Social isolation and stress related hormones modulate the stimulating effect of ethanol in preweanling rats

C57BL/6J offspring mice reared by a single-mother exhibit, compared to mice reared in a biparental parenting structure, distinct neural activation patterns and heightened ethanol-induced anxiolysis

RATIONALE

Parenting experiences with caregivers play a key role in neurodevelopment. We recently reported that adolescents reared by a single-mother (SM) display an anxiety-prone phenotype and drink more alcohol, compared to peers derived from a biparental (BP) rearing condition.

OBJECTIVES

To investigate if SM and BP offspring infant mice exhibit differential sensitivity to ethanol-induced locomotor activity and differential activity patterns in brain areas related to anxiety response. We also analyzed anxiety response and ethanol-induced anxiolysis in SM and BP adolescents.

METHODS

Mice reared in SM or BP conditions were assessed for (a) ethanol-induced locomotor activity at infancy, (b) central expression of Fos-like proteins (likely represented mostly by FosB, a transcription factor that accumulates after chronic stimuli exposure and serves as a molecular marker of neural plasticity) and cathecolaminergic activity, and (c) anxiety-like behavior and ethanol-induced anxiolysis in adolescence.

RESULTS

Infant mice were sensitive to the stimulating effects of 2.0 g/kg alcohol, regardless parenting structure. SM mice exhibited, relative to BP mice, a significantly greater number of Fos-like positive cells in the central amygdala and basolateral amygdala nuclei. Ethanol treatment, but not parenting condition, induced greater activation of dopaminergic neurons in ventral tegmental area. SM, but not BP, adolescent mice were sensitive to ethanol-induced anxiolysis.

CONCLUSIONS

These results highlight the complex relationship between parenting experiences and neurodevelopment. The SM parenting may result in greater neural activation patterns in brain areas associated with anxiety response, potentially contributing to increased basal anxiety and alcohol sensitivity.

Moderate ethanol exposure during early ontogeny of the rat alters respiratory plasticity, ultrasonic distress vocalizations, increases brain catalase activity, and acetaldehyde-mediated ethanol intake

Early ontogeny of the rat (late gestation and postnatal first week) is a sensitive period to ethanol’s positive reinforcing effects and its detrimental effects on respiratory plasticity. Recent studies show that acetaldehyde, the first ethanol metabolite, plays a key role in the modulation of ethanol motivational effects. Ethanol brain metabolization into acetaldehyde via the catalase system appears critical in modulating ethanol positive reinforcing consequences. Catalase system activity peak levels occur early in the ontogeny. Yet, the role of ethanol-derived acetaldehyde during the late gestational period on respiration response, ultrasonic vocalizations (USVs), and ethanol intake during the first week of the rat remains poorly explored. In the present study, pregnant rats were given a subcutaneous injection of an acetaldehyde-sequestering agent (D-penicillamine, 50 mg/kg) or saline (0.9% NaCl), 30 min prior to an intragastric administration of ethanol (2.0 g/kg) or water (vehicle) on gestational days 17–20. Respiration rates (breaths/min) and apneic episodes in a whole-body plethysmograph were registered on postnatal days (PDs) 2 and 4, while simultaneously pups received milk or ethanol infusions for 40-min in an artificial lactation test. Each intake test was followed by a 5-min long USVs emission record. On PD 8, immediately after pups completed a 15-min ethanol intake test, brain samples were collected and kept frozen for catalase activity determination. Results indicated that a moderate experience with ethanol during the late gestational period disrupted breathing plasticity, increased ethanol intake, as well brain catalase activity. Animals postnatally exposed to ethanol increased their ethanol intake and exerted differential affective reactions on USVs and apneic episodes depending on whether the experience with ethanol occur prenatal or postnatally. Under the present experimental conditions, we failed to observe, a clear role of acetaldehyde mediating ethanol’s effects on respiratory plasticity or affective states, nevertheless gestational acetaldehyde was of crucial importance in determining subsequent ethanol intake affinity. As a whole, results emphasize the importance of considering the participation of acetaldehyde in fetal programming processes derived from a brief moderate ethanol experience early in development, which in turn, argues against “safe or harmless” ethanol levels of exposure.

