Hippocampal long-term potentiation is reduced by chronic opiate treatment and can be restored by re-exposure to opiates

Impaired hippocampal synaptic plasticity and NR2A/2B expression ratio in remifentanil withdrawal rats

Remifentanil is a kind of synthetic opioid which has gained wide clinical acceptance by anesthesiologists. In this study, we attempted to test whether withdrawal effects on learning mechanisms can be triggered by repeated low-dose remifentanil treatment. Male Sprague-Dawley (SD) rats were subjected to remifentanil (50μg/kgs.c.) twice per day at 12h intervals for 15 days. When the animals of remifentanil group were withdrawn from remifentanil at 10h after the last injection, changes in open field test, Morris water maze test (MWM) and synaptic efficacy were examined in each group. We demonstrated that repeated exposure to 50μg/kg remifentanil produced enhanced locomotor activity indicating that a remifentanil addiction animal model in rats was established. MWM results showed that exposure to remifentanil had no influence on the spatial cognition. After withdrawal of remifentanil rats showed impaired spatial cognition. In electrophysiology test, remifentanil group rats showed a trend for a rightward shift of input/output relationship and significant deficits in maintenance of STP and LTP. Immunohistochemistry results demonstrated increased NR2A/NR2B ratio that should be included depression of LTP. In the whole-cell patch-clamp recording, after elimination from remifentanil incubation, mEPSC frequency was down regulated in hippocampal CA1 neurons, indicating that basal synaptic transmission were affected by remifentanil withdrawal. Taken together, the current findings demonstrate that the remifentanil withdrawn rats exhibit obvious impairment of hippocampus-dependent memory and synaptic plasticity. Increased hippocampal NR2A/NR2B expression ratio and the changes of basal synaptic transmission may participate in the impairment of LTP.

Behavioural and Electrophysiological Evidence of Impaired Learning and Memory in Male Sprague Dawley Rats following Subchronic Exposure to Standardised Methanolic Extract of Mitragyna speciosa Korth

BACKGROUND

Mitragyna speciosa (MS) or ketum is primarily found in Southeast Asia, particularly in northern Malaysia and Thailand. The medicinal value of this plant has attracted significant attention from both herbal medicine practitioners and scientists worldwide. Despite having illegal consumption status, the plant merits study. We conducted a series of experiments to test our hypothesis that ketum impairs both learning and memory in rats.

METHODS

Ketum leaves were extracted using methanol and standardised for the amount of its pure compound, mitragynine. Rats were divided into groups for a passive avoidance task and long-term potentiation (LTP) extracellular recording. In the extracellular recording condition, rats were grouped into control, MS100 (100 mg/kg of ketum extract), MS200 (200 mg/kg of ketum extract), and MS500 (500 mg/kg of ketum extract) groups. An additional group that received morphine was included in the passive avoidance task (10 mg/kg), and there were six animals per group. Rats received daily treatments orally for 28 days for both experiments.

RESULT

Using a passive avoidance task, our data revealed that the rats' memory significantly increased with increasing doses of MS compared to the morphine-treated group. Our findings from LTP recordings showed that LTP was fully blocked by the higher doses of MS.

CONCLUSION

We speculate on the possibility that additional factors were involved in the passive avoidance task because it was an in vivo animal study, while the LTP experiment solely involved brain slices.

Effect of acute fentanyl treatment on synaptic plasticity in the hippocampal CA1 region in rats

Postoperative cognitive dysfunction (POCD), mainly characterized by short-term decline of learning and memory, occurs after operations under anesthesia. However, the underlying mechanisms are poorly understood. The μ-opioid receptors (MOR) are highly expressed in interneurons of hippocampus, and is believed to be critical for the dysfunction of synaptic plasticity between hippocampal neurons. Therefore, we investigated the effect of fentanyl, a strong agonist of MOR and often used for anesthesia and analgesia in clinical settings, on hippocampal synaptic plasticity in the Schaffer-collateral CA1 pathway during acute exposure and washout in vitro. Our results revealed that acute fentanyl exposure (0.01, 0.1, 1 μM) dose-dependently increased the field excitatory postsynaptic potentials (fEPSPs), which was prevented by pre-administration of picrotoxin (50 μM) or MOR antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2 (CTOP, 10 μM). While fentanyl exposure-increased fEPSPs amplitude was prevented by picrotoxin [an inhibitor of γ-aminobutyric acid receptor (GABAR)] treatment or fentanyl washout, pretreatment of picrotoxin failed to prevent the fentanyl-impaired long-term potentiation (LTP) of synaptic strength as well as the fentanyl-enhanced long-term depression (LTD). These results demonstrated that fentanyl acute exposure and washout increases hippocampal excitability in the Schaffer-collateral CA1 pathway, depending on disinhibiting interneurons after MOR activation. In addition, fentanyl acute exposure and washout modulated synaptic plasticity, but the inhibitory activation was not critical. Elucidating the detailed mechanisms for synaptic dysfunction after fentanyl exposure and washout may provide insights into POCD generation after fentanyl anesthesia.

A Review Study on the Effect of Iranian Herbal Medicines on Opioid Withdrawal Syndrome

Addiction is a chronic and recurring disease that recurrence phenomenon is the most important challenge in treatment of this disease. Recent experiences have shown that synthetic drugs have undesirable side effects. Recent studies on medicinal plants have shown that they might be effective in treatment of different stages of addiction with lower side effects and costs. The aim of this study was to review the effects of medicinal plants in the treatment of morphine addiction in experimental animals. In this review article, by using keywords of morphine, withdrawal, and plants or herbal medicine in databases of indexing cites, desired articles were obtained since 1994. Inclusion criteria for selecting articles were the articles related to application of medicinal plants in decreasing symptoms resulting from morphine withdrawal were selected. Results of this study on experimental studies have shown that medicinal plants such as Trachyspermum copticum L and Melissa officinalis decrease the symptoms of withdrawal syndrome in a dose-dependent. Also, medicinal plants like Avena sativa, Hypericum perforatu, Passiflora incarnate, Valeriana officinalis, Satureja hortensis L, and Mentha piperita can have effects on behavior, emotions, and other problems of addicts, decreasing withdrawal symptoms. Results of this study showed that medicinal plants can be effective in controlling deprivation, decreasing dependency creation, and possibly detoxification of opioid addicts.

