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. Author manuscript; available in PMC: 2017 Dec 27.
Published in final edited form as: Alcohol Clin Exp Res. 2009 Oct 23;34(1):81–89. doi: 10.1111/j.1530-0277.2009.01069.x

Alcohol, Cocaine, and Brain Stimulation-Reward in C57Bl6/J and DBA2/J Mice

Eric W Fish 1, Thorfinn T Riday 1, Megan M McGuigan 1, Sara Faccidomo 1, Clyde W Hodge 1, C J Malanga 1
PMCID: PMC5744672  NIHMSID: NIHMS170478  PMID: 19860803

Abstract

Background

Pleasure and reward are critical features of alcohol drinking that are difficult to measure in animal studies. Intracranial self-stimulation (ICSS) is a behavioral method for studying the effects of drugs directly on the neural circuitry that underlies brain reward. These experiments had 2 objectives: first, to establish the effects of alcohol on ICSS responding in the C57Bl6/J (C57) and DBA2/J (DBA) mouse strains; and second, to compare these effects to those of the psychostimulant cocaine.

Methods

Male C57 and DBA mice were implanted with unipolar stimulating electrodes in the lateral hypothalamus and conditioned to spin a wheel for reinforcement by the delivery of rewarding electrical stimulation (i.e., brain stimulation-reward or BSR). Using the curve-shift method, the BSR threshold (θ0) was determined immediately before and after oral gavage with alcohol (0.3, 0.6, 1.0, 1.7 g/kg) or water. Blood alcohol concentration (BAC) was measured to determine the influence of alcohol metabolism on BSR threshold. Separately, mice were administered cocaine (1.0, 3.0, 10.0, 30.0 mg/kg) or saline intraperitoneally.

Results

In C57 mice, the 0.6 g/kg dose of alcohol lowered BSR thresholds by about 20%, during the rising (up to 40 mg/dl), but not falling, phase of BAC. When given to the DBA mice, alcohol lowered BSR thresholds over the entire dose range; the largest reduction was by about 50%. Cocaine lowered BSR thresholds in both strains. However, cocaine was more potent in DBA mice than in C57 mice as revealed by a leftward shift in the cocaine dose–response curve. For both alcohol and cocaine, effects on BSR threshold were dissociable from effects on operant response rates.

Conclusions

In C57 and DBA mice, reductions in BSR threshold reflect the ability of alcohol to potentiate the neural mechanisms of brain reward. The DBA mice are more sensitive to the reward-potentiating effects of both alcohol and cocaine, suggesting that there are mouse strain differences in the neural mechanisms of brain reward that can be measured with the ICSS technique.

Keywords: Ethanol, Psychomotor Stimulant, Intracranial Self-Stimulation, Behavioral Genetics, Dopamine

The pleasure of mild intoxication is one reason why people drink alcohol. This state is associated with rising blood alcohol levels, relaxation, conviviality, increased interest in rewarding activities, and in some individuals, motor stimulation and activation (King et al., 2002; Lukas and Mendelson, 1988; Pohorecky, 1977; Williams, 1966). However, when more severe intoxication develops, or as blood alcohol levels fall, the opposite effects ensue, namely dysphoria, sedation, and withdrawal from rewarding activities (Smith et al., 1975; Williams, 1966). The neural mechanisms for the pleasurable effects of alcohol are thought to involve the mesolimbic motor circuitry (Weiss and Porrino, 2002) interfacing motivation and behavior. As blood alcohol levels rise, there is rapid activation of neurons that project from the ventral tegmental area (VTA) to portions of the ventral striatum and frontal cortex (Gilman et al., 2008; Williams-Hemby and Porrino, 1997). These projections are essential for naturally motivated behaviors (e.g., feeding, drinking, and social activities; Wise, 2005) and key substrates for the actions of many drugs of abuse.

One behavioral technique for studying drug effects on mesolimbic circuitry in nonhuman animals is intracranial self-stimulation (ICSS) (Kornetsky and Bain, 1992; Wise, 2002). In ICSS, animals respond to be reinforced by the delivery of rewarding electrical current directly into the brain (i.e., brain stimulation-reward, BSR) (Olds and Milner, 1954). Drugs that potentiate the activation of the mesolimbic system and are pleasurable when used by humans reduce the amount of current that is necessary to maintain responding (Bauco and Wise, 1994; Esposito and Kornetsky, 1977; Gilliss et al., 2002; Kenny et al., 2009; Malanga et al., 2008; Wise and Munn, 1993). Thus, reductions in the BSR threshold are thought to reflect pleasurable activation of the mesolimbic reward system. On the other hand, drugs and withdrawal states that humans find aversive increase the BSR threshold in rodents (Johnson et al., 2008; Kenny et al., 2006; Kornetsky and Bain, 1992; Schulteis et al., 1995). Unlike other behavioral techniques for studying the rewarding and reinforcing effects of drugs, ICSS reflects mesolimbic activity during an intoxicated state. In addition, ICSS bypasses factors that potentially confound drug self-administration studies such as taste palatability, hunger, and thirst because the drug is given by an experimenter.