A Zebrafish Model of Neurotoxicity by Binge-Like Methamphetamine Exposure

Hyperthermia is a common confounding factor for assessing the neurotoxic effects of methamphetamine (METH) in mammalian models. The development of new models of methamphetamine neurotoxicity using vertebrate poikilothermic animals should allow to overcome this problem. The aim of the present study was to develop a zebrafish model of neurotoxicity by binge-like methamphetamine exposure. After an initial testing at 20 and 40 mg/L for 48 h, the later METH concentration was selected for developing the model and the effects on the brain monoaminergic profile, locomotor, anxiety-like and social behaviors as well as on the expression of key genes of the catecholaminergic system were determined. A concentration- and time-dependent decrease in the brain levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT) was found in METH-exposed fish. A significant hyperactivity was found during the first hour of exposure, followed 3 h after by a positive geotaxis and negative scototaxis in the novel tank and in the light/dark paradigm, respectively. Moreover, the behavioral phenotype in the treated fish was consistent with social isolation. At transcriptional level, th1 and slc18a2 (vmat2) exhibited a significant increase after 3 h of exposure, whereas the expression of gfap, a marker of astroglial response to neuronal injury, was strongly increased after 48 h exposure. However, no evidences of oxidative stress were found in the brain of the treated fish. Altogether, this study demonstrates the suitability of the adult zebrafish as a model of METH-induced neurotoxicity and provides more information about the biochemical and behavioral consequences of METH abuse.

Ethanol's disruptive effects upon early breathing plasticity and blood parameters associated with hypoxia and hypercapnia

Early ethanol exposure affects respiratory neuroplasticity; a risk factor associated with the Sudden Infant Death Syndrome. High and chronic ethanol doses exert long-lasting effects upon respiratory rates, apneic episodes and ventilatory processes triggered by hypoxia. The present study was performed in 3-9-day-old rat pups. Respiratory processes under normoxic and hypoxic conditions were analyzed in pups intoxicated with different ethanol doses which were pre-exposed or not to the drug. A second major goal was to examine if acute and/or chronic early ethanol exposure affects blood parameters related with hypercapnic or hypoxic states. In Experiment 1, at postnatal day 9, animals previously treated with ethanol (2.0 g/kg) or vehicle (0.0 g/kg) were tested sober or intoxicated with 0.75, 1.37 or 2.00 g/kg ethanol. The test involved sequential air conditions defined as initial normoxia, hypoxia and recovery normoxia. Motor activity was also evaluated. In Experiment 2, blood parameters indicative of possible hypoxic and hypercapnic states were assessed as a function of early chronic or acute experiences with the drug. The main results of Experiment 1 were as follows: i) ethanol's depressant effects upon respiratory rates increased as a function of sequential treatment with the drug (sensitization); ii) ethanol inhibited apneic episodes even when employing the lowest dose at test (0.75 g/kg); iii) the hyperventilatory response caused by hypoxia negatively correlated with the ethanol dose administered at test; iv) ventilatory long-term facilitation (LTF) during recovery normoxia was observed in pups pre-exposed to the drug and in pups that received the different ethanol doses at test; v) self-grooming increased in pups treated with either 1.37 or 2.00 g/kg ethanol. The main result of Experiment 2 indicated that acute as well as chronic ethanol exposure results in acidosis-hypercapnia. The results indicate that early and brief experiences with ethanol are sufficient to affect different respiratory plasticity processes as well as blood biomarkers indicative of acidosis-hypercapnia. An association between the LTF process and the acidosis-hypercapnic state caused by ethanol seems to exist. The mentioned experiences with the drug are sufficient to result in an anomalous programming of respiratory patterns and metabolic conditions.

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.

Early ethanol pre-exposure alters breathing patterns by disruptions in the central respiratory network and serotonergic balance in neonate rats

Early ethanol exposure alters neonatal breathing plasticity. Respiratory EtOH's effects are attributed to central respiratory network disruptions, particularly in the medullary serotonin (5HT) system. In this study we evaluated the effects of neonatal pre-exposure to low/moderate doses upon breathing rates, activation patterns of brainstem's nuclei and expression of 5HT 2A and 2C receptors. At PD9, breathing frequencies, tidal volumes and apneas were examined in pups pre-exposed to vehicle or ethanol (2.0 g/kg) at PDs 3, 5 and 7. This developmental stage is equivalent to the 3^rd human gestational trimester, characterized by increased levels of synaptogenesis. Pups were tested under sobriety or under the state of ethanol intoxication and when subjected to normoxia or hypoxia. Number of c-Fos and 5HT immunolabelled cells and relative mRNA expression of 5HT 2A and 2C receptors were quantified in the brainstem. Under normoxia, ethanol pre-exposed pups exhibited breathing depressions and a high number of apneas. An opposite phenomenon was found in ethanol pre-treated pups tested under hypoxia where an exacerbated hypoxic ventilatory response (HVR) was observed. The breathing depression was associated with an increase in the neural activation levels of the raphe obscurus (ROb) and a high mRNA expression of the 5HT 2A receptor in the brainstem while desactivation of the ROb and high activation levels in the solitary tract nucleus and area postrema were associated to the exacerbated HVR. In summary, early ethanol experience induces respiratory disruptions indicative of sensitization processes. Neuroadaptive changes in central respiratory areas under consideration appear to be strongly associated with changes in their respiratory plasticity.

