Morphine blood levels, dependence, and regulation of hippocampal subgranular zone proliferation rely on administration paradigm

The Effect of Antinociceptive Dose of Morphine on Cell Therapy in Rats with Spinal Cord Injury

Spinal cord injury (SCI) is a sensory-motor injury. Today, combined treatments such as cell therapy along with drug therapy and their interactions are of interest. Morphine is an opioid drug used to relieve intolerable pain. This study aims to evaluate the impact of an antinociceptive dose of morphine (with minimal tolerance/dependence but effective pain relief) on cell therapy in SCI. The antinociceptive dose of morphine was determined in rats with SCI through the Hargreaves and naloxone-induced morphine withdrawal tests. The rats were then allocated to 5 groups: laminectomy, SCI, SCI + Morphine, SCI + cell therapy, SCI + Morphine + cell therapy. The antinociceptive dose (5 mg/kg) was administered on days 1, 4, 10, and 13 (i.p.) post-SCI. On day 7, Neural-like stem cells derived from adipose tissue were transplanted intraspinally into the injured animals, and they were monitored for 12 weeks. The outcomes were assessed using the BBB test, somatosensory evoked potential (SSEP), and histology. The BBB test indicated that morphine significantly hindered functional recovery post-cell transplantation compared to animals receiving only cell therapy (p < 0.05). In the SSEP test, the analysis of amplitude and latency of waves did not reveal a significant difference (p > 0.05). The histological results showed that cell therapy reduced the cavity size post-SCI, while morphine had no significant impact on it. Morphine at the antinociceptive dose significantly impairs motor recovery despite cell therapy. Nonetheless, there was no significant difference between groups in terms of sensory pathway outcomes.

Effect of morphine administration after status epilepticus on epileptogenesis in rats

INTRODUCTION

Morphine is widely used in patients and has been reported to alter seizure threshold, but its role in the development of epilepsy is unknown. In this study, role of morphine administration in the development of epilepsy using the status epilepticus (SE) model was determined in rats.

METHODS

Rats experiencing SE with lithium-pilocarpine (LiP) were randomized into four groups- saline, morphine low dose (5 mg/kg, s.c.), morphine high dose (5-20 mg/kg, s.c.), and naloxone (1 mg/kg, s.c.). Treatments were started 90 min after termination of SE and repeated twice daily for next three days. Rats were video monitored daily for 21 days to determine onset and frequency of spontaneous convulsive seizures (SS).

RESULTS

Morphine in low doses increased frequency of SS (1.51 ± 0.15 vs LiP 0.60 ± 0.12 seizures/rat/day, p-value = 0.0026) and seizures occurred during handling (SDH) (0.08 ± 0.02 vs LiP control 0.01 ± 0.01) (p-value = 0.0018). In high doses, no significant change in SS and SDH was found as compared to LiP. No effect of morphine on the onset of SS and percentage of rats experienced SS was found. No effect of naloxone per se was found on SS.

CONCLUSION

Morphine administration after SE does not affect epileptogenesis as no change in the onset of SS and percentage of rats experiencing SS was found. However, it might alter the susceptibility and frequency of SS. As no other study is available with a similar finding, it needs further evaluation.

Zebrafish (Danio rerio) larvae show behavioral and embryonic development defects when exposed to opioids at embryo stage

Opioid abuse continues to plague society, and in recent years, there has been an epidemic, leading to increased addiction and death. It is poorly understood how prenatal opioid use affects the lives of children. The aim of this work was to evaluate the effect of early embryonic codeine or morphine exposure in zebrafish (Danio rerio), examining gastrulation progression (epiboly), teratogenic effects, mortality and locomotor behavior response to light/dark cycles. Zebrafish embryos were exposed to codeine or morphine (designated C or M) at 1, 5 or 10 mg/L (designated 01, 05 or 10, respectively) from 3 to 24 h postfertilization (hpf) or from 3 to 48 hpf (designated -24 or - 48 for 1 or 2 days of exposure, respectively). The C10-24, C01-48, C05-48 and C10-48 groups showed significantly smaller eyes than control larvae at 7 days postfertilization (dpf). Locomotor behavior of control larvae in light/dark cycles showed greater swimming time and distance in dark cycles. Two-day codeine exposure produced strong effects, showing no significant response due to light/dark cycles in distance moved. Morphine exposed groups showed similar effects as observed in 2-day codeine exposed groups, showing less large movement activity and also no significant difference between inactive duration in response to light/dark cycles. In conclusion, we observed low teratogenic effects and mortality effects. Animals exposed to high levels and higher exposure times of opioids were hypoactive, relative to controls, in the dark period. Future studies will be needed to understand the neural defects producing behavior changes.

