Morphine treatment during juvenile isolation increases social activity and opioid peptides release in the adult rat

Endogenous Opioids at the Intersection of Opioid Addiction, Pain, and Depression: The Search for a Precision Medicine Approach

Opioid addiction and overdose are at record levels in the United States. This is driven, in part, by their widespread prescription for the treatment of pain, which also increased opportunity for diversion by sensation-seeking users. Despite considerable research on the neurobiology of addiction, treatment options for opioid abuse remain limited. Mood disorders, particularly depression, are often comorbid with both pain disorders and opioid abuse. The endogenous opioid system, a complex neuromodulatory system, sits at the neurobiological convergence point of these three comorbid disease states. We review evidence for dysregulation of the endogenous opioid system as a mechanism for the development of opioid addiction and/or mood disorder. Specifically, individual differences in opioid system function may underlie differences in vulnerability to opioid addiction and mood disorders. We also review novel research, which promises to provide more detailed understanding of individual differences in endogenous opioid neurobiology and its contribution to opioid addiction susceptibility.

A history of chronic morphine exposure during adolescence increases despair-like behaviour and strain-dependently promotes sociability in abstinent adult mice

A crucial issue in treating opiate addiction, a chronic relapsing disorder, is to maintain a drug-free abstinent state. Prolonged abstinence associates with mood disorders, strongly contributing to relapse. In particular, substance use disorders occurring during adolescence predispose to depression later in adulthood. Using our established mouse model of opiate abstinence, we characterized emotional consequences into adulthood of morphine exposure during adolescence. Our results indicate that morphine treatment in adolescent mice has no effect on anxiety-like behaviours in adult mice, after abstinence. In contrast, morphine treatment during adolescence increases behavioural despair in adult mice. We also show that morphine exposure strain-dependently enhances sociability in adult mice. Additional research will be required to understand where and how morphine acts during brain maturation to affect emotional and social behaviours into adulthood.

Opioid receptors: distinct roles in mood disorders

The roles of opioid receptors in pain and addiction have been extensively studied, but their function in mood disorders has received less attention. Accumulating evidence from animal research reveals that mu, delta and kappa opioid receptors (MORs, DORs and KORs, respectively) exert highly distinct controls over mood-related processes. DOR agonists and KOR antagonists have promising antidepressant potential, whereas the risk-benefit ratio of currently available MOR agonists as antidepressants remains difficult to evaluate, in addition to their inherent abuse liability. To date, both human and animal studies have mainly examined MORs in the etiology of depressive disorders, and future studies will address DOR and KOR function in established and emerging neurobiological aspects of depression, including neurogenesis, neurodevelopment, and social behaviors.

Effects of group exposure on single injection-induced behavioral sensitization to drugs of abuse in mice

BACKGROUND

Behavioral sensitization in rodents is hypothesized to reflect neuronal adaptations that are related to drug addiction in humans. We evaluated the effects of group exposure on the acute hyperlocomotion and behavioral sensitization induced by four drugs of abuse in C57BL/6 mice: methylenedioxymethamphetamine (MDMA), d-amphetamine, morphine and ethanol.

METHODS

In the priming session, animals received an ip injection of one of the drugs of abuse and were exposed to an open field either individually or in groups of four. Seven days later, we assessed behavioral sensitization in the challenge session. All animals received an ip injection of the same drug and were exposed to the open field in the same social conditions described for the priming session. Locomotion and social interaction were quantified during each session.

RESULTS

Acute MDMA, morphine and ethanol, but not d-amphetamine, increased social interaction. However, group exposure only potentiated MDMA-induced hyperlocomotion. After a challenge injection of each drug, there was no sensitization to the facilitating effect of MDMA, morphine or ethanol on social interaction, but locomotion sensitization developed to all drugs of abuse except ethanol. This sensitization was potentiated by group exposure in MDMA-treated animals, attenuated in morphine-treated animals and not modified in d-amphetamine-treated animals. Acute MDMA enhanced body contact and peaceful following, while acute morphine and ethanol increased social sniffing.

CONCLUSIONS

These results provide preclinical evidence showing that while different drugs of abuse affect different components of social interaction, the neuronal adaptations related to drug dependence can be critically and specifically influenced by group exposure.

