Prenatal exposure to morphine alters analgesic responses and preference for sweet solutions in adult rats

Perigestational Opioid Exposure Alters Alcohol-Driven Reward Behaviors in Adolescent Rats

Every fifteen minutes, a baby is born in the U.S. experiencing neonatal opioid withdrawal syndrome (NOWS). Since 2004, the rate of NOWS has increased 7-fold. Clinical studies have established intrauterine exposure to drugs of abuse as a risk factor for adverse health outcomes in adult life, including the propensity for future illicit drug use. Despite extensive knowledge about common mechanisms of action in the neural circuitry that drives opioid and alcohol reward, there is little data on the risks that those born with NOWS face regarding alcohol use later in life. Here, we investigate the impact of perigestational opioid exposure (POE) on the mesolimbic reward system of male and female Sprague Dawley rats at postnatal and adolescent ages. Our laboratory has developed a clinically relevant model for morphine exposure spanning pre-conception to the first week of life. Using this model, we found that POE increased alcohol consumption in female rats under noncontingent conditions, and inversely, reduced alcohol consumption in both male and female rats during operant conditioning sessions. Operant responding was also reduced for sucrose, suggesting that the impact of POE on reward-seeking behaviors is not limited to drugs of abuse. Expression of µ-opioid receptors was also significantly altered in the nucleus accumbens and medial habenula, regions previously shown to play a significant role in reward/aversion circuitry.

Significance Statement

Early life exposure to opioids is known to alter future drug behavior in rats. In the present study, female rats exposed to morphine via their mothers throughout and after pregnancy exhibited increased alcohol consumption when allowed to consume freely. During operant conditioning, however, male and female rats exposed to gestational morphine decreased consumption of alcohol as well as sucrose. We also observed that gestational morphine exposure altered µ-opioid receptor expression in reward-related brain regions. Our study provides the first evidence of changes in alcohol-directed reward behavior in a gestational opioid exposure rat model.

Differential effects of acute and prolonged morphine withdrawal on motivational and goal-directed control over reward-seeking behavior

Opioid addiction is a relapsing disorder marked by uncontrolled drug use and reduced interest in normally rewarding activities. The current study investigated the impact of spontaneous withdrawal from chronic morphine exposure on emotional, motivational, and cognitive processes involved in regulating the pursuit and consumption of natural food rewards in male rats. In Experiment 1, rats experiencing acute morphine withdrawal lost weight and displayed somatic signs of drug dependence. However, hedonically-driven sucrose consumption was significantly elevated, suggesting intact and potentially heightened emotional reward processing. In Experiment 2, rats undergoing acute morphine withdrawal displayed reduced motivation when performing an effortful response for palatable food reward. Subsequent reward devaluation testing revealed that acute withdrawal also disrupted their ability to exert flexible goal-directed control over their reward-seeking behavior. Specifically, morphine-withdrawn rats displayed insensitivity to reward devaluation both when relying on prior action-outcome learning and when given direct feedback about the consequences of their actions. In Experiment 3, rats tested after prolonged morphine withdrawal displayed heightened rather than diminished motivation for food rewards and retained their ability to engage in flexible goal-directed action selection. However, brief re-exposure to morphine was sufficient to impair motivation and disrupt goal-directed action selection, though in this case insensitivity to reward devaluation was only observed in the presence of morphine-paired context cues and in the absence of response-contingent feedback. We suggest that these opioid-withdrawal induced deficits in motivation and goal-directed control may contribute to addiction by interfering with the pursuit of adaptive alternatives to drug use.

Molecular and long-term behavioral consequences of neonatal opioid exposure and withdrawal in mice

Introduction

Infants exposed to opioids in utero are at high risk of exhibiting Neonatal Opioid Withdrawal Syndrome (NOWS), a combination of somatic withdrawal symptoms including high pitched crying, sleeplessness, irritability, gastrointestinal distress, and in the worst cases, seizures. The heterogeneity of in utero opioid exposure, particularly exposure to polypharmacy, makes it difficult to investigate the underlying molecular mechanisms that could inform early diagnosis and treatment of NOWS, and challenging to investigate consequences later in life.

Methods

To address these issues, we developed a mouse model of NOWS that includes gestational and post-natal morphine exposure that encompasses the developmental equivalent of all three human trimesters and assessed both behavior and transcriptome alterations.