Behavioral effects of SGK1 knockout in VTA and dopamine neurons

Drugs of abuse cause significant neuroadaptations within the ventral tegmental area (VTA), with alterations in gene expression tied to changes in reward behavior. Serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription, catalytic activity, and phosphorylation are upregulated in the VTA by chronic cocaine or morphine treatment, positioning SGK1 as a critical mediator of reward behavior. Using transgenic mouse models, we investigated the effect of SGK1 knockout in the VTA and in dopamine (DA) neurons to evaluate the necessity of protein expression for natural and drug reward behaviors. SGK1 knockdown in the VTA did not impact reward behaviors. Given VTA cellular heterogeneity, we also investigated a DA neuron-specific SGK1 knockout (KO). DA SGK1 KO significantly decreased body weight of adult mice as well as increased general locomotor activity; however, reward behaviors were similarly unaltered. Given that SGK1 mutants virally overexpressed in the VTA are capable of altering drug-associated behavior, our current results suggest that changes in SGK1 protein signaling may be distinct from expression. This work yields novel information on the impact of SGK1 deletion, critical for understanding the role of SGK1 signaling in the central nervous system and evaluating SGK1 as a potential therapeutic target for treatment of substance use disorders.

Indices of dentate gyrus neurogenesis are unaffected immediately after or following withdrawal from morphine self-administration compared to saline self-administering control male rats

Opiates - including morphine - are powerful analgesics with high abuse potential. In rodents, chronic opiate exposure or self-administration negatively impacts hippocampal-dependent function, an effect perhaps due in part to the well-documented opiate-induced inhibition of dentate gyrus (DG) precursor proliferation and neurogenesis. Recently, however, intravenous (i.v.) morphine self-administration (MSA) was reported to enhance the survival of new rat DG neurons. To reconcile these disparate results, we used rat i.v. MSA to assess 1) whether a slightly-higher dose MSA paradigm also increases new DG neuron survival; 2) how MSA influences cells in different stages of DG neurogenesis, particularly maturation and survival; and 3) if MSA-induced changes in DG neurogenesis persist through a period of abstinence. To label basal levels of proliferation, rats received the S-phase marker bromodeoxyuridine (BrdU, i.p.) 24 -h prior to 21 days (D) of i.v. MSA or saline self-administration (SSA). Either immediately after SA (0-D) or after 4 weeks in the home cage (28-D withdrawal), stereology was used to quantify DG proliferating precursors (or cells in cell cycle; Ki67+ cells), neuroblast/immature neurons (DCX+ cells), and surviving DG granule cells (BrdU+ cells). Analysis revealed the number of DG cells immunopositive for these neurogenesis-relevant markers was similar between MSA and SSA rats at the 0-D or 28-D timepoints. These negative data highlight the impact experimental parameters, timepoint selection, and quantification approach have on neurogenesis results, and are discussed in the context of the large literature showing the negative impact of opiates on DG neurogenesis.

Non-nociceptive roles of opioids in the CNS: opioids' effects on neurogenesis, learning, memory and affect

Mortality due to opioid use has grown to the point where, for the first time in history, opioid-related deaths exceed those caused by car accidents in many states in the United States. Changes in the prescribing of opioids for pain and the illicit use of fentanyl (and derivatives) have contributed to the current epidemic. Less known is the impact of opioids on hippocampal neurogenesis, the functional manipulation of which may improve the deleterious effects of opioid use. We provide new insights into how the dysregulation of neurogenesis by opioids can modify learning and affect, mood and emotions, processes that have been well accepted to motivate addictive behaviours.

Determination of circuit-specific morphological adaptations in ventral tegmental area dopamine neurons by chronic morphine

Chronic opiate exposure induces neuroadaptations in the mesocorticolimbic system including ventral tegmental area (VTA) dopamine (DA) neurons, whose soma size is decreased following opiate exposure. Yet it is now well documented that VTA DA neurons are heterogeneous, with notable differences between VTA DA neurons based on their projection target. Therefore, we sought to determine whether chronic morphine induced similar changes in the morphology of VTA DA neurons that project to the nucleus accumbens (NAc) and prefrontal cortex (PFC). We utilized Cre-dependent retrograde viral vectors in DA Cre driver lines to label VTA DA neurons that projected to NAc and PFC and assessed neuronal soma size. Consistent with previous data, the soma size of VTA DA neurons that projected to the NAc medial shell was decreased following morphine exposure. However, soma size of VTA DA neurons that projected to the NAc core was unaltered by morphine. Interestingly, morphology of PFC-projecting VTA DA neurons was also altered by morphine, but in this case soma size was increased compared to sham controls. Differences in basal soma size were also noted, suggesting stable differences in projection-specific morphology in addition to drug-induced changes. Together, these data suggest morphine-induced changes in VTA DA morphology occur within distinct VTA DA populations and that study of opiate-induced structural plasticity of individual VTA DA subcircuits may be critical for understanding addiction-related behavior.

