Sara Palm

Differences in voluntary ethanol consumption in Wistar rats from five different suppliers

Understanding the mechanism of action of ethanol and the neurobiological substrates for alcohol use disorders is challenging. In search of this knowledge, it is imperative to use valid animal experimental models. The Wistar rat is one example of a commonly used strain that also exert foundation stock for several rat lines selectively bred for high and low voluntary ethanol intake. Different studies report varying ethanol intake in Wistar rats posing the question of whether this is because of the methodological differences or the rat strain. The purpose of this study was therefore to compare voluntary ethanol intake in Wistar rats from five different suppliers. Rats from B&K Universal, UK (BK); Charles River, Germany; Harlan Laboratories, IN (Hsd); Harlan Laboratories, The Netherlands (RccHan™); and Taconic, Denmark were exposed to a three-bottle free-choice paradigm with intermittent 24 h access to 5 and 20% ethanol and water three times per week for 6 weeks. A general finding was that the RccHan™ rats differed significantly from the other groups. At the end of the experiment, the RccHan™ group had the highest median ethanol intake of 3.85 g/kg/24 h, whereas the BK rats had the lowest intake of 1.84 g/kg/24 h. The preference for ethanol was also different throughout the experiment. At the end of the experiment, the RccHan™ rats had the highest preference of approximately 80%, whereas the BK rats had the lowest preference around 25%. During the 6-week drinking period, only the Hsd rats increased their ethanol intake, as evidenced by a significant increase of 5% ethanol intake. Although all rats are of Wistar origin, they display profound differences in voluntary ethanol consumption depending on the supplier. The choice of Wistar can therefore have implications for the outcome and make comparisons between studies difficult. The present findings highlight the supplier as an important parameter to consider when planning and performing preclinical animal studies in the field of alcohol research.

Differences in basal and ethanol-induced levels of opioid peptides in Wistar rats from five different suppliers

One major cause for discrepancies in results from animal experimental studies is the use of different animal strains and suppliers. We have previously reported that Wistar rats from five different suppliers display profound differences in ethanol intake and behavior. One of the neurobiological processes that could be underlying these differences is the endogenous opioid system, which has been implicated in the rewarding and reinforcing effects of alcohol. We therefore hypothesized that the differences between the supplier groups would also be evident in the endogenous opioid system. Radioimmunoassay was used to determine the levels of the opioid peptides Met-enkephalin-Arg(6)Phe(7) and dynorphin B in several brain areas of ethanol-drinking and ethanol naïve Wistar rats from five different suppliers. In the ethanol naïve animals, differences between the supplier groups were found in the pituitary gland, hypothalamus, frontal cortex, dorsal striatum and hippocampus. In the ethanol-drinking rats, differences were found in the same structures, with the addition of medial prefrontal cortex and substantia nigra. Correlations between ethanol intake and peptide levels were also found in several of the areas examined. The structures in which differences were found have all been implicated in the transition from drug use to addiction and these differences may lead to different propensities and vulnerability to this transition. Because the endogenous opioids have been suggested to be involved in a number of neurobiological disorders the results do not only have implications for research on alcohol or drug addiction, but many other fields as well.

Alcohol-Induced Changes in Opioid Peptide Levels in Adolescent Rats Are Dependent on Housing Conditions

Background Endogenous opioids are implicated in the mechanism of action of alcohol and alcohol affects opioids in a number of brain areas, although little is known about alcohols effects on opioids in the adolescent brain. One concern, in particular when studying young animals, is that alcohol intake models often are based on single housing that may result in alcohol effects confounded by the lack of social interactions. The aim of this study was to investigate short- and long-term alcohol effects on opioids and the influence of housing conditions on these effects. Methods In the first part, opioid peptide levels were measured after one 24-hour session of single housing and 2-hour voluntary alcohol intake in adolescent and adult rats. In the second part, a model with a cage divider inserted during 2-hour drinking sessions was tested and the effects on opioids were examined after 6 weeks of adolescent voluntary intake in single-and pair-housed rats, respectively. Results The effects of single housing were age specific and affected Met-enkephalin-Arg6Phe7 (MEAP) in particular. In adolescent rats, it was difficult to distinguish between effects induced by alcohol and single housing, whereas alcohol-specific effects were seen in dynorphin B (DYNB), beta-endorphin (BEND), and MEAP levels in adults. Voluntary drinking affected several brain areas and the majority of alcohol-induced effects were not dependent on housing. However, alcohol effects on DYNB and BEND in the amygdala were dependent on housing. Housing alone affected MEAP in the cingulate cortex. Conclusions Age-specific housing- and alcohol-induced effects on opioids were found. In addition, prolonged voluntary alcohol intake under different housing conditions produced several alcohol-induced effects independent of housing. However, housing-dependent effects were found in areas implicated in stress, emotionality, and alcohol use disorder. Housing condition and age may therefore affect the reasons and underlying mechanisms for drinking and could potentially affect the outcome of a number of end points in research on alcohol intake.

Risk-assessment and risk-taking behavior predict potassium- and amphetamine-induced dopamine response in the dorsal striatum of rats.