Memory impairment and reduced exploratory behavior in mice after administration of systemic morphine

In the present study, the effects of morphine were examined on tests of spatial memory, object exploration, locomotion, and anxiety in male ICR mice. Administration of morphine (15 or 30 mg/kg, intraperitoneally (i.p.)) induced a significant decrease in Y-maze alternations compared to saline vehicle-treated mice. The reduced Y-maze alternations induced by morphine were completely blocked by naloxone (15 mg/kg) or β-funaltrexamine (5 mg/kg) but not by norbinaltorphimine (5 mg/kg) or naltrindole (5 mg/kg), suggesting that the morphine-induced spatial memory impairment was mediated predominantly by μ-opioid receptors (MOPs). Significant spatial memory retrieval impairments were observed in the Morris water maze (MWM) in mice treated with morphine (15 mg/kg) or scopolamine (1 mg/kg), but not with naloxone or morphine plus naloxone. Reduced exploratory time was observed in mice after administration of morphine (15 mg/kg), in a novel-object exploration test, without any changes in locomotor activity. No anxiolytic-like behavior was observed in morphine-treated mice in the elevated plus maze. A significant reduction in buried marbles was observed in morphine-treated mice measured in the marble-burying test, which was blocked by naloxone. These observations suggest that morphine induces impairments in spatial short-term memory and retrieval, and reduces exploratory behavior, but that these effects are not because of overall changes in locomotion or anxiety.

Opioid addiction and withdrawal differentially drive long-term depression of inhibitory synaptic transmission in the hippocampus

Addictive behavior is increasingly accepted as a drug-associated pathological memory in which the hippocampus is profoundly engaged. It has been well documented that adaptations of synaptic plasticity of excitatory transmission in the hippocampus may contribute to opioid addiction. However, it remains unknown whether and how adaptive changes of synaptic plasticity of inhibitory transmission in the hippocampus occurs during opioid abuse. Here, we reported that a single in vivo morphine exposure (SM) did not affect inhibitory long-term depression (I-LTD) in the hippocampus, compared with saline control; while repeated morphine exposure (RM) abolished this I-LTD. Interestingly, opioid withdrawal for 3-5 days after repeated (RMW), but not a single morphine exposure (SMW), largely enhanced I-LTD. More importantly, the I-LTD in single morphine treatment is dependent on presynaptic mechanism since it can be blocked by AM251, a selective cannabinoid receptor 1 antagonist. While the large I-LTD in RMW group is dependent on combinatorial presynaptic and postsynaptic mechanisms since it can be blocked by co-application of AM251 and L-type calcium channel blocker LaCl₃. Thus, these results demonstrate that opioid use and withdrawal drive the dynamics of presynaptic and postsynaptic I-LTD expression in the hippocampus that may contribute to the persistent behavioral changes during opioid abuse.

Imbalance of HCN1 and HCN2 expression in hippocampal CA1 area impairs spatial learning and memory in rats with chronic morphine exposure

The hyperpolarization-activated cyclic-nucleotide-gated non-selective cation (HCN) channels play a vital role in the neurological basis underlying nervous system diseases. However, the role of HCN channels in drug addiction is not fully understood. In the present study, we investigated the expression of HCN1 and HCN2 subunits in hippocampal CA1 and the potential molecular mechanisms underlying the modulation of HCN channels in rats with chronic morphine exposure with approaches of electrophysiology, water maze, and Western blotting. We found that chronic morphine exposure (5 mg/kg, sc, for 7 days) caused an inhibition of long-term potentiation (LTP) and impairment of spatial learning and memory, which is associated with a decrease in HCN1, and an increase in HCN2 on cell membrane of hippocampal CA1 area. Additional experiments showed that the imbalance of cell membrane HCN1 and HCN2 expression under chronic morphine exposure was related to an increase in expression of TPR containing Rab8b interacting protein (TRIP8b) (1a-4) and TRIP8b (1b-2), and phosphorylation of protein kinase A (PKA) and adaptor protein 2 μ2 (AP2 μ2). Our results demonstrate the novel information that drug addiction-induced impairment of learning and memory is involved in the imbalance of HCN1 and HCN2 subunits, which is mediated by activation of TRIP8b (1a-4), TRIP8b (1b-2), PKA and AP2 μ2.

GIRK Channels: A Potential Link Between Learning and Addiction

The ability of drug-associated cues to reinitiate drug craving and seeking, even after long periods of abstinence, has led to the hypothesis that addiction represents a form of pathological learning, in which drugs of abuse hijack normal learning and memory processes to support long-term addictive behaviors. In this chapter, we review evidence suggesting that G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels are one mechanism through which numerous drugs of abuse can modulate learning and memory processes. We will examine the role of GIRK channels in two forms of experience-dependent long-term changes in neuronal function: homeostatic plasticity and synaptic plasticity. We will also discuss how drug-induced changes in GIRK-mediated signaling can lead to changes that support the development and maintenance of addiction.

Self administration of oxycodone alters synaptic plasticity gene expression in the hippocampus differentially in male adolescent and adult mice

Abuse and addiction to prescription opioids such as oxycodone (a short-acting Mu opioid receptor (MOP-r) agonist) in adolescence is a pressing public health issue. We have previously shown differences in oxycodone self-administration behaviors between adolescent and adult C57BL/6J mice and expression of striatal neurotransmitter receptor genes, in areas involved in reward. In this study, we aimed to determine whether oxycodone self-administration differentially affects genes regulating synaptic plasticity in the hippocampus of adolescent compared to adult mice, since the hippocampus may be involved in learning aspects associated with chronic drug self administration. Hippocampus was isolated for mRNA analysis from mice that had self administered oxycodone (0.25 mg/kg/infusion) 2h/day for 14 consecutive days or from yoked saline controls. Gene expression was analyzed with real-time polymerase chain reaction (PCR) using a commercially available "synaptic plasticity" PCR array containing 84 genes. We found that adolescent and adult control mice significantly differed in the expression of several genes in the absence of oxycodone exposure, including those coding for mitogen-activated protein kinase, calcium/calmodulin-dependent protein kinase II gamma subunit, glutamate receptor, ionotropic AMPA2 and metabotropic 5. Chronic oxycodone self administration increased proviral integration site 1 (Pim1) and thymoma viral proto-oncogene 1 mRNA levels compared to controls in both age groups. Both Pim1 and cadherin 2 mRNAs showed a significant combined effect of Drug Condition and Age × Drug Condition. Furthermore, the mRNA levels of both cadherin 2 and cAMP response element modulators showed an experiment-wise significant difference between oxycodone and saline control in adult but not in adolescent mice. Overall, this study demonstrates for the first time that chronic oxycodone self-administration differentially alters synaptic plasticity gene expression in the hippocampus of adolescent and adult mice.