Dentate gyrus μ-opioid receptor-mediated neurogenic processes are associated with alterations in morphine self-administration

Adult hippocampal dentate gyrus (DG) neural stem cells (NSCs) continuously undergo proliferation and differentiation, producing new functional neurons that remodel existing synaptic circuits. Although proliferation of these adult DG NSCs has been implicated in opiate dependence, whether NSC neuronal differentiation and subsequent dendritogenesis are also involved in such addictive behavior remains unknown. Here, we ask whether opiate exposure alters differentiation and dendritogenesis of DG NSCs and investigate the possibility that these alterations contribute to opiate addiction. We show that rat morphine self-administration (MSA), a paradigm that effectively mimics human opiate addiction, increases NSC neuronal differentiation and promotes neuronal dendrite growth in the adult DG. Further, we demonstrate that the μ-opioid receptor (MOR) is expressed on DG NSCs and that MSA leads to a two-fold elevation of endogenous MOR levels in doublecortin expressing (DCX⁺) NSC progenies in the rat DG. MOR expression is also detected in the cultured rat NSCs and morphine treatment in vitro increases NSC neuronal differentiation and dendritogenesis, suggesting that MOR mediates the effect of morphine on NSC neuronal differentiation and maturation. Finally, we show that conditional overexpression of MOR in DG NSCs under a doxycycline inducible system leads to facilitation of the acquisition of MSA in rats, without affecting the extinction process. We advocate that targeting MOR selectively in the DG NSC population might offer a novel therapeutic intervention for morphine addiction.

Dentate gyrus neurogenesis ablation via cranial irradiation enhances morphine self-administration and locomotor sensitization

Adult dentate gyrus (DG) neurogenesis is important for hippocampal-dependent learning and memory, but the role of new neurons in addiction-relevant learning and memory is unclear. To test the hypothesis that neurogenesis is involved in the vulnerability to morphine addiction, we ablated adult DG neurogenesis and examined morphine self-administration (MSA) and locomotor sensitization. Male Sprague-Dawley rats underwent hippocampal-focused, image-guided X-ray irradiation (IRR) to eliminate new DG neurons or sham treatment (Sham). Six weeks later, rats underwent either MSA (Sham = 16, IRR = 15) or locomotor sensitization (Sham = 12, IRR = 12). Over 21 days of MSA, IRR rats self-administered ~70 percent more morphine than Sham rats. After 28 days of withdrawal, IRR rats pressed the active lever 40 percent more than Sham during extinction. This was not a general enhancement of learning or locomotion, as IRR and Sham groups had similar operant learning and inactive lever presses. For locomotor sensitization, both IRR and Sham rats sensitized, but IRR rats sensitized faster and to a greater extent. Furthermore, dose-response revealed that IRR rats were more sensitive at a lower dose. Importantly, these increases in locomotor activity were not apparent after acute morphine administration and were not a byproduct of irradiation or post-irradiation recovery time. Therefore, these data, along with other previously published data, indicate that reduced hippocampal neurogenesis confers vulnerability for multiple classes of drugs. Thus, therapeutics to specifically increase or stabilize hippocampal neurogenesis could aid in preventing initial addiction as well as future relapse.

Chronic pain and adult hippocampal neurogenesis: translational implications from preclinical studies

Adult hippocampal neurogenesis (ahNG) occurs in the human brain. Adult generated neurons have been proposed to functionally contribute to relevant hippocampal functions such as learning and memory, mood regulation, and stress response. Learning, environmental enrichment, and physical exercise exert positive effects on ahNG. In parallel, these proneurogenic stimuli have been shown to ameliorate cognitive performance and/or depressive-like behavior in animal models. Conversely, aging, social isolation, and chronic stress exert negative effects on ahNG. Interestingly, reduction of hippocampal neurogenesis is suggested to potentially contribute to cognitive decline and mood alterations associated with aging and several neuropsychiatric disorders. Clinical observation demonstrates that patients affected by chronic pain often exhibit increased anxiety and depression, impaired cognitive flexibility, and memory capacities. As of today, our understanding of the molecular and cellular events that may underlie the comorbidity of chronic pain, depression, and cognitive impairment is limited. Herein we review recent preclinical data suggesting that chronic pain may induce profound changes in hippocampal plasticity, including reduced ahNG. We discuss the possibility that deregulated hippocampal neurogenesis in chronic pain may, at least in part, contribute to cognitive and mood alterations. Based on this hypothesis, the mechanisms underlying chronic pain-associated changes in hippocampal neurogenesis and related functions need to be addressed experimentally. One interesting feature of ahNG is its susceptibility to pharmacological modulation. Again, based on preclinical data we discuss the possibility that, at least in principle, distinct analgesic drugs commonly used in chronic pain states (typical and atypical opiates, α2δ ligands, and acetyl-l-carnitine) may differentially impact ahNG and that this aspect could be taken into account to reduce and/or prevent the potential risk of cognitive and emotional side effects in the clinical setting.