Biological intersection of sex, age, and environment in the corticotropin releasing factor (CRF) system and alcohol

The neuropeptide corticotropin-releasing factor (CRF) is critical in neural circuit function and behavior, particularly in the context of stress, anxiety, and addiction. Despite a wealth of preclinical evidence for the efficacy of CRF receptor 1 antagonists in reducing behavioral pathology associated with alcohol exposure, several clinical trials have had disappointing outcomes, possibly due to an underappreciation of the role of biological variables. Although he National Institutes of Health (NIH) now mandate the inclusion of sex as a biological variable in all clinical and preclinical research, the current state of knowledge in this area is based almost entirely on evidence from male subjects. Additionally, the influence of biological variables other than sex has received even less attention in the context of neuropeptide signaling. Age (particularly adolescent development) and housing conditions have been shown to affect CRF signaling and voluntary alcohol intake, and the interaction between these biological variables is particularly relevant to the role of the CRF system in the vulnerability or resilience to the development of alcohol use disorder (AUD). Going forward, it will be important to include careful consideration of biological variables in experimental design, reporting, and interpretation. As new research uncovers conditions in which sex, age, and environment play major roles in physiological and/or pathological processes, our understanding of the complex interaction between relevant biological variables and critical signaling pathways like the CRF system in the cellular and behavioral consequences of alcohol exposure will continue to expand ultimately improving the ability of preclinical research to translate to the clinic. This article is part of the special issue on Neuropeptides.

Sexual dimorphism in the neural impact of stress and alcohol

Alcohol use disorder is a widespread mental illness characterized by periods of abstinence followed by recidivism, and stress is the primary trigger of relapse. Despite the higher prevalence of alcohol use disorder in males, the relationship between stress and behavioral features of relapse, such as craving, is stronger in females. Given the greater susceptibility of females to stress-related psychiatric disorders, understanding sexual dimorphism in the relationship between stress and alcohol use is essential to identifying better treatments for both male and female alcoholics. This review addresses sex differences in the impact of stressors on alcohol drinking and seeking in rodents and humans. As these behavioral differences in alcohol use and relapse originate from sexual dimorphism in neuronal function, the impact of stressors and alcohol, and their interaction, on molecular adaptations and neural activity in males and females will also be discussed. Together, the data reviewed herein, arising from a symposium titled "Sex matters in stress-alcohol interactions" presented at the Fourth Volterra Conference on Stress and Alcohol, will highlight the importance of identifying sex differences to improve treatments for comorbid stress and alcohol use disorder in both sexes.

Social Factors in Ethanol Sensitization

Behavioral sensitization is a neuroadaptive process characterized by an increase in a particular behavior after repeated exposure to drugs or other stimuli, such as stress. Sensitization can also be extended to neurochemical and neuroendocrine sensitization. Several factors can influence sensitization to the effects of ethanol. For instance, stress is an important component in addiction that can strengthen ethanol-induced behaviors. In animal models, stressful situations can be induced by alterations in social aspects of the animal's environment, such as maternal separation, social conflicts, and housing conditions. Social conflict models involve acute, chronic or intermittent interaction of an animal to a conspecific and can occur at any stage of life, including preweaning, adolescence or adulthood. These events can influence ethanol-induced behavioral sensitization in different ways, such as increases in locomotion, drug reward, and drug-taking behaviors. On the other hand, environmental enrichment can produce a protective phenotype against drug-related behaviors. In this chapter, we discuss findings regarding consequences of social stress and environmental enrichment on sensitization to ethanol.

Brief ethanol exposure and stress-related factors disorganize neonatal breathing plasticity during the brain growth spurt period in the rat

RATIONALE

The effects of early ethanol exposure upon neonatal respiratory plasticity have received progressive attention given a multifactorial perspective related with sudden infant death syndrome or hypoxia-associated syndromes. The present preclinical study was performed in 3-9-day-old pups, a stage in development characterized by a brain growth spurt that partially overlaps with the 3rd human gestational trimester.

METHODS

Breathing frequencies and apneas were examined in pups receiving vehicle or a relatively moderate ethanol dose (2.0 g/kg) utilizing a whole body plethysmograph. The experimental design also considered possible associations between drug administration stress and exteroceptive cues (plethysmographic context or an artificial odor). Ethanol exposure progressively exerted a detrimental effect upon breathing frequencies. A test conducted at PD9 when pups were under the state of sobriety confirmed ethanol's detrimental effects upon respiratory plasticity (breathing depression).

