Opioid antagonists block the acquisition of ethanol-mediated conditioned tactile preference in infant rats

The Left-Right Side-Specific Neuroendocrine Signaling from Injured Brain: An Organizational Principle

A neurological dogma is that the contralateral effects of brain injury are set through crossed descending neural tracts. We have recently identified a novel topographic neuroendocrine system (T-NES) that operates via a humoral pathway and mediates the left-right side-specific effects of unilateral brain lesions. In rats with completely transected thoracic spinal cords, unilateral injury to the sensorimotor cortex produced contralateral hindlimb flexion, a proxy for neurological deficit. Here, we investigated in acute experiments whether T-NES consists of left and right counterparts and whether they differ in neural and molecular mechanisms. We demonstrated that left- and right-sided hormonal signaling is differentially blocked by the δ-, κ- and µ-opioid antagonists. Left and right neurohormonal signaling differed in targeting the afferent spinal mechanisms. Bilateral deafferentation of the lumbar spinal cord abolished the hormone-mediated effects of the left-brain injury but not the right-sided lesion. The sympathetic nervous system was ruled out as a brain-to-spinal cord-signaling pathway since hindlimb responses were induced in rats with cervical spinal cord transections that were rostral to the preganglionic sympathetic neurons. Analysis of gene-gene co-expression patterns identified the left- and right-side-specific gene co-expression networks that were coordinated via the humoral pathway across the hypothalamus and lumbar spinal cord. The coordination was ipsilateral and disrupted by brain injury. These findings suggest that T-NES is bipartite and that its left and right counterparts contribute to contralateral neurological deficits through distinct neural mechanisms, and may enable ipsilateral regulation of molecular and neural processes across distant neural areas along the neuraxis.

Left-Right Side-Specific Neuropeptide Mechanism Mediates Contralateral Responses to a Unilateral Brain Injury

Neuropeptides are implicated in control of lateralized processes in the brain. A unilateral brain injury (UBI) causes the contralesional sensorimotor deficits. To examine whether opioid neuropeptides mediate UBI induced asymmetric processes we compared effects of opioid antagonists on the contralesional and ipsilesional hindlimb responses to the left-sided and right-sided injury in rats. UBI induced hindlimb postural asymmetry (HL-PA) with the contralesional hindlimb flexion, and activated contralesional withdrawal reflex of extensor digitorum longus (EDL) evoked by electrical stimulation and recorded with EMG technique. No effects on the interossei (Int) and peroneaus longus (PL) were evident. The general opioid antagonist naloxone blocked postural effects, did not change EDL asymmetry while uncovered cryptic asymmetry in the PL and Int reflexes induced by UBI. Thus, the spinal opioid system may either mediate or counteract the injury effects. Strikingly, effects of selective opioid antagonists were the injury side-specific. The μ-antagonist β-funaltrexamine (FNA) and κ-antagonist nor-binaltorphimine (BNI) reduced postural asymmetry after the right but not left UBI. In contrast, the δ-antagonist naltrindole (NTI) inhibited HL-PA after the left but not right-side brain injury. The opioid gene expression and opioid peptides were lateralized in the lumbar spinal cord, and coordination between expression of the opioid and neuroplasticity-related genes was impaired by UBI that together may underlie the side-specific effects of the antagonists. We suggest that mirror-symmetric neural circuits that mediate effects of left and right brain injury on the contralesional hindlimbs are differentially controlled by the lateralized opioid system.

The offspring of alcohol-exposed sires exhibit heightened ethanol intake and behavioral alterations in the elevated plus maze

Research suggests that addictive traits are indeed heritable, but very few preclinical studies have explored transgenerational effects of paternal alcohol exposure. The present study addressed this gap in knowledge. We explored whether offspring of ethanol-exposed sires would be more likely to accept ethanol than descendants of water-exposed and control sires. We also investigated whether the second generation of ethanol-exposed descendants would accept ethanol more than controls and were more or less likely to experience anxiety-like behavior in behavioral assessments. We exposed male rats to repeated binge doses of alcohol (4 g/kg/day across 8 days), water, or left them untreated and mated them with untreated females. We then bred the offspring of these rats to test transgenerational effects of paternal alcohol exposure. We tested 14-day-old offspring from the first and second filial generation for their acceptance of ethanol and water, and measured anxiety-like behavior in 38-day-old, second-generation offspring using an elevated plus maze. The results indicate that offspring of ethanol-exposed sires increase ethanol acceptance in the first generation compared to untreated controls, whereas in the second-generation increased ethanol acceptance vs. these controls is seen in descendants of both ethanol- and vehicle-treated sires. At adolescence, the second generation of rats derived from alcohol-exposed sires exhibited significantly more time spent in the open arms and significantly more arm entries than any other group. The present study suggests that parental ethanol exposure is associated with lingering effects in the infant and adolescent offspring. The second filial generation was also found to be affected, albeit similarly by grandparental ethanol exposure or by the stress of the vehicle administration.

Ipsilesional versus contralesional postural deficits induced by unilateral brain trauma: a side reversal by opioid mechanism

Unilateral traumatic brain injury and stroke result in asymmetric postural and motor deficits including contralateral hemiplegia and hemiparesis. In animals, a localized unilateral brain injury recapitulates the human upper motor neuron syndrome in the formation of hindlimb postural asymmetry with contralesional limb flexion and the asymmetry of hindlimb nociceptive withdrawal reflexes. The current view is that these effects are developed due to aberrant activity of motor pathways that descend from the brain into the spinal cord. These pathways and their target spinal circuits may be regulated by local neurohormonal systems that may also mediate effects of brain injury. Here, we evaluate if a unilateral traumatic brain injury induces hindlimb postural asymmetry, a model of postural deficits, and if this asymmetry is spinally encoded and mediated by the endogenous opioid system in rats. A unilateral right-sided controlled cortical impact, a model of clinical focal traumatic brain injury was centred over the sensorimotor cortex and was observed to induce hindlimb postural asymmetry with contralateral limb flexion. The asymmetry persisted after complete spinal cord transection, implicating local neurocircuitry in the development of the deficits. Administration of the general opioid antagonist naloxone and μ-antagonist β-funaltrexamine blocked the formation of postural asymmetry. Surprisingly, κ-antagonists nor-binaltorphimine and LY2444296 did not affect the asymmetry magnitude but reversed the flexion side; instead of contralesional (left) hindlimb flexion the ipsilesional (right) limb was flexed. The postural effects of the right-side cortical injury were mimicked in animals with intact brain via intrathecal administration of the opioid κ-agonist (2)-(trans)-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidiny)-cyclohexyl]benzeneacetamide that induced hindlimb postural asymmetry with left limb flexion. The δ-antagonist naltrindole produced no effect on the contralesional (left) flexion but inhibited the formation of the ipsilesional (right) limb flexion in brain-injured rats that were treated with κ-antagonist. The effects of the antagonists were evident before and after spinal cord transection. We concluded that the focal traumatic brain injury-induced postural asymmetry was encoded at the spinal level, and was blocked or its side was reversed by administration of opioid antagonists. The findings suggest that the balance in activity of the mirror symmetric spinal neural circuits regulating contraction of the left and right hindlimb muscles is controlled by different subtypes of opioid receptors; and that this equilibrium is impaired after unilateral brain trauma through side-specific opioid mechanism.

