Elevated concentrations of ethanol in plasma do not suppress voluntary ethanol consumption in C57BL mice

Mouse breathalyzer

BACKGROUND

The development of a relatively simple, noninvasive method for estimating blood ethanol concentrations in mice will be useful in behavioral studies related to alcoholism. This study validated such a method.

METHODS

The apparatus consists of a body chamber fitted with a head stock through which the mouse head protrudes. This was fitted against a water-jacketed head-space chamber surrounding the mouse's head. Rebreathed air maintained at 37 degrees C in the head-space chamber was removed using a peristaltic pump and loaded into a 1-ml injection loop. Ethanol in the sample was quantified using gas chromatography. To validate this method, ethanol levels in breath samples were compared against those in tail blood samples collected immediately after the breath samples. Breath samples were collected at 5, 10, 20, 40, 80, 120, and 160 minutes after ethanol (0.4, 0.8, 1.2, 1.6, 2.4, and 3.2 g/kg) was administered to male C57BL/6J mice.

RESULTS

Breath and blood ethanol levels were well correlated (r(2) = 0.96) across time points on the descending ethanol-time curve at doses below 2.4 g/kg. Correlation for these doses on the ascending portion of the curve had greater variance, but was still well correlated (r(2) = 0.92).

CONCLUSIONS

The mouse breathalyzer is an accurate, convenient, noninvasive and well-tolerated method for estimating blood ethanol concentrations in mice across a range of behaviorally relevant concentrations below 2.4 g/kg, especially on the descending limb of the ethanol-time curve. Although this procedure requires a gas chromatograph in the animal facility, the ability to estimate ethanol concentrations quickly and easily will be especially useful in behavioral studies where repeated blood sampling is not feasible.

Chronic Ethanol Consumption by C57BL/6 Mice Promotes Tolerance to Its Interoceptive Cues and Increases Extracellular Dopamine, an Effect Blocked by Naltrexone

BACKGROUND

C57BL/6 (B6) mice voluntarily consume ethanol. Although preingestive factors might be accountable, the fact that B6 mice voluntarily consume sufficient ethanol to set the conditions for an ethanol-deprivation effect suggest that post-ingestive pharmacological induced changes also occur. In this study, we determined the amounts of ethanol voluntarily consumed by B6 mice and associated blood ethanol levels (BEL), the effects of this consumption on extracellular dopamine (DA) and how this was altered by naltrexone, as well as on its interoceptive discriminative cues.

METHODS

In experiment 1, the amounts of 12% ethanol consumed at 2, 4, and 6 hr into the active phase of the circadian cycle and associated BEL were determined. In experiment 2, dialysate samples were collected for 1 hr to establish basal DA levels. Mice were then injected with saline or naltrexone (6 mg/kg) and given access to water and 12% ethanol or to water only, and samples were collected at 20-min intervals for the next 2 hr. In experiment 3, mice were trained to discriminate ethanol's interoceptive cues via operant techniques, and half were given 3 weeks access to ethanol and water, the other half water only. Ethanol-consuming and water control mice were again tested for their ability to discriminate the drug's interoceptive cues.

RESULTS

Mice ingested nearly 6 g/kg of ethanol and attained BEL near 100 mg/100 mL by 6 hr into the active phase. Ethanol intake at 2-hr into the dark phase was approximately 2.5 g/kg, and increased DA to approximately 100% above basal levels. Naltrexone reduced ethanol consumption and blocked the DA increase. Ethanol consumption for 3 weeks attenuated its discriminative cues.

CONCLUSIONS

B6 mice voluntarily consume sufficient ethanol (1) to produce intoxicating BEL; (2) to increase DA levels in nucleus accumbens, an effect blocked by naltrexone; and (3) to attenuate its discriminative cues.

Blood alcohol concentrations after scheduled access in high-alcohol-preferring mice

