Morphine-induced changes of adenylate and guanylate cyclase in locus ceruleus, periaqueductal gray, and substantia nigra in rats

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Hypertensive Effect of Downregulation of the Opioid System in Mouse Model of Different Activity of the Endogenous Opioid System

The opioid system is well-known for its role in modulating nociception and addiction development. However, there are premises that the endogenous opioid system may also affect blood pressure. The main goal of the present study was to determine the impact of different endogenous opioid system activity and its pharmacological blockade on blood pressure. Moreover, we examined the vascular function in hyper- and hypoactive states of the opioid system and its pharmacological modification. In our study, we used two mouse lines which are divergently bred for high (HA) and low (LA) swim stress-induced analgesia. The obtained results indicated that individuals with low endogenous opioid system activity have higher basal blood pressure compared to those with a hyperactive opioid system. Additionally, naloxone administration only resulted in the elevation of blood pressure in HA mice. We also showed that the hypoactive opioid system contributes to impaired vascular relaxation independent of endothelium, which corresponded with decreased guanylyl cyclase levels in the aorta. Together, these data suggest that higher basal blood pressure in LA mice is a result of disturbed mechanisms in vascular relaxation in smooth muscle cells. We believe that a novel mechanism which involves endogenous opioid system activity in the regulation of blood pressure will be a promising target for further studies in hypertension development.

The role of reactive oxygen species in morphine addiction of SH-SY5Y cells

AIMS

The alteration of ROS level is frequently observed in the course of morphine addiction, and ROS is proverbially involved in this process. This study aims to explore the relationship among morphine addiction, reactive oxygen species (ROS) and expression of μ-opioid receptor (MOR) in differentiated SH-SY5Y cells.

MAIN METHODS

SH-SY5Y cells were induced to differentiation by treatment with retinoic acid (RA); the activity of lactate dehydrogenase (LDH) and the nitro blue tetrazolium (NBT) reduction were assessed by spectrophotometry. Intracellular reactive oxygen species (ROS) was measured with the 2,7-dichlorofluorescin diacetate (DCFH-DA) assay. Cellular cAMP was determined by using a competitive protein binding kit. The mRNA expression of μ-opioid receptor (MOR) was evaluated by qRT-PCR.

KEY FINDINGS

Morphine-induced ROS are generated in a concentration- and time-dependent manner and inhibited by naloxone. Exogenous oxidants increase the level of ROS and aggravate morphine addiction, while the exogenous antioxidants efficiently reverse these effects. Morphine decreases the mRNA level of MOR in a concentration-dependent manner. And the mRNA level of MOR is remarkably reduced in the presence of exogenous oxidants and effectively promoted by antioxidants.

SIGNIFICANCE

This study indicates that ROS can affect morphine addiction through involving MOR. Treatment with ROS scavenging can serve as a medical therapy for morphine addiction.

Hydrogen sulfide inhibits opioid withdrawal-induced pain sensitization in rats by down-regulation of spinal calcitonin gene-related peptide expression in the spine

Hyperalgesia often occurs in opioid-induced withdrawal syndrome. In the present study, we found that three hourly injections of DAMGO (a μ-opioid receptor agonist) followed by naloxone administration at the fourth hour significantly decreased rat paw nociceptive threshold, indicating the induction of withdrawal hyperalgesia. Application of NaHS (a hydrogen sulfide donor) together with each injection of DAMGO attenuated naloxone-precipitated withdrawal hyperalgesia. RT-PCR and Western blot analysis showed that NaHS significantly reversed the gene and protein expression of up-regulated spinal calcitonin gene-related peptide (CGRP) in naloxone-treated animals. NaHS also inhibited naloxone-induced cAMP rebound and cAMP response element-binding protein (CREB) phosphorylation in rat spinal cord. In SH-SY5Y neuronal cells, NaHS inhibited forskolin-stimulated cAMP production and adenylate cyclase (AC) activity. Moreover, NaHS pre-treatment suppressed naloxone-stimulated activation of protein kinase C (PKC) α, Raf-1, and extracellular signal-regulated kinase (ERK) 1/2 in rat spinal cord. Our data suggest that H2S prevents the development of opioid withdrawal-induced hyperalgesia via suppression of synthesis of CGRP in spine through inhibition of AC/cAMP and PKC/Raf-1/ERK pathways.