Learning experiences comprising central ethanol exposure in rat neonates: Impact upon respiratory plasticity and the activity of brain catalase

Fetal ethanol exposure represents a risk factor for sudden infant death syndrome, and the respiratory effects of fetal ethanol exposure promote hypoxic ischemic consequences. This study analyzes central ethanol's effects upon breathing plasticity during an ontogenetic stage equivalent to the human third gestational trimester. Ethanol's unconditioned breathing effects and their intervention in learning processes were examined. Since central ethanol is primarily metabolized via the catalase system, we also examined the effects of early history with the drug upon this system. During postnatal days 3, 5, and 7 (PDs 3-7), pups were intracisternally administered with vehicle or ethanol (300 mg%). They were tested in a plethysmograph scented or not scented with ethanol odor. The state of intoxication attenuated the onset of apneas, a phenomenon that is suggestive of ethanol's anxiolytic effects given the state of arousal caused by the novel environment and the stress of ethanol administration. At PD9, pups were evaluated when sober under sequential air conditions (initial-normoxia, hypoxia, and recovery-normoxia), with or without the presence of ethanol odor. Initial apneic episodes increased when ethanol intoxication was previously associated with the odor. Pups then ingested ethanol, and brain catalase activity was determined. Pre-exposure to ethanol intoxication paired with the odor of the drug resulted in heightened enzymatic activity. Central ethanol exposure appears to exert antianxiety effects that attenuate apneic disruptions. However, during withdrawal, the cues associated with such effects elicit an opposite reaction. The activity of the catalase system was also dependent upon learning processes that involved the association of environmental stimuli and ethanol intoxication.

Maternal manipulation during late gestation (GDs 17-20) enhances ethanol consumption and promotes changes and opioid mRNA expression in infant rats

Fetal ethanol experience generates learning and memories capable to increase ethanol consummatory behaviors during infancy. Opioid system seems to be involved in mediating those alcohol-related behaviors. In this work, we proposed to study the impact of prenatal exposure to a moderate ethanol dose, upon ingestion of the drug and possible ethanol-induced molecular changes on opioid precursor peptides (POMC, Pro-enk and Pro-DYN) and receptors (MOR, DOR and KOR) mRNA expression, in hypothalamus. Pregnant rats received during gestational days (GDs) 17-20, a daily intragastric (i.g.) administration with 2g/kg ethanol or water. A third group of dams was left undisturbed during pregnancy (Unmanipulated group). Intake test was conducted at postnatal days (PDs) 14-15. Three groups of pups were performed: control (no intake test), water (vehicle) and 5% ethanol. At the end of intake test blood samples were taken to quantify blood ethanol concentrations (BECs) and hypothalamus sections were obtained to perform qRT-PRC assessment of opioid precursor peptides and receptors. The analysis of the consummatory responses (% of consumption) and pharmacokinetic profiles (BECs) suggested that maternal manipulation induced by i.g. intubations, during the last four days of gestation (whenever ethanol or water), are sufficient to induce infantile ethanol intake during infancy. Gene expression from the hypothalamus of unmanipulated group revealed that infantile ingestive experiences with ethanol can down-regulate expression of mRNA Pro-Dyn and up-regulate mRNA expression of MOR and KOR. Finally, MOR mRNA expression was attenuated by prenatal i.g. manipulation in pups exposed to 5% ethanol.

Effects of sex on ethanol conditioned place preference, activity and variability in C57BL/6J and DBA/2J mice

Previous studies of ethanol drinking in rodents have shown greater intake in females than in males, but the reasons behind this difference are unknown. To address one possible interpretation of the drinking difference, these studies tested the hypothesis that female and male mice differ in sensitivity to the rewarding effects of ethanol using the conditioned place preference (CPP) procedure. To increase the generalizability of the results, sex differences were examined in two inbred mouse strains known to differ in their sensitivity to ethanol reward: C57BL/6J (B6) and DBA/2J (D2). Mice were conditioned in an unbiased CPP procedure using either 1 or 2 g/kg ethanol. To detect possible differences in learning rate, they were tested once at the midpoint of conditioning and again after conditioning ended. As expected, CPP was stronger with 2 g/kg than with 1 g/kg, and D2 mice generally showed stronger CPP than B6 mice. However, there were no sex differences in the rate of CPP acquisition or in CPP magnitude, suggesting no sex difference in ethanol reward sensitivity as indexed by CPP. Nevertheless, there were sex differences in locomotor activity. B6 females were generally more active than B6 males during CPP acquisition whereas D2 females were slightly less active than D2 males during both CPP acquisition and preference testing. Unexpectedly, female mice showed more variability than males in the behavioral measures recorded in these studies, encouraging greater attention to variability in the design, analysis and interpretation of future studies of sex differences in mice.