Opiate Analgesics as Negative Modulators of Adult Hippocampal Neurogenesis: Potential Implications in Clinical Practice

During the past decade, studies of the mechanisms and functional implications of adult hippocampal neurogenesis (ahNG) have significantly progressed. At present, it is proposed that adult born neurons may contribute to a variety of hippocampal-related functions, including specific cognitive aspects and mood regulation. Several groups focussed on the factors that regulate proliferation and fate determination of adult neural stem/progenitor cells (NSC/NPC), including clinically relevant drugs. Opiates were the first drugs shown to negatively impact neurogenesis in the adult mammalian hippocampus. Since that initial report, a vast array of information has been collected on the effect of opiate drugs, by either modulating proliferation of stem/progenitor cells or interfering with differentiation, maturation and survival of adult born neurons. The goal of this review is to critically revise the present state of knowledge on the effect of opiate drugs on the different developmental stages of ahNG, as well as the possible underlying mechanisms. We will also highlight the potential impact of deregulated hippocampal neurogenesis on patients undergoing chronic opiate treatment. Finally, we will discuss the differences in the negative impact on ahNG among clinically relevant opiate drugs, an aspect that may be potentially taken into account to avoid long-term deregulation of neural plasticity and its associated functions in the clinical practice.

Morphine Modulates Adult Neurogenesis and Contextual Memory by Impeding the Maturation of Neural Progenitors

The regulation of adult neurogenesis by opiates has been implicated in modulating different addiction cycles. At which neurogenesis stage opiates exert their action remains unresolved. We attempt to define the temporal window of morphine’s inhibition effect on adult neurogenesis by using the POMC-EGFP mouse model, in which newborn granular cells (GCs) can be visualized between days 3–28 post-mitotic. The POMC-EGFP mice were trained under the 3-chambers conditioned place preference (CPP) paradigm with either saline or morphine. We observed after 4 days of CPP training with saline, the number of EGFP-labeled newborn GCs in sub-granular zone (SGZ) hippocampus significantly increased compared to mice injected with saline in their homecage. CPP training with morphine significantly decreased the number of EGFP-labeled GCs, whereas no significant difference in the number of EGFP-labeled GCs was observed with the homecage mice injected with the same dose of morphine. Using cell-type selective markers, we observed that morphine reduced the number of late stage progenitors and immature neurons such as Doublecortin (DCX) and βIII Tubulin (TuJ1) positive cells in the SGZ but did not reduce the number of early progenitors such as Nestin, SOX2, or neurogenic differentiation-1 (NeuroD1) positive cells. Analysis of co-localization between different cell markers shows that morphine reduced the number of adult-born GCs by interfering with differentiation of early progenitors, but not by inducing apoptosis. In addition, when NeuroD1 was over-expressed in DG by stereotaxic injection of lentivirus, it rescued the loss of immature neurons and prolonged the extinction of morphine-trained CPP. These results suggest that under the condition of CPP training paradigm, morphine affects the transition of neural progenitor/stem cells to immature neurons via a mechanism involving NeuroD1.

Effect of Opioid on Adult Hippocampal Neurogenesis

During the past decade, the study of the mechanisms and functional implications of adult neurogenesis has significantly progressed. Many studies focus on the factors that regulate proliferation and fate determination of adult neural stem/progenitor cells, including addictive drugs such as opioid. Here, we review the most recent works on opiate drugs' effect on different developmental stages of adult hippocampal neurogenesis, as well as the possible underlying mechanisms. We conclude that opiate drugs in general cause a loss of newly born neural progenitors in the subgranular zone of dentate gyrus, by either modulating proliferation or interfering with differentiation and maturation. We also discuss the consequent impact of regulation of adult neurogenesis in animal's opioid addiction behavior. We further look into the future directions in studying the convergence between the adult neurogenesis field and opioid addiction field, since the adult-born granular cells were shown to play a role in neuroplasticity and may help to reduce the vulnerability to drug craving and relapse.

Alterations of neuronal precursor cells in stages of human adult neurogenesis in heroin addicts

BACKGROUND

Adult neurogenesis has been shown to occur throughout life and different brain pathologies were demonstrated to be associated with altered neurogenesis. Here, an impact of heroin addiction on neurogenesis in humans is hypothesised.

METHODS

Post mortem hippocampal specimens of drug addicts with known heroin abuse and a group of non-addictive control subjects were analysed, using antibodies indicating different stages of neurogenesis. The subgranular zone of the dentate gyrus was examined qualitatively and quantitatively.

RESULTS

The data indicate (i) a decreased number of neural precursor cells, (ii) accompanied by low rates of proliferation and (iii) a marked loss of dendritic trees in targeting cells in heroin fatalities. (iv) The age-dependent increase of differentiating cells in the healthy controls was not observed in the addicts. Additionally, double immunofluorescence labelling indicated the precursor nature of Musashi-1 positive cells in the human subgranular zone of the dentate gyrus.

CONCLUSIONS

Present data firstly demonstrate the influence of drug addiction with known heroin abuse on different developmental stages of progenitors in the dentate gyrus. The patterns of antibody staining suggest a distinct inhibition of neurogenesis at the stage of neural precursor cells and revealed morphological changes in targeting cells in cases of heroin addicts as compared to healthy controls. These alterations could be considerable for memory and cognitive deficits as well as addictive behaviour in chronic drug abusers and may give rise to specific pro-neurogenic therapies.