Brain-derived neurotrophic factor and addiction: Pathological versus therapeutic effects on drug seeking

Many abused drugs lead to changes in endogenous brain-derived neurotrophic factor (BDNF) expression in neural circuits responsible for addictive behaviors. BDNF is a known molecular mediator of memory consolidation processes, evident at both behavioral and neurophysiological levels. Specific neural circuits are responsible for storing and executing drug-procuring motor programs, whereas other neural circuits are responsible for the active suppression of these "seeking" systems. These seeking-circuits are established as associations are formed between drug-associated cues and the conditioned responses they elicit. Such conditioned responses (e.g. drug seeking) can be diminished either through a passive weakening of seeking- circuits or an active suppression of those circuits through extinction. Extinction learning occurs when the association between cues and drug are violated, for example, by cue exposure without the drug present. Cue exposure therapy has been proposed as a therapeutic avenue for the treatment of addictions. Here we explore the role of BDNF in extinction circuits, compared to seeking-circuits that "incubate" over prolonged withdrawal periods. We begin by discussing the role of BDNF in extinction memory for fear and cocaine-seeking behaviors, where extinction circuits overlap in infralimbic prefrontal cortex (PFC). We highlight the ability of estrogen to promote BDNF-like effects in hippocampal-prefrontal circuits and consider the role of sex differences in extinction and incubation of drug-seeking behaviors. Finally, we examine how opiates and alcohol "break the mold" in terms of BDNF function in extinction circuits.

Effects of voluntary and treadmill exercise on spontaneous withdrawal signs, cognitive deficits and alterations in apoptosis-associated proteins in morphine-dependent rats

Chronic exposure to morphine results in cognitive deficits and alterations of apoptotic proteins in favor of cell death in the hippocampus, a brain region critically involved in learning and memory. Physical activity has been shown to have beneficial effects on brain health. In the current work, we examined the effects of voluntary and treadmill exercise on spontaneous withdrawal signs, the associated cognitive defects, and changes of apoptotic proteins in morphine-dependent rats. Morphine dependence was induced through bi-daily administrations of morphine (10mg/kg) for 10 days. Then, the rats were trained under two different exercise protocols: mild treadmill exercise or voluntary wheel exercise for 10 days. After exercise training, their spatial learning and memory and aversive memory were examined by a water maze and by an inhibitory avoidance task, respectively. The expression of the pro-apoptotic protein Bax and the anti-apoptotic protein Bcl-2 in the hippocampus were determined by immunoblotting. We found that chronic exposure to morphine impaired spatial and aversive memory and remarkably suppressed the expression of Bcl-2, but Bax expression remained constant. Both voluntary and treadmill exercise alleviated memory impairment, increased the expression of Bcl-2 protein, and only the later suppressed the expression of Bax protein in morphine-dependent animals. Moreover, both exercise protocols diminished the occurrence of spontaneous morphine withdrawal signs. Our findings showed that exercise reduces the spontaneous morphine-withdrawal signs, blocks the associated impairment of cognitive performance, and overcomes morphine-induced alterations in apoptotic proteins in favor of cell death. Thus, exercise may be a useful therapeutic strategy for cognitive and behavioral deficits in addict individuals.

Increased small conductance calcium-activated potassium type 2 channel-mediated negative feedback on N-methyl-D-aspartate receptors impairs synaptic plasticity following context-dependent sensitization to morphine

BACKGROUND

Hippocampal long-term potentiation (LTP) is impaired following repeated morphine administration paired with a novel context. This procedure produces locomotor sensitization that can be abolished by blocking calcium (Ca(2+))-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) in the hippocampus. However, the mechanisms underlying LTP impairment remain unclear. Here, we investigate the role of N-methyl-D-aspartate receptors (NMDARs), AMPARs, and small conductance Ca(2+)-activated potassium type 2 (SK2) channels in LTP induction after context-dependent sensitization to morphine.

METHODS

Mice were treated with saline or escalating doses of morphine (5, 8, 10, and 15 mg/kg) every 12 hours in a locomotor activity chamber and a challenge dose of 5 mg/kg morphine was given 1 week later. After the challenge, the hippocampi were removed to assay phosphatase 2A (PP2A) activity, NMDAR, and SK2 channel synaptic expression or to perform electrophysiological recordings.

RESULTS

Impaired hippocampal LTP, which accompanied morphine-induced context-dependent sensitization, could not be restored by blocking Ca(2+)-permeable AMPARs. Context-dependent sensitization to morphine altered hippocampal NMDAR subunit composition and enhanced the SK2 channel-mediated negative feedback on NMDAR. Increased PP2A activity observed following context-dependent sensitization suggests that the potentiated SK2 channel effect on NMDAR was mediated by increased SK2 sensitivity to Ca(2+). Finally, inhibition of SK2 channel or PP2A activity restored LTP.

CONCLUSIONS

Our studies demonstrate that the SK2 channel-NMDAR feedback loop plays a role in opiate-induced impairment of hippocampal plasticity and that the positive modulation of SK2 channels occurs via increases in PP2A activity. This provides further evidence that small conductance Ca(2+)-activated potassium channels play a role in drug-induced plasticity.

Single and repeated ultra-rapid detoxification prevents cognitive impairment in morphine addicted rats: a privilege for single detoxification

BACKGROUND

Opioids have been shown to affect learning and memory processes. Different protocols of morphine withdrawal can substantially vary in their success to prevent opioid induced impairments of cognitive performance. In the present study, we report the effects of single and repetitive ultra-rapid detoxification (URD) on spatial learning and memory in morphine addicted rats.

METHODS

Morphine (10 mg/kg) was intraperitoneally (IP) injected in male rats once a day over one week and after which they were detoxified with naloxone administration under anesthesia. For the repetitive procedure, a second one week morphine treatment with a second subsequent detoxification was performed. Control groups received an equivalent volume of saline injections. Spatial learning and memory was evaluated using the Morris water maze (MWM) task.

FINDINGS

Both protocols of morphine administration resulted in a severe spatial memory impairment that could be significantly prevented by both single and repetitive URD. However, memory abilities in animals treated with repetitive URD were still significantly lower than in animals of the corresponding control group. Alterations in motor activity or sensory-motor coordination between morphine treated and control animals could be ruled out by comparing swimming speed and visible platform performances that were not different between groups. Thus, URD and, specifically single URD, can prevent the spatial memory impairments in addicted rats.

CONCLUSION

As opioid addiction is an extending and serious concern in many societies, these findings may have clinical values and therapeutic implications for patients who experience multiple opioid relapses.

Effect of Chronic Morphine Consumption on Synaptic Plasticity of Rat's Hippocampus: A Transmission Electron Microscopy Study

It is well known that the synapses undergo some changes in the brain during the course of normal life and under certain pathological or experimental circumstances. One of the main goals of numerous researchers has been to find the reasons for these structural changes. In the present study, we investigated the effects of chronic morphine consumption on synaptic plasticity, postsynaptic density thickness, and synaptic curvatures of hippocampus CA1 area of rats. So for reaching these goals, 24 N-Mary male rats were randomly divided into three groups, morphine (n = 8), placebo (n = 8), and control (n = 8) groups. In the morphine group, complex of morphine (0.1, 0.2, 0.3, and 0.4) mg/mL and in the placebo (sucrose) group complex of sucrose (% 0.3) were used for 21 days. After the end of drug treatment the animals were scarified and perfused intracardinally and finally the CA1 hippocampal samples were taken for ultrastructural studies, and then the obtained data were analyzed by SPSS and one-way analysis of variance. Our data indicated that synaptic numbers per nm(3) change significantly in morphine group compared to the other two groups (placebo and control) (P < 0.001) and also statistical analysis revealed a significant difference between groups in terms of thickness of postsynaptic density (P < 0.001) and synaptic curvature (P < 0.007). It seems that morphine dependence in rats plays a main role in the ultrastructural changes of hippocampus.

