Neonatal withdrawal following pre- and postnatal exposure to methadone in the rat

Rapastinel accelerates loss of withdrawal signs after repeated morphine and blunts relapse to conditioned place preference

The purpose of the present study was to evaluate the efficacy of rapastinel, an allosteric modulator of NMDA receptor function, to accelerate the loss of opioid withdrawal symptoms and blunt or prevent relapse to morphine conditioned place preference (CPP) in rats. Two studies were conducted. In study 1, adult and adolescent male and female rats were treated with increasing doses of morphine (5 mg/kg, bid to 25 mg/kg bid) for 5 days. On day 6 animals were treated with naloxone (1 mg/kg) and withdrawal was assessed. They were then treated with saline or rapastinel (5 mg/kg) on days 6 and 8, and withdrawal was assessed on day 9. Rapastinel treated animals exhibited significantly lower levels of withdrawal signs on day 9. No sex or age differences were observed. In Study 2, CPP for morphine was established in adult rats (males and females) by 4 daily pairings with saline and morphine (am/pm alternation). They were tested for CPP on day 5, and then treated with rapastinel (5 mg/kg) or saline daily on days 6-10 of extinction. On day 11 they received a final dose of rapastinel or saline followed by extinction trial. On day 12, animals received 1 mg/kg of morphine and were tested for relapse. Rapastinel did not affect extinction of CPP, but rapastinel-treated animals spent significantly less time in the previously morphine-paired side than saline-treated animals during the relapse trial. These findings of accelerated loss of withdrawal signs and blunted relapse to CPP suggest that rapastinel could provide an adjunctive therapy for opioid dependence during initiation of pharmacotherapy for opioid dependence.

Sex Differences in Behavioral and Brainstem Transcriptomic Neuroadaptations following Neonatal Opioid Exposure in Outbred Mice

The opioid epidemic led to an increase in the number of neonatal opioid withdrawal syndrome (NOWS) cases in infants born to opioid-dependent mothers. Hallmark features of NOWS include weight loss, severe irritability, respiratory problems, and sleep fragmentation. Mouse models provide an opportunity to identify brain mechanisms that contribute to NOWS. Neonatal outbred Swiss Webster Cartworth Farms White (CFW) mice were administered morphine (15 mg/kg, s.c.) twice daily from postnatal day 1 (P1) to P14, an approximation of the third trimester of human gestation. Female and male mice underwent behavioral testing on P7 and P14 to determine the impact of opioid exposure on anxiety and pain sensitivity. Ultrasonic vocalizations (USVs) and daily body weights were also recorded. Brainstems containing pons and medulla were collected during morphine withdrawal on P14 for RNA sequencing. Morphine induced weight loss from P2 to P14, which persisted during adolescence (P21) and adulthood (P50). USVs markedly increased at P7 in females, emerging earlier than males. On P7 and P14, both morphine-exposed female and male mice displayed hyperalgesia on the hot plate and tail-flick assays, with females showing greater hyperalgesia than males. Morphine-exposed mice exhibited increased anxiety-like behavior in the open-field arena on P21. Transcriptome analysis of the brainstem, an area implicated in opioid withdrawal and NOWS, identified pathways enriched for noradrenergic signaling in females and males. We also found sex-specific pathways related to mitochondrial function and neurodevelopment in females and circadian entrainment in males. Sex-specific transcriptomic neuroadaptations implicate unique neurobiological mechanisms underlying NOWS-like behaviors.

Maternal continuous oral oxycodone self-administration alters pup affective/social communication but not spatial learning or sensory-motor function

BACKGROUND

The broad use/misuse of prescription opioids during pregnancy has resulted in a surge of infants with Neonatal Opioid Withdrawal Syndrome (NOWS). Short-term irritability and neurological complications are its hallmarks, but the long-term consequences are unknown.

METHODS

A newly-developed preclinical model of oxycodone self-administration enables adult female rats to drink oxycodone (∼10/mg/kg/day) before and during pregnancy, and after delivery, and to maintain normal liquid intake, titrate dosing, and avoid withdrawal.