Effects of addictive drugs on adult neural stem/progenitor cells

Neural stem/progenitor cells (NSPCs) undergo a series of developmental processes before giving rise to newborn neurons, astrocytes and oligodendrocytes in adult neurogenesis. During the past decade, the role of NSPCs has been highlighted by studies on adult neurogenesis modulated by addictive drugs. It has been proven that these drugs regulate the proliferation, differentiation and survival of adult NSPCs in different manners, which results in the varying consequences of adult neurogenesis. The effects of addictive drugs on NSPCs are exerted via a variety of different mechanisms and pathways, which interact with one another and contribute to the complexity of NSPC regulation. Here, we review the effects of different addictive drugs on NSPCs, and the related experimental methods and paradigms. We also discuss the current understanding of major signaling molecules, especially the putative common mechanisms, underlying such effects. Finally, we review the future directions of research in this area.

Morphine and cocaine increase serum- and glucocorticoid-inducible kinase 1 activity in the ventral tegmental area

Drugs of abuse modulate the function and activity of the mesolimbic dopamine circuit. To identify novel mediators of drug-induced neuroadaptations in the ventral tegmental area (VTA), we performed RNA sequencing analysis on VTA samples from mice administered repeated saline, morphine, or cocaine injections. One gene that was similarly up-regulated by both drugs was serum- and glucocorticoid-inducible kinase 1 (SGK1). SGK1 activity, as measured by phosphorylation of its substrate N-myc downstream regulated gene (NDRG), was also increased robustly by chronic drug treatment. Increased NDRG phosphorylation was evident 1 but not 24 h after the last drug injection. SGK1 phosphorylation itself was similarly modulated. To determine the role of increased SGK1 activity on drug-related behaviors, we over-expressed constitutively active (CA) SGK1 in the VTA. SGK1-CA expression reduced locomotor sensitization elicited by repeated cocaine, but surprisingly had the opposite effect and promoted locomotor sensitization to morphine, without affecting the initial locomotor responses to either drug. SGK1-CA expression did not significantly affect morphine or cocaine conditioned place preference, although there was a trend toward increased conditioned place preference with both drugs. Further characterizing the role of this kinase in drug-induced changes in VTA may lead to improved understanding of neuroadaptations critical to drug dependence and addiction. We find that repeated, but not acute, morphine or cocaine administration induces an increase in serum- and glucocorticoid-inducible kinase (SGK1) gene expression and activity in the ventral tegmental area (VTA). This increase in SGK1 activity may play a role in drug-dependent behaviors and suggests a novel signaling cascade for potential intervention in drug dependence and addiction.

Opiate-induced molecular and cellular plasticity of ventral tegmental area and locus coeruleus catecholamine neurons

The study of neuronal adaptations induced by opiate drugs is particularly relevant today given their widespread prescription and nonprescription use. Although much is known about the acute actions of such drugs on the nervous system, a great deal of work remains to fully understand their chronic effects. Here, we focus on longer-lasting adaptations that occur in two catecholaminergic brain regions that mediate distinct behavioral actions of opiates: ventral tegmental area (VTA) dopaminergic neurons, important for drug reward, and locus coeruleus (LC) noradrenergic neurons, important for physical dependence and withdrawal. We focus on changes in cellular, synaptic, and structural plasticity in these brain regions that contribute to opiate dependence and addiction. Understanding the molecular determinants of this opiate-induced plasticity will be critical for the development of better treatments for opiate addiction and perhaps safer opiate drugs for medicinal use.

Adult hippocampal neurogenesis in the pathogenesis of addiction and dual diagnosis disorders

BACKGROUND

As knowledge deepens about how new neurons are born, differentiate, and wire into the adult mammalian brain, growing evidence depicts hippocampal neurogenesis as a special form of neuroplasticity that may be impaired across psychiatric disorders. This review provides an integrated-evidence based framework describing a neurogenic basis for addictions and addiction vulnerability in mental illness.

METHODS

Basic studies conducted over the last decade examining the effects of addictive drugs on adult neurogenesis and the impact of neurogenic activity on addictive behavior were compiled and integrated with relevant neurocomputational and human studies.

RESULTS

While suppression of hippocampal neurogenic proliferation appears to be a universal property of addictive drugs, the pathophysiology of addictions involves neuroadaptative processes within frontal-cortical-striatal motivation circuits that the neurogenic hippocampus regulates via direct projections. States of suppressed neurogenic activity may simultaneously underlie psychiatric and cognitive symptoms, but also confer or signify hippocampal dysfunction that heightens addiction vulnerability in mental illness as a basis for dual diagnosis disorders.

CONCLUSIONS

Research on pharmacological, behavioral and experiential strategies that enhance adaptive regulation of hippocampal neurogenesis holds potential in advancing preventative and integrative treatment strategies for addictions and dual diagnosis disorders.