Age-dependent and strain-dependent influences of morphine on mouse social investigation behavior

Opioid-coded neural circuits play a substantial role in how individuals respond to drugs of abuse. Most individuals begin using such drugs during adolescence and within a social context. Several studies indicate that adolescent mice exhibit a heightened sensitivity to the effects of morphine, a prototypical opiate drug, but it is unclear whether these developmental differences are related to aspects of motivated behavior. Moreover, exposure to opioids within the rodent brain can alter the expression of social behavior, yet little is known about whether this relationship changes as a function of development or genetic variation. In this study, we conducted a series of experiments to characterize the relationship between genetic background, adolescent development and morphine-induced changes in mouse social investigation (SI). At two time points during adolescent development [postnatal days (PD) 25 and 45], social interactions of test mice of the gregarious C57BL/6J (B6) strain were more tolerant to the suppressive effects of morphine [effective dose 50 (ED50)=0.97 mg/kg and 2.17 mg/kg morphine, respectively] than test mice from the less social BALB/cJ (BALB) strain (ED50=0.61 mg/kg and 0.91 mg/kg morphine, respectively). By contrast, this strain-dependent difference was not evident among adult mice on PD 90 (ED50=1.07 mg/kg and 1.41 mg/kg morphine for BALB and B6 mice, respectively). An additional experiment showed that the ability of morphine to alter social responsiveness was not directly related to drug-induced changes in locomotor behavior. Finally, administration of morphine to stimulus mice on PD 25 reduced social investigation of test mice only when individuals were from the B6 genetic background. Overall, these results indicate that alterations in endogenous opioid systems are related to changes in SI that occur during adolescence, and that morphine administration may mimic rewarding aspects of social encounter.

Effects of fluoxetine and PCPA on isolation-induced morphine self-administration and startle reactivity

The present study investigated the effects of the SSRI fluoxetine and the serotonin synthesis blocker--parachlorophenylalanine (PCPA) on morphine self-administration and startle reactivity in rats subjected to social isolation during adulthood. Adult Wistar rats were housed individually or in pairs for 21days. They were treated with fluoxetine, PCPA, or vehicle and tested for their startle response and intake of a morphine solution (0.5mg/ml). Socially restricted rats consumed significantly more morphine solution (but not water) than rats living in pairs, in both one-bottle and in two-bottle tests. They also showed significantly higher startle response amplitude. Daily fluoxetine treatment (5mg/kg i.p.) counteracted these behavioral alterations induced by isolation housing while PCPA treatment (200mg/kg for 3 consecutive days) further exacerbated it. Social isolation may increase morphine self-administration and emotional reactivity in the startle box by affecting serotonin. Antidepressants (such as fluoxetine) may normalize or stabilize serotonin function and restore the behavioral changes produced by isolation.

Social isolation increases morphine intake: behavioral and psychopharmacological aspects

Environmental and situational factors are important determinants of recreational drug use in humans. We aimed to develop a reliable animal model for studying the effects of environmental variables on drug-seeking behavior using the 'social isolation/social restriction' paradigm. Adult Wistar rats housed in short-term isolation (21 days) consumed significantly more morphine solution (0.5 mg/ml) than rats living in pairs, both in one-bottle and in two-bottle tests. No differences were found in their water consumption. This effect was observed in both males and females and the results were also replicated after reversal of housing conditions. We also found that as little as 60-min of daily social-physical interaction with another rat was sufficient to completely abolish the increase in morphine consumption in socially restricted animals. We discuss some possible interpretations for these effects. These results indicate that environmental and situational factors influence drug intake in laboratory rats as they do in humans, and thus may be of interest in studying drug-seeking behavior in humans.

Long-term opiate effects on amphetamine-induced dopamine release in the nucleus accumbens core and conditioned place preference

Withdrawal following chronic exposure to opiates or other drugs of abuse, administered as frequent doses, or a chronic infusion can cause reductions in mesolimbic dopamine (DA) transmission. However, mesolimbic DA transmission can be enhanced by opiates or psychostimulants administered intermittently as a single daily injection. Both enhanced and attenuated responsiveness of the mesolimbic DA system may have important implications for substance abuse disorders. Previous studies have shown that procedures that use electrical stimulation or drug treatments to augment neurotransmitter release are more effective for demonstrating declines in mesolimbic DA transmission that persist for extended periods following opiate withdrawal. The present study evaluated the effects of pretreatment with noncontingent morphine on amphetamine-induced DA release in the nucleus accumbens core and conditioned place preference (CPP). Morphine pretreatment was administered as a constant infusion, which was gradually increased to a dose of 50 mg/kg/day over a 1-week period in Wistar rats. At 10 days after cessation of morphine pretreatment, baseline dialysate DA levels in the nucleus accumbens core were unchanged, but amphetamine-induced increases in DA were attenuated by greater than 50% in morphine-pretreated animals. Morphine pretreatment did not modify locomotor activity during conditioning sessions, expressed as absolute values or change in activity counts between saline and morphine injections. Place preference, conditioned by two morphine pairings at 10 and 11 days after the onset of opiate withdrawal, was enhanced by opiate pretreatment between 12 and 33 days after the onset of withdrawal. In conclusion, morphine pretreatment delivered as a constant infusion can have pronounced and long-lasting effects on DA release and CPP, which may have important implications for drug-seeking behavior and treatment of substance abuse disorders.