RESULTS

Oxycodone was detected in the serum of mothers and pups. Growth parameters in dams and pups and litter mass and size were similar to controls. There were no differences in paw retraction latency to a thermal stimulus between Oxycodone and Control pups at postnatal (PN) 2 or PN14. Oxycodone and Control pups had similar motor coordination, cliff avoidance, righting time, pivoting, and olfactory spatial learning from PN3 through PN13. Separation-induced ultrasonic vocalizations at PN8 revealed higher call frequency in Oxycodone pups relative to Control pups (p<0.031; Cohen's d=1.026). Finally, Oxycodone pups displayed withdrawal behaviors (p's<0.029; Cohen's d's>0.806), and Oxycodone males only vocalized more than Control pups in the first minute of testing (p's<0.050; Cohen's d's>.866). Significant effects were corroborated by estimation plots.

CONCLUSIONS

Our rat model of oral oxycodone self-administration in pregnancy shows exacerbated affect/social communication in pups in a sex-dependent manner but spared cognition and sensory-motor behaviors. This preclinical model reproduces selective aspects of human opioid use during pregnancy, enabling longitudinal analysis of how maternal oxycodone changes emotional behavior in the offspring.

Automated quantification of opioid withdrawal in neonatal rat pups using Ethovision® XT software

Chronic prenatal exposure to opioids often causes fetal opioid dependence that leads to neonatal opioid withdrawal syndrome (NOWS) shortly after delivery. Rat models of NOWS often require quantifying neonatal withdrawal behaviors using time-consuming, labor-intensive manual scoring methods. The goal of this study was to automate quantification of opioid withdrawal in neonatal rat pups. Accordingly, we used the animal behavior software Ethovision® XT to analyze archived videos of rat pups subjected to precipitated opioid withdrawal testing on postnatal day 0. We compared results obtained from Ethovision® XT with those previously obtained from manual scoring. Two endpoints reported by Ethovision® XT, Distance Moved (cm) and Movement Duration (s), had strong positive linear relationships with manually derived global withdrawal scores (GWS; R² > 0.73). Sensitivity and specificity of each endpoint to discriminate presence and absence of low-grade withdrawal were assessed by receiver operator characteristic curve analysis, which indicated that Distance Moved and Movement Duration had excellent accuracy (AUC > 0.90). Finally, we analyzed main and interaction effects of prenatal treatment (with vehicle or mu opioid receptor full agonists) and postnatal challenge (with saline or an opioid receptor antagonist) on each endpoint and determined they were similar for the manual and automated methods. These results show that Ethovision® XT software can reliably quantify opioid withdrawal in neonatal rat pups with non-inferiority to manual scoring even in videos that were not originally purposed and optimized for Ethovision® XT analysis. This faster and less labor-intensive method of analysis is expected to accelerate progress in preclinical studies of 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.

Prenatal exposure to methadone or buprenorphine impairs cognitive performance in young adult rats

BACKGROUND

Concerns have been raised about the use of opioid maintenance treatment (OMT) during pregnancy and negative effects for the offspring. While neonatal outcomes and short-term effects are relatively well described, studies examining long-term effects in adolescents and adults are absent. The aim of the present study was to examine effects on learning and memory in young adult rats prenatally exposed to methadone or buprenorphine.

METHODS

Female rats were implanted with a 28-day osmotic minipump delivering methadone (10 mg/kg/day), buprenorphine (1 mg/kg/day) or vehicle 5 days prior to mating. To examine possible effects on cognitive functioning, young adult offspring were included in three different behavioral tests that examine recognition memory, nonspatial, and spatial learning and memory. In addition, offspring growth and maternal behavior after birh were investigated.

RESULTS

Prenatal exposure to methadone or buprenorphine caused impaired recognition memory and nonspatial reference learning and memory in young adult rats compared with the vehicle-treated group. Methadone-exposed offspring, but not the buprenorphine-exposed, also showed reduced long-term spatial memory. We did not observe any changes in maternal behavior or offspring growth after prenatal exposure to methadone or buprenorphine, suggesting that the impaired cognitive functioning is due to the opioid exposure rather than reduced maternal caregiving.

CONCLUSION

The present findings of long-term cognitive impairments in methadone- and buprenorphine-exposed offspring points to a negative impact of OMT on neurobiological development.