NeuroD modulates opioid agonist-selective regulation of adult neurogenesis and contextual memory extinction

Addictive drugs, including opioids, modulate adult neurogenesis. In order to delineate the probable implications of neurogenesis on contextual memory associated with addiction, we investigated opioid agonist-selective regulation of neurogenic differentiation 1 (NeuroD) activities under the conditioned place preference (CPP) paradigm. Training mice with equivalent doses of morphine and fentanyl produced different CPP extinction rates without measurable differences in the CPP acquisition rate or magnitude. Fentanyl-induced CPP required much longer time for extinction than morphine-induced CPP. We observed a parallel decrease in NeuroD activities and neurogenesis after morphine-induced CPP, but not after fentanyl-induced CPP. Increasing NeuroD activities with NeuroD-lentivirus (nd-vir) injection at the dentate gyrus before CPP training reversed morphine-induced decreases in NeuroD activities and neurogenesis, and prolonged the time required for extinction of morphine-induced CPP. On the other hand, decreasing NeuroD activities via injection of miRNA-190-virus (190-vir) reversed the fentanyl effect on NeuroD and neurogenesis and shortened the time required for extinction of fentanyl-induced CPP. Another contextual memory task, the Morris Water Maze (MWM), was affected similarly by alteration of NeuroD activities. The reduction in NeuroD activities either by morphine treatment or 190-vir injection decreased MWM task retention, while the increase in NeuroD activities by nd-vir prolonged MWM task retention. Thus, by controlling NeuroD activities, opioid agonists differentially regulate adult neurogenesis and subsequent contextual memory retention. Such drug-related memory regulation could have implications in eventual context-associated relapse.

Astrocytes are a neural target of morphine action via glucocorticoid receptor-dependent signaling

Chronic opioid use leads to the structural reorganization of neuronal networks, involving genetic reprogramming in neurons and glial cells. Our previous in vivo studies have revealed that a significant fraction of the morphine-induced alterations to the striatal transcriptome included glucocorticoid (GC) receptor (GR)-dependent genes. Additional analyses suggested glial cells to be the locus of these changes. In the current study, we aimed to differentiate the direct transcriptional effects of morphine and a GR agonist on primary striatal neurons and astrocytes. Whole-genome transcriptional profiling revealed that while morphine had no significant effect on gene expression in both cell types, dexamethasone significantly altered the transcriptional profile in astrocytes but not neurons. We obtained a complete dataset of genes undergoing the regulation, which includes genes related to glucose metabolism (Pdk4), circadian activity (Per1) and cell differentiation (Sox2). There was also an overlap between morphine-induced transcripts in striatum and GR-dependent transcripts in cultured astrocytes. We further analyzed the regulation of expression of one gene belonging to both groups, serum and GC regulated kinase 1 (Sgk1). We identified two transcriptional variants of Sgk1 that displayed selective GR-dependent upregulation in cultured astrocytes but not neurons. Moreover, these variants were the only two that were found to be upregulated in vivo by morphine in a GR-dependent fashion. Our data suggest that the morphine-induced, GR-dependent component of transcriptome alterations in the striatum is confined to astrocytes. Identification of this mechanism opens new directions for research on the role of astrocytes in the central effects of opioids.

BDNF is a negative modulator of morphine action

Brain-derived neurotrophic factor (BDNF) is a key positive regulator of neural plasticity, promoting, for example, the actions of stimulant drugs of abuse such as cocaine. We discovered a surprising opposite role for BDNF in countering responses to chronic morphine exposure. The suppression of BDNF in the ventral tegmental area (VTA) enhanced the ability of morphine to increase dopamine (DA) neuron excitability and promote reward. In contrast, optical stimulation of VTA DA terminals in nucleus accumbens (NAc) completely reversed the suppressive effect of BDNF on morphine reward. Furthermore, we identified numerous genes in the NAc, a major target region of VTA DA neurons, whose regulation by BDNF in the context of chronic morphine exposure mediated this counteractive function. These findings provide insight into the molecular basis of morphine-induced neuroadaptations in the brain's reward circuitry.

Role for mTOR signaling and neuronal activity in morphine-induced adaptations in ventral tegmental area dopamine neurons

While the abuse of opiate drugs continues to rise, the neuroadaptations that occur with long-term drug exposure remain poorly understood. We describe here a series of chronic morphine-induced adaptations in ventral tegmental area (VTA) dopamine neurons, which are mediated via downregulation of AKT-mTORC2 (mammalian target of rapamycin complex-2). Chronic opiates decrease the size of VTA dopamine neurons in rodents, an effect seen in humans as well, and concomitantly increase the excitability of the cells but decrease dopamine output to target regions. Chronic morphine decreases mTORC2 activity, and overexpression of Rictor, a component of mTORC2, prevents morphine-induced changes in cell morphology and activity. Further, local knockout of Rictor in VTA decreases DA soma size and reduces rewarding responses to morphine, consistent with the hypothesis that these adaptations represent a mechanism of reward tolerance. Together, these findings demonstrate a novel role for AKT-mTORC2 signaling in mediating neuroadaptations to opiate drugs of abuse.