Effects of voluntary exercise on hippocampal long-term potentiation in morphine-dependent rats

This study was designed to examine the effect of voluntary exercise on hippocampal long-term potentiation (LTP) in morphine-dependent rats. The rats were randomly distributed into the saline-sedentary (Sal/Sed), the dependent-sedentary, the saline-exercise (Sal/Exc), and the dependent-exercise (D/Exc) groups. The Sal/Exc and the D/Exc groups were allowed to freely exercise in a running wheel for 10 days. The Sal/Sed and the morphine-sedentary groups were kept sedentary for the same extent of time. Morphine (10 mg/kg) was injected bi-daily (12 h interval) during 10 days of voluntary exercise. On day 11, 2h after the morphine injection, the in vivo LTP in the dentate gyrus of the hippocampus was examined. The theta frequency primed bursts were delivered to the perforant path for induction of LTP. Population spike (PS) amplitude and the field excitatory post-synaptic potentials (fEPSP) slope were measured as indices of increase in synaptic efficacy. Chronic morphine increased the mean basal EPSP, and augmented PS-LTP. Exercise significantly increased the mean baseline EPSP and PS responses, and augmented PS-LTP in both saline and morphine-treated groups. Moreover, the increase of PS-LTP in the morphine-exercise group was greater (22.5%), but not statistically significant, than that of the Sal/Exc group. These results may imply an additive effect between exercise and morphine on mechanisms of synaptic plasticity. Such an interaction between exercise and chronic morphine may influence cognitive functions in opiate addicts.

The effect of chronic thienorphine administration on long-term potentiation and synaptic structure in rat hippocampus

Thienorphine is a new nonselective partial agonist of opioid receptors, which is currently under a Phase II clinical trial in China as a new treatment for opioid dependence. In this study, we compared the effect of thienorphine with morphine on long-term potentiation (LTP) in the lateral perforant path (LPP)-granule cell synapse of the rat dentate gyrus (DG). Furthermore, the effect of thienorphine on the synaptic structure of the CA1 hippocampal region and the expression of synaptophysin was investigated. Results indicated interesting differences between thienorphine and morphine on the modulation of hippocampal synaptic plasticity. Chronic thienorphine treatment facilitated LTP in the LPP-DG cell synapses more than chronic morphine treatment. Morphometric measurement and analysis showed that chronic thienorphine administration decreased the length of the active zone and reduced the thickness of CA1 postsynaptic densities compared with the saline group (control), but were elevated compared with the morphine group. Furthermore, the expression of hippocampal synaptophysin was increased with chronic thienorphine administration but reduced with chronic morphine treatment. Taken together, our study clearly demonstrates that chronic thienorphine treatment enhances LTP, modulates hippocampal synaptic structure, and increases the expression of hippocampal synaptophysin. Therefore, further study is warranted to investigate thienorphine as a new treatment for opioid dependence.

Decreased mitochondrial DNA copy number in the hippocampus and peripheral blood during opiate addiction is mediated by autophagy and can be salvaged by melatonin

Drug addiction is a chronic brain disease that is a serious social problem and causes enormous financial burden. Because mitochondrial abnormalities have been associated with opiate addiction, we examined the effect of morphine on mtDNA levels in rat and mouse models of addiction and in cultured cells. We found that mtDNA copy number was significantly reduced in the hippocampus and peripheral blood of morphine-addicted rats and mice compared with control animals. Concordantly, decreased mtDNA copy number and elevated mtDNA damage were observed in the peripheral blood from opiate-addicted patients, indicating detrimental effects of drug abuse and stress. In cultured rat pheochromocytoma (PC12) cells and mouse neurons, morphine treatment caused many mitochondrial defects, including a reduction in mtDNA copy number that was mediated by autophagy. Knockdown of the Atg7 gene was able to counteract the loss of mtDNA copy number induced by morphine. The mitochondria-targeted antioxidant melatonin restored mtDNA content and neuronal outgrowth and prevented the increase in autophagy upon morphine treatment. In mice, coadministration of melatonin with morphine ameliorated morphine-induced behavioral sensitization, analgesic tolerance and mtDNA content reduction. During drug withdrawal in opiate-addicted patients and improvement of protracted abstinence syndrome, we observed an increase of serum melatonin level. Taken together, our study indicates that opioid addiction is associated with mtDNA copy number reduction and neurostructural remodeling. These effects appear to be mediated by autophagy and can be salvaged by melatonin.

Inhibition of Activity of GABA Transporter GAT1 by δ-Opioid Receptor

Analgesia is a well-documented effect of acupuncture. A critical role in pain sensation plays the nervous system, including the GABAergic system and opioid receptor (OR) activation. Here we investigated regulation of GABA transporter GAT1 by δOR in rats and in Xenopus oocytes. Synaptosomes of brain from rats chronically exposed to opiates exhibited reduced GABA uptake, indicating that GABA transport might be regulated by opioid receptors. For further investigation we have expressed GAT1 of mouse brain together with mouse δOR and μOR in Xenopus oocytes. The function of GAT1 was analyzed in terms of Na⁺-dependent [³H]GABA uptake as well as GAT1-mediated currents. Coexpression of δOR led to reduced number of fully functional GAT1 transporters, reduced substrate translocation, and GAT1-mediated current. Activation of δOR further reduced the rate of GABA uptake as well as GAT1-mediated current. Coexpression of μOR, as well as μOR activation, affected neither the number of transporters, nor rate of GABA uptake, nor GAT1-mediated current. Inhibition of GAT1-mediated current by activation of δOR was confirmed in whole-cell patch-clamp experiments on rat brain slices of periaqueductal gray. We conclude that inhibition of GAT1 function will strengthen the inhibitory action of the GABAergic system and hence may contribute to acupuncture-induced analgesia.