Results

Opioid exposure throughout all three human equivalent trimesters delayed developmental milestones and produced acute withdrawal phenotypes in mice reminiscent of those observed in infants. We also uncovered different patterns of gene expression depending on the duration and timing of opioid exposure (3-trimesters, in utero only, or the last trimester equivalent only). Opioid exposure and subsequent withdrawal affected social behavior and sleep in adulthood in a sex-dependent manner but did not affect adult behaviors related to anxiety, depression, or opioid response.

Discussion

Despite marked withdrawal and delays in development, long-term deficits in behaviors typically associated with substance use disorders were modest. Remarkably, transcriptomic analysis revealed an enrichment for genes with altered expression in published datasets for Autism Spectrum Disorders, which correlate well with the deficits in social affiliation seen in our model. The number of differentially expressed genes between the NOWS and saline groups varied markedly based on exposure protocol and sex, but common pathways included synapse development, the GABAergic and myelin systems, and mitochondrial function.

Effect of early embryonic exposure to morphine on defects in the GABAergic system of day-old chicks

Embryonic morphine exposure (EME) leads to abnormal brain development and behavior in the offspring, and the functional alteration of γ-aminobutyric acid (GABA) system is considered to be one of the important mechanisms. To mimic the problem of susceptibility of human gestational drug abuse on addictive drugs in offspring, we administered morphine exposure on days 5-8 and 13-16 of chicken embryo development and examined the functions of GABA neurons and their receptors in postnatal chicks by neuroelectrophysiology, immunohistochemistry and behavioral methods. We found that morphine exposure during embryonic stages 5-8 (MorphineE5-8) significantly reduced the incidence of spontaneous inhibitory postsynaptic potentiation (IPSP) and the induction of evoked IPSP and the mean amplitude of GABA_A agonist muscimol-induced response in the intermediate medial interstitial (IMM) region, compared to naïve controls or saline-exposed chicks. The results of immunocytochemistry further suggest that MorphineE5-8 decreased the synaptic density of GAD-expressing sites in the IMM, while increased the expression of the GABA_A receptor subtype γ2 isoform. Behavioral results found that Morphine5-8 treatment de-inhibited morphine-induced psychomotor responses in postnatal chicks. Morphine exposure at embryonic stages 13-16 (MorphineE13-16) showed no significant changes in the above indicators compared to the saline group. Evidence suggests that early embryonic morphine exposure leads to defects in GABAergic function in the IMM, which in turn alters the responsiveness of postnatal chicks to addictive drugs. These results will help to understand the GABA mechanisms by which embryonic addictive drug exposure contributes to offspring susceptibility to addiction.

Circadian rhythms of locomotor activity in rats: Data on the effect of morphine administered from the early stages of embryonic development until weaning

The circadian clock generates behavioural and physiological rhythms to maximize the efficacy of organismal functions. The circadian system with a major circadian pacemaker in the suprachiasmatic nucleus of the hypothalamus develops gradually and its proper function in adulthood depends on an appropriate neurochemical milieu during ontogeny [1]. Locomotor activity is under direct control by the circadian clock, and alterations in its rhythmicity indicate changes of circadian clock function. We evaluated circadian parameters of locomotor rhythms of adult male Wistar rats born to mothers that were exposed to a stable dose of 0.1 mg/ml of morphine in drinking water (36 ml water on average/day/each rat) from embryonic day 10 (E10) until weaning at postnatal day 28 (P28). Increasing the dose of morphine in drinking water was used to evaluate the changes in the rhythmic gene expression in the suprachiasmatic nucleus and in the livers of young rats at P20 [2]. At P90, we started measurement of endogenous rhythmicity for 12 days in constant darkness (DD), then we applied a 15 min light pulse at circadian time 15 (CT15) and followed the animals for the next 15 days in DD. We evaluated the magnitude of light-induced phase shift and compared the circadian parameters of free-running rhythmicity in the intervals before and after the light pulse. All data were also compared between morphine-exposed animals (M group) and controls (C group) that were not exposed to morphine. An unpaired t-test confirmed a significantly longer light-induced phase delay in M group compared with C group, a prolonged circadian period in M group in the interval after the light pulse, and greater amplitude for C group in the first interval, i.e. before the light pulse. No change in total activity counts between groups was confirmed.