N-acetylcysteine mitigates acute opioid withdrawal behaviors and CNS oxidative stress in neonatal rats

BACKGROUND

Neonatal abstinence syndrome (NAS) is a significant problem. Opioid withdrawal induces oxidative stress and disrupts glutamate and glutathione homeostasis. We hypothesized that N-acetylcysteine (NAC) administered during acute opioid withdrawal in neonatal rats would decrease withdrawal behaviors and normalize CNS glutathione and glutamate.

METHODS

Osmotic minipumps with methadone (opioid dependent, OD) and saline (Sham) were implanted into Sprague Dawley dams 7 days prior to delivery. Pups were randomized to receive either naloxone plus saline or NAC (50-100 mg/kg), administered on postnatal day (PND) 7. We performed MR spectroscopy on PND6-7 before, 30 min, and 120 min after withdrawal. On PND7, we assessed withdrawal behaviors for 90 min after naloxone administration and summed scores during peak withdrawal period.

RESULTS

Mean summed behavioral scores were significantly different between groups (χ² (2) = 10.49, p = 0.005) but not different between NAC/NAL/OD and Sham (p = 0.14): SAL/NAL/OD = 17.2 ± 4.2 (n = 10); NAC/NAL/OD = 11.3 ± 5.6 (n = 9); Sham = 6.5 ± 0.6 (n = 4). SAL/NAL/OD pups had decreased glutathione at 120 min (p = 0.01), while NAC/NAL/OD pups maintained pre-withdrawal glutathione (p = 0.26).

CONCLUSION

In antenatal OD, NAC maintains CNS glutathione and mitigates acute opioid withdrawal in neonatal rats. This is the first study to demonstrate acute opioid withdrawal neurochemical changes in vivo in neonatal OD. NAC is a potential novel treatment for NAS.

High Accumulation of Methadone Compared with Buprenorphine in Fetal Rat Brain after Maternal Exposure

Experimental animal studies are valuable in revealing a causal relationship between prenatal exposure to opioid maintenance treatment (OMT) and subsequent effects; however, previous animal studies of OMT during pregnancy have been criticized for their lack of clinical relevance because of their use of high drug doses and the absence of pharmacokinetic data. Hence, the aim of this study was to determine blood and brain concentrations in rat dams, fetuses, and offspring after continuous maternal exposure to methadone or buprenorphine during gestation and to examine the offspring for neonatal outcomes and withdrawal symptoms. Female rats were implanted with a 28-day osmotic minipump delivering methadone (10 mg/kg per day), buprenorphine (1 mg/kg per day) or vehicle 5 days before mating. Continuous exposure to methadone or buprenorphine induced stable blood concentrations in the dams of 0.25 ± 0.02 µM and 5.65 ± 0.16 nM, respectively. The fetal brain concentration of methadone (1.89 ± 0.35 nmol/g) was twice as high as that in the maternal brain, whereas the fetal brain concentration of buprenorphine (20.02 ± 4.97 pmol/g) was one-third the maternal brain concentration. The opioids remained in the offspring brain several days after the exposure ceased. Offspring prenatally exposed to methadone, but not buprenorphine, displayed reduced body weight and length and increased corticosterone levels. No significant changes in ultrasonic vocalizations were revealed. Our data in rat fetuses and neonates indicate that OMT with buprenorphine may be a better choice than methadone during pregnancy. SIGNIFICANCE STATEMENT: Concern has been raised about the use of opioid maintenance treatment during pregnancy because of the important role of the endogenous opioid system in brain development. Here, we show that the methadone concentration in the fetal rat brain was twice as high as that in the maternal brain, whereas the buprenorphine concentration was one-third the maternal concentration. Furthermore, buprenorphine allowed more favorable birth outcomes, suggesting that buprenorphine may be a better choice during pregnancy.