RESULTS

Pre-exposure to the drug also resulted in a highly disorganized respiratory response following a hypoxic event, i.e., heightened apneic episodes. Associative processes involving drug administration procedures and placement in the plethysmographic context also affected respiratory plasticity. Pups that experienced intragastric administrations in close temporal contiguity with such a context showed diminished hyperventilation during hypoxia. In a 2nd test conducted at PD9 while pups were intoxicated and undergoing hypoxia, an attenuated hyperventilatory response was observed. In this test, there were also indications that prior ethanol exposure depressed breathing frequencies during hypoxia and a recovery normoxia phase.

CONCLUSION

As a whole, the results demonstrated that brief ethanol experience and stress-related factors significantly disorganize respiratory patterns as well as arousal responses linked to hypoxia in neonatal rats.

Maternal separation on the ethanol-preferring adult rat liver structure

Background and rationale for the study. We designed to test whether there is interaction of maternal separation (MS) on the ethanol-preferring rats liver structure. The UCh rat pups were separated daily from their mothers during the stress hyporesponsive period (SHRP), between four and 14 days-old, always at the same time for four hours in a cage containing eight subdivisions, one for each pup. Subsequently, rats that presented the highest (UChB) and the lowest (UChA) ethanol (EtOH) consumption were selected to the study. Both UChB and UChA rats received 10% (v/v) EtOH and distilled water ad libitum until the end of the experiment (120 days-old). The liver was collected to histological routine for morphometric and stereological analyses, and immunohistochemistry.

RESULTS

There was an interaction of MS and EtOH on the liver: increased liver mass, peritubular vessels, stellate cell numbers, steatosis and cell death, decreased necrosis, sinusoidal capillary diameters and cell proliferation. While there was a decrease in FSH, testosterone and 5α-di-hidrotestosterone, and increasing corticosterone and cholesterol.

CONCLUSIONS

There is interaction of MS and EtOH on the liver structure, dependent on the amount of EtOH intake. Furthermore, the interaction of stress and drugs can increase or decrease their effects on the liver or indirectly via hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes.

Effects of ethanol exposure in a familiar or isolated context during infancy on ethanol intake during adolescence

Early exposure to ethanol affects ethanol intake later in life. This early experience encompasses exposure to social stimuli and the pharmacological and orosensory properties of ethanol. The specific contribution of each type of stimulus to subsequent ethanol intake remains unknown. We assessed the intake of various concentrations of ethanol in a familiar or isolated context during infancy and the lingering effects of this experience on ethanol intake during adolescence. On postnatal day 3 (PD3), PD7, and PD11, rats were given 5% ethanol or water in a nursing or isolated context (Experiments 1 and 2). Intake tests (ethanol vs. water) were conducted during adolescence. Experiment 2 matched the amount of fluid ingested during infancy in both contexts and subsequently tested ethanol consumption during adolescence. The results revealed a facilitative effect of the nursing context on fluid intake during the tests in infancy. Pups stimulated with ethanol but not water in the isolated context exhibited an increase in ethanol consumption during adolescence. This effect disappeared when the isolated infants were matched to receive the same amount of ethanol ingested by their nursed counterparts. In Experiment 3, isolated infant rats were exposed to different ethanol concentrations (.0%, 2.5%, 5.0%, and 10.0%), and drug consumption was tested during adolescence. This exposure increased adolescent ethanol intake, regardless of the alcohol concentration (Experiment 3). The common denominators that resulted in enhanced ethanol intake during adolescence were preexposure to ethanol via active consumption of the drug that induced a low-to-moderate level of intoxication in an isolated context.

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.

Pharmacological characterization of the nociceptin/orphanin FQ receptor on ethanol-mediated motivational effects in infant and adolescent rats

Activation of nociceptin/orphanin FQ (NOP) receptors attenuates ethanol drinking and prevents relapse in adult rodents. In younger rodents (i.e., infant rats), activation of NOP receptors blocks ethanol-induced locomotor activation but does not attenuate ethanol intake. The aim of the present study was to extend the analysis of NOP modulation of ethanol's effects during early ontogeny. Aversive and anxiolytic effects of ethanol were measured in infant and adolescent rats via conditioned taste aversion and the light-dark box test; whereas ethanol-induced locomotor activity and ethanol intake was measured in adolescents only. Before these tests, infant rats were treated with the natural ligand of NOP receptors, nociceptin (0.0, 0.5 or 1.0 μg) and adolescent rats were treated with the specific agonist Ro 64-6198 (0.0, 0.1 or 0.3 mg/kg). The activation of NOP receptors attenuated ethanol-induced anxiolysis in adolescents only, and had no effect on ethanol's aversive effects. Administration of Ro 64-6198 blocked ethanol-induced locomotor activation but did not modify ethanol intake patterns. The attenuation of ethanol stimulating and anxiolytic effect by activation of NOP receptors indicates a modulatory role of this receptor on ethanol effects, which is expressed early in ontogeny.