Consequences of Parental Opioid Exposure on Neurophysiology, Behavior, and Health in the Next Generations

Substance abuse and the ongoing opioid epidemic represents a large societal burden. This review will consider the long-term impact of opioid exposure on future generations. Prenatal, perinatal, and preconception exposure are reviewed with discussion of both maternal and paternal influences. Opioid exposure can have long-lasting effects on reproductive function, gametogenesis, and germline epigenetic programming, which can influence embryogenesis and alter the developmental trajectory of progeny. The potential mechanisms by which preconception maternal and paternal opioid exposure produce deleterious consequences on the health, behavior, and physiology of offspring that have been identified by clinical and animal studies will be discussed. The timing, nature, dosing, and duration of prenatal opioid exposure combined with other important environmental considerations influence the extent to which these manipulations affect parents and their progeny. Epigenetic inheritance refers to the transmission of environmental insults across generations via mechanisms independent of the DNA sequence. This topic will be further explored in the context of prenatal, perinatal, and preconception opioid exposure for both the maternal and paternal lineage.

Maternal morphine intake during pregnancy and lactation affects the circadian clock of rat pups

Early-life morphine exposure causes a variety of behavioural and physiological alterations observed later in life. In the present study, we investigated the effects of prenatal and early postnatal morphine on the maturation of the circadian clockwork in the suprachiasmatic nucleus and the liver, and the rhythm in aralkylamine N-acetyltransferase activity in the pineal gland. Our data suggest that the most affected animals were those born to control, untreated mothers and cross-fostered by morphine-exposed dams. These animals showed the highest mesor and amplitude in the rhythm of Per2, Nr1d1 but not Per1 gene expression in the suprachiasmatic nuclei (SCN) and arrhythmicity in AA-NAT activity in the pineal gland. In a similar pattern to the rhythm of Per2 expression in the SCN, they also expressed Per2 in a higher amplitude rhythm in the liver. Five of seven specific genes in the liver showed significant differences between groups in their expression. A comparison of mean relative mRNA levels suggests that this variability was caused mostly by cross-fostering, animals born to morphine-exposed dams that were cross-fostered by control mothers and vice versa differed from both groups of natural mothers raising offspring. Our data reveal that the circadian system responds to early-life morphine administration with significant changes in clock gene expression profiles both in the SCN and in the liver. The observed differences between the groups suggest that the dose, timing and accompanying stress events such as cross-fostering may play a role in the final magnitude of the physiological challenge that opioids bring to the developing circadian clock.

Enriched environment and social isolation differentially modulate addiction-related behaviors in male offspring of morphine-addicted dams: The possible role of μ-opioid receptors and ΔFosB in the brain reward pathway

Prenatal opioids exposure negatively affects the neurobehavioral abilities of children born from dependence dams. Adolescent housing conditions can buffer the detrimental impacts of early life experiences or contradictory can worsen individual psychosocial functions. The present study investigated the effects of maternal morphine dependence and different rearing conditions on behaviors and protein expression in brain reward circuits of male pups. Female Wistar rats a week before conception, during pregnancy and lactation were injected twice daily with escalating doses of morphine or saline. On a postnatal day 21, male pups were weaned and subjected to three different environments for two months: standard (STD), isolated (ISO), or enriched environment (EE). The anxiety and drug-related reward were measured using elevated plus maze, open field test, and conditioned place preference. Western blotting was used to determine the protein level of ΔFosB and μ-opioid receptor proteins in the striatum and the midbrain of male offspring, respectively. Results showed that maternal morphine administration dramatically increased anxiety-like and morphine place preference behaviors in offspring. Also, ISO condition aggravated these behavioral outcomes. While, rearing in EE could attenuate anxiety and morphine conditioning in pups. At molecular levels, maternal morphine exposure and social isolation markedly increased both of ΔFosB and μ-opioid receptor proteins expression. However, rearing in the EE declined ΔFosB protein expression. Together, these findings help to elucidate long lasting impacts of early life morphine exposure and rearing environment on the behavioral and molecular profile of addicted individuals.

Maternal Morphine Exposure and Post-Weaning Social Isolation Impair Memory and Ventral Striatum Dopamine System in Male Offspring: Is an Enriched Environment Beneficial?

Maternal opioids abuse has some deleterious consequences on next generations. Besides, children's rearing conditions can affect the behavioral states and brain plasticity in their later life. In the present study, we investigated the effects of maternal morphine (MOR) treatment and post-weaning rearing conditions on memory, pain threshold, and the ventral striatum dopaminergic activity in male offspring. Female Wistar rats were treated twice daily either with escalating doses of MOR or with normal saline (NS) one week before mating, during pregnancy and lactation. After weaning, the male pups were assigned to six groups and then raised for an 8-week period under three different conditions: standard (STD), isolated (ISO) or enriched environment (EE). The behavioral tests, including passive avoidance task, novel object recognition, and tail-flick test, were also performed. Moreover, the ventral striatum dopamine's content (DA), mRNA expressions of dopamine receptor 1(D1R) and dopamine receptor 2 (D2R), and dopamine transporter (DAT) were evaluated. The obtained data showed that maternal MOR exposure and post-weaning social isolation could dramatically impair memory in offspring, while EE could reverse these adverse outcomes. Moreover, results of tail flick latency indicated the increased pain threshold in EE animals. At molecular level, maternal MOR injections and social isolation reduced DA levels and altered expressions of D1R, D2R, and DAT within the ventral striatum of these male offspring. However, post-weaning EE partially buffered these changes. Our finding signified the effects of maternal MOR exposure and social isolation on the behaviors and neurochemistry of brain in next generation, and it also provided evidence on reversibility of these alterations following EE.