Conditioned effects of ethanol on the immune system

Several studies indicate that the immune system can be subjected to classical conditioning. Acute ethanol intoxication significantly modulates several pro-inflammatory cytokines (e.g. interleukins-1 and 6 [IL-1β and IL-6, respectively] and tumor necrosis factor alpha [TNFα])) in several brain areas, including amygdala (AMG), paraventricular nucleus (PVN), and hippocampus (HPC). It is unknown, however, whether cues associated with ethanol can elicit conditioned alterations in cytokine expression. The present study analyzed, in male Sprague-Dawley rats, whether ethanol-induced changes in the central cytokine response may be amenable to conditioning. In Experiments 1 and 2, the rats were given one or two pairings between a distinctive odor (conditional stimulus, CS) and the post-absorptive effects of a high (3.0 or 4.0 g/kg, Experiments 1 and 2, respectively) ethanol dose. Neither of these experiments revealed conditioning of IL-6, IL-1β, or TNFα, as measured via mRNA levels. Yet, re-exposure to the lemon-odor CS in Experiment 1 significantly increased C-Fos levels in the PVN. In Experiment 3, the rats were given four pairings between an odor CS and a moderate ethanol dose (2.0 g/kg), delivered intraperitoneally (i.p.) or intragastrically (i.g.). Re-exposure to the odor CS significantly increased IL-6 levels in HPC and AMG, an effect only evident in paired rats administered ethanol i.p. Overall, this study suggests that ethanol exposure can regulate the levels of IL-6 at HPC and AMG via classical conditioning mechanisms. These ethanol-induced, conditioned alterations in cytokine levels may ultimately affect the intake and motivational effects of ethanol. Impact statement This study examines, across three experiments, whether odor cues associated with ethanol exposure can condition changes in cytokine expression. The analysis of ethanol-induced conditioning of immune responses is a novel niche that can help understand the transition from social drinking to alcohol abuse and dependence. Ethanol-induced conditioning of the immune system could likely exacerbate neuroinflammation and drug-related toxicity, which in turn may facilitate further engagement in ethanol intake. The main new finding of the present study was that, after four pairings of ethanol's unconditioned effects and a distinctive odor, the latter CS increased IL-6 levels in HPC and AMG. This suggests that ethanol's effects upon IL-6 in HPC and AMG may come under conditioned control, particularly after repeated pairings between distinctive odor cues and ethanol's effects. This article advances our knowledge of conditioned increases in cytokine responses, which should help understand the mechanisms underlying alcohol use, abuse, and relapse.

Behavioral sensitization to ethanol: Neural basis and factors that influence its acquisition and expression

Ethanol-induced behavioral sensitization (EBS) was first described in 1980, approximately 10 years after the phenomenon was described for psychostimulants. Ethanol acts on γ-aminobutyric acid (GABA) and glutamate receptors as an allosteric agonist and antagonist, respectively, but it also affects many other molecular targets. The multiplicity of factors involved in the behavioral and neurochemical effects of ethanol and the ensuing complexity may explain much of the apparent disparate results, found across different labs, regarding ethanol-induced behavioral sensitization. Although the mesocorticolimbic dopamine system plays an important role in EBS, we provide evidence of the involvement of other neurotransmitter systems, mainly the glutamatergic, GABAergic, and opioidergic systems. This review also analyses the neural underpinnings (e.g., induction of cellular transcription factors such as cyclic adenosine monophosphate response element binding protein and growth factors, such as the brain-derived neurotrophic factor) and other factors that influence the phenomenon, including age, sex, dose, and protocols of drug administration. One of the reasons that make EBS an attractive phenomenon is the assumption, firmly based on empirical evidence, that EBS and addiction-related processes have common molecular and neural basis. Therefore, EBS has been used as a model of addiction processes. We discuss the association between different measures of ethanol-induced reward and EBS. Parallels between the pharmacological basis of EBS and acute motor effects of ethanol are also discussed.

Brief prenatal ethanol exposure alters behavioral sensitivity to the kappa opioid receptor agonist (U62,066E) and antagonist (Nor-BNI) and reduces kappa opioid receptor expression

BACKGROUND

Approximately 10 to 15% of women consume alcohol (ethanol [EtOH]) during pregnancy in the United States. Even low amounts of EtOH consumption during pregnancy can elicit long-term consequences. Prenatal experience with as few as 3 drinks has been associated with increase problem drinking in adulthood. Such effects are corroborated in rodents; however, the underlying neural adaptations contributing to this effect are not clear. In the current set of experiments, we investigated whether changes in EtOH responding following prenatal EtOH exposure involved kappa opioid receptor activation and expression.

METHODS

Sprague-Dawley rats were prenatally exposed to low levels of alcohol (1.0 g/kg) during late gestation (gestational days 17 to 20 [GD17-20]) via intragastric intubation of pregnant dams. Following birth, EtOH intake, kappa- and mu-opioid-induced place conditioning, and kappa opioid receptor expression in mesolimbic brain regions were assessed in infant rats (postnatal days 14 to 15 [PD14-15]) that were offspring of dams given EtOH, vehicle, or untreated, during pregnancy.

RESULTS

Animals exposed to prenatal alcohol drank more alcohol later in life and exhibited significant changes in the kappa opioid system. While control subjects found kappa opioid activation aversive, animals exposed to EtOH prenatally exhibited either no aversion or appetitive responding. Further analysis revealed that synaptosomal kappa opioid receptor expression was significantly decreased in brain areas implicated in responding to EtOH.

CONCLUSIONS

Overall, these data suggest that prenatal EtOH affects kappa opioid function and expression and that these changes may be involved in increased drinking later in life.

Relationship between ethanol-induced activity and anxiolysis in the open field, elevated plus maze, light-dark box, and ethanol intake in adolescent rats

It is yet unclear if ethanol-induced motor stimulation in the open field (OF) merely reflects psychomotor stimulating effects of the drug or if this stimulation is driven or modulated by ethanol's antianxiety properties. In the present study, adolescent rats were administered with different ethanol doses or remained untreated. They were sequentially assessed in the OF, elevated plus maze (EPM), and light-dark box (LDB) and then assessed for ethanol intake. The aims were to assess the relationship between measures of ethanol-induced activity and anxiolysis, analyze ethanol intake as a function of prior ethanol exposure, and associate behavioral responsiveness in these apparatus with ethanol intake during adolescence. The results suggested that the enhanced exploration of the OF observed after 2.5 and 3.25 g/kg ethanol reflected a motor-stimulating effect that appeared to be relatively independent of anxiolysis. The 1.25 g/kg dose induced motor stimulation in the OF and anti-anxiety effects in the EPM, but these effects were relatively independent. The 0.5 g/kg ethanol dose exerted significant anxiolytic effects in the EPM in the absence of stimulating effects in the OF. A multivariate regression analysis indicated that adolescents with a higher frequency of rearing behavior in the OF, higher percentage of open arm entries in the EPM, and lower propensity to enter the central area of the OF exhibited greater ethanol intake. These results indicate that the OF is a valid procedure for the measurement of ethanol-induced stimulation, and provide information toward characterizing subpopulations of adolescents at risk for initiating alcohol drinking.