In Utero Exposure to Norbuprenorphine, a Major Metabolite of Buprenorphine, Induces Fetal Opioid Dependence and Leads to Neonatal Opioid Withdrawal Syndrome

Buprenorphine is the preferred treatment of opioid use disorder during pregnancy but can cause fetal opioid dependence and neonatal opioid withdrawal syndrome (NOWS). Notably, withdrawal severity is independent of maternal buprenorphine dose, suggesting that interindividual variance in pharmacokinetics may influence risk and severity of NOWS. Using a rat model of NOWS, we tested the hypothesis that clinically relevant doses of the active metabolite norbuprenorphine (NorBUP) can induce in utero opioid dependence, manifested as naltrexone-precipitated withdrawal signs in the neonate. Pregnant Long-Evans rats were implanted with 14-day osmotic minipumps containing vehicle, morphine (positive control), or NorBUP (0.3-10 mg/kg per day) on gestation day 9. By 12 hours post-delivery, an intraperitoneal injection of the opioid antagonist naltrexone (1 or 10 mg/kg) or saline was administered to pups. Precipitated withdrawal signs were graded by raters blinded to treatment conditions. In a separate group, NorBUP concentrations in maternal and fetal blood and brain on gestation day 20 were determined by liquid chromatography-tandem mass spectrometry. Steady-state maternal blood concentrations of NorBUP in dams infused with 1 or 3 mg/kg per day were comparable to values reported in pregnant humans treated with buprenorphine (1.0 and 9.6 ng/ml, respectively), suggesting a clinically relevant dosing regimen. At these doses, NorBUP increased withdrawal severity in the neonate as shown by an evaluation of 10 withdrawal indicators. These findings support the possibility that NorBUP contributes to fetal opioid dependence and NOWS following maternal buprenorphine treatment during pregnancy.

Preferential Delivery of an Opioid Antagonist to the Fetal Brain in Pregnant Mice

Prolonged fetal exposure to opioids results in neonatal abstinence syndrome (NAS), a major medical problem requiring intensive care and increased hospitalization times for newborns with NAS. Multiple strategies are currently available to alleviate withdrawal in infants with NAS. To prevent NAS caused by opioid maintenance programs in pregnant women, blocking fetal dependence without compromising the mother's opiate therapy is desirable. Here we tested in pregnant mice whether a peripherally selective opioid antagonist can preferentially enter the fetal brain and, thereby, in principle, selectively protect the fetus. We show using mass spectrometry that 6β-naltrexol, a neutral opioid antagonist with very limited ability to cross the blood-brain barrier (BBB), readily crosses the placental barrier and enters the fetal brain at high levels, although it is relatively excluded from the maternal brain. Furthermore, owing to the late development of the BBB in postnatal mice, we show that 6β-naltrexol can readily enter the juvenile mouse brain until at least postnatal day 14. Taking advantage of this observation, we show that long-term exposure to morphine starting in the second postnatal week causes robust and quantifiable dependence behaviors that are suppressed by concomitant administration of 6β-naltrexol with much greater potency (ID50 0.022-0.044 mg/kg, or 1/500 the applied dose of morphine) than previously demonstrated for either the suppression of central nervous system opioid effects or the induction of withdrawal in adults. These results indicate that peripherally selective opioid antagonists capable of penetrating the placenta may be beneficial for preventing or reducing neonatal dependence and NAS in a dose range that should not interfere with maternal opioid maintenance.

The effect of intermittent alcohol vapor or pulsatile heroin on somatic and negative affective indices during spontaneous withdrawal in Wistar rats

RATIONALE

Once dependent on alcohol or opioids, negative affect may accompany withdrawal. Dependent individuals are hypothesized to "self-medicate" in order to cope with withdrawal, which promotes escalated alcohol and drug use.

OBJECTIVES

The current study aimed to develop a reliable animal model to assess symptoms that occur during spontaneous alcohol and opioid withdrawal.

METHODS

Dependence was induced using intermittent alcohol exposure or pulsatile heroin delivery and assessed for the presence of withdrawal symptoms during acute withdrawal by measuring somatic signs, behavior in the forced swim test (FST), and air-puff-induced 22-kHz ultrasonic vocalizations (USVs). Additional animals subjected to 8 weeks of alcohol vapor exposure were evaluated for altered somatic signs, operant alcohol self-administration, and 22-kHz USV production, as well as performance in the elevated plus maze (EPM).

RESULTS

During spontaneous withdrawal from pulsatile heroin or intermittent alcohol vapor, animals displayed increased somatic withdrawal signs, FST immobility, and 22-kHz USV production but did not show any behavioral change in the EPM unless the duration of alcohol exposure was extended to 4 weeks. Following 8 weeks of alcohol vapor exposure, animals displayed somatic withdrawal signs, escalated alcohol self-administration, and increased 22-kHz USVs.