Enduring consequences of perinatal fentanyl exposure in mice

Opioid use by pregnant women is an understudied consequence associated with the opioid epidemic, resulting in a rise in the incidence of neonatal opioid withdrawal syndrome (NOWS) and lifelong neurobehavioral deficits that result from perinatal opioid exposure. There are few preclinical models that accurately recapitulate human perinatal drug exposure and few focus on fentanyl, a potent synthetic opioid that is a leading driver of the opioid epidemic. To investigate the consequences of perinatal opioid exposure, we administered fentanyl to mouse dams in their drinking water throughout gestation and until litters were weaned at postnatal day (PD) 21. Fentanyl-exposed dams delivered smaller litters and had higher litter mortality rates compared with controls. Metrics of maternal care behavior were not affected by the treatment, nor were there differences in dams' weight or liquid consumption throughout gestation and 21 days postpartum. Twenty-four hours after weaning and drug cessation, perinatal fentanyl-exposed mice exhibited signs of spontaneous somatic withdrawal behavior and sex-specific weight fluctuations that normalized in adulthood. At adolescence (PD 35), they displayed elevated anxiety-like behaviors and decreased grooming, assayed in the elevated plus maze and sucrose splash tests. Finally, by adulthood (PD 55), they displayed impaired performance in a two-tone auditory discrimination task. Collectively, our findings suggest that perinatal fentanyl-exposed mice exhibit somatic withdrawal behavior and change into early adulthood reminiscent of humans born with NOWS.

Prenatal Opioid Exposure Enhances Responsiveness to Future Drug Reward and Alters Sensitivity to Pain: A Review of Preclinical Models and Contributing Mechanisms

The opioid crisis has resulted in an unprecedented number of neonates born with prenatal opioid exposure (POE); however, the long-term effects of POE on offspring behavior and neurodevelopment remain relatively unknown. The advantages and disadvantages of the various preclinical POE models developed over the last several decades are discussed in the context of clinical and translational relevance. Although considerable and important variability exists among preclinical models of POE, the examination of these preclinical models has revealed that opioid exposure during the prenatal period contributes to maladaptive behavioral development as offspring mature including an altered responsiveness to rewarding drugs and increased pain response. The present review summarizes key findings demonstrating the impact of POE on offspring drug self-administration (SA), drug consumption, the reinforcing properties of drugs, drug tolerance, and other reward-related behaviors such as hypersensitivity to pain. Potential underlying molecular mechanisms which may contribute to this enhanced addictive phenotype in POE offspring are further discussed with special attention given to key brain regions associated with reward including the striatum, prefrontal cortex (PFC), ventral tegmental area (VTA), hippocampus, and amygdala. Improvements in preclinical models and further areas of study are also identified which may advance the translational value of findings and help address the growing problem of POE in clinical populations.

Prospective associations between parental substance use and youth highly processed food intake in the Fragile Families and Child Wellbeing Study

BACKGROUND

Given growing evidence of overlap in characteristics of addictive substances and highly processed foods (e.g., ice cream), transdiagnostic approaches may be appropriate. Prior work indicates youth with parents who use addictive substances are at risk for greater substance use. The current study tested hypotheses that parental substance use behaviors would prospectively predict greater youth highly processed food intake [but not minimally processed food intake (e.g., fruit)].

METHODS

The Fragile Families and Child Wellbeing Study was a longitudinal birth cohort study of youth (N = 4,898) born in large U.S. cities in 1998-2000. The current study was an archival data analysis using parental substance use data collected at youth birth and when youth were age 3, and youth diet data collected when youth were ages 5, 9, and 15.