Central effects of ethanol interact with endogenous mu-opioid activity to control isolation-induced analgesia in maternally separated infant rats

Endogenous opioid activity plays an important role in ethanol consumption and reinforcement in infant rats. Opioid systems are also involved in mediation and regulation of stress responses. Social isolation is a stressful experience for preweanling rats and changes the effects of ethanol through opioid-dependent mechanisms. The present study assessed effects of intracisternal (i.c.) administration of a selective mu-opioid antagonist (CTOP) and i.p. administration of a nonspecific opioid antagonist (naloxone) on voluntary intake and behavior in socially isolated 12-day-old (P12) pups treated with 0.5 g/kg ethanol. Voluntary intake of 0.1% saccharin or water, locomotion, rearing activity, paw licking and grooming were assessed during short-term isolation from littermates (STSI; 8-min duration). Thermal nociceptive reactivity was measured before and after this intake test, with normalized differences between pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). Results indicated several effects of social isolation and ethanol mediated through the mu-opioid system. Effects of low dose ethanol (0.5 g/kg) and voluntary consumption of saccharin interacted with endogenous mu-opioid activity associated with STSI. Blockade of mu-opioid receptors on saccharin consumption and paw licking-grooming affected intoxicated animals. Low dose ethanol and ingestion of saccharin blunted effects of CTOP on rearing behavior and nociceptive reactivity. Central injections of CTOP stimulated paw licking and grooming dependent on ethanol dose and type of fluid ingested. Ethanol selectively increased saccharin intake during STSI in females, naloxone and CTOP blocked ethanol-mediated enhancement of saccharin intake. We suggest that enhancement of saccharin intake by ethanol during STSI is the product of synergism between isolation-induced mu-opioid activity that increases the pup's sensitivity to appetitive taste stimulation and the anxiolytic effects of 0.5 g/kg ethanol that decreases behaviors otherwise competing with independent ingestive activity.

Age-dependent effects of stress on ethanol-induced motor activity in rats

RATIONALE

It is important to study age-related differences that may put adolescents at risk for alcohol-related problems. Adolescents seem less sensitive to the aversive effects of ethanol than adults. Less is known of appetitive effects of ethanol and stress modulation of these effects.

OBJECTIVES

This study aims to describe the effects of acute social or restraint stress on ethanol-precipitated locomotor activity (LMA), in adolescent and adult rats. Effects of activation of the kappa system on ethanol-induced LMA were also evaluated.

METHODS

Adolescent or adult rats were restrained for 90 min, exposed to social deprivation stress for 90 or 180 min or administered with the kappa agonist U62,066E before being given ethanol, and assessed for LMA.

RESULTS

Adolescents were significantly more sensitive to the stimulating, and less sensitive to the sedative, effects of ethanol than adults. Basal locomotion was significantly increased by social deprivation stress in adult, but not in adolescent, rats. U62,066E significantly reduced basal and ethanol-induced locomotion in the adolescents. Corticosterone and progesterone levels were significantly higher in adolescents than in adults.

CONCLUSIONS

Adolescents exhibit greater sensitivity to ethanol-induced LMA and reduced sensitivity to ethanol-induced motor sedation than adult rats. Ethanol's effects on motor activity were not affected by acute stress. Unlike adults, adolescents were insensitive to acute restraint and social deprivation stress but exhibited motor depression after activation of the endogenous kappa opioid receptor system.

Prenatal ethanol exposure increases ethanol intake and reduces c-Fos expression in infralimbic cortex of adolescent rats

Prenatal ethanol exposure significantly increases later predisposition for alcohol intake, but the mechanisms associated with this phenomenon remain hypothetical. This study analyzed (Experiment 1) ethanol intake in adolescent inbred WKAH/Hok Wistar rats prenatally exposed to ethanol (2.0g/kg) or vehicle, on gestational days 17-20. Subsequent Experiments (2, 3 and 4) tested several variables likely to underlie the effect of gestational ethanol on adolescent ethanol preference, including ethanol-induced locomotor activation (LMA), ethanol-induced emission of ultrasonic vocalizations (USVs) after exposure to a rough exteroceptive stimulus, and induction of the immediate early gene C-fos in brain areas associated with processing of reward stimuli and with the retrieval and extinction of associative learning. Prenatal ethanol induced a two-fold increase in ethanol intake. Adolescents exhibited significant ethanol-induced LMA, emitted more aversive than appetitive USVs, and postnatal ethanol administration significantly exacerbated the emission of USVs. These effects, however, were not affected by prenatal ethanol. Adolescents prenatally exposed to ethanol as fetuses exhibited reduced neural activity in infralimbic cortex (but not in prelimbic cortex or nucleus accumbens core or shell), an area that has been implicated in the extinction of drug-mediated associative memories. Ethanol metabolism was not affected by prenatal ethanol. Late gestational exposure to ethanol significantly heightened drinking in the adolescent offspring of an inbred rat strain. Ethanol-induced LMA and USVs were not associated with differential ethanol intake due to prenatal ethanol exposure. Prenatal ethanol, however, altered basal neural activity in the infralimbic prefrontal cortex. Future studies should analyze the functionality of medial prefrontal cortex after prenatal ethanol and its potential association with predisposition for heightened ethanol intake.

