Blockade of peripheral σ 1 -receptors increases morphine-induced antinociception.

JM Entrena2, C Sánchez-Fernández1, R González-Cano1, A Artacho1, D Zamanillo3, JM Baeyens1. 1University of Granada, Department of Pharmacology, Biomedical Research Center and Institute of Neurosciences, 18012, Spain, 2University of Granada, Scientific Instrumentation Center, 18100, Spain, 3Laboratorios Esteve, Department of Pharmacology, Drug Discovery and Preclinical Development, 08041, Spain

Antecedents

Several studies showed that the intracerebroventricular administration of σ1-receptor antisense oligodeoxynucleotides and of haloperidol, a non-selective σ1-receptor antagonist, enhances morphine antinociception against heat-induced pain. Together, these studies suggest the presence of a tonically active anti-opioid σ1 system in the central nervous system (CNS). Morphine also produces antinociception through peripheral opioid receptors. However, it is unknown whether the blockade of σ1-receptor outside the CNS is able to increase the peripherally-mediated opioid antinociception. Therefore, our aim was to study the local antinociceptive effect of morphine when injected into the hind paw alone and associated to several selective σ1-receptor antagonists. Moreover, we compared the peripheral antinociception induced by morphine in wild-type and σ1-receptor knockout (σ1-KO) mice.

Methods

Experiments were performed in CD-1 wild-type (WT) and σ1-KO mice weighing 25-30 g. Non-punctate nociceptive mechanical stimuli (450 g pressure) were applied alternatively to both hind-paws with a rounded tip cone-shaped paw-presser Analgesimeter (Ugo Basile, Italia) until the mouse showed a struggling behavior or 50 s had passed (cut-off time). The antinociceptive effect of intraplantar (i.pl.) administration of morphine and morphine co-administered i.pl. with several selective σ1-receptor antagonists [BD-1063 (12.5-200 µg/20µl), BD-1047 (50 µg/20µl), NE-100 (50 µg/20µl), S1RA (100 µg/20µl)], was tested 5 minutes after injection into the right hind paw. The density of µ opioid receptors was measured with [3H]DAMGO (15 nM) binding assays in hind-paw skin from WT and σ1-KO mice.

Results

Morphine (50, 100 and 200 µg, i.pl.) produced no antinociception in WT mice (struggle response latencies were: 1.66 ± 0.17, 1.29 ± 0.10 and 1.58 ± 0.20 s, respectively); however, it produced a dose-dependent antinociceptive effect in σ1-KO mice (14.5 ± 1.12, 26.42 ± 3.49 and 48.69 ± 3.93 s; respectively). This effect was locally-mediated because antinociception was observed in the injected paw but not in the non-injected paw of σ1-KO mice (1.12 ± 0.08, 1.5 ± 0.22 and 1.13 ± 0.09 s). Statistically significant differences between the values obtained in the injected and non-injected hind paws in σ1-KO mice. [3H]DAMGO binding characteristics were the same in hind-paw skin from WT (0.030 ± 0.002 pmol/mg of protein) and σ1-KO mice (0.0296 ± 0.001 pmol/mg of protein), which indicates that there were no differences in the peripheral µ-opioid receptors between both types of mice. In wild-type mice the antinociceptive effect of morphine (100 µg, i.pl.) was potentiated by co-administration with all the σ1-receptor antagonists evaluated. In contrast, none of the σ1-receptor antagonists modified morphine-induced antinociception in σ1-KO mice. Statistical analysis was carried out using two-way ANOVA followed by the Bonferroni test. The differences between values were considered to be significant when the value of p was below 0.05.

Conclusion

These results suggest that blockade of peripheral σ1 receptors increase the peripherally-mediated antinociceptive effect of morphine.

Supported by Junta-Andalucía (CTS-109), MEC (SAF2006-06122) and MEC-FPU (CSF & RGC).