg) and nimodipine (1 microg) co-administered intraspinally in mice was observed using the tail flick test. It was compared to the analgesic effect of these drugs (morphine - 250 microg subcutaneously; nimodipine - 100 microg intraperitoneally) after systemic administration. Nimodipine is highly lipophilic and readily crosses the blood brain barrier. Addition of nimodipine to morphine potentiated the analgesic response of the latter when administered through the intraspinal route but not when administered through systemic route. It may be due to direct inhibitory effect of morphine and nimodipine on neurons of superficial laminae of the spinal cord after binding to mu -opioid receptors and L-type calcium channels respectively.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.