To date, ICSS experiments investigating the reward-potentiating effects of alcohol have been performed exclusively in rats. Early studies (e.g., Vrtunski et al., 1973) relied solely on maximal response rates as the behavioral index of reward and suggested that alcohol can increase responding for BSR. More recent studies reveal that self- or experimenter-administered alcohol can lower BSR thresholds without changing response rates, indicating a potentiation of brain reward circuitry (Eiler et al., 2007; Lewis and June, 1994; Moolten and Kornetsky, 1990). Others have found increases or no effect on stimulation thresholds (Carlson and Lydic, 1976; Ghosh et al., 1991; Schaefer and Michael, 1987). Given the extensive use of mice in behavioral and molecular neurobiological alcohol research, studies using ICSS to investigate alcohol reward in this species are highly desirable.

One of the most consistent murine contributions to alcohol studies is the pronounced difference between the C57Bl6/J (C57) and DBA2/J (DBA) strains. The C57 strain drinks much more alcohol in both voluntary and schedule-controlled conditions (McClearn and Rodgers, 1959; Risinger et al., 1998) but is generally less sensitive than the DBA strain to other measures of alcohol reward, including motor stimulation (Crabbe et al., 1982), sensitization (Phillips et al., 1994), conditioned place preference (Cunningham et al., 1992) and the acquisition of alcohol discriminative stimulus cues (Shelton and Grant, 2002). The DBA strain does not drink significant amounts of alcohol, which is due in part, to taste or olfactory sensitivity (Belknap et al., 1977; Grahame and Cunningham, 1997; McClearn and Rodgers, 1959), but shows more dopaminergic activation by alcohol, as measured by increases in VTA cell firing (Brodie and Appel, 2000) and stimulated dopamine release in the nucleus accumbens (Kapasova and Szumlinski, 2008). However, these strain differences are not limited to alcohol, as DBA mice also appear to be more sensitive to the rewarding effects of cocaine (Cazala, 1976; Orsini et al., 2005; van der Veen et al., 2007).

The current study was performed to determine the effects of acute alcohol and cocaine administration on BSR thresholds in C57 and DBA mice calculated by the “curve-shift” method of ICSS (Carlezon and Chartoff, 2007; Miliaressis et al., 1986). This behavioral measure of the reactivity of the mesolimbic circuitry informs about how these mice experience the acute intoxicated state. Lowering of BSR thresholds is interpreted as potentiation of mesolimbic activity that is perceived as rewarding, whereas raising BSR thresholds is interpreted as depreciation of the rewarding value of electrical stimulation. Alcohol and cocaine were compared to determine if the hypothesized strain differences were specific to alcohol or reflected generalized differences in the responsiveness of the mesolimbic dopamine system to drugs of abuse with different pharmacological mechanisms of action. Blood alcohol concentrations measured at multiple time points after administration allow correlation of BSR thresholds with the rising or falling phases of intoxication.

MATERIALS AND METHODS

Mice

Male C57Bl6/J (n = 26) and DBA2/J (n = 35) mice weighing at least 22 g were purchased from Jackson Labs (Bar Harbor, ME) and initially housed in groups of 4 in clear, polycarbonate cages (28 × 17 × 14 cm) lined with cob bedding and covered with stainless steel wire lids. Cages were changed with fresh bedding once a week. Food (Purina rodent chow) and tap water were freely available. The vivarium was 21 ± 1°C, 30 to 40% humidity, and on 12-hour dark/light cycle (lights off at 8:00 AM). All procedures were approved by the Institutional Animal Care and Use Committee (IACUC) of the University of North Carolina and conducted according to the Guide for the Care and Use of Laboratory Animals (NIH publication No. 85–23, revised 1996).

After 1 week of acclimation to the laboratory, mice were anesthetized with ketamine (120 mg/kg) and xylazine (18 mg/kg) (Sigma, St Louis, MO) and stereotaxically implanted with an insulated monopolar stainless steel electrode (0.1 mm diameter, Plastics One, Roanoke, VA) aimed at the right medial forebrain bundle at the level of the lateral hypothalamus (LH) (AP −1.2; ML −1.0; DV −5.0), using coordinates from (Paxinos and Franklin, 1996). The electrode was attached to a stainless steel screw that served as the electrical ground and was mounted on the skull with dental cement. Postoperatively, the mice recovered for 1 hour on a heating pad and were housed individually for the duration of the experiment.