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.

Participation of the nociceptin/orphanin FQ receptor in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

Activation of nociceptin/orphanin FQ (NOP) receptors seems to attenuate ethanol-induced reinforcement in adult rodents. Since early ethanol exposure results in later increased responsiveness to ethanol, it is important to analyze NOP receptor modulation of ethanol-related behaviors during early ontogeny. By measuring NOP involvement in ethanol intake and ethanol-induced locomotor activation, we analyzed the specific participation of NOP receptors on these ethanol-related behaviors in two-week-old rats. In each experiment animals were pre-treated with the endogenous ligand for this receptor (nociceptin/orphanin FQ at 0.0, 0.5, 1.0 or 2.0 μg) or a selective NOP antagonist (J-113397 at 0.0, 0.5, 2.0 or 5.0 mg/kg). Results indicated that activation of the nociceptin receptor system had no effect on ethanol or water intake, while blockade of the NOP receptor has an unspecific effect on consummatory behavior: J-113397 increased ethanol (at a dose of 0.5 mg/kg) and water intake (at 0.5 and 5.0 mg/kg). Ethanol-mediated locomotor stimulation was attenuated by activation of the NOP system (nociceptin at 1.0 and 2.0 μg). Nociceptin had no effect on basal locomotor activity. Blockade of NOP receptors did not modify ethanol-induced locomotor activation. Contrary to what has been reported for adult rodents, nociceptin failed to suppress intake of ethanol in infants. Attenuation of ethanol-induced stimulation by activation of NOP receptor system suggests an early role of this receptor in this ethanol-related behavior.

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.

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.

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 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.

Mianserin, but not ondansetron, reduces the locomotor stimulating effect of ethanol in preweanling rats

During infancy rats are highly sensitive to the locomotor stimulating effect of ethanol, an effect particularly observed when they are tested during the rising phase of the blood ethanol curve and in a novel environment. According to a recent study infant rats require some degree of stress to get stimulated after being challenged with ethanol. Ethanol-induced stimulation in preweanling rats required the activation of CRH-1 receptors. Considering these antecedents, we explored modulation of the acute stimulating effect of ethanol (2.5g/kg) by two anxiolytic drugs, Mianserin (2.5 or 5mg/kg) and Ondansetron (1 or 3mg/kg). Mianserin attenuated the stimulating effect of ethanol at a dose that did not affect locomotor activity in water-controls, likely acting through 5-HT2 receptors, while Ondansetron, a 5-HT3 antagonist, did not affect this response. These results are consistent with recent findings indicating that one of the mechanisms by which the CRH-1 receptor modulates anxiety depends on sensitization of the 5-HT2 receptor antagonist, and highlight the importance of stress as a modulator of the effects of ethanol during early developmental stages.

Maternal separation as a model of brain-gut axis dysfunction

RATIONALE

Early life stress has been implicated in many psychiatric disorders ranging from depression to anxiety. Maternal separation in rodents is a well-studied model of early life stress. However, stress during this critical period also induces alterations in many systems throughout the body. Thus, a variety of other disorders that are associated with adverse early life events are often comorbid with psychiatric illnesses, suggesting a common underlying aetiology. Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is thought to involve a dysfunctional interaction between the brain and the gut. Essential aspects of the brain-gut axis include spinal pathways, the hypothalamic pituitary adrenal axis, the immune system, as well as the enteric microbiota. Accumulating evidence suggest that stress, especially in early life, is a predisposing factor to IBS.

OBJECTIVE

The objective of this review was to assess and compile the most relevant data on early life stress and alterations at all levels of the brain gut axis.

RESULTS

In this review, we describe the components of the brain-gut axis individually and how they are altered by maternal separation. The separated phenotype is characterised by alterations of the intestinal barrier function, altered balance in enteric microflora, exaggerated stress response and visceral hypersensitivity, which are all evident in IBS.

CONCLUSION

Thus, maternally separated animals are an excellent model of brain-gut axis dysfunction for the study of disorders such as IBS and for the development of novel therapeutic interventions.