Mu-opioid Receptor-deficient Mice Research Articles

The effects of kratom on restraint–stress-induced analgesia and its mechanisms of action

Ethnopharmacological relevanceMitragyna speciosa and its extracts are called kratom (dried leaves, extract). They contain several alkaloids with an affinity for different opioid receptors. They are used in traditional medicine for the treatment of different diseases, as a substitute by opiate addicts, and to mitigate opioid withdrawal symptoms. Apart from their medical properties, they are used to enhance physical endurance and as a means of overcoming stress. PurposeThe aim of this study was to determine the mechanisms underlying the effects of kratom on restraint-stress-induced analgesia which occurs during or following exposure to a stressful or fearful stimulus. MethodsTo gain further insights into the action of kratom on stress, we conducted experiments using restraint stress as a test system and stress-induced analgesia as a test parameter. Using transgenic mu opioid-receptor (MOR) deficient mice, we studied the involvement of this receptor type. We used nor-binaltorphimine (BNT), an antagonist at kappa opioid receptors (KOR), to study functions of this type of receptor. Membrane potential assay was also employed to measure the intrinsic activity of kratom in comparison to U50,488, a highly selective kappa agonist. ResultsTreatment with kratom diminished stress-induced analgesia in wildtype and MOR knockout animals. Pretreatment of MOR deficient mice with BNT resulted in similar effects. In comparison to U50,488, kratom exhibited negligible intrinsic activity at KOR alone. ConclusionsThe results suggest that the use of kratom as a pharmacological tool to mitigate withdrawal symptoms is related to its action on KOR.

Inhibition of hematopoietic progenitor cell growth by Tyr-MIF, an endogenous opiate modulator, and its degradation products

There is increasing evidence that neuronal factors can affect hematopoietic cell proliferation. Endogenous opioids with specificity for several opioid receptor classes were tested for their ability to inhibit murine and human hematopoietic progenitor cell proliferation. Tyr-MIF, an opioid tetrapeptide ( H-Tyr-Pro-Leu-Gly-NH 2), demonstrated a dose-dependent inhibition of colony formation at concentrations <10 uM, inhibiting M-CSF and G-CSF-responsive progenitor cells equally. Tyr-MIF did not inhibit the number of colonies responsive to recombinant interleukin 3 (rmIL-3) or recombinant murine granulocyte–macrophage colony stimulating factor (rmGM-CSF), but significantly reduced colony size of GM-CSF responsive colonies. Colony formation by human low density and CD34 + marrow cells in response to G-CSF was also inhibited by Tyr-MIF and was more sensitive to inhibition than murine progenitor cells. Colony formation by single CD34 + cells was also inhibited by Tyr-MIF, indicating an effect directly on progenitor cells. Incubation of marrow cells in liquid culture and removal of Tyr-MIF prior to quantitating progenitor cell proliferation demonstrated that opioid-induced inhibition was reversible. The inhibitory effect of Tyr-MIF was not blocked by naloxone, a mu receptor specific antagonist, or diminished in mu opioid receptor deficient mice. HPLC analysis of cell-free culture medium containing Tyr-MIF showed no presence of the parent peptide after 24 h while progenitor cell inhibitory activity was retained. Analysis of potential degradation products of Tyr-MIF indicated that only H-Gly-NH 2 or H-Gly-NH 2 containing peptides inhibited colony forming unit (CFU) proliferation. These results indicate that Tyr-MIF is a reversible inhibitor of mature hematopoietic progenitor cell proliferation, and that this effect is most likely mediated by the degradation product H-Gly-NH 2. Potential applications including protection of myeloid cells after cytosuppresive therapy are discussed.

Rewarding effects of ethanol and cocaine in mu opioid receptor-deficient mice.

To investigate the role of mu opioid receptors in the reinforcing effects of psychotropic drugs, the voluntary ethanol intake and ethanol- and cocaine-induced conditioned place preference in mu opioid receptor-deficient mice and their wild-type counterpartners was tested. Moreover, dopamine D1 and D2 receptor binding was measured. It was found that ethanol intake was significantly lower in deficient mice. Conditioned place preference in wild-type animals was induced with 5.0 mg/kg cocaine and this dose was ineffective in the knockouts. In this group conditioned place preference occurred after injection of 10.0 mg/kg cocaine. Cocaine induced a similar increase in locomotor activity in both groups of mice. There was no difference in dopamine D1 receptor binding, whereas dopamine D2 receptor binding was significantly lower in the hippocampus of deficient animals. This suggests that interaction between opioid systems and dopaminergic systems may account for the differences in responding to the drugs.

Lack of dependence and rewarding effects of deltorphin II in mu-opioid receptor-deficient mice

Lack of dependence and rewarding effects of deltorphin II in mu-opioid receptor-deficient mice

Morphine self-administration in mu-opioid receptor-deficient mice.

Morphine-induced place preference was demonstrated recently in wild-type mice, whereas this conditioned behaviour was not observed in mu-opioid receptor-deficient mice. In the present study, we investigated locomotor effects of subcutaneously (s.c.) injected morphine as well as intracerebroventricular (i.c.v.) morphine self-administration in mu-opioid receptor-knockout mice. After s.c. morphine injection, locomotor activity significantly increased in wild-type animals. As expected, in the self-administration test the rate of self-administration constantly increased in wild-type mice reflecting reward effects of morphine. This increase was independent of locomotor/motor activity. In contrast, self-administration rates and locomotor/motor activity significantly decreased in the receptor-deficient animals. It was shown that this aversive effect might partly be due to kappa-opioid receptor interaction.