Development of procedures yielding substantial blood alcohol concentrations during voluntary access to an alcohol solution in mice is necessary to further characterize genetic and neurobiologic mechanisms underlying alcohol self-administration. Although, in experimental situations, some populations of mice readily drink an alcohol solution, results from previous studies have not typically revealed high blood alcohol concentrations after voluntary access, probably because of the high alcohol metabolism rate in mice. Toward development of a murine drinking model, 36 selectively bred high-alcohol-preferring mice of both sexes were subjected to a 30-min scheduled-access procedure by using saccharin fading to gradually introduce an alcohol solution. Mice had ad libitum access to food and water 24 h a day. The alcohol solution was available 1 h after the start of the dark part of the cycle for 30 min per day, 5 days per week. After complete removal of saccharin from the drinking tubes, mice consistently drank 1.4 g/kg of a 10% [volume/volume (vol./vol.)] alcohol solution in 30 min. Analysis of tail blood samples, taken immediately after the end of the 30-min access period, indicated blood alcohol concentrations were tightly correlated with alcohol intakes (range, 6-130 mg/dl; average, nearly 60 mg/dl). A concentration-response function of 10%, 12%, 15%, 18%, and 21% (vol./vol.) alcohol solutions indicated an inverted U-shaped relation between alcohol intake and alcohol concentration, with peak intake of greater than 1.75 g/kg per 30 min when a 15% alcohol solution was available. No sex differences were seen. These findings indicate the utility of this procedure in obtaining pharmacologically relevant blood alcohol concentrations after voluntary oral self-administration of an alcohol solution in mice.

An investigation of the role of 5-HT(2C) receptors in modifying ethanol self-administration behaviour

We have previously reported that the 5-HT uptake blocker and releaser, dexfenfluramine, attenuates ethanol intake, and that this may be mediated via a 5-HT(2C) receptor mechanism. Our goals were to further determine the contribution made by this receptor subtype in mediating the reduction in ethanol self-administration induced by dexfenfluramine using the selective 5-HT(2C) antagonist, SB242,084. Additionally, we wanted to compare dexfenfluramine's effects on ethanol motivated responding with those elicited by the 5-HT(2C) receptor agonist Ro60-0175. In male Wistar rats trained to self-administer a 12% w/v ethanol solution on an FR-4 schedule, both dexfenfluramine (0.05--2.5 mg/kg ip) and Ro60-0175 (0.1--1 mg/kg sc) produced a significant dose-dependent reduction in ethanol self-administration, which was reversed by SB242,084 (0.5 mg/kg ip). Interestingly, SB242,084 alone (0.1--1 mg/kg ip) significantly increased ethanol motivated responding in both high and low ethanol drinking animals. While dexfenfluramine had no effect on ethanol's kinetic profile, the selective 5-HT(2C) agents used had opposing effects, with the agonist Ro60-0175 decreasing and the antagonist SB242,084 increasing blood ethanol levels. Since there were incongruent drug effects on ethanol self-administration and blood ethanol levels, these data support a role for 5-HT(2C) receptors in modifying ethanol intake independent of their effects on blood ethanol kinetics. Furthermore, 5-HT(2C) receptors may exert a tonic control over ethanol self-administration behaviour, since agonist and antagonist administration had opposing effects on this behaviour.

Electrophysiological response to neuropeptide Y (NPY): in alcohol-naive preferring and non-preferring rats

Electroencephalograms (EEGs) and event-related potentials (ERPs) to auditory stimuli were recorded following intracerebroventricular administration of neuropeptide Y (saline, NPY: 1.0, 3.0 nmol) in two lines of rats that have been genetically selected for alcohol preferring (P) or non-preferring (NP) behaviors. Previous studies have demonstrated that NPY has a distinct electrophysiological profile that is similar to that of ethanol. In outbred Wistar rats, both NPY and ethanol produced highly significant decreases in the amplitude and increases in the latency of the N1 component of the ERP to all three auditory stimuli. Because the N1 has been associated with attention, these data suggest that both NPY and alcohol may diminish attentional processes. In the present study, NPY-induced decreases in N1 amplitude were also found, but only to the frequently presented tone. This suggests that both P and NP rats may have attenuated responses to NPY's effects on attention/arousal. Like outbred Wistars, P and NP rats were also found to have significant NPY-induced increases in N1 latency in the cortex and hippocampus. However, in the amygdala, while P rats evidenced increases in N1 latency and decreases in N1 amplitudes, NP rats displayed the opposite effects. Spectral analysis revealed that NPY also produced differential EEG responses in P and NP rats. In previous studies in outbred Wistar rats NPY has been found to produce slowing of delta (1-2 Hz) frequencies at the 1-nmol dose and reductions in power, particularly in the higher frequencies in the amygdala, at the 3-nmol dose. This electrophysiological profile is not unlike what is seen following alcohol and benzodiazepines and is associated with anxiolysis. P rats were found to have this general pattern of EEG responses to NPY but attenuated suggesting that they may have reduced responses to electrophysiological measures of the anxiolytic effects of NPY. In contrast, NP rats had NPY-induced EEG effects in amygdala and frontal cortex that were opposite to those seen in P rats. These opposing responses to NPY tended to produce a "normalization" of the power differences that existed between the two rat lines at baseline. Taken together with previous findings that P rats have decreased NPY concentrations in limbic and frontal cortical sites, these data suggest that differences in the regulation of NPY neurons may contribute to the expression of behavioral preference for ethanol consumption in these rat lines.