RESULTS

Over and above the influence of covariates including family income, prenatal maternal illicit drug use significantly predicted 0.34, 0.23, and 1.32 servings increase in youth sugary food/beverage intake at ages 5, 9, and 15, respectively, and 0.25 servings increase in youth snack foods or chips intake at age 9. Prenatal maternal illicit drug use did not significantly predict youth fruit and vegetable intake at any age. Less consistent and weaker significant prospective associations emerged between postnatal maternal substance use and youth diet, and between paternal substance use and youth diet.

CONCLUSIONS

Prenatal exposure to illicit drugs may present transdiagnostic risk for greater youth highly processed food intake and substance use. Future causal and mechanistic research is warranted.

Clinical and basic research investigations into the long-term effects of prenatal opioid exposure on brain development

Coincident with the opioid epidemic in the United States has been a dramatic increase in the number of children born with neonatal abstinence syndrome (NAS), a form of withdrawal resulting from opioid exposure during pregnancy. Many research efforts on NAS have focused on short-term care, including acute symptom treatment and weaning of the infants off their drug dependency prior to authorizing their release. However, investigations into the long-term effects of prenatal opioid exposure (POE) on brain development, from the cellular to the behavioral level, have not been as frequent. Given the importance of the perinatal period for human brain development, opioid-induced disturbances in the formation and function of nascent synaptic networks and glia have the potential to impact brain connectivity and cognition long after the drug supply is cutoff shortly after birth. In this review, we will summarize the current state of NAS research, bringing together findings from human studies and preclinical animal models to highlight what is known about how POE can induce significant, prolonged deficits in brain structure and function. With rates of NAS continuing to rise, particularly in regions that already face substantial socioeconomic challenges, we speculate as to the most promising avenues for future research to alleviate this growing multigenerational threat.

Modeling prenatal opioid exposure in animals: Current findings and future directions

The past decade has seen a drastic rise in the number of infants exposed to opioids in utero. It is unclear what lasting effect this exposure may have on these children. Animal models of prenatal opioid exposure may provide insight into potential areas of vulnerability. The present review summarizes the findings across animal models of prenatal opioid exposure, including exposure to morphine, methadone, buprenorphine, and oxycodone. Details regarding the drug, doses, and duration of treatment, as well as key findings, are summarized in tables with associated references. Finally, significant gaps in the current preclinical literature and future directions are discussed.

Sugar in Infants, Children and Adolescents: A Position Paper of the European Society for Paediatric Gastroenterology, Hepatology and Nutrition Committee on Nutrition

The consumption of sugars, particularly sugar-sweetened beverages (SSBs; beverages or drinks that contain added caloric sweeteners (ie, sucrose, high-fructose corn syrup, fruit juice concentrates), in European children and adolescents exceeds current recommendations. This is of concern because there is no nutritional requirement for free sugars, and infants have an innate preference for sweet taste, which may be modified and reinforced by pre- and postnatal exposures. Sugar-containing beverages/free sugars increase the risk for overweight/obesity and dental caries, can result in poor nutrient supply and reduced dietary diversity, and may be associated with increased risk of type 2 diabetes mellitus, cardiovascular risk, and other health effects. The term "free sugars," includes all monosaccharides/disaccharides added to foods/beverages by the manufacturer/cook/consumer, plus sugars naturally present in honey/syrups/unsweetened fruit juices and fruit juice concentrates. Sugar naturally present in intact fruits and lactose in amounts naturally present in human milk or infant formula, cow/goat milk, and unsweetened milk products is not free sugar. Intake of free sugars should be reduced and minimised with a desirable goal of <5% energy intake in children and adolescents aged ≥2 to 18 years. Intake should probably be even lower in infants and toddlers <2 years. Healthy approaches to beverage and dietary consumption should be established in infancy, with the aim of preventing negative health effects in later childhood and adulthood. Sugar should preferably be consumed as part of a main meal and in a natural form as human milk, milk, unsweetened dairy products, and fresh fruits, rather than as SSBs, fruit juices, smoothies, and/or sweetened milk products. Free sugars in liquid form should be replaced by water or unsweetened milk drinks. National Authorities should adopt policies aimed at reducing the intake of free sugars in infants, children and adolescents. This may include education, improved labelling, restriction of advertising, introducing standards for kindergarten and school meals, and fiscal measures, depending on local circumstances.