Effect of Aqueous Extract of Crocus sativus L. on Morphine-Induced Memory Impairment

In the present study, the effect of aqueous extracts of saffron on morphine-induced memory impairment was investigated. On the training trial, the mice received an electric shock when the animals were entered into the dark compartment. Twenty-four and forty-eight hours later, the time latency for entering the dark compartment was recorded and defined as the retention trial. The mice were divided into (1) control, (2) morphine which received morphine before the training in the passive avoidance test, (3–5) three groups treated by 50, 150 and 450 mg/kg of saffron extract before the training trial, and (6 and 7) the two other groups received 150 and 450 mg/kg of saffron extract before the retention trial. The time latency in morphine-treated group was lower than control (P < 0.01). Treatment of the animals by 150 and 450 mg/kg of saffron extract before the training trial increased the time latency at 24 and 48 hours after the training trial (P < 0.05 and P < 0.01). Administration of both 150 and 450 mg/kg doses of the extract before retention trials also increased the time latency (P < 0.01). The results revealed that the saffron extract attenuated morphine-induced memory impairment.

Effect of c-Jun N-terminal kinase (JNK)/p38 mitogen-activated protein kinase (p38 MAPK) in morphine-induced tau protein hyperphosphorylation

Opioids have been widely used in clinical practice as potent pain relievers for centuries. However, opioids have many deleterious effects. It has been reported that opioid increases tau protein phosphorylation. Hyperphosphorylation of tau is also a pathological feature of Alzheimer's disease and other chronic neurodegenerative disorders. However, the underlying mechanism by which opioids enhance tau phosphorylation is not yet known. In this study, we treated rat embryo cortical neurons with morphine and observed its effect on tau phosphorylation. We found that morphine induced tau hyperphosphorylation and increased levels of phospho-JNK and phospho-p38; these effects were blocked by pretreatment with naloxone. Inhibition of JNK by SP600125 significantly reduced tau hyperphosphorylation in neurons treated with morphine. Similarly, SB203580, an antagonist of p38 MAPK, abolished tau hyperphosphorylation in neurons treated with morphine. Our data suggest that JNK/p38 MAPK, activated by morphine in an opioid receptor-dependent manner, may lead to tau hyperphosphorylation.

Chronic morphine exposure affects contrast response functions of V1 neurons in cats

Opiates disrupt neural functions in many brain areas, including visual cortex. Previous studies have indicated substantial changes of many neuronal response properties induced by chronic morphine exposure in the visual information processing system. However, it remains unclear whether neuronal contrast coding is also affected. To investigate this issue, we measured the contrast response functions (CRFs) of V1 neurons in chronic morphine-treated and saline-treated cats by using extra-cellular single-unit recording techniques. Our results indicated significantly lower contrast sensitivity in morphine-treated cats than in saline-treated cats and V1 neurons in morphine-treated cats exhibited enhanced maximum visual responses, higher baseline responses and lower signal-to-noise ratios compared with saline-treated cats. These findings provide some neurobiological evidence for the morphine-mediated degenerations of the visual cortex, which could underlie the opiate-induced deficits in visual function.

Aβ damages learning and memory in Alzheimer's disease rats with kidney-yang deficiency

Previous studies demonstrated that Alzheimer's disease was considered as the consequence produced by deficiency of Kidney essence. However, the mechanism underlying the symptoms also remains elusive. Here we report that spatial learning and memory, escape, and swimming capacities were damaged significantly in Kidney-yang deficiency rats. Indeed, both hippocampal Aβ(40) and 42 increases in Kidney-yang deficiency contribute to the learning and memory impairments. Specifically, damage of synaptic plasticity is involved in the learning and memory impairment of Kidney-yang deficiency rats. We determined that the learning and memory damage in Kidney-yang deficiency due to synaptic plasticity impairment and increases of Aβ(40) and 42 was not caused via NMDA receptor internalization induced by Aβ increase. β-Adrenergic receptor agonist can rescue the impaired long-term potential (LTP) in Kidney-yang rats. Taken together, our results suggest that spatial learning and memory inhibited in Kidney-yang deficiency might be induced by Aβ increase and the decrease of β(2) receptor function in glia.

Chronic Opium Treatment Can Differentially Induce Brain and Liver Cells Apoptosis in Diabetic and Non-diabetic Male and Female Rats

It has been shown that some opium derivatives promote cell death via apoptosis. This study was designed to examine the influence of opium addiction on brain and liver cells apoptosis in male and female diabetic and non-diabetic Wistar rats. This experimental study was performed on normal, opium-addicted, diabetic and diabetic opium-addicted male and female rats. Apoptosis was evaluated by TUNEL and DNA fragmentation assays. Results of this study showed that apoptosis in opium-addicted and diabetic opium-addicted brain and liver cells were significantly higher than the both normal and diabetic rats. In addition, we found that apoptosis in brain cells of opium-addicted and diabetic opium-addicted male rats were significantly higher than opium-addicted and diabetic opium-addicted female, whereas apoptosis in liver cells of opium-addicted and diabetic opium-addicted female rats were significantly higher than opium-addicted and diabetic opium-addicted male. Overall, these results indicate that opium probably plays an important role in brain and liver cells apoptosis, therefore, leading neurotoxicity and hepatotoxicity. These findings also in away possibly means that male brain cells are more susceptible than female and interestingly liver of females are more sensitive than males in induction of apoptosis by opium.

Voluntary exercise ameliorates cognitive deficits in morphine dependent rats: the role of hippocampal brain-derived neurotrophic factor

Chronic exposure to opiates impairs spatial learning and memory. Given the well-known beneficial effects of voluntary exercise on cognitive functions, we investigated whether voluntary exercise would ameliorate the cognitive deficits that are induced by morphine dependence. If an effect of exercise was observed, we aimed to investigate the possible role of hippocampal brain-derived neurotrophic factor (BDNF) in the exercise-induced enhancement of learning and memory in morphine-dependent rats. The rats were injected with bi-daily doses (10mg/kg, at 12h intervals) of morphine over a period of 10 days of voluntary exercise. Following these injections, a water maze task was performed twice a day for five consecutive days, followed by a probe trial 2 days later. A specific BDNF inhibitor (TrkB-IgG chimera) was used to block the hippocampal BDNF action during the 10 days of voluntary exercise. We found that voluntary exercise blocked the ability of chronic morphine to impair spatial memory retention. A blockade of the BDNF action blunted the exercise-induced improvement of spatial memory in the dependent rats. Moreover, the voluntary exercise diminished the severity of the rats' dependency on morphine. This study demonstrates that voluntary exercise ameliorates, via a TrkB-mediated mechanism, the cognitive deficits that are induced by chronic morphine. Thus, voluntary exercise might be a potential method to ameliorate some of the deleterious behavioral consequences of the abuse of morphine and other opiates.