Apparatus and Procedure

Behavioral testing began 1 week after electrode implantation. Mice were placed in a sound-attenuated operant conditioning chamber (16 × 14 × 13, Med Associates, St Albans, VT) with a grid floor (ENV-005A; Med Associates), wheel manipulandum (ENV-113AM; Med Associates), and house light (ENV-315W; Med Associates). Each chamber was connected to a computer interface and running software (MED-PC for Windows, version 4.1; Med Associates) that recorded wheel spins, controlled the light, and issued the delivery of electrical current (brain stimulation-reward, BSR) through a stimulator (PHM-150B/2; Med Associates). Electrodes were connected to the stimulator through a swivel commutator and insulated wire (Plastics One). Wheel spinning was shaped by the delivery of a brief (500 ms) unipolar cathodal square-wave current at a frequency of 158 (pulse width = 100 μs) and illumination of the house light (500 ms), to reinforce each successive approximation of a quarter turn of the wheel. Once the mouse readily spun the wheel, each quarter turn was reinforced by electrical stimulation. Responses made during the 500 ms stimulation period were recorded but earned no additional stimulation. Current was adjusted for each individual mouse and held constant throughout the experiment at the lowest intensity that maintained at least 40 responses/min (−65 to −180 μA).

After optimizing current intensity, the stimulation frequency was decreased in a series of 15 discrete 0.05 log10 steps. The mice responded for each frequency in 1-minute intervals beginning with a 10-second phase during which reinforcing stimulation was presented 5 times noncontingently. For the next 50 seconds, stimulation was freely available. At the end of the interval the next descending step began. During the conditioning phase, each series was presented 4 times (60-minute session) and the stimulation frequency range was adjusted so that each mouse responded for only the 5 to 7 highest frequencies. The threshold to maintain responding (θ0) was defined as the x-intercept of the least-squares regression line through the frequencies that sustained 20, 30, 40, 50, and 60% of the maximal response rate and was calculated using custom-designed software (courtesy of William A. Carlezon, McLean Hospital, Belmont, MA). This method of threshold determination is less sensitive to changes in maximal response rates than are other calculations (e.g., frequency maintaining half-maximal responding, EF50; Miliaressis et al., 1986). When BSR thresholds varied by less than 10% on 3 consecutive days, the mice were habituated to oral and intraperitoneal injections and the drug testing phases began.

Each test session consisted of a 45-minute preinjection period during which the mice responded for 3 series of 15 descending stimulation frequencies. The mice were removed from the chamber, injected, and returned immediately for a 60-minute postinjection period during which the mice responded for 4 series of 15 descending stimulation frequencies were presented. Preinjection BSR thresholds were calculated from the average of the second and third series (the first series was considered a “warm-up” and was discarded) and used as daily baselines for comparison to the postinjection BSR thresholds calculated for each 15-minute series after drug or vehicle injection.

Drugs

Ethyl alcohol (Pharmaco-Aaper, Brookfield, CT) solutions were prepared w/v in tap water and injected orally via a stainless steel feeding tube in a volume of 1 ml/100 g body weight. Cocaine hydrochloride (Sigma) was dissolved in 0.9% saline and injected i.p. through a 30 gauge needle in a volume of 1 ml/100 g body weight (calculated as free base). Drug injections were given in a counter balanced sequence on alternating days separated by a vehicle (water or saline) injection. Each mouse received each alcohol dose twice. Following the alcohol dose-effect determination, the same mice received each cocaine dose once. Blood alcohol levels were measured in separate mice (n = 15 of each strain) through tail blood collected 5-, 15-, 30-, and 60-minute after oral administration of the 0.6 or 1.7 g/kg dose. Plasma was separated and 5 μl was processed using the AM1 alcohol analyzer (Analox Instruments, Lunenberg, MA).

Histology

At the end of the experiment, mice were deeply anesthetized with sodium pentobarbital (120 mg/kg) and intracardially perfused with 0.9% saline followed by 4% paraformaldehyde in 0.1 M PBS. The brains were removed and sectioned (50 μm) on a sliding microtome and stained with cresyl violet Nissl stain and viewed under low-powered (4×) light microscopy to determine the location of the most ventral electrode tip placements.