CONCLUSIONS

These paradigms provide consistent methods to evaluate the behavioral ramifications, and neurobiological substrates, of alcohol and opioid dependence during spontaneous withdrawal. As immobility in the FST and percent open-arm time in the EPM were dissociable, with 22-kHz USVs paralleling immobility in the FST, assessment of air-puff-induced 22-kHz USVs could provide an ethologically valid alternative to the FST.

Subsequent anxiety-related behavior in rats exposed to low-dose methadone during gestation, lactation or both periods consecutively

In order to assess the long-term behavioral consequences of exposing rats to methadone during gestation, lactation or both periods consecutively, pregnant Wistar dams were provided with drinking water containing approximately 2.39 mg/kg/day methadone. Soon after birth, litters of offspring were assigned to methadone-naïve foster mothers. Half of these foster mothers were then provided with drinking water containing methadone (approximately 2.86 mg/kg/day), while the other half received unadulterated water. Maternal weight gain, pregnancy duration, litter sizes, sex ratios and average pup weights were recorded. Following weaning on postnatal day (PND) 28, individual rats were weighed and inspected for physical abnormalities and stress reactions at PND20, 60 and 120. At these same ages, observations were also made of the rats' behavior in an emergence apparatus, and an open field. Apart from a smaller number of full-term pregnancies, there were no effects of any type of methadone treatment on physical measurements recorded at any age. Nor were there any behavioral effects of gestational methadone experienced on its own. However, methadone experienced during lactation (without gestational exposure) decreased emergence speed at PND30, and for all testing ages combined, increased open-field ambulation (males only), walking, rearing and occupancy of the center of the apparatus. Exposure to methadone during both gestation and lactation decreased emergence latencies at PND30 and, for all ages combined, decreased ambulation (males only), center occupancy and defecation. The subsequent behavioral effects of methadone were largely confined to lactational exposure and, when combined with gestational exposure, suggested increased anxiety.

Changing mechanisms of opiate tolerance and withdrawal during early development: animal models of the human experience

Human infants may be exposed to opiates through placental transfer from an opiate-using mother or through the direct administration of such drugs to relieve pain (e.g., due to illness or neonatal surgery). Infants of many species show physical dependence and tolerance to opiates. The magnitude of tolerance and the nature of withdrawal differ from those of the adult. Moreover, the mechanisms that contribute to the chronic effects of opiates are not well understood in the infant but include biological processes that are both common to and distinct from those of the adult. We review the animal research literature on the effects of chronic and acute opiate exposure in infants and identify mechanisms of withdrawal and tolerance that are similar to and different from those understood in adults. These mechanisms include opioid pharmacology, underlying neural substrates, and the involvement of other neurotransmitter systems. It appears that brain circuitry and opioid receptor types are similar but that NMDA receptor function is immature in the infant. Intracellular signaling cascades may differ but data are complicated by differences between the effects of chronic versus acute morphine treatment. Given the limited treatment options for the dependent infant patient, further study of the biological functions that are altered by chronic opiate treatment is necessary to guide evidenced-based treatment modalities.

The effects of maternally administered methadone, buprenorphine and naltrexone on offspring: review of human and animal data

Most women using heroin are of reproductive age with major risks for their infants. We review clinical and experimental data on fetal, neonatal and postnatal complications associated with methadone, the current "gold standard", and compare these with more recent, but limited, data on developmental effects of buprenorphine, and naltrexone. Methadone is a micro-opioid receptor agonist and is commonly recommended for treatment of opioid dependence during pregnancy. However, it has undesired outcomes including neonatal abstinence syndrome (NAS). Animal studies also indicate detrimental effects on growth, behaviour, neuroanatomy and biochemistry, and increased perinatal mortality. Buprenorphine is a partial micro-opioid receptor agonist and a kappa-opioid receptor antagonist. Clinical observations suggest that buprenorphine during pregnancy is similar to methadone on developmental measures but is potentially superior in reducing the incidence and prognosis of NAS. However, small animal studies demonstrate that low doses of buprenorphine during pregnancy and lactation lead to changes in offspring behaviour, neuroanatomy and biochemistry. Naltrexone is a non-selective opioid receptor antagonist. Although data are limited, humans treated with oral or sustained-release implantable naltrexone suggest outcomes potentially superior to those with methadone or buprenorphine. However, animal studies using oral or injectable naltrexone have shown developmental changes following exposure during pregnancy and lactation, raising concerns about its use in humans. Animal studies using chronic exposure, equivalent to clinical depot formulations, are required to evaluate safety. While each treatment is likely to have maternal advantages and disadvantages, studies are urgently required to determine which is optimal for offspring in the short and long term.