Effects of environmental enrichment during abstinence in morphine dependent parents on anxiety, depressive-like behaviors and voluntary morphine consumption in rat offspring

Chronic morphine exposure during puberty increased morphine-induced rewarding effects and sensitization in the next generation. Given the well-known beneficial effects of environmental enrichment on the severity of physical and psychological dependence on morphine, we examined effects of enriched environment during morphine abstinence in morphine dependent parental rats before mating on the anxiety and depressive-like behaviors, and voluntary morphine consumption in their offspring. Paternal and/or maternal rats were injected with bi-daily doses (10mg/kg, 12h intervals) of morphine for 14days followed by rearing in a standard environment (SE) or enriched environment (EE) during 30days of morphine abstinence before mating. The pubertal male and female rat offspring were tested for anxiety (the elevated plus maze- EPM) and depression (sucrose preference test-SPT), and voluntary morphine consumption using a two-bottle choice (TBC) paradigm. The results showed that EE experience in morphine-dependent both parents result in an increase in the percentage of time spent into open arms/time spent on both arms using EPM in male offspring, higher levels of sucrose preference in female offspring and lower levels of voluntary morphine consumption in male and female offspring. Thus, EE experience in morphine-dependent both parents reduced anxiety, depressive-like behavior and also the voluntary morphine consumption in their offspring during puberty which may prevent the vulnerability of the next generation to drug abuse.

Beneficial effects of co-treatment with dextromethorphan on prenatally methadone-exposed offspring

Background

Heroin use among young women of reproductive age has drawn much attention around the world. Although methadone is widely used in maintenance therapy for heroin/morphine addiction, the long-term effects of prenatal exposure to methadone and preventative therapy remain unclear. For revealing this question, female pregnant Sprague–Dawley rats were sub-grouped to receive (1) vehicle, (2) methadone 5 mg/kg at embryonic day 3 (E3) and then 7 mg/kg from E4 to E20, (3) dextromethorphan (DM) 3 mg/kg, and (4) methadone + DM (the rats received methadone followed by DM treatment), subcutaneously, twice a day from E3 to E20. The body weight, natural withdrawal, pain sensitivity, ED50, conditioned place preference and water maze were conducted at different postnatal stages (P1 to P79) of offspring. The quantitative real-time RT-PCR and electrophysiology were also used to measure the gene expression of opioid receptors in the spinal cord and changes of LTP/LTD in the hippocampus, separately.

Results

Prenatal exposure to methadone or DM did not affect survival rate, body weight, water maze and LTP or LTD of offspring. However, prenatal methadone significantly increased the withdrawal symptoms, pain sensitivity, addiction liability and decreased the mRNA expression of pain related opioid receptors. Co-administration of DM with methadone in the maternal rats effectively prevented these abnormalities of offspring induced by methadone.

Conclusions

Our study clearly showed that co-administration of dextromethorphan with methadone in the maternal rats prevented the adverse effects induced by prenatal methadone exposure. It implies that dextromethorphan may have a potential to be used in combination with methadone for maintenance treatment in pregnant heroin-addicted women to prevent the adverse effects induced by methadone on offspring.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0126-2) contains supplementary material, which is available to authorized users.

Behavioral effects of perinatal opioid exposure

Opioids are among the world's oldest known drugs used mostly for pain relief, but recreational use is also widespread. A particularly important problem is opioid exposure in females, as their offspring can also be affected. Adverse intrauterine and postnatal environments can affect offspring development and may lead to various disabilities later in life. It is clear that repetitive painful experiences, such as randomly occurring invasive procedures during neonatal intensive care, can permanently alter neuronal and synaptic organization and therefore later behavior. At the same time, analgesic drugs can also be harmful, inducing neuronal apoptosis or withdrawal symptoms in the neonate and behavioral alterations in adulthood. Hence, risk-benefit ratios should be taken into consideration when pain relief is required during pregnancy or in neonates. Recreational use of opioids can also alter many aspects of life. Intrauterine opioid exposure has many toxic effects, inducing poor pregnancy outcomes due to underdevelopment, but it is believed that later negative consequences are more related to environmental factors such as a chaotic lifestyle and inadequate prenatal care. One of the crucial components is maternal care, which changes profoundly in addicted mothers. In substance-dependent mothers, pre- and postnatal care has special importance, and controlled treatment with a synthetic opioid (e.g., methadone) could be beneficial. We aimed to summarize and compare human and rodent data, as it is important to close the gap between scientific knowledge and societal policies. Special emphasis is given to gender differences in the sensitivity of offspring to perinatal opioid exposure.