Deficient plasticity in the hippocampus and the spiral of addiction: focus on adult neurogenesis

Addiction is a complex neuropsychiatric disorder which causes disruption at multiple levels, including cognitive, emotional, and behavioral domains. Traditional biological theories of addiction have focused on the mesolimbic dopamine pathway and the nucleus accumbens as anatomical substrates mediating addictive-like behaviors. More recently, we have begun to recognize the engagement and dynamic influence of a much broader circuitry which encompasses the frontal cortex, the amygdala, and the hippocampus. In particular, neurogenesis in the adult hippocampus has become a major focus of attention due to its ability to influence memory, motivation, and affect, all of which are disrupted in addiction. First, I summarize toxicological data that reveal strongly suppressive effects of drug exposure on adult hippocampal neurogenesis. Then, I discuss the impact of deficient neurogenesis on learning and memory function, stress responsiveness and affective behavior, as they relate to addiction. Finally, I examine recent behavioral observations that implicate neurogenesis in the adult hippocampus in the emergence and maintenance of addictive behavior. The evidence reviewed here suggests that deficient neurogenesis is associated with several components of the downward spiraling loop that characterizes addiction, including elevated sensitivity to drug-induced reward and reinforcement, enhanced neurohormonal responsiveness, emergence of a negative affective state, memory impairment, and inflexible behavior.

Prescription opioid analgesics rapidly change the human brain

Chronic opioid exposure is known to produce neuroplastic changes in animals; however, it is not known if opioids used over short periods of time and at analgesic dosages can similarly change brain structure in humans. In this longitudinal, magnetic resonance imaging study, 10 individuals with chronic low back pain were administered oral morphine daily for 1 month. High-resolution anatomical images of the brain were acquired immediately before and after the morphine administration period. Regional changes in gray matter volume were assessed on the whole brain using tensor-based morphometry, and those significant regional changes were then independently tested for correlation with morphine dosage. Thirteen regions evidenced significant volumetric change, and degree of change in several of the regions was correlated with morphine dosage. Dosage-correlated volumetric decrease was observed primarily in the right amygdala. Dosage-correlated volumetric increase was seen in the right hypothalamus, left inferior frontal gyrus, right ventral posterior cingulate, and right caudal pons. Follow-up scans that were conducted an average of 4.7 months after cessation of opioids demonstrated many of the morphine-induced changes to be persistent. In a separate study, 9 individuals consuming blinded placebo capsules for 6 weeks evidenced no significant morphologic changes over time. The results add to a growing body of literature showing that opioid exposure causes structural and functional changes in reward- and affect-processing circuitry. Morphologic changes occur rapidly in humans during new exposure to prescription opioid analgesics. Further research is needed to determine the clinical impact of those opioid-induced gray matter changes.

Opioid modulation of cell proliferation in the ventricular zone of adult zebra finches (Taenopygia guttata)

Besides modulating pain, stress, physiological functions, motivation, and reward, the opioid system has been implicated in developmental and adult mammalian neurogenesis and gliogenesis. In adult male songbirds including zebra finches, neurons generated from the ventricular zone (VZ) of the lateral ventricles are incorporated throughout the telencephalon, including the song control nuclei, HVC, and area X. Although the endogenous opioid met-enkephalin is present in neurons adjacent to the VZ and is upregulated in song control regions during singing, it is not known whether the opioid system can modulate adult neurogenesis/gliogenesis in zebra finches. We used quantitative RT-PCR and in situ hybridization to demonstrate that μ- and δ-opioid receptors are expressed by the VZ of adult male zebra finches. Treating cultured VZ cells from male birds with the opioid antagonist naloxone led to an increase in cell proliferation measured by 5-bromo-2-deoxyuridine incorporation, whereas administering met-enkephalin had the opposite effect, compared with saline-treated cultures. Systemically administering naloxone (2.5 mg/kg body wt) to adult male zebra finches for 4 d also led to a significant increase in cell proliferation in the ventral VZ of these birds, compared with saline-treated controls. Our results show that cell proliferation is augmented by naloxone in the VZ adjacent to the anterior commissure, suggesting that the endogenous opioids modulate adult neurogenesis/gliogenesis by inhibiting cell proliferation in songbirds.

Spinal mediators that may contribute selectively to antinociceptive tolerance but not other effects of morphine as revealed by deletion of GluR5