Drug dependence and the endogenous opioid system

The discovery of endogenous opioids has markedly influenced the research on the biology of drug dependence. Evidence has been presented that these brain substances are self-administered by laboratory animals. This finding, among others, has led to the hypothesis that endogenous opioids are involved in reinforcing habits, including dependence on drugs of abuse. The course of drug dependence is presented as a continuum from no drug use via controlled use to an actual dependence on the drug. Specific brain opioid systems belonging to four conceptualized brain circuits are described to be involved during the different phases of the drug dependence continuum. More recent research to delineate the role of endogenous opioid systems in drug dependence has focussed on genetic research in humans and animals. Among others, the findings obtained from studies of opioid receptor and opioid peptide precursor knockout mice provided further support for a role of endogenous opioid systems in drug dependence, in agreement with previous pharmacological studies.

Opioid modulation of taste hedonics within the ventral striatum

There is a long-standing interest in the role of endogenous opioid peptides in feeding behavior and, in particular, in the modulation of food reward and palatability. Since drugs such as heroin, morphine, alcohol, and cannabinoids, interact with this system, there may be important common neural substrates between food and drug reward with regard to the brain's opioid systems. In this paper, we review the proposed functional role of opioid neurotransmission and mu opiate receptors within the nucleus accumbens and surrounding ventral striatum. Opioid compounds, particularly those selective for the mu receptor, induce a potent increase in food intake, sucrose, salt, saccharin, and ethanol intake. We have explored this phenomenon with regard to macronutrient selection, regional specificity, role of output structures, Fos mapping, analysis of motivational state, and enkephalin gene expression. We hypothesize that opioid-mediated mechanisms within ventral striatal medium spiny neurons mediate the affective or hedonic response to food ('liking' or food 'pleasure'). A further refinement of this hypothesis is that activation of ventral striatal opioids specifically encodes positive affect induced by tasty and/or calorically dense foods (such as sugar and fat), and promotes behaviors associated with this enhanced palatability. It is proposed that this brain mechanism was beneficial in evolutionary development for ensuring the consumption of relatively scarce, high-energy food sources. However, in modern times, with unlimited supplies of high-calorie food, it has contributed to the present epidemic of obesity.

Individual housing during the play period results in changed responses to and consequences of a psychosocial stress situation in rats

In the present study, the consequences of individual rearing during the play period on adult responses to aggression were investigated in a complex social setting. In a group of either socially or individually reared rats, an aggressor was repeatedly introduced. Separate control groups of individually and socially reared rats were not exposed to an aggressor. To allow an interpretation of the altered reactivity to aggression in terms of (in)efficient or (in)adequate behavior, not only the responses in the presence of the aggressor but also the consequences of the altered reactivity on subsequent behavior in the absence of the aggressor were observed. We demonstrated that a higher number of injuries, more agitation of the aggressor, and more ultrasonic distress vocalizations accompanied the altered responses of individually reared rats in the presence of the aggressor. In the absence of the aggressor, individually reared rats displayed less social stress-reducing behaviors (play and social grooming) than socially reared rats. It was concluded that individually reared rats display a less adequate response to aggression in this social context.

Decision support system for overall welfare assessment in pregnant sows A: model structure and weighting procedure

The problem of how to objectively assess the overall welfare status of animals under farming conditions has contributed to an ongoing debate that has hampered actual decision making on animal welfare. For this reason we constructed a model based on the assumed hierarchical organization of the animals' needs for overall welfare assessment in the case of pregnant sows. This model is implemented in a computer-based decision support system that takes a description of a housing and management system as input and produces a welfare score as output. A formalized procedure was used to construct the model for welfare assessment in pregnant sows on the basis of available scientific knowledge. This SOWEL (from SOw WELfare) model contains 37 attributes that describe the welfare-relevant properties of housing and management systems. In the decision support system these attributes are linked to scientific statements and a list of needs to provide a scientific basis for welfare assessment. Weighting factors that represent the relative importance of the attributes are derived from the scientific statements about the various welfare performance criteria that have been measured by scientists. The welfare score is calculated as the weighted average score. All information in the decision support system is stored in tables in a relational database such that newly available knowledge and insights can be incorporated to refine the model. The model has been developed in line with several existing models but it differs from these models in that it is the first to provide a formalized procedure to explicate the reasoning steps involved in welfare assessment based on available scientific knowledge.

Endogenous opioids and reward

The discovery of endogenous opioids has markedly influenced the research on the biology of addiction and reward brain processes. Evidence has been presented that these brain substances modulate brain stimulation reward, self-administration of different drugs of abuse, sexual behaviour and social behaviour. There appears to be two different domains in which endogenous opioids, present in separate and distinct brain regions, are involved. One is related to the modulation of incentive motivational processes and the other to the performance of certain behaviours. It is concluded that endogenous opioids may play a role in the vulnerability to certain diseases, such as addiction and autism, but also when the disease is present, such as alcoholism.