Hydrogen sulfide attenuates opioid dependence by suppression of adenylate cyclase/cAMP pathway

AIMS

The best-established mechanism of opioid dependence is the up-regulation of adenylate cyclase (AC)/cAMP pathway, which was reported to be negatively regulated by hydrogen sulfide (H2S), a novel endogenous neuromodulator. The present study was, therefore, designed to determine whether H2S is able to attenuate the development of opioid dependence via down-regulating AC/cAMP pathway.

RESULTS

We demonstrated that application of sodium hydrosulphide (NaHS) and GYY4137, two donors of H2S, significantly alleviated naloxone-induced robust withdrawal jumping (the most sensitive and reliable index of opioid physical dependence) in morphine-treated mice. Repeated treatment with NaHS inhibited the up-regulated protein expression of AC in the striatum of morphine-dependent mice. Furthermore, NaHS also attenuated morphine/naloxone-elevated mRNA levels of AC isoform 1 and 8, production of cAMP, and phosphorylation of cAMP response element-binding protein (CREB) in mice striatum. These effects were mimicked by the application of exogenous H2S or over-expression of cystathione-β-synthase, an H2S -producing enzyme, in SH-SY5Y neuronal cells on treatment with [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin, a selective μ-opioid receptor agonist. Blockade of extracellular-regulated protein kinase 1/2 (ERK1/2) with its specific inhibitor attenuated naloxone-induced CREB phosphorylation. Pretreatment with NaHS or stimulation of endogenous H2S production also significantly suppressed opioid withdrawal-induced ERK1/2 activation in mice striatum or SH-SY5Y cells.

INNOVATION

H2S treatment is important in prevention of the development of opioid dependence via suppression of cAMP pathway in both animal and cellular models.

CONCLUSION

Our data suggest a potential role of H2S in attenuating the development of opioid dependence, and the underlying mechanism is closely related to the inhibition of AC/cAMP pathway.

Pioglitazone potentiates development of morphine-dependence in mice: possible role of NO/cGMP pathway

Peroxizome proliferator-activated receptor gamma (PPARγ) is highly expressed in the central nervous system where it modulates numerous gene transcriptions. Nitric oxide synthase (NOS) expression could be modified by simulation of PPARγ which in turn activates nitric oxide (NO)/soluble guanylyl-cyclase (sGC)/cyclic guanosine mono phosphate (cGMP) pathway. It is well known that NO/cGMP pathway possesses pivotal role in the development of opioid dependence and this study is aimed to investigate the effect of PPARγ stimulation on opioid dependence in mice as well as human glioblastoma cell line. Pioglitazone potentiated naloxone-induced withdrawal syndrome in morphine dependent mice in vivo. While selective inhibition of PPARγ, neuronal NOS or GC could reverse the pioglitazone-induced potentiation of morphine withdrawal signs; sildenafil, a phosphodiesterase-5 inhibitor amplified its effect. We also showed that nitrite levels in the hippocampus were significantly elevated in pioglitazone-treated morphine dependent mice. In the human glioblastoma (U87) cell line, rendered dependent to morphine, cAMP levels did not show any alteration after chronic pioglitazone administration while cGMP measurement revealed a significant rise. We were unable to show a significant alteration in neuronal NOS mRNA expressions by pioglitazone in mice hippocampus or U87 cells. Our results suggest that pioglitazone has the ability to enhance morphine-dependence and to augment morphine withdrawal signs. The possible pathway underlying this effect is through activation of NO/GC/cGMP pathway.