Certain personality types and behavioral traits display high correlations to drug use and an increased level of dopamine in the reward system is a common denominator of all drugs of abuse. Dopamine response to drugs has been suggested to correlate with some of these personality types and to be a key factor influencing the predisposition to addiction. This study investigated if behavioral traits can be related to potassium- and amphetamine-induced dopamine response in the dorsal striatum, an area hypothesized to be involved in the shift from drug use to addiction. The open field and multivariate concentric square field™ tests were used to assess individual behavior in male Wistar rats. Chronoamperometric recordings were then made to study the potassium- and amphetamine-induced dopamine response in vivo. A classification based on risk-taking behavior in the open field was used for further comparisons. Risk-taking behavior was correlated between the behavioral tests and high risk takers displayed a more pronounced response to the dopamine uptake blocking effects of amphetamine. Behavioral parameters from both tests could also predict potassium- and amphetamine-induced dopamine responses showing a correlation between neurochemistry and behavior in risk-assessment and risk-taking parameters. In conclusion, the high risk-taking rats showed a more pronounced reduction of dopamine uptake in the dorsal striatum after amphetamine indicating that this area may contribute to the sensitivity of these animals to psychostimulants and proneness to addiction. Further, inherent dopamine activity was related to risk-assessment behavior, which may be of importance for decision-making and inhibitory control, key components in addiction.

Effects of Rearing Conditions on Behaviour and Endogenous Opioids in Rats with Alcohol Access during Adolescence

Causal links between early-life stress, genes and later psychiatric diagnoses are not possible to fully address in human studies. Animal models therefore provide an important complement in which conditions can be well controlled and are here used to study and distinguish effects of early-life stress and alcohol exposure. The objective of this study was to investigate the impact of rearing conditions on behaviour in young rats and if these changes could be followed over time and to examine interaction effects between early-life environment and adolescent alcohol drinking on behaviour and immunoreactive levels of the opioid peptides dynorphin B, met-enkephalin-Arg6Phe7 and beta-endorphin. We employed a rodent model, maternal separation, to study the impact of rearing conditions on behaviour, voluntary alcohol consumption and alcohol-induced effects. The consequences of short, 15 min (MS 15), and long, 360 min (MS 360), maternal separation in combination with adolescent voluntary alcohol consumption on behaviour and peptides were examined. A difference in the development of risk taking behaviour was found between the MS15 and MS360 while the development of general activity was found to differ between intake groups. Beta-endorphin levels in the pituitary and the periaqueductal gray area was found to be higher in the MS15 than the MS360. Adolescent drinking resulted in higher dynorphin B levels in the hippocampus and higher met-enkephalin-Arg6Phe7 levels in the amygdala. Amygdala and hippocampus are involved in addiction processes and changes in these brain areas after adolescent alcohol drinking may have consequences for cognitive function and drug consumption behaviour in adulthood. The study shows that individual behavioural profiling over time in combination with neurobiological investigations provides means for studies of causality between early-life stress, behaviour and vulnerability to psychiatric disorders.

Dopamine release dynamics change during adolescence and after voluntary alcohol intake.

Adolescence is associated with high impulsivity and risk taking, making adolescent individuals more inclined to use drugs. Early drug use is correlated to increased risk for substance use disorders later in life but the neurobiological basis is unclear. The brain undergoes extensive development during adolescence and disturbances at this time are hypothesized to contribute to increased vulnerability. The transition from controlled to compulsive drug use and addiction involve long-lasting changes in neural networks including a shift from the nucleus accumbens, mediating acute reinforcing effects, to recruitment of the dorsal striatum and habit formation. This study aimed to test the hypothesis of increased dopamine release after a pharmacological challenge in adolescent rats. Potassium-evoked dopamine release and uptake was investigated using chronoamperometric dopamine recordings in combination with a challenge by amphetamine in early and late adolescent rats and in adult rats. In addition, the consequences of voluntary alcohol intake during adolescence on these effects were investigated. The data show a gradual increase of evoked dopamine release with age, supporting previous studies suggesting that the pool of releasable dopamine increases with age. In contrast, a gradual decrease in evoked release with age was seen in response to amphetamine, supporting a proportionally larger storage pool of dopamine in younger animals. Dopamine measures after voluntary alcohol intake resulted in lower release amplitudes in response to potassium-chloride, indicating that alcohol affects the releasable pool of dopamine and this may have implications for vulnerability to addiction and other psychiatric diagnoses involving dopamine in the dorsal striatum.

Alcohol and Endogenous Opioids

The endogenous opioid system is involved in alcohol-induced reward and development of alcohol use disorder (AUD). Alcohol exposure induces a number of changes in the endogenous opioid system, both after acute and chronic intake. Opioid agonists and antagonists can modulate alcohol consumption and opioid antagonists are therefore used in pharmacotherapy of AUD. There are three different receptor types—mu-, delta- and kappa-opioid receptors—with different affinity for the three opioid peptides: beta-endorphin, enkephalin, and dynorphin. The three systems also mediate different effects. Positive effects are generally attributed to the endorphins and enkephalins, while negative effects are mediated by dynorphins. Dysfunction in any or all of these systems contribute to development of AUD. Genetic and environmental factors interact to cause long-term changes in endogenous opioid function, which can affect vulnerability to AUD and efficacy of treatment with opioid antagonists.