Corticotropin releasing factor (CRF): studies in alcohol preferring and non-preferring rats

Electroencephalographic (EEG) responses to corticotropin releasing factor (CRF) as well as CRF concentrations in several brain regions were measured in two lines of rats which have been genetically selected for alcohol preferring (P) or non-preferring (NP) behaviors. Fifteen rats were implanted with chronic electrodes and EEG spectra were evaluated following intracerebroventricular (ICV) administration of CRF (0.15 nmol) or saline. P rats demonstrated a significantly increased EEG response to CRF in the theta frequency range (ANOVA: PREF x DRUG 4-6 Hz, P less than 0.03; 6-8 Hz, P less than 0.05) in frontal cortex. A significantly lower concentration of CRF was found in the P rats in hypothalamus (P less than 0.02), amygdala (P less than 0.003), prefrontal cortex (P less than 0.01), and cingulate cortex (P less than 0.02). The finding that P rats had an increased response to exogenously administered CRF, taken together with decreased CRF concentrations, suggests that CRF receptors may be up-regulated in these animals. Differences in the regulation of CRF neurons may contribute to the expression of behavioral preference for ethanol consumption in these rat lines.

Electrophysiological response to ethanol in P and NP rats

Event-related potentials (ERPs) have been successfully used in human subjects to evaluate alcoholics as well as those at risk for the future development of alcoholism. In the present study, two lines of rats, those with a preference for ethanol consumption (P) and those not preferring (NP) to drink ethanol were studied using ERP-producing stimuli. Rats were implanted with electrodes in the frontal cortex and dorsal hippocampus (DHPC). A passive auditory "oddball" paradigm was used to record ERP responses following saline and two doses (0.5, 1.0 g/kg) of ethanol. P and NP rats differed under the saline condition in that P rats had smaller N1-like ERP components and larger P2 waves in both cortex and hippocampus. P and NP rats were also found to differ in response to ethanol administration. NP rats evidenced dose-dependent reductions in ERP component amplitudes such as the N1 recorded from cortical sites. P rats did not have such reductions in N1 amplitudes and in fact, displayed increased N1 amplitudes in hippocampal sites. These studies provide further electrophysiological evidence that rats with a genetically influenced preference for ethanol consumption differ from nonpreferring rats at baseline and have a less intense depressant or more stimulating response to ethanol challenge.

Toward an analogue of alcoholism in mice: analysis of nongenetic variance in consumption of alcohol

Drinking behavior of the isogenic mouse strain C57BL/6J was analyzed into nongenetic components: stochastic fluctuations, responses to fluctuations in the current environment, and persistent differences between individual animals. The latter accounted for the major part of the variance. The variance was neither increased by differences in diet during the postweaning rapid growth period (prior to assay for drinking choice) nor diminished by uniformity of treatment during this period, suggesting that significant differentiation had occurred prior to weaning. The large variance between animals could be explained by assuming that the genetic role in consumption of alcohol by C57BL mice is permissive--a relative insensitivity to the aversive orosensory and pharmacological effects of 10% alcohol--rather than a specific drug-seeking predisposition.

Essential fatty acids, prostaglandins, and alcoholism: an overview

Essential fatty acids (EFAs) are major structural components of the brain and through their effects on membrane properties can influence nerve conduction, transmitter release, and transmitter action. Prostaglandins (PGs) derived from EFAs have profound behavioral effects and are also able to modify conduction and transmitter function. Effects of alcohol on EFAs and PGs are therefore good candidates for explaining at least some of the actions of alcohol on brain function. Ethanol has three main known actions on EFA and PG metabolism: it reduces blood linoleic acid levels and induces or exaggerates EFA deficiency states; it blocks metabolism of linoleic acid to EFA metabolites which are known to be important in brain structure; and it enhances conversion of the linoleic acid metabolite, dihomo-gamma-linolenic acid, to PGE1. This review demonstrates that some of the short-term behavioral effects of ethanol and some of its long-term adverse effects on brain, liver, and other tissues may be partly explicable in terms of ethanol actions on EFA and PG metabolism. Modification of such metabolism by dietary and other means has already been shown to influence the effects of alcohol and alcohol withdrawal in both humans and animals. This promises to be a fruitful source of investigation with substantial implications for the understanding and treatment of alcoholism.