μ-Opioid blockade reduces ethanol effects on intake and behavior of the infant rat during short-term but not long-term social isolation

Numerous findings in adult and infant rats have shown that the endogenous opioid system is involved in control of ethanol consumption and its reinforcing effects. Opioid systems are also involved in reactivity to social isolation with several factors (age, duration, and type of isolation) affecting this modulation. The present study investigated the effects of a selective mu-opioid antagonist CTOP (0, 0.1, 0.5mg/kg), ethanol (0, 0.5 g/kg), and the interaction of the two drugs on the behavioral consequences of two types of social isolation given to preweanling rats: 1) short-term social isolation from littermates (STSI, duration 8 min) and 2) relatively long-term (5h) isolation (LTSI) from the dam and littermates. Voluntary intake of saccharin, locomotion, rearing activity, paw licking, and grooming were assessed during an 8-min. intake test. Thermal nociceptive reactivity was also measured before and after the testing session with normalized differences in pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). The results indicate that pharmacological blockade of mu-opioid receptors by CTOP substantially attenuated ethanol's anxiolytic effects on the developing rat's reactions to social isolation. Some of these stress-attenuating effects of CTOP were observed only in animals exposed to short-term isolation (STSI) but not in pups isolated for 5h (LTSI). Ethanol selectively increased saccharin intake during STSI in females and CTOP blocked this effect. Ethanol decreased the magnitude of analgesia associated with STSI but had no effect on pain reactivity during LTSI. CTOP by itself did not affect IIA or saccharin intake in sober animals. The findings of the present experiments indicate that the anxiolytic effects of 0.5 g/kg ethanol on pups exposed to STSI are modulated by endogenous opioid activity.

Role of mu, delta and kappa opioid receptors in ethanol-reinforced operant responding in infant rats

We recently observed that naloxone, a non-specific opioid antagonist, attenuated operant responding to ethanol in infant rats. Through the use of an operant conditioning technique, we aimed to analyze the specific participation of mu, delta, and kappa opioid receptors on ethanol reinforcement during the second postnatal week. In Experiment 1, infant rats (PDs 14-17) were trained to obtain 5, 7.5, 10, or 15% ethanol, by operant nose-poking. Experiment 2 tested blood ethanol levels (BELs) attained by operant behavior. In Experiment 3, at PDs 16-18, rats received CTOP (mu antagonist: 0.1 or 1.0 mg/kg), naltrindole (delta antagonist: 1.0 or 5.0 mg/kg) or saline before training. In Experiment 4, rats received nor-binaltorphimine (kappa antagonist: 10.0 or 30.0 mg/kg, a single injection after completion of PD 15 operant training), spiradoline mesylate (kappa agonist: 1.0 or 5.0 mg/kg; at PDs 16-18) or saline (PDs 16-18), before the conditioning. Experiments 5 and 6 assessed possible side effects of opioid drugs in locomotor activity (LA) and conditioned taste aversion (CTA). Ethanol at 7.5 and 10% promoted the highest levels of operant responding. BELs were 12-15 mg/dl. In Experiment 3 naltrindole (dose-response effect) and CTOP (the lowest dose) were effective in decreasing operant responding. Nor-binaltorphimine at 10.0 mg/kg and spiradoline at 5.0 mg/kg also blocked ethanol responding. The effects of opioid drugs on ethanol reinforcement cannot be explained by effects on LA or CTA. Even though particular aspects of each opioid receptor require further testing, a fully functional opioid system seems to be necessary for ethanol reinforcement, during early ontogeny.

Ethanol-induced locomotor activity in adolescent rats and the relationship with ethanol-induced conditioned place preference and conditioned taste aversion

Adolescent rats exhibit ethanol-induced locomotor activity (LMA), which is considered an index of ethanol's motivational properties likely to predict ethanol self-administration, but few studies have reported or correlated ethanol-induced LMA with conditioned place preference (CPP) by ethanol at this age. The present study assessed age-related differences in ethanol's motor stimulating effects and analyzed the association between ethanol-induced LMA and conventional measures of ethanol-induced reinforcement. Experiment 1 compared ethanol-induced LMA in adolescent and adult rats. Subsequent experiments analyzed ethanol-induced CPP and conditioned taste aversion (CTA) in adolescent rats evaluated for ethanol-induced LMA. Adolescent rats exhibit a robust LMA after high-dose ethanol. Ethanol-induced LMA was fairly similar across adolescents and adults. As expected, adolescents were sensitive to ethanol's aversive reinforcement, but they also exhibited CPP. These measures of ethanol reinforcement, however, were not related to ethanol-induced LMA. Spontaneous LMA in an open field was, however, negatively associated with ethanol-induced CTA.

Ontogenetic differences in ethanol's motivational properties during infancy

Pairing a conditioned stimulus (CS) with ethanol generally produces aversion for that CS in adult rodents. However, infant rats (PD1-PD3) exposed to ethanol demonstrate appetitive reinforcement to ethanol (Nizhnikov, Varlinskaya, Petrov, & Spear, 2006; Petrov, Varlinskaya, & Spear, 2003). This sensitivity to the appetitive properties of ethanol during infancy may be transient, as during the second postnatal week rat pups tend to exhibit conditioned aversions to flavors paired with ethanol. The present study examined changes in the motivation properties of ethanol through ontogeny and the neurobiology underlying these changes. Rat pups were exposed to a taste conditioning procedure on PD4 or PD12. Rat pups were intraorally infused with 2.5% of their body weight of saccharin solution (0.1%) and immediately after injected intraperitoneolly (i.p.) with one of six doses of ethanol (0.0-2.0 g/kg). A day later pups were given saccharine infusions and percent body weight gain was used as an index of ethanol's reinforcing effects. PD4 pups expressed appetitive reinforcement to ethanol, as indicated by greater saccharin intake, as compared to control counterparts and to the older PD12 pups. Subsequent experiments revealed that PD4 pups were less sensitive to the aversive properties of the drug than PD12 pups. The older pups found high doses of ethanol aversive while PD4 rat pups did not condition aversions to this dose of ethanol after a single trial. A similar pattern of results was observed between the low doses of ethanol and the highest doses of a kappa opioid agonist. The PD12 animals did not condition to the kappa opioid agonist, while the younger rats expressed an appetitive response. These results illustrate an ontogenetic change in the motivational properties of ethanol, with sensitivity to its appetitive properties declining and responsiveness to the aversive properties increasing with age during early infancy.