Tolerance, opioid-induced allodynia and withdrawal associated allodynia in infant and young rats

Our laboratory has previously characterized age-dependent changes in nociception upon acute morphine withdrawal. This study characterizes changes in mechanical and thermal nociception following acute, intermittent, or continuous morphine administration in infant (postnatal days 5-8) and young (postnatal days 19-21) rats. Morphine was given as a single acute administration (AM), intermittently twice a day for 3 days (IM), or continuously for 72 h via pump (CM). AM did not produce long-term changes in mechanical or thermal nociception in either infant or young rats. CM produced changes in mechanical nociception that included the development of tolerance, opioid-induced mechanical allodynia and withdrawal-associated mechanical allodynia in young rats, but only tolerance and a prolonged withdrawal-associated mechanical allodynia in infant rats. IM produced withdrawal-associated mechanical allodynia in both infant and young rats. Measuring paw withdrawal responses to thermal stimuli, infant and young rats showed tolerance without opioid-induced thermal hyperalgesia or withdrawal-associated thermal hyperalgesia following CM. In contrast to CM, withdrawal-associated thermal hyperalgesia was seen in both ages following IM. In conclusion, CM versus IM differentially modified mechanical and thermal nociception, suggesting that opioid-dependent thermal hyperalgesia and mechanical allodynia can be dissociated from each other in infant and young rats. Furthermore, tolerance, opioid-induced hypersensitivity, and withdrawal-associated hypersensitivity are age-specific and may be mediated by distinct mechanisms.

Role of endothelin (ETA) receptors in neonatal morphine withdrawal

We have previously demonstrated role of central endothelin (ET) receptors in neonatal morphine tolerance. The present study was conducted to investigate involvement of central ET receptors in neonatal rat morphine withdrawal. The aim was to determine activation of G-proteins coupled to opioid and ET receptors by morphine and ET ligands in neonatal rat brains during morphine withdrawal. Pregnant female rats were rendered tolerant to morphine by chronic exposure to morphine pellets over 7 days. Withdrawal was induced on day 8 by removal of pellets. Rat pups were delivered by cesarean section 24 h after pellet removal. G-protein stimulation induced by morphine; ET-1; ETA receptor antagonist, BMS182874; and ETB receptor agonist, IRL1620, was determined in the brain of neonatal rats undergoing morphine withdrawal by [35S]GTPgammaS binding assay. Morphine-induced maximal stimulation of G-protein in morphine withdrawal group (83.60%) was significantly higher compared to placebo control group (66.81%). EC50 value for ET-1-induced G-protein stimulation during morphine withdrawal (170.60 nM) was higher than control (62.5 nM). BMS182874, did not stimulate GTP binding in control but significantly increased maximal stimulation of G-proteins in morphine withdrawal (86.07%, EC50 = 31.25 nM). IRL1620-induced stimulation of G-proteins was similar in control and morphine withdrawal. The present findings indicate involvement of central ETA receptors in neonatal morphine withdrawal.

Neonatal animal models of opiate withdrawal

The symptoms of opiate withdrawal in infants are defined as neonatal abstinence syndrome (NAS). NAS is a significant cause of morbidity in term and preterm infants. Factors, such as polysubstance abuse, inadequate prenatal care, nutritional deprivation, and the biology of the developing central nervous system contribute to the challenge of evaluating and treating opiate-induced alterations in the newborn. Although research on the effects of opiates in neonatal animal models is limited, the data from adult animal models have greatly contributed to understanding and treating opiate tolerance, addiction, and withdrawal in adult humans. Yet the limited neonatal data that are available indicate that the mechanisms involved in these processes in the newborn differ from those in adult animals, and that neonatal models of opiate withdrawal are needed to understand and develop effective treatment regimens for NAS. In this review, the behavioral and neurochemical evidence from the literature is presented and suggests that mechanisms responsible for opiate tolerance, dependence, and withdrawal differ between adult and neonatal models. Also reviewed are studies that have used neonatal rodent models, the authors' preliminary data based on the use of neonatal rat and mouse models of opiate withdrawal, and other neonatal models that have been proposed for the study of neonatal opiate withdrawal.