An experimental model of self-intoxication in C57 mice

Male C57BL/6J mice offered unrestricted access to food, water and 10% ethanol, exhibited obvious intoxication when treated with the alcohol dehydrogenase inhibitor, 4-methylpyrazole (4MP) by chronic infusion. Plasma concentrations of ethanol ranged from 156 +/- 43 mg/dl at midday to 254 +/- 31 mg/dl at midnight producing a twenty-fold increase in the total exposure to blood alcohol. Illness sufficiently severe to require intervention occurred in five of the ten mice in the experimental group, while controls treated with 4MP and offered only water to drink displayed no adverse effects. The continuation of drinking despite life-threatening toxicity suggests these mice failed to make an association between the consumption of ethanol and its consequences.

Voluntary consumption of ethanol and its consequences in C57 mice treated with 4-methylpyrazole

Daily injections of the alcohol dehydrogenase inhibitor 4-methylpyrazole (4MP) were administered to C57BL/6J mice offered continuous free access to food, water and 10% v/v ethanol. There was a significant correlation (r = -0.82) between the rate of ethanol consumption during pretreatment and the effect of 4MP on subsequent intake. Mice drinking more than 2.5 g/kg per day decreased their intake, while subjects drinking less than this amount increased the quantity of ethanol self-administered. The elevated concentrations of plasma ethanol which resulted from voluntary consumption were sufficient to produce intoxication but did not induce physical dependence. Presenting mice with 10% ethanol as their only fluid or offering them a choice of water and saccharin-sweetened ethanol increased intake but failed to raise plasma ethanol to the concentrations observed in mice offered unflavored ethanol and water, and treated with 4MP. The evidence suggests that plasma ethanol does not limit voluntary drinking in untreated mice and that concentrations of 135 to 250 mg/dl are not avoided by C57 mice in a free-choice situation.

Toward an analogue of alcoholism in mice: scale factors in the model

Mice of the C57BL strain, given continuous access to 10% alcohol and plain water, with unlimited food and no stress, frequently drink enough alcohol to produce intoxicating levels in the blood. Nevertheless, this behavior does not appear to replicate the essential features of human alcoholism since the drinking lacks serious toxic effects and the intoxication occurs only as transient episodes in association with homeostatic consumption of fluid and food. It is suggested that continuous monitoring of intake and estimation of the concentration of alcohol in blood, which are now technically feasible, will permit distinction between alcoholic-type drinking and a simple licking for the flavor of alcohol in beverage concentration.

Initial sensitivity and acute tolerance to ethanol in the P and NP lines of rats

We recently reported that selectively bred, alcohol-preferring (P) and alcohol-nonpreferring (NP) rats differ in sensitivity to a single sedative-hypnotic dose of ethanol, as measured by performance in the jump test. The present study examines the contributions of initial sensitivity and acute tolerance development to this difference. Initial sensitivity, assessed by brain alcohol content upon loss of the aerial righting reflex, was not significantly different between P and NP groups given 3 g ethanol/kg body weight intraperitoneally. Acute tolerance was indexed from blood alcohol concentrations (BAC) upon recovery of jumping performance following two successive ethanol doses. Practiced P and NP rats were required to jump 35 cm to a descending platform following the IP injection of 2.0 g ethanol/kg. The NP group took significantly longer (74 min) than the P (33 min) group whereupon BAC1 of NP rats (234 mg%) was significantly lower than that of P rats (250 mg%). A second injection (1.0 g/kg) was given immediately after the animals reached the 35 cm criterion. Again, NP rats took significantly longer (124 min) than P rats (52 min) to jump 35 cm and BAC2 of NP animals was lower (295 mg%) than that of P rats (343 mg%). The difference between BAC2 and BAC1, the measure of tolerance development, was significantly larger for P rats (90 mg%) than for NP rats (61 mg%). No significant differences in blood ethanol elimination were observed between the groups. The data indicate no difference in initial sensitivity between P and NP animals but that P rats develop acute tolerance more rapidly and/or to a greater degree than do NP rats. The results are consistent with a relationship in these selectively bred lines of rats between alcohol preference and the development of acute tolerance.