Several groups maintain that morphine tolerance and dependence correlate with increased activity of protein kinases ERK1/2 and P38 MAPK and PKC as well as elevated levels of the neuropeptides dynorphin (DYN), substance P (sP), and calcitonin gene-related peptide (CGRP) in spinal cord dorsal horn (SCDH). They demonstrate that tolerance and dependence can be prevented, and sometimes reversed, by constitutive genetic deletion or pharmacological inhibition of these factors. Recently, we showed that mice with a constitutive deletion of the GluR5 subunit of kainate receptors (GluR5 KO) are not different from wild type (WT) littermates with respect to baseline nociceptive thresholds as well as acute morphine antinociception, morphine physical dependence and conditioned place preference. However, unlike WT, GluR5 KO mice do not develop antinociceptive tolerance following systemic morphine administration. In this report, we examined levels of these mediators in SCDH of WT and GluR5 KO mice following subcutaneous implantation of placebo or morphine pellets. Surprisingly, spinal DYN and CGRP, along with phosphorylated ERK2 (pERK2), P38 (pP38) and PKCgamma (pPKCgamma) are elevated by deletion of GluR5. Additionally, chronic systemic morphine administration increased spinal pERK2, pP38 and pPKCgamma levels in both tolerant WT and non-tolerant GluR5 KO mice. In contrast, while morphine increased spinal DYN and CGRP in WT mice, DYN remained unchanged and CGRP was reduced in GluR5 KO mice. These observations suggest that spinal ERK2, P38 and PKCgamma are likely involved in multiple adaptive responses following systemic morphine administration, whereas DYN and CGRP may contribute selectively to the development of antinociceptive tolerance.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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 neurologic 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).

Effect of chronic morphine on the dentate gyrus neurogenic microenvironment

Opiates, such as morphine, decrease neurogenesis in the postnatal hippocampal subgranular zone (SGZ) by inhibiting progenitor proliferation and maturation. However, it is not known how morphine influences the growth factors and vasculature that encompass the neurogenic SGZ microenvironment. We examined morphine's effect on pro- and anti-proliferative factors in the dentate gyrus (DG; Experiment 1) as well as the DG neurovasculature itself (Experiment 2). For Experiment 1, mice were implanted with subcutaneous sham or morphine pellets (0 and 48 h) and were decapitated 24 or 96 h later. One brain hemisphere was postfixed to examine proliferation by immunohistochemistry, and a DG-enriched sample was dissected from the other hemisphere to examine the neurogenic microenvironment via immunoblotting for known pro- and anti-proliferative factors. Consistent with previous results, morphine decreased the number of proliferating cells in the SGZ, as the number of Ki67-immunoreactive (IR) cells was decreased at 96 h. Morphine did not alter DG levels of the pro-proliferative factor brain-derived neurotrophic factor, anti-proliferative factor interleukin-1 beta, or their receptors TrkB and IL1R1 at either time point. However, morphine increased the pro-proliferative factor vascular endothelial growth factor (VEGF) at 96 h. Given that VEGF is also a potent angiogenic factor, Experiment 2 examined whether the morphine-induced increase in VEGF correlated with altered DG neurovasculature. Mice were implanted with morphine pellets as in Experiment 1, and 2 h before perfusion (24 or 96 h) were administered bromodeoxyuridine (BrdU; intraperitoneal, 150 mg/kg). Tissue was co-stained for BrdU and the endothelial cell marker endoglin to enable examination of DG vessels and proximity of BrdU-IR cells to endoglin-IR vessels. At 96 h, endoglin-IR vessel area and perimeter were increased, but proximity of BrdU-IR cells to endoglin-IR vessels remained unchanged. These data suggest that following chronic morphine exposure, factors within the neurogenic microenvironment are maintained or upregulated to compensate for decreased SGZ proliferation.

Time course of morphine's effects on adult hippocampal subgranular zone reveals preferential inhibition of cells in S phase of the cell cycle and a subpopulation of immature neurons

Opiates, such as morphine, decrease neurogenesis in the adult hippocampal subgranular zone (SGZ), raising the possibility that decreased neurogenesis contributes to opiate-induced cognitive deficits. However, there is an incomplete understanding of how alterations in cell cycle progression and progenitor maturation contribute to this decrease. The present study examined how morphine regulates progenitor cell cycle, cell death and immature SGZ neurons (experiment 1) as well as the progression of SGZ progenitors through key stages of maturation (experiment 2). In experiment 1, mice received sham or morphine pellets (s.c., 0 and 48 h) and bromodeoxyuridine (BrdU) 2 h prior to sacrifice (24, 72 or 96 h). Morphine decreased both the number of S phase and total cycling cells, as there were fewer cells immunoreactive (IR) for the S phase marker BrdU and the cell cycle marker Ki67. The percentage of Ki67-IR cells that were BrdU-IR was decreased after 24 but not 96 h of morphine, suggesting a disproportionate effect on S phase cells relative to all cycling cells at this time point. Cell death (activated caspase-3 counts) was increased after 24 but not 96 h. In experiment 2, nestin-green fluorescent protein (GFP) mice given BrdU 1 day prior to morphine or sham surgery (0 and 48 h, sacrifice 96 h) had fewer Ki67-IR cells, but no change in BrdU-IR cell number, suggesting that this population of BrdU-IR cells was less sensitive to morphine. Interestingly, examination of key stages of progenitor cell maturation revealed that morphine increased the percent of BrdU-IR cells that were type 2b and decreased the percent that were immature neurons. These data suggest that chronic morphine decreases SGZ neurogenesis by inhibiting dividing cells, particularly those in S phase, and progenitor cell progression to a more mature neuronal stage.