Chronic tolerance to the locomotor stimulating effect of ethanol in preweanling rats as a function of social stress

During early stages of development rats are highly sensitive to the locomotor stimulating effect of relatively high ethanol doses, an effect strongly modulated by social stress. This ethanol effect can be modulated by pharmacological treatments that also can attenuate the development of ethanol-induced locomotor sensitization in mice. By the end of the preweanling period the mechanisms underlying sensitization induced by psychostimulants are functional. The aim of the present study was to analyze the locomotor response to ethanol in preweanling rats as a function of repeated exposure to the drug under two different social conditions. Subjects were treated with ethanol between postnatal days 15 and 18 after being isolated for four hours (Experiment 1a) or simply residing in their home-cage (Experiment 1b). After two days of withdrawal locomotor response to ethanol was assessed in both social conditions. In Experiment 2 naïve rats were tested in terms of ethanol-induced activation of the locomotor response in both social conditions. Results from the present study showed no evidence of locomotor sensitization in preweanling rats in any of the social conditions. Instead we observed behavioral tolerance to the stimulating effect of ethanol in animals trained in the home-cage condition, in which subjects trained with ethanol showed sedation in response to ethanol at testing. Overall these results contribute to the understanding of the sensitivity of rats to the acute and chronic locomotor response to ethanol in an ontogenetic period characterized by high sensitivity to ethanol.

Odor-avoidance or odor-preference induced by amphetamine in the infant rat depending on the dose and testing modality

By the second postnatal week of life infant rats can acquire taste avoidance induced by amphetamine. Psychostimulant drugs supports appetitive and aversive learning in adult rats. Their appetitive effects are more likely to become associated with contextual cues, while the aversive ones have been consistently found in taste aversion learning. To explain this paradox, it has been proposed that rats would avoid a taste that predicts a change in their homeostasis because this species cannot vomit. In this study we assessed the motivational properties of amphetamine in preweanling rats by means of an odor conditioning preparation, which enables the analysis of the hedonic value of the memory by means of a consumption test or in terms of locomotor approach to the odor. Results indicate that regardless of the amphetamine dose (1 or 5 mg/kg), when animals were evaluated in the intake test, subjects avoided the odor. However, the outcome in the locomotor avoidance test varied as a function of the amphetamine dose. Rats trained with the low dose (1 mg/kg) showed odor preference, while the highest amphetamine dose (5 mg/kg) induced odor avoidance. When LiCl was employed as an unconditioned stimulus (US), rats showed avoidance in the intake and locomotor activity tests. These data indicate that amphetamine, like other drugs of abuse, supports appetitive conditioning in preweanling rats. Interestingly, infant rats expressed conditioned odor avoidance or preference depending on the dose and testing modality. Results were discussed considering current theories of avoidance learning induced by rewarding drugs.

The role of social isolation in ethanol effects on the preweanling rat

The present experiments investigated the effects of acute ethanol exposure on voluntary intake of 0.1% saccharin or water as well as behavioral and nociceptive reactivity in 12-day-old (P12) rats exposed to differing levels of isolation. The effects of ethanol emerged only during short-term social isolation (STSI) with different patterns observed in males and females and in pups exposed to saccharin or water. The 0.5g/kg ethanol dose selectively increased saccharin intake in females, decreased rearing activity in males and attenuated isolation-induced analgesia (IIA) in all water-exposed pups. Ingestion of saccharin decreased IIA, and the 0.5g/kg ethanol dose further reduced IIA. The 1.0g/kg ethanol dose, administered either intragastrically or intraparentionally, also decreased IIA in P12 females, but not in P9 pups. A significant correlation between voluntary saccharin intake and baseline nociceptive reactivity was revealed in saline injected animals, saccharin intake was inversely correlated with behavioral activation and latency of reaction to noxious heat after 0.5g/kg ethanol in females. The 0.5g/kg ethanol dose did not affect plasma corticosterone (CORT) measured 5h after maternal separation or 20min after ethanol injection. Female pups CORT level was inversely correlated with magnitude of IIA that accompanied the first episode of STSI (pretest isolation) 1.5-2h before CORT measurement. The present findings suggest that the anxiolytic properties of ethanol are responsible for enhancement of saccharin intake during STSI. Furthermore, differential reactivity of P12 males and females to STSI plays an important role in ethanol effects observed at this age.

Early role of the κ opioid receptor in ethanol-induced reinforcement

Effects of early ethanol exposure on later ethanol intake emphasize the importance of understanding the neurobiology of ethanol-induced reinforcement early in life. Infant rats exhibit ethanol-induced appetitive conditioning and ethanol-induced locomotor activation, which have been linked in theory and may have mechanisms in common. The appetitive effects of ethanol are significantly modulated by μ and δ opioid receptors, whereas μ but not δ receptors are involved in the motor stimulant effects of ethanol during early development. The involvement of the κ opioid receptor (KOR) system in the motivational effects of ethanol has been much less explored. The present study assessed, in preweanling (infant) rats, the modulatory role of the KOR system in several paradigms sensitive to ethanol-induced reinforcement. Kappa opioid activation and blockade were examined in second-order conditioned place preference with varied timing before conditioning and with varied ethanol doses. The role of KOR on ethanol-induced locomotion and ethanol-induced taste conditioning was also explored. The experiments were based on the assumption that ethanol concurrently induces appetitive and aversive effects and that the latter may be mediated by activation of kappa receptors. The main result was that blockade of kappa function facilitated the expression of appetitive ethanol reinforcement in terms of tactile and taste conditioning. The effects of kappa activation on ethanol conditioning seemed to be independent from ethanol's stimulant effects. Kappa opioid activation potentiated the motor depressing effects of ethanol but enhanced motor activity in control subjects. Overall, the results support the hypothesis that a reduced function of the KOR system in nondependent subjects should attenuate the aversive consequences of ethanol.

Naloxone attenuation of ethanol-reinforced operant responding in infant rats in a re-exposure paradigm

RATIONALE

Early ethanol exposure promotes ethanol reinforcement, mediated perhaps by ethanol's motivational effects. The opioid system mediates ethanol reinforcement, at least in part.

OBJECTIVES

Modulation of consummatory and seeking behaviors by the opioid system was tested in terms of ethanol or sucrose operant self-administration.

METHODS

Wistar-derived infant rats were tested in an operant conditioning task. (1) Infants were trained on postnatal days (PDs) 14-17 to obtain 5% sucrose and 3.75% ethanol or water, and evaluated in an extinction session at PD 18. (2) Ethanol (3.75%) was used as reinforcer. At PDs 16-17, 6 h before operant task, pups were re-exposed to ethanol after naloxone injection (0 or 1 mg/kg). (3) Sucrose (5%) acted as reinforcer. Pups were re-exposed to sucrose after naloxone injection. (4) A PD 18 re-exposure trial in which pups were injected with naloxone and re-exposed to ethanol was added.