Anxiety profile in morphine-dependent and withdrawn rats: effect of voluntary exercise

Withdrawal from chronic opiates is associated with an increase in anxiogenic-like behaviours, but the anxiety profile in the morphine-dependent animals is not clear. Thus, one of the aims of the present study was to examine whether morphine-dependent rats would increase the expression of anxiogenic-like behaviours in novel and stressful conditions. Additionally, recent studies have shown that voluntary exercise can reduce anxiety levels in rodents. Therefore, another aim of this study was to examine the effect of voluntary exercise on the anxiety profile in both morphine-dependent animals and animals experiencing withdrawal. Rats were injected with bi-daily doses (10 mg/kg, at 12 h intervals) of morphine over a period of 10 days in which they were also allowed voluntary exercise. Following these injections, anxiety-like behaviours were tested in the elevated plus-maze (EPM) model and the light/dark (L/D) box. We found reductions in time spent in, and entries into, the EPM open arms and reductions in time spent in the lit side of the L/D box for both sedentary morphine-dependent and withdrawn rats as compared to the sedentary control groups. The exercising morphine-dependent and withdrawn rats exhibited an increase in EPM open arm time and entries and L/D box lit side time as compared with the sedentary control groups. We conclude that voluntary exercise decreases the severity of the anxiogenic-like behaviours in both morphine-dependent and withdrawn rats. Thus, voluntary exercise could be a potential natural method to ameliorate some of the deleterious behavioural consequences of opiate abuse.

RACK1 affects morphine reward via BDNF

Chronic morphine addiction may trigger functional changes in the mesolimbic dopamine system, which is believed to be the neurobiological substrate of opiate addiction. Brain derived neurotrophic factor (BDNF) has been implicated in addiction-related pathology in animal studies. Our previous studies have shown that RACK1 is involved in morphine reward in mice. The recent research indicates nuclear RACK1 by localizing at the promoter IV region of the BDNF gene and the subsequent chromatin modifications leads to the activation of the promoter and transcription of BDNF. The present study was designed to investigate if shRACK1 (a short hairpin RNA of RACK1) could reverse the mice's behavioral responses to morphine and BDNF expression in hippocampus and prefrontal cortex. No significant changes were observed in vehicle-infused mice which received no morphine treatment (CONC) and shRACK1-infused mice which received no morphine treatment (CONR), whereas vehicle-infused mice preceded the morphine injection (MIC) showed increased BDNF expression in hippocampus and prefrontal cortex, as compared to vehicle-infused mice which received no morphine treatment (CONC). Intracerebroventricular shRACK1 treatment reversed these, and in fact, ShRACK1-infused mice preceded the morphine injection (MIR) showed reduced BDNF expression in hippocampus and prefrontal cortex, as compared to MIC. In the conditioned place preference (CPP) test, inactivating RACK1 markedly reduces morphine-induced conditioned place preference. Non-specific changes in CPP could not account for these effects since general CPP of shRACK1- and vehicle-infused animals was not different. Combined behavioral and molecular approaches have support the possibility that the RACK1-BDNF system plays an important role in the response to morphine-induced reward.

Analgesia and sedation after pediatric cardiac surgery

In recent years, the importance of appropriate intra-operative anesthesia and analgesia during cardiac surgery has become recognized as a factor in postoperative recovery. This includes the early perioperative management of the neonate undergoing radical surgery and more recently the care surrounding fast-track and ultra fast-track surgery. However, outside these areas, relatively little attention has focused on postoperative sedation and analgesia within the pediatric intensive care unit (PICU). This reflects perceived priorities of the primary disease process over the supporting structure of PICU, with a generic approach to sedation and analgesia that can result in additional morbidities and delayed recovery. Management of the marginal patient requires optimisation of not only cardiac and other attendant pathophysiology, but also every aspect of supportive care. Individualized sedation and analgesia strategies, starting in the operating theater and continuing through to hospital discharge, need to be regarded as an important aspect of perioperative care, to speed the process of recovery.

Addiction and brain reward and antireward pathways

Addictive drugs have in common that they are voluntarily self-administered by laboratory animals (usually avidly), and that they enhance the functioning of the reward circuitry of the brain (producing the 'high' that the drug user seeks). The core reward circuitry consists of an 'in-series' circuit linking the ventral tegmental area, nucleus accumbens and ventral pallidum via the medial forebrain bundle. Although originally believed to simply encode the set point of hedonic tone, these circuits are now believed to be functionally far more complex, also encoding attention, expectancy of reward, disconfirmation of reward expectancy, and incentive motivation. 'Hedonic dysregulation' within these circuits may lead to addiction. The 'second-stage' dopaminergic component in this reward circuitry is the crucial addictive-drug-sensitive component. All addictive drugs have in common that they enhance (directly or indirectly or even transsynaptically) dop-aminergic reward synaptic function in the nucleus accumbens. Drug self-administration is regulated by nucleus accumbens dopamine levels, and is done to keep nucleus accumbens dopamine within a specific elevated range (to maintain a desired hedonic level). For some classes of addictive drugs (e.g. opiates), tolerance to the euphoric effects develops with chronic use. Postuse dysphoria then comes to dominate reward circuit hedonic tone, and addicts no longer use drugs to get high, but simply to get back to normal ('get straight'). The brain circuits mediating the pleasurable effects of addictive drugs are anatomically, neurophysiologically and neurochemically different from those mediating physical dependence, and from those mediating craving and relapse. There are important genetic variations in vulnerability to drug addiction, yet environmental factors such as stress and social defeat also alter brain-reward mechanisms in such a manner as to impart vulnerability to addiction. In short, the 'bio-psycho-social' model of etiology holds very well for addiction. Addiction appears to correlate with a hypodopaminergic dysfunctional state within the reward circuitry of the brain. Neuroimaging studies in humans add credence to this hypothesis. Credible evidence also implicates serotonergic, opioid, endocannabinoid, GABAergic and glutamatergic mechanisms in addiction. Critically, drug addiction progresses from occasional recreational use to impulsive use to habitual compulsive use. This correlates with a progression from reward-driven to habit-driven drug-seeking behavior. This behavioral progression correlates with a neuroanatomical progression from ventral striatal (nucleus accumbens) to dorsal striatal control over drug-seeking behavior. The three classical sets of craving and relapse triggers are (a) reexposure to addictive drugs, (b) stress, and (c) reexposure to environmental cues (people, places, things) previously associated with drug-taking behavior. Drug-triggered relapse involves the nucleus accumbens and the neurotransmitter dopamine. Stress-triggered relapse involves (a) the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and the neurotransmitter corticotrophin-releasing factor, and (b) the lateral tegmental noradrenergic nuclei of the brain stem and the neurotransmitter norepinephrine. Cue-triggered relapse involves the basolateral nucleus of the amygdala, the hippocampus and the neurotransmitter glutamate. Knowledge of the neuroanatomy, neurophysiology, neurochemistry and neuropharmacology of addictive drug action in the brain is currently producing a variety of strategies for pharmacotherapeutic treatment of drug addiction, some of which appear promising.