Opiate physical dependence and N-methyl-D-aspartate receptors

The present review focused the involvement of N-methyl-D-aspartate (NMDA) receptors in morphine physical dependence. The increased levels of extracellular glutamate, NMDA receptor zeta subunit (NR1) mRNA, NMDA receptor epsilon 1 subunit (NR2A) protein, phosphorylated Ca(2+)/calmodulin kinase II (p-CaMKII) protein, c-fos mRNA, c-Fos protein, are observed in the specific brain areas of mice and/or rats showing signs of naloxone-precipitated withdrawal. In preclinical and clinical studies, a variety of NMDA receptor antagonists and pretreatment with an antisense oligonucleotide of the NR1 have been reported to inhibit the development, expression and/or maintenance of opiate physical dependence. In contrast to data obtained in adult animals, NMDA receptor antagonists are neither effective in blocking the development of opiate dependence nor the expression of opiate withdrawal in neonatal rats. In the NMDA receptor-deficient mice, the NR2A knockout mice show the marked loss of typical withdrawal abstinence behaviors precipitated by naloxone. The rescue of NR2A protein by electroporation into the nucleus accumbens of NR2A knockout mice reverses the loss of abstinence behaviors. The activation of CaMKII and increased expression of c-Fos protein in the brain of animals with naloxone-precipitated withdrawal syndrome are prevented by NMDA receptor antagonists, whereas the increased levels of extracellular glutamate are not prevented by them. These findings indicate that glutamatergic neurotransmission at the NMDA receptor site contributes to the development, expression and maintenance of opiate dependence, and suggest that NMDA receptor antagonists may be a useful adjunct in the treatment of opiate dependence.

Evidence that morphine tolerance may be regulated by endothelin in the neonatal rat

BACKGROUND

Opioids are widely used in the neonatal intensive care units for analgesia and sedation. Management of tolerance and withdrawal symptoms in neonates remains a major challenge.

OBJECTIVES

The present study investigates the involvement of a central endothelin (ET) mechanism in the development of tolerance to morphine in neonatal rats.

METHODS

Pregnant female rats were rendered tolerant to morphine and rat pups were delivered at term by cesarean section. The affinity (Kd) and density (Bmax) of ET receptors was determined by [125I]ET-1 binding in the brains of neonatal rats. Changes in G-protein stimulation were determined in placebo and morphine-tolerant neonatal rats by [35S]-guanosine-5'-o-(3-thio)triphosphate ([35S]GTPgammaS)-binding assay.

RESULTS

Morphine tolerance did not affect the characteristics (affinity and density) of the ET receptors in the neonatal rat brains. Morphine as well as ET-1 produced significantly lower (p < 0.05) maximal stimulation of [35S]GTPgammaS binding in morphine-tolerant neonatal rats compared to the placebo group. The ETA receptor antagonist, BMS182874, produced significantly higher stimulation of G proteins in the morphine-tolerant compared to the placebo group. The ETB receptor agonist, IRL1620, produced a similar effect in both placebo and morphine-tolerant rats.

CONCLUSIONS

This is the first report indicating the involvement of the G-protein-coupled ETA receptor in neonatal morphine tolerance.

Decreased venous return is neither sufficient nor necessary to elicit ultrasonic vocalizations of infant rat pups

It has been proposed that all ultrasonic vocalizations (USVs) in young rats are by-products of a cardiovascular response to decreased venous return, the abdominal compression reaction. To test the hypothesis, venous return was decreased in infant rats while USV and cardiovascular measures were monitored. Neither injection of the vasodilator sodium nitroprusside nor blood withdrawal from the superior vena cava or carotid artery elicited USV from pups in their home cage. Thus, decreased venous return by itself is not sufficient to elicit USV. To test whether venous return is a necessary mechanism for USV production, 5% dextrose in water or blood was infused intravenously into isolated pups that were producing USV. This artificial increase of venous return did not affect the rate of USV.