RESULTS

Sucrose and ethanol promoted higher levels of operant responding than water during training and extinction. Re-exposure to ethanol preceded by naloxone decreased nose-poking. A similar profile was observed towards sucrose. No seeking behavior was observed in pups re-exposed to ethanol following naloxone injection during PDs 16-18.

CONCLUSIONS

Self-administration of ethanol was established in terms of operant responding in preweanling rats with no previous exposure to the drug. Pairing of naloxone with ethanol, at a point separate in time from operant responding, reduced ethanol reinforcement. This indicated participation of the opioid system in ethanol reinforcement. This effect seems not to be unique to ethanol but also is observable when sucrose acts as reinforcer.

Early maternal separation affects ethanol-induced conditioning in a nor-BNI insensitive manner, but does not alter ethanol-induced locomotor activity

Early environmental stress significantly affects the development of offspring. This stress has been modeled in rats through the maternal separation (MS) paradigm, which alters the functioning of the HPA axis and can enhance ethanol intake at adulthood. Infant rats are sensitive to ethanol's reinforcing effects, which modulate ethanol seeking and intake. Little is known about the impact of MS on sensitivity to ethanol's appetitive and aversive effects during infancy. The present study assessed ethanol-induced conditioned place preference established through second-order conditioning (SOC), spontaneous or ethanol-induced locomotor activity and ethanol intake in preweanling rats that experienced normal animal facility rearing (AFR) or daily episodes of maternal separation (MS) during postnatal days 1-13 (PDs 1-13). Low-ethanol dose (0.5 g/kg) induced appetitive conditioned place preference (via SOC) in control rats given conventional rearing but not in rats given maternal separation in early infancy, whereas 2.0 g/kg ethanol induced aversive conditioned place preference in the former but not the latter. The administration of a kappa antagonist at PD 1 or immediately before testing did not alter ethanol-induced reinforcement. High (i.e., 2.5 and 2.0 g/kg) but not low (i.e., 0.5 g/kg) ethanol dose induced reliable motor stimulation, which was independent of early maternal separation. Ethanol intake and blood alcohol levels during conditioning were unaffected by rearing conditions. Pups given early maternal separation had lower body weights than controls and showed an altered pattern of exploration when placed in an open field. These results indicate that, when assessed in infant rats, earlier maternal separation alters the balance between the appetitive and aversive motivational effects of ethanol but has no effect on the motor activating effects of the drug.

Ethanol-mediated appetitive conditioning in infant rats, but not corticosterone release, is dependent on route of ethanol administration

A recent study found appetitive reinforcement in infant rats given 1.0 but not 2.0 g/kg ethanol and only when ethanol was delivered intragastrically (i.g., but not if intraperitoneally, i.p.; Nizhnikov, Pautassi, Truxell, & Spear [2009] Alcohol 43, 347-358). Corticosterone release could modulate ethanol's motivational effects. The goal of this study was to replicate the differential capability of i.g. vs. i.p. ethanol to induce conditioning and to find hormonal correlates underlying this phenomenon. Experiment 1 confirmed that 1.0 g/kg ethanol induced conditioned preference in infant rats when given i.g. but not i.p. In Experiment 2 corticosterone was assessed at 20, 40, 60, or 120 min after ethanol (0.0, 0.5, 1.0, and 2.0 g/kg, i.g. or i.p.). Route of administration failed to alter corticosterone release. The 2.0 g/kg, but not 0.5 or 1.0 g/kg, ethanol dose evoked heightened corticosterone release. The results confirm the differing motivational effects associated with i.g. and i.p. ethanol. These effects do not seem to be related to differential corticosterone responsiveness.

Implicit versus explicit associative learning and experimentally induced placebo hypoalgesia

The present study examined whether 1) placebo hypoalgesia can be generated through implicit associative learning (ie, conditioning in the absence of conscious awareness) and 2) the magnitude of placebo hypoalgesia changes when expectations about pain are made explicit. The temperature of heat pain stimuli was surreptitiously lowered during conditioning trials for the placebo cream and the magnitude of the placebo effect was assessed during a subsequent set of trials when the temperature was the same for both placebo and control conditions. To assess whether placebo hypoalgesia could be generated from an implicit tactile stimulus, a 2 × 2 design was used with direction of cream application as one factor and verbal information about which cream was being applied as the second factor. A significant placebo effect was observed when participants received verbal information about which cream was being applied but not following implicit conditioning alone. However, 87.5% of those who showed a placebo response as the result of implicit conditioning were able to accurately guess the order of cream application during the final trial, despite a lack of awareness about the sensory manipulation and low confidence in their ratings, suggesting implicit learning in some participants. In summary, implicit associative learning was evident in some participants but it was not sufficient to produce a placebo effect suggesting some level of explicit expectation or cognitive mediation may be necessary. Notably, the placebo response was abolished when expectations were made explicit, suggesting a delicate interplay between attention and expectation.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

High ethanol dose during early adolescence induces locomotor activation and increases subsequent ethanol intake during late adolescence

Adolescent initiation of ethanol consumption is associated with subsequent heightened probability of ethanol use disorders. The present study examined the relationship between motivational sensitivity to ethanol initiation in adolescent rats and later ethanol intake. Experiment 1 determined that ethanol induces locomotor activation shortly after administration but not if tested at a later post-administration interval. In Experiment 2, adolescent rats were assessed for ethanol-induced locomotor activation on postnatal Day 28. These animals were then evaluated for ethanol-mediated conditioned taste aversion and underwent a 16-day-long ethanol intake protocol. Ethanol-mediated aversive effects were unrelated to ethanol locomotor stimulation or subsequent ethanol consumption patterns. Ethanol intake during late adolescence was greatest in animals initiated to ethanol earliest at postnatal Day 28. Females that were more sensitive to ethanol's locomotor-activating effects showed a transient increase in ethanol self-administration. Blood ethanol concentrations during initiation were not related to ethanol-induced locomotor activation. Adolescent rats appeared sensitive to the locomotor-stimulatory effects of ethanol. Even brief ethanol exposure during adolescence may promote later ethanol intake.