Antinociceptive actions of peripheral glucose administration

The effects of intraperitoneal (ip) D-glucose administration on antinociception were studied in male Long-Evans rats. Rats were assessed for antinociception using the hot-water tail-withdrawal procedure (54±0.2 °C) to determine if peripheral administration of D-glucose (300, 560, or 720 mg/kg) would enhance morphine-mediated antinociception (MMA) (1.0, 3.0, 4.2, 5.6, and 10.0mg/kg cumulative-dosing regime) and if D-glucose (560, 720, or 1000 mg/kg) alone could produce antinociceptive activity that was naloxone (0.32 mg/kg) reversible. Additionally, the actions of D-glucose on MMA were compared with a stereoisomer, L-glucose, which is not metabolized. The results of these studies demonstrate that peripheral administration of D-glucose significantly enhances MMA and that D-glucose alone produces antinociceptive actions that are potentially mediated by the endogenous opioid system. Furthermore, L-glucose failed to have an effect on MMA suggesting that the alterations in antinociception seen with D-glucose are not due to stressors such as osmolality or injection. The current studies provide evidence that D-glucose alteration of antinociception is not simply a response to taste or gustation.

Enhancement of tolerance development to morphine in rats prenatally exposed to morphine, methadone, and buprenorphine

Background

Abuse of addictive substances is a serious problem that has a significant impact on areas such as health, the economy, and public safety. Heroin use among young women of reproductive age has drawn much attention around the world. However, there is a lack of information on effects of prenatal exposure to opioids on their offspring. In this study, an animal model was established to study effects of prenatal exposure to opioids on offspring.

Methods

Female pregnant Sprague-Dawley rats were sub-grouped to receive (1) vehicle, (2) 2-4 mg/kg morphine (1 mg/kg increment per week), (3) 7 mg/kg methadone, and (4) 3 mg/kg buprenorphine, subcutaneously, once or twice a day from E3 to E20. The experiments were conducted on animals 8-12 weeks old and with body weight between 250 and 350 g.

Results

Results showed that prenatal exposure to buprenorphine caused higher mortality than other tested substance groups. Although we observed a significantly lower increase in body weight in all of the opioid-administered dams, the birth weight of the offspring was not altered in all treated groups. Moreover, no obvious behavioral abnormality or body-weight difference was noted during the growing period (8-12 weeks) in all offspring. When the male offspring received morphine injection twice a day for 4 days, the prenatally opioid-exposed rats more quickly developed a tolerance to morphine (as shown by the tail-flick tests), most notably the prenatally buprenorphine-exposed offspring. However, the tolerance development to methadone or buprenorphine was not different in offspring exposed prenatally to methadone or buprenorphine, respectively, when compared with that of the vehicle controlled group. Similar results were also obtained in the female animals.

Conclusions

Animals prenatally exposed to morphine, methadone, or buprenorphine developed tolerance to morphine faster than their controlled mates. In our animal model, prenatal exposure to buprenorphine also resulted in higher mortality and much less sensitivity to morphine-induced antinociception than prenatal exposure to morphine or methadone. This indicates that buprenorphine in higher doses may not be an ideal maintenance drug for treating pregnant women. This study provides a reference in selecting doses for clinical usage in treating pregnant heroin addicts.

Peri, pre and postnatal morphine exposure: exposure-induced effects and sex differences in the behavioural consequences in rat offspring

This study investigated the behavioural consequences of peri, pre and postnatal morphine (MO) exposure in rats. From gestational day 1 dams were treated with either saline or MO subcutaneously once a day (5 mg/kg on the first 2 days, 10 mg/kg subsequently). Spontaneous locomotor activity in a new environment (habituation) and antinociceptive effects of MO were measured separately in male and female pups after weaning and also in late adolescence or adulthood. The rewarding effect of MO was assessed by conditioned place preference in adult animals. Both exposure-induced and sex differences were observed. A significant delay in habituation to a new environment and decreased sensitivity to the antinociceptive effect of MO were found in male offspring of MO-treated dams. In contrast, the place preference induced by MO was enhanced in the MO-exposed adult animals and this effect was more marked in females. Prenatal exposure to MO resulted in more marked changes than the postnatal exposure through maternal milk. The results indicate that a medium MO dose administered once-daily results in long-term consequences in offspring and may make them more vulnerable to MO abuse in adulthood.

Differential effects of prenatal morphine exposure on analgesia produced by vaginocervical stimulation or systemic morphine administration in adult rats

The present study investigated the effects of prenatal morphine exposure on the magnitude of analgesia produced by vaginocervical stimulation (VS) or systemic morphine injection in adult rats. In Experiment 1, an acute subcutaneous morphine (1 mg/kg) injection induced a 124% greater increase in tail-flick latency (TFL) in adult rats exposed prenatally to saline than to morphine. By contrast, in Experiment 2, VS induced a 196% greater increase in TFL in adult rats exposed prenatally to morphine than to saline. Female rats exposed prenatally to morphine also had a greater VS-produced increase in vocalization threshold (VOC-T) to tail shock than those exposed prenatally to saline. Thus, the present study demonstrates that prenatal morphine exposure exerts diametrically opposite effects on analgesia that is produced in adulthood by morphine or VS, attenuating the former while potentiating the latter. These findings provide evidence that the mechanisms underlying the two types of analgesia differ fundamentally.

