Morphine withdrawal modifies antinociceptive effects of acute morphine in rats

The effect of chronic stress on the immunogenicity and immunoprotection of the M6-TT vaccine in female mice

Active vaccination is an effective therapeutic option to reduce the reinforcing effects of opioids. Several studies showed that chronic stress affects the immune system decreasing the efficiency of some vaccines. Heroin withdrawal is a stressor and it is a stage in which the patient who abuses heroin is vulnerable to stress affects the immune response and consequently its immunoprotective capacity, then, the objective was to determine the effect of heroin-withdrawal and heroin-withdrawal plus immobilization, on the immune (immunogenicity) and protective response (behavioral response) of morphine-6-hemisuccinate-tetanus toxoid (M6-TT) vaccine in animals of two inbred mice strains with different sensitivity to drug-opioid and stress. Female BALB/c and C57Bl/6 inbred mice were immunized with the M6-TT. A solid-phase antibody-capture ELISA was used to monitor antibody titer responses after each booster dose in vaccinated animals. During the vaccination period, the animals were subjected to two different stress conditions: drug-withdrawal (DW) and immobilization (IMM). The study used tail-flick testing to evaluate the heroin-induced antinociceptive effects. Additionally, heroin-induced locomotor activity was evaluated. Stress decreased the heroin-specific antibody titer generated by the M6-TT vaccine in the two inbred mouse strains evaluated. In the two stress conditions, the antibody titer was not able to decrease the heroin-induced antinociceptive effects and locomotor activity. These findings suggest that stress decreases the production of antibodies and the immunoprotective capacity of the M6-TT vaccine. This observation is important to determine the efficacy of active vaccination as a potential therapy for patients with opioid drug use disorder, since these patients during drug-withdrawal present stress disorders, which could affect the efficacy of therapy such as active vaccination.

Astrocytic NDRG2 is involved in glucocorticoid-mediated diabetic mechanical allodynia

AIMS

The present study aims to test whether astrocytes contribute to glucocorticoid-mediated diabetic mechanical allodynia.

METHODS

Streptozotocin (STZ)-induced diabetic rats were used in our study. The intrathecal operation was performed 21 days after the onset of diabetes. Diabetic mechanical allodynia was present 28 d after the onset of diabetes, and the mechanical threshold was tested using von Frey filaments. Immunohistochemistry, including immunofluorescent histochemical staining, was performed to observe the morphology of the spinal dorsal horn (SDH). Western blot analysis was employed as a semi-quantitative assay of the expression levels of GFAP and NDRG2 associated with diabetic mechanical allodynia.

RESULTS

Diabetic rats displayed mechanical allodynia and activated astrocytes in the SDH 28 days after the onset of diabetes. This allodynia was attenuated by intrathecal administration of the astrocyte-specific inhibitor l-α-aminoadipate. In parallel, intrathecal injection of RU486, a glucocorticoid receptor antagonist, inhibited the activation of astrocytes in the SDH, alleviating the diabetes-induced mechanical allodynia. Furthermore, we found that dorsal horn astrocytes express abundant N-myc downstream-regulated gene 2 (NDRG2), which contributes to astrocyte reactivity. NDRG2 was over-expressed in activated astrocytes in diabetic rats with mechanical allodynia. Intrathecal injection of RU486 prevented the over-expression of NDRG2, which reversed the astrocyte reactivity and diabetic tactile allodynia.

CONCLUSIONS

These results suggest that glucocorticoid-mediated over-expression of NDRG2 may contribute to the activation of dorsal horn astrocytes, which play a crucial role in diabetic mechanical allodynia. Thus, inhibiting glucocorticoid receptors and/or astrocyte reactivity in the SDH may be a therapeutic strategy for treating diabetic tactile allodynia.