Social, thermal, and temporal influences on isolation-induced and maternally potentiated ultrasonic vocalizations of rat pups

Sensory and temporal factors have been demonstrated to be involved in the regulation of isolation-induced ultrasonic vocalizations (USV) of young rats. Sensory cues include thermal, olfactory, and tactile modalities. Temporal factors include the time spent in isolation. The goal of the present research was to examine the interaction of these factors in both isolation-induced and maternally potentiated USV. Maternal potentiation of USV occurs when a brief interaction with the dam, even a passive (anesthetized) dam, elicits an augmented vocal response to a subsequent isolation, with rates of USV in rat pups well above those emitted in standard isolation tests. We found that passive maternal potentiation of USV did occur under all conditions tested. Neither a 30-min prior isolation nor high ambient temperature prevented an increase in USV rate over the rate of the original isolation. After 30-min isolation at warm temperatures when the rate of USV had fallen to zero, the pups increased vocalization in the presence of the dam as well as in the subsequent isolation. Temporal and thermal factors also interacted significantly in regulating the level of the USV emitted by the pups during the first isolation, in the presence of the anesthetized dam, and during the second isolation.

Opiate withdrawal during development: are NMDA receptors indispensable?

Despite decades of research, the mechanisms that underlie opiate tolerance, dependence and withdrawal remain elusive. Evidence accumulated over the past ten years suggests that the NMDA receptor plays a central role in mediating the neuroplasticity induced by chronic opiate administration in adult animals. Yet, during ontogeny, the NMDA receptor complex undergoes qualitative developmental changes, which renders some of the basic assumptions for a role of the NMDA receptor in opiate withdrawal invalid in infants. Recent data indicate that NMDA receptor antagonists are not effective in blocking morphine tolerance, dependence and withdrawal in the neonatal rat. Roles for other glutamate receptor types (e.g. metabotropic glutamate receptors) have also been proposed recently. In this article, the latest evidence that characterizes the dynamic roles of glutamate receptors in these phenomena during ontogeny will be discussed.

Inhibition of morphine withdrawal by the NMDA receptor antagonist MK-801 in rat is age-dependent

This study investigated the effects of the NMDA receptor antagonist MK-801 on the development of morphine dependence in 7-, 14-, and 21-day-old rat pups. For 6.5 days, starting at 1, 8, or 15 days of age, rats were pretreated with MK-801 (0.03 or 0.1 mg/kg, bid) or saline; 15 min later, morphine sulfate (10 mg/kg) or saline was injected to induce opiate dependence. On the afternoon of the seventh day, pups were injected with MK-801 (0.1 mg/kg) or saline and 15 min later with naltrexone (1 mg/kg) to precipitate withdrawal. Pups were then placed in a warm chamber with the litter and their behavior scan-sampled every 15 sec for a total of 15 min. MK-801 failed to inhibit morphine withdrawal in the 7-day-old rat, but did attenuate the development of morphine dependence in both the 14- and 21-day-old rats. These results suggest that the NMDA receptor is not functionally active in opiate withdrawal until around the second to third week of postnatal life in the rat and that there exists a transition period for the NMDA receptor to play a role in the development of opiate dependence and withdrawal.

Opiate withdrawal in the fetal rat: a behavioral profile

Offspring of women exposed to opiate drugs such as heroin and methadone during pregnancy have a high incidence of morbidity and mortality. Infants also show opiate withdrawal. In this study, we examined the behavioral effects of precipitated withdrawal in morphine-dependent fetal rats at gestational day (GD) 20. The dam was implanted on GD 14 with a pellet containing 75.0 mg of morphine. On GD 20, the dam underwent chemoyelotomy at L1/L2. The uterine horns were externalized and four subject fetuses were selected for behavioral observation, two from each uterine horn. The fetus was then injected subcutaneously with either saline or naltrexone (1.0 mg/kg) and the behaviors of the fetus recorded every 15 sec for 20 min. The results show that naltrexone injected fetuses that had been chronically exposed to morphine demonstrated increased limb and mouth movements, face wiping, and body curls, and spent less time quiet as compared with control fetuses. These results indicate that a morphine withdrawal-like syndrome occurs in the fetal rat.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied 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 and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.