Delta opioid receptors colocalize with corticotropin releasing factor in hippocampal interneurons

The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect, likely playing a critical role in the interaction between stress and drug addiction. Prior study findings suggest that the stress-related neuropeptide corticotropin releasing factor (CRF) and the delta opioid receptor (DOR) may localize to similar neuronal populations within HF lamina. Here, hippocampal sections of male and cycling female adult Sprague-Dawley rats were processed for immunolabeling using antisera directed against the DOR and CRF peptide, as well as interneuron subtype markers somatostatin or parvalbumin, and analyzed by fluorescence and electron microscopy. Both DOR- and CRF-labeling was observed in interneurons in the CA1, CA3, and dentate hilus. Males and normal cycling females displayed a similar number of CRF immunoreactive neurons co-labeled with DOR and a similar average number of CRF-labeled neurons in the dentate hilus and stratum oriens of CA1 and CA3. In addition, 70% of DOR/CRF dual-labeled neurons in the hilar region co-labeled with somatostatin, suggesting a role for these interneurons in regulating perforant path input to dentate granule cells. Ultrastructural analysis of CRF-labeled axon terminals within the hilar region revealed that proestrus females have a similar number of CRF-labeled axon terminals that contain DORs compared to males but an increased number of CRF-labeled axon terminals without DORs. Taken together, these findings suggest that while DORs are anatomically positioned to modulate CRF immunoreactive interneuron activity and CRF peptide release, their ability to exert such regulatory activity may be compromised in females when estrogen levels are high.

Long-term methadone treatment impairs novelty preference in rats both when present and absent in brain tissue

Behavioral consequences of long-term methadone treatment have received little attention either in humans or experimental animals. In this work, we show that methadone (2.5-10 mg/kg) administered (sc) once daily for three weeks with repeated withdrawal on Saturday and Sunday impairs the novelty preference in rats. One hour after the last injection, when methadone was still present in brain tissue, the rats were too affected by the sedative effects of the drug to perform the test. This was confirmed by an almost total lack of locomotor activity or exploratory behavior. One day after the last injection, the methadone treated rats showed a 70% reduction (p < 0.05) in novelty preference compared to rats administered saline. No methadone was detected in the brain tissue at this time. Moreover, there were no differences in locomotor activity or total exploratory behavior between the groups, indicating a specific impairment of cognitive functioning. In brain tissue, the methadone concentration versus time profile was shifted to the left after long-term treatment, indicating a change in uptake and distribution of the drug. The area under the two concentration versus time curves was, however, similar. Methadone disappeared completely from the brain within one day. Together, these results suggest that long-term methadone treatment may have a negative impact on cognitive functioning in rats, regardless of whether methadone is present in brain tissue.

Abnormal synaptic plasticity in basolateral amygdala may account for hyperactivity and attention-deficit in male rat exposed perinatally to low-dose bisphenol-A

If the pregnant and lactating female rats are exposed to environmental levels of bisphenol-A (BPA), their male offspring will display hyperactivity and attention-deficit. In patients with attention-deficit/hyperactivity disorder (ADHD), the size of the amygdala is reported to be reduced. This study examined functional alterations in the basolateral amygdala (BLA) of the postnatal 28-day-old male offspring exposed perinatally to BPA (BPA-rats). We specifically focused on the synaptic properties of GABAergic/dopaminergic systems in the BLA. A single electrical stimulation of the capsule fibers evoked multispike responses with an enhanced primary population spikes (1st-PS) in the BPA-rats. A single train of high-frequency stimulation of the fibers induced NMDA receptor (NMDAR) dependent long-term potentiation (LTP) in BPA-rats, but not in control rats. Also, paired-pulse inhibition (PPI, GABA-dependent) in control rats was reversed to paired-pulse facilitation (PPF) in BPA-rats. Perfusion of slices obtained from BPA-rats with the GABA(A) receptor (GABA(A)R) agonist muscimol blocked the multispike responses and LTP, and recovered PPI. By contrast, the dopamine D1 receptor antagonist SCH23390 abolished LTP and attenuated the increased amplitude of 1st-PS in BPA-rats. Conversely, blockade of GABA(A)R by bicuculline could produce the multispike responses and PPF in BLA in control rats. Furthermore, in BLA the infusion of SCH23390, muscimol or the NMDAR blocker MK801 ameliorated the hyperactivity and improved the deficits in attention. These findings suggest that the perinatal exposure to BPA causes GABAergic disinhibition and dopaminergic enhancement, leading to an abnormal cortical-BLA synaptic transmission and plasticity, which may be responsible for the hyperactivity and attention-deficit in BPA-rats. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Implication of Substance P in myocardial contractile function during ischemia in rats

Evidence suggests that substance P (SP) participates in the pathology of acute myocardial ischemia and infarction but the profiles of the peptide in regulation of cardiac functions are still elusive. The aim of this study was to investigate the role of substance P in regulation of cardiac functions and its association with adrenergic mechanism in acute myocardial ischemia and infarction with rodent models. The experiments were carried out in Sprague-Dawley rats. SP and norepinephrine were significantly up-regulated in myocardium at 15min, 30min and 60min of coronary artery occlusion. Pretreatment of the rats with a specific antagonist of neurokinin-1 receptor, D-SP, significant increased+dp/dt and decreased -dp/dt, compared with the controls, pretreated with 0.9% saline. Pretreatment of the isolated CAO hearts with substance P (10(-7)mol/L) significantly increased left ventricular end diastolic pressure. SP producing no effects on cardiac functions when given alone to isolated (non-CAO) heart caused significant attenuation of the changes in the contractility and diastolic functions induced by norepinephrine, when given with norepinephrine. SP attenuated the increase in the activity of PKA provoked by norepinephrine in cultured myocytes. In conclusion, the findings may indicate SP regulates cardiac functions via modulation of adrenergic activity, through suppression of over-activation of PKA.

Hormonal regulation of delta opioid receptor immunoreactivity in interneurons and pyramidal cells in the rat hippocampus