Differential tolerance to morphine antinociception in assays of pain-stimulated vs. pain-depressed behavior in rats

In preclinical research on pain and analgesia, noxious stimuli can stimulate expression of some behaviors (e.g. withdrawal reflexes) and depress others (e.g. feeding, locomotion, and positively reinforced operant responding). Tolerance to morphine antinociception is a robust and reliable phenomenon in preclinical assays of pain-stimulated behavior, but development of morphine tolerance in assays of pain-depressed behavior has not been studied. This study compared morphine antinociceptive tolerance in parallel assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats. Intraperitoneal injection of dilute lactic acid served as a noxious stimulus to stimulate a stretching response in one group of rats and to depress operant responding for electrical brain stimulation (intracranial self-stimulation; ICSS) in another group of rats. Antinociception produced by morphine (1.0 mg/kg) was determined after a regimen of chronic treatment with either saline or morphine in separate subgroups of rats in each procedure. In rats receiving chronic saline, acid alone stimulated a stretching response and depressed ICSS, and both acid effects were blocked by 1.0 mg/kg morphine. Rats receiving chronic morphine displayed hyperalgesic responses to the acid noxious stimulus in both procedures. Complete tolerance developed to morphine antinociception in the assay of acid-stimulated stretching, but morphine retained full antinociceptive effectiveness in the assay of acid-depressed ICSS. These results suggest that morphine antinociception in an assay of pain-depressed behavior is relatively resistant to tolerance. More broadly, these results suggest that antinociceptive tolerance can develop at different rates or to different degrees for different measures of antinociception.

Stress-induced pain: a target for the development of novel therapeutics

Although current therapeutics provide relief from acute pain, drugs used for treatment of chronic pain are typically less efficacious and limited by adverse side effects, including tolerance, addiction, and gastrointestinal upset. Thus, there is a significant need for novel therapies for the treatment of chronic pain. In concert with chronic pain, persistent stress facilitates pain perception and sensitizes pain pathways, leading to a feed-forward cycle promoting chronic pain disorders. Stress exacerbation of chronic pain suggests that centrally acting drugs targeting the pain- and stress-responsive brain regions represent a valid target for the development of novel therapeutics. This review provides an overview of how stress modulates spinal and central pain pathways, identifies key neurotransmitters and receptors within these pathways, and highlights their potential as novel targets for therapeutics to treat chronic pain.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Opiate withdrawal modifies synaptic plasticity in subicular-nucleus accumbens pathway in vivo

Subiculum receives output of hippocampal CA1 neurons and projects glutamatergic synapses onto nucleus accumbens (NAc), the subicular-NAc pathway linking memory and reward system. It is unknown whether morphine withdrawal influences synaptic plasticity in the subicular-NAc pathway. Here, we recorded the field excitatory postsynaptic potential (EPSP) within the shell of NAc by stimulating ventral subiculum in anesthetized adult rats. We found that high frequency stimulation (HFS, 200 Hz) induced long-term potentiation (LTP) but low frequency stimulation (LFS, 1 Hz) failed to induce long-term depression (LTD) in control animals. However, behavioral stress enabled LFS to induce a reliable LTD (sLTD) that was dependent on the glucocorticoid receptors. Both LTP and sLTD were prevented by the N-methyl-d-aspartate receptor antagonist AP-5. After repeated morphine treatment for 12 days, acute withdrawal (12 h) impaired LTP but had no effect on sLTD; prolonged withdrawal (4 days) restored the LTP but impaired the sLTD. Remarkably, basal synaptic efficacy reflected by baseline EPSP amplitude was potentiated in acute withdrawal but was depressed in prolonged withdrawal. Thus, acute and prolonged opiate withdrawal may cause endogenous LTP and LTD in the subicular-NAc pathway that occludes the subsequent induction of synaptic plasticity, demonstrating adaptive changes of the NAc functions during opiate withdrawal.

Stress evoked by opiate withdrawal facilitates hippocampal LTP in vivo

Stress impairs hippocampal long-term potentiation (LTP), but it is unknown whether the stress evoked by opiate withdrawal has the same effect. Here the authors report that opiate withdrawal for 4 days does not influence basal synaptic transmission, but results in a greatly increased LTP in hippocampal CA1 area in anesthetized rats. Elevated-platform stress enabled a large LTP in rats withdrawn for only 18 h, but the glucocorticoid receptor antagonist RU38486 (twice per day for 3 days) prevented the large LTP on 4 days withdrawal. Moreover, 4 days withdrawal enhanced the NMDAR-mediated EPSCs, in which the NR2A-containing NMDAR-mediated EPSC was increased but the NR2B-containing NMDAR-mediated EPSC was decreased. These results suggest that adaptive changes of the NMDAR and glucocorticoid receptor functions during 4 days of opiate withdrawal may enable stress to facilitate hippocampal LTP, potentially contributing to the opiate withdrawal experience-dependent modifications of hippocampal functions.