Potentiation by nitric oxide synthase inhibitor and calcium channel blocker of aspartame-induced antinociception in the mouse formalin test

By applying a 12 day regimen of the non-calorific sweetener, aspartame, in combination with representative compounds of the calcium channel blocker and nitric oxide synthase inhibitor, we tried to investigate using a formalin-test in mice the relative role of aspartame on pain and its mechanism of action. Verapamil (2, 3.5, 5, 7.5 mg/kg) induced significant (P < 0.01) antinociception in both phases of the formalin test. L-Nitro-arginine-methyl-ester (L-NAME) at the doses used, induced significant (P < 0.01) antinociception in early phase (1, 2, 5, 10 mg/kg) and late phase (5, 10 mg/kg). Twelve days of treatment in animals by aspartame (0.16% w/v) significantly induced antinociception in both phases of the formalin test. Both verapamil (5 mg/kg) and L-NAME (10 mg/kg) significantly (P < 0.01) potentiated aspartame-induced antinociception in both phases of formalin test. The present findings support the hypothesis that the activation of NMDA receptors by aspartame modulates pain-related behaviour via a nitric oxide/cGMP/glutamate release cascade. It is concluded that aspartame would be a good analgesic agent if it would be used in combination with a calcium channel blocker or NOS inhibitor.

Social behavior of juvenile rats after in utero exposure to morphine: dose-time-effect relationship

In the present study, the effects of morphine exposure in utero on social behavior in juvenile male rats was investigated. Pinning, a measure for play behavior, and social grooming of the offspring were measured at postnatal day 21. The subjects were offspring of Wistar rat dams given sc. injections of 1 or 10 mg/kg body weight morphine HCl daily from gestational days 8 (GD8)-GD 21 and control dams injected daily with saline. Pinning and social grooming of the morphine-treated offspring were significantly elevated compared to saline controls. The doses of morphine used neither affected the gestation of pregnant mother rats nor sensorimotor development of the juvenile rats. Prenatal exposure to morphine of 10 mg/kg daily increased both pinning and social grooming, prenatal exposure to a lower dose of 1 mg/kg increased pinning behavior but not social grooming in the offspring. To study the importance of the gestational period, offspring of dams given 10 mg/kg body weight morphine HCl from GD8-GD15 and saline from GD16-parturition or morphine from GD16-parturition and saline from GD8-GD15 was tested. Pinning was only increased when morphine exposure occurred during the third week of gestation, social grooming was increased when morphine exposure had been in the second week of gestation. Subcutaneous administration of 1 mg/kg naltrexone 1 h before the test significantly decreased play behavior in control rats, but not in animals prenatally exposed to morphine. From these experiments we conclude that the long term effect of in utero exposure to morphine on play behavior is established by affecting the endogenous opioid system.

Effects of prenatal morphine exposure on NK cytotoxicity and responsiveness to LPS in rats

Prenatal exposure to opiates can adversely affect fetal development, resulting in long-term growth retardation and impairments in physiological and behavioral functions. In the present study we studied long-term effects of prenatal morphine exposure on immune functions, including the activity of natural killer (NK) cells and the febrile and behavioral responses to lipopolysaccharide (LPS). Pregnant Fischer 344 rats were given increasing doses of morphine in slow release emulsion during gestational days 12-18. Control rats were injected with vehicle and were either pair fed to morphine rats or fed ad lib. Postnatal experiments were conducted when offspring were 10-12 weeks old. Compared to both control groups, rats prenatally exposed to morphine exhibited: 1) suppressed cytotoxic activity of NK cells; 2) reduced LPS-induced fever measured by a biotelemetric system; 3) reduced hyperalgesia measured by the hot-plate test at 30 min, and augmented hypoalgesia at 2-6 h post-LPS; 4) higher open-field activity in saline-treated animals, and more pronounced suppression of activity in LPS-injected animals; 5) LPS-induced reduction of food consumption, body weight, and social exploration, which did not differ from the reduction observed in control animals. These findings indicate that prenatal exposure to morphine induces long-term impairment of host-defense mechanisms, which may render the offspring more susceptible to infectious diseases.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.