An acetaldehyde-sequestering agent inhibits appetitive reinforcement and behavioral stimulation induced by ethanol in preweanling rats

Ethanol's motivational consequences have been related to the actions of acetaldehyde, a metabolic product of ethanol oxidation. The present study assessed the role of acetaldehyde in the motivational effects of ethanol on preweanling rats. In Experiment 1 pups (postnatal days 13-14, PD 13-14) were given systemic administration of D-penicillamine (DP, a drug that sequesters acetaldehyde: 0, 25, 50 or 75 mg/kg) before pairings of 1.0 g/kg ethanol and a rough surface (sandpaper, conditioned stimulus, CS). At test, pups given sandpaper-ethanol pairings exhibited greater preference for the CS than unpaired controls, but this preference was not expressed by pups given DP. Pre-training administration of 25 or 50 mg/kg DP completely blocked the expression of ethanol-mediated appetitive conditioning. D-penicillamine did not alter blood ethanol levels. Subsequent experiments revealed that ethanol-induced activation was blocked by central (intra-cisterna magna injections, volume: 1 μl, dose: 0 or 75 μg) but not systemic treatment with DP (0, 25, 50 or 75 mg/kg; ip). These results indicate that: (a) preweanling rats are sensitive to the reinforcing effect of ethanol, and (b) that this effect is associated with the motor activating effect of the drug. These effects seem to be mediated by the first metabolite of ethanol, acetaldehyde.

Naloxone blocks ethanol-mediated appetitive conditioning and locomotor activation in adolescent rats

Age-related differences in ethanol sensitivity could put adolescents at risk for developing alcohol-related problems. Little information exists, however, about adolescent sensitivity to ethanol's appetitive effects and the neurobiological mechanisms underlying ethanol reinforcement during this developmental stage. The present study assessed the role of the opioid system in adolescent rats in an appetitive second-order schedule of ethanol reinforcement and ethanol-induced locomotor stimulation. On postnatal day 32 (PD32), animals were pretreated with the general opioid antagonist naloxone (0.0, 0.75, 1.50, or 2.5 mg/kg) and then given pairings of ethanol (0.0 or 2.0 g/kg, intragastrically) with intraoral pulses of water (conditioned stimulus 1 [CS₁], first-order conditioning phase). CS₁ delivery occurred 30-45 min after ethanol administration when the effect of ethanol was assumed to be appetitive. On PD33, adolescents were exposed to CS₁ (second-order conditioning phase) while in a chamber featuring distinctive exteroceptive cues (CS₂). Preference for CS₂ was then tested. Adolescents given CS₁-ethanol pairings exhibited greater preference for CS₂ than controls, indicating ethanol-mediated reinforcement, but only when not pretreated with naloxone. Blood alcohol levels during conditioning were not altered by naloxone. Experiment 2 revealed that ethanol-induced locomotor activation soon after administration, and naloxone dose-dependently suppressed this stimulating effect. The present study indicates that adolescent rats are sensitive to ethanol's reinforcing and locomotor-stimulating effects. Both effects of ethanol appear to be mediated by endogenous opioid system activation.

Genetic and environmental influences on ethanol consumption: perspectives from preclinical research

BACKGROUND

Alcohol use disorders (abuse and dependence, AUD) are multifactorial phenomena, depending on the interplay of environmental and genetic variables.

METHOD

This review describes current developments in animal research that may help (a) develop gene therapies for the treatment of alcoholism, (b) understand the permissive role of stress on ethanol intake, and (c) elucidate why exposure to ethanol early in life is associated with a greater risk of AUD.

RESULTS

The polymorphisms found in liver alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) affect the elimination of ethanol and the susceptibility to ethanol intake. A highly active ADH protects against alcoholism, an effect related to a presteady state burst in arterial acetaldehyde. Social stressors, such as repeated early maternal separation or social defeat, exert a permissive effect on ethanol intake, perhaps by altering the normal development of the hypothalamic-pituitary-adrenal axis. Ethanol exposure during gestation, infancy, or adolescence increases the likelihood of AUD later in life. Early perception of ethanol's positive and negative (anti-anxiety) reinforcing effects may play a role in this phenomenon.

CONCLUSIONS

The review underscores the advantages of using preclinical animal models of AUD and highlights points of intersection between the topics to help design a more integrated approach for the study of alcohol-related problems.

Differential role of mu, delta and kappa opioid receptors in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

The opioid system modulates ethanol intake and reinforcement in adult and preweanling rodents. While adult heterogeneous rats normally do not show ethanol-mediated locomotor stimulation, preweanling rats show it quite clearly. We recently observed that naloxone, a non-specific opioid antagonist, attenuated ethanol-induced locomotor activation in preweanling rats. In the present study we tested the role of specific opioid receptors (mu, delta and kappa) in ethanol-mediated locomotor stimulation and ethanol intake. In Experiment 1 13-day-old rats received naloxonazine (mu antagonist: 0, 7.5 or 15 mg/kg), naltrindole (delta antagonist: 0, 2 or 4 mg/kg) or nor-binaltorphimine (kappa antagonist: 0, 2, 4 or 8 mg/kg) before an intragastric administration of ethanol (0 or 2.5 g/kg), and subsequent locomotor activity assessment. In Experiment 2, the same opioid antagonists were administered on postnatal days 13 and 14 before consumption of ethanol (6%), saccharin (0.05%) or distilled water. In Experiment 1 only naloxonazine reduced ethanol-mediated locomotor stimulation. None of the opioid antagonists affected locomotor activity in water controls. In Experiment 2 naloxonazine and naltrindole suppressed ingestion of all the solutions tested. Similar to what has been reported in adult rodents, mu-opioid receptors seem to modulate ethanol-activating effects during early ontogeny. Hence, there seems to be a partial overlap of neurochemical mechanisms involved in the rewarding and stimulating effects of ethanol in preweanling rats. Mu-receptor antagonists reduced both ethanol-induced activity and ethanol intake, but it is unclear whether the latter effect is specific to ethanol or only a reflection of an effect on consummatory behavior generally, since mu and delta receptor antagonists also suppressed ingestion of water and saccharin.