Clinical and preclinical studies indicate that women and men differ in relapse vulnerability to drug-seeking behavior during abstinence periods. As relapse is frequently triggered by exposure of the recovered addict to objects previously associated with drug use and the formation of these associations requires memory systems engaged by the hippocampal formation (HF), studies exploring ovarian hormone modulation of hippocampal function are warranted. Previous studies revealed that ovarian steroids alter endogenous opioid peptide levels and trafficking of mu opioid receptors in the HF, suggesting cooperative interaction between opioids and estrogens in modulating hippocampal excitability. However, whether ovarian steroids affect the levels or trafficking of delta opioid receptors (DORs) in the HF is unknown. Here, hippocampal sections of adult male and normal cycling female Sprague-Dawley rats were processed for quantitative immunoperoxidase light microscopy and dual label fluorescence or immunoelectron microscopy using antisera directed against the DOR and neuropeptide Y (NPY). Consistent with previous studies in males, DOR-immunoreactivity (-ir) localized to select interneurons and principal cells in the female HF. In comparison to males, females, regardless of estrous cycle phase, show reduced DOR-ir in the granule cell layer of the dentate gyrus and proestrus (high estrogen) females, in particular, display reduced DOR-ir in the CA1 pyramidal cell layer. Ultrastructural analysis of DOR-labeled profiles in CA1 revealed that while females generally show fewer DORs in the distal apical dendrites of pyramidal cells, proestrus females, in particular, exhibit DOR internalization and trafficking towards the soma. Dual label studies revealed that DORs are found in NPY-labeled interneurons in the hilus, CA3, and CA1. While DOR colocalization frequency in NPY-labeled neuron somata was similar between animals in the hilus, proestrus females had fewer NPY-labeled neurons that co-labeled with DOR in stratum oriens of CA1 and CA3 when compared to males. Ultrastructural analysis of NPY-labeled axon terminals within stratum radiatum of CA1 revealed that NPY-labeled axon terminals contain DORs that are frequently found at or near the plasma membrane. As no differences were noted by sex or estrous cycle phase, DOR activation on NPY-labeled axon terminals would inhibit GABA release probability equally in males and females. Taken together, these findings suggest that ovarian steroids can impact hippocampal function through direct effects on DOR levels and trafficking in principal cells and broad indirect effects through reductions in DOR-ir in NPY-labeled interneurons, particularly in CA1.

Mu opioid receptor activation normalizes temporo-ammonic pathway driven inhibition in hippocampal CA1

The hippocampus of the mammalian brain is important for the formation of long-term memories. Hippocampal-dependent learning can be affected by a number of neurotransmitters including the activation of μ-opioid receptors (MOR). It has been shown that MOR activation can alter synaptic plasticity and network oscillations in the hippocampus, both of which are thought to be important for the encoding of information and formation of memories. One hippocampal oscillation that has been correlated with learning and memory formation is the 4-10 Hz theta rhythm. During theta rhythms, inputs to hippocampal CA1 from CA3 (Schaffer collaterals, SC) and the entorhinal cortex (perforant path) can integrate at different times within an individual theta cycle. Consequently, when excitatory inputs in the stratum lacunosum-moleculare (the temporo-ammonic pathway (TA), which includes the perforant path) are stimulated approximately one theta period before SC inputs, the TA can indirectly inhibit SC inputs. This inhibition is due to the activation of postsynaptic GABA(B) receptors on CA1 pyramidal neurons. Importantly, MOR activation has been shown to suppress GABA(B) inhibitory postsynaptic potentials in CA1 pyramidal neurons. Therefore, we examined how MOR activation affects the integration between TA inputs and SC inputs in hippocampal CA1. To do this we used voltage-sensitive dye imaging and whole cell patch clamping from acute hippocampal slices taken from young adult rats. Here we show that MOR activation has no effect on the integration between TA and SC inputs when activation of the TA precedes SC by less than one half of a theta cycle (<75 ms). However, MOR activation completely blocked the inhibitory action of TA on SC inputs when TA stimulation occurred approximately one theta cycle before SC activation (>150 ms). This MOR suppression of TA driven inhibition occurred in both the SC input layer of hippocampal CA1 (stratum radiatum) and the output layer of CA1 pyramidal neurons (stratum pyramidale). Thus MOR activation can have profound effects on the temporal integration between two primary excitatory pathways to hippocampal CA1 and subsequently the resultant output from CA1 pyramidal neurons. These data provide important information for understanding how acute or chronic MOR activation may affect the integration of activity within hippocampal CA1 during theta rhythm.

Ameliorating effect of histamine on impairment of cued fear extinction induced by morphine withdrawal in histidine decarboxylase gene knockout mice

AIM

Histamine plays an important role in morphine addiction and memory-dependent behavior. However, little is known about the effect of histamine on the impairment of memory after morphine withdrawal. This study was designed to investigate the effect of histamine on memory impairment induced by morphine withdrawal in histidine decarboxylase knockout (HDC-KO) and wild-type (WT) mice.

METHODS

WT and HDC-KO mice were given subcutaneous morphine or saline twice daily for 5 consecutive days. The mice received a cued or contextual fear conditioning session 7 days after the last injection. During subsequent days, mice received 4 cued or contextual extinction sessions (one session per day). Western blot was used to assess extracellular signal-regulated kinase (ERK) phosphorylation in the amygdala and hippocampus.

RESULTS

Morphine withdrawal did not affect the acquisition of cued or contextual fear responses. It impaired cued but not contextual fear extinction. The acquisition of cued and contextual fear responses was accelerated in HDC-KO mice. Histamine deficiency aggravated the impairment of cued fear extinction induced by morphine withdrawal, whereas histamine (icv, 5 μg/mouse) reversed this effect. Morphine withdrawal decreased ERK phosphorylation in the amygdala after cued fear extinction, especially in HDC-KO mice.

CONCLUSION

These results suggest that morphine withdrawal specifically impairs cued fear extinction and histamine ameliorates this impairment. Its action might be mediated by the modulation of ERK phosphorylation in the amygdala. Histamine should be explored for possible roles in the prevention or treatment of morphine abuse and relapse.

Extracellular matrix plasticity and GABAergic inhibition of prefrontal cortex pyramidal cells facilitates relapse to heroin seeking

Successful treatment of drug addiction is hampered by high relapse rates during periods of abstinence. Neuroadaptation in the medial prefrontal cortex (mPFC) is thought to have a crucial role in vulnerability to relapse to drug seeking, but the molecular and cellular mechanisms remain largely unknown. To identify protein changes that contribute to relapse susceptibility, we investigated synaptic membrane fractions from the mPFC of rats that underwent 21 days of forced abstinence following heroin self-administration. Quantitative proteomics revealed that long-term abstinence from heroin self-administration was associated with reduced levels of extracellular matrix (ECM) proteins. After extinction of heroin self-administration, downregulation of ECM proteins was also present in the mPFC, as well as nucleus accumbens (NAc), and these adaptations were partially restored following cue-induced reinstatement of heroin seeking. In the mPFC, these ECM proteins are condensed in the perineuronal nets that exclusively surround GABAergic interneurons, indicating that ECM adaptation might alter the activity of GABAergic interneurons. In support of this, we observed an increase in the inhibitory GABAergic synaptic inputs received by the mPFC pyramidal cells after the re-exposure to heroin-conditioned cues. Recovering levels of ECM constituents by metalloproteinase inhibitor treatment (FN-439; i.c.v.) prior to a reinstatement test attenuated subsequent heroin seeking, suggesting that the reduced synaptic ECM levels during heroin abstinence enhanced sensitivity to respond to heroin-conditioned cues. We provide evidence for a novel neuroadaptive mechanism, in which heroin self-administration-induced adaptation of the ECM increased relapse vulnerability, potentially by augmenting the responsivity of mPFC GABAergic interneurons to heroin-associated stimuli.