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This article is part of the supplement: 15th Scientific Symposium of the Austrian Pharmacological Society (APHAR)

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Spinal interaction between μ and δ opioid receptors in naive and morphine-tolerant rats

Pál Riba1*, Kornél P Király1, Tamás Friedmann1, Mahmoud Al-Khrasani1, Melinda Sobor3 and Susanna Fürst12

Author Affiliations

1 Department of Pharmacology and Pharmacotherapy, Semmelweis University, 1089 Budapest, Hungary

2 Hungarian Academy of Sciences-Semmelweis University Neuropsychopharmacological Research Group, 1089 Budapest, Hungary

3 National Institute of Pharmacy, 1051 Budapest, Hungary

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BMC Pharmacology 2009, 9(Suppl 2):A48  doi:10.1186/1471-2210-9-S2-A48

The electronic version of this article is the complete one and can be found online at:

Published:12 November 2009

© 2009 Riba et al; licensee BioMed Central Ltd.


The role of δ opioid receptors in opioid antinociception and tolerance development is still unclear. In the spinal cord of morphine-tolerant mice δ receptor ligands given intrathecally (i.t.) differently influenced the antinociceptive effect of the μ agonist D-Ala2-methyl-glycinol (DAMGO). The δ1 agonist D-Pen2,5-enkephalin (DPDPE) inhibited, the δ2 agonist deltorphin II did not alter, and the δ antagonist cha-TIPPψ potentiated the effect of DAMGO. We hypothesized that during the development of morphine tolerance the formation of μ-δ heterodimers may contribute to the spinal μ opioid tolerance. Delta ligands may affect the dimer formation differently. Those, like DPDPE may facilitate the dimer formation, hence inhibit the antinociceptive effect of DAMGO by causing virtual μ receptor down-regulation. Ligands that do not affect the dimer formation do not influence antinociception but ligands with the presumed capability of disconnecting the dimers may decrease the spinal tolerance to DAMGO. The δ ligand profile in morphine-tolerant rats, were also studied.


Male Wistar rats (150-200 g) were treated with subcutaneous (s.c) morphine twice daily for four days with increasing doses (50, 100, 200, 200 μmol/kg). On the fifth day the antinociceptive effect (rat tail flick test) of DAMGO was measured alone and combined with a fixed dose of δ ligands given i.t.: DPDPE, Ile3,5-deltorphin II, cha-TIPPψ and naltrindole, respectively.


The repeated treatment with morphine resulted in approximately three to six-fold shift of the ED50 value of DAMGO compared to that of naive rats. Both in naive control and morphine-tolerant rats all ligands except naltrindole potentiated the antinociceptive effect of i.t. DAMGO (two to five-fold). In the tolerant rats the potentiation restored the potency of DAMGO to the control level.


Delta ligands behave differently in rats than in mice. One possible explanation could be a higher basal density of the μ-δ heterodimers in rats. The inhibitory action of naltrindole on the antinociceptive effect of DAMGO could be explained by its relatively low μ/δ selectivity as well as by the different effect on the μ-δ heterodimer. The difference in the DPDPE effect in morphine-tolerant rats and mice requires further clarification.


This work was supported by the Hungarian grants OTKA K-60999 and ETT-441/2006, and a Bolyai Fellowship of the Hungarian Academy of Sciences and Faculty of General Medicine, Semmelweis University.