Statistical Analysis

To compare baseline BSR thresholds between the C57 and DBA strains, thresholds were calculated as the total charge delivered in Coulombs (μA × μs) and analyzed with an unpaired t-test. All data on BSR thresholds and maximum response rates were expressed as a percent change from the preinjection baseline for each day. Multiple drug and vehicle determinations were averaged into a single value after initial analysis of variance (ANOVAs) revealed no significant effects of replication. Two-way ANOVAs with one between factor (strain) and one within factor (dose or time) with post hoc Dunnett’s tests were used to compare the effects of different cocaine and alcohol doses to their respective vehicle treatments and blood alcohol content after the 0.6 or 1.7 g/kg alcohol dose.

RESULTS

The electrode placements are shown in Fig. 1. Both strains of mice were conditioned to spin the wheel within 2 test sessions, most within the first minutes of the first session. There were qualitative differences between the strains during the acquisition phase. The 11 C57 mice implanted with the electrodes spun the wheel to be reinforced by BSR. The DBA mice were initially more reactive to the electrical stimulation and lower intensities (i.e., <−100 μA) were used to shape wheel spinning. Eleven of the DBA mice with more rostral MFB placements demonstrated aversive reactions (i.e., flattened body posture, retropulsion, jumping, and avoidance of the wheel) even to low intensity stimulation and were not studied further. The remaining 9 DBA mice with more caudal placements performed the operant at a slightly lower maximum response rate than did the C57 mice (63.3 ± 3.8 vs. 81.2 ± 9.0 r/50 s, respectively), but this difference was not statistically significant (p = 0.10). Importantly, both strains responded in a frequency dependent manner (Fig. 2) and the BSR threshold, as expressed as total charge delivered in Coulombs, was similar between the 2 strains (Fig. 2, inset).

Fig. 1.

Fig. 1

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Placement of intracranial self-stimulation electrodes in C57Bl6/J and DBA2/J mice. All electrodes were aimed at the right medial forebrain bundle at the level of the lateral hypothalamus. For clarity, placements for DBA and C57 mice are shown on the left and right, respectively. Circles represent the electrode tip as visualized by manual inspection of Nissl-stained brain sections. Black circles denote C57 mice. Filled gray circles denote DBA mice that responded for brain stimulation-reward. Open circles denote DBA mice that did not respond for brain stimulation-reward.

Fig. 2.

Fig. 2

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Responding for BSR in C57Bl6/J and DBA2/J mice. Stimulus intensity (μA) was adjusted for each individual mouse to achieve a maximum response rate of at least 40 responses/50 s (y-axis) and ranged from −65 to −180 μA between mice. The frequency of stimulation (Hz) varied across 1-minute trials in a descending sequence of discrete 0.05 log10 steps (x-axis). Between mice, the highest stimulation frequencies ranged between 158 and 100 Hz and the lowest stimulation frequencies ranged between 32 and 19 Hz. Data are expressed as the mean number of responses (± SEM, vertical lines). Filled black circles and filled gray triangles represent data from C57 and DBA mice, respectively. Inset: The amount of charge in Coulombs (μA × μs) required to maintain responding in C57 and DBA mice at BSR threshold (θ0). Data are expressed as the mean threshold (± SEM, vertical lines) for C57 (filled black bars) and DBA (filled gray bars) mice.

Alcohol Dose-Effects

Alcohol produced dose-dependent effects on ICSS responding that depended on the strain and time after injection. Each 15-minute time interval after alcohol administration was analyzed separately. In the first 15-minute interval there was a significant interaction between alcohol dose and mouse strain on θ0 [F(4,99) = 8.7; p < 0.001; Fig. 3A far left panel] and maximum response rate [MAX; F(4,99) = 9.6; p < 0.001; Fig. 3B far left panel]. In C57 mice, the 0.6 g/kg alcohol dose lowered θ0, while in the DBA mice the 0.6 to 1.7 g/kg doses dose dependently lowered θ0. θ0 was lower in DBA than in C57 mice after the 1.0 and 1.7 g/kg doses. The 1.0 and 1.7 g/kg doses increased MAX in the DBA mice but did not affect MAX in the C57 mice. MAX response rate was higher in the DBA mice than in the C57 mice after the 0.6 to 1.7 g/kg doses.

Fig. 3.

Fig. 3

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Dose–response relationship for alcohol on BSR thresholds and maximum response rates in C57Bl/6J and DBA2/J mice. BSR thresholds (θ0, Figure A, y-axis) and maximum response rates (MAX, Figure B, y-axis) were measured before and after oral injection and are expressed as mean (± 1 SEM, vertical lines) percent change from the preinjection baseline for C57 (n = 11, circles) and DBA (n = 9, triangles) mice given water (V, open symbols) or alcohol (0.3, 0.6, 1.0, 1.7 g/kg, filled symbols). Each panel (left to right) represents the effects on BSR thresholds and MAX rates during the four 15-minute intervals following the injection. Asterisks denote significance (p < 0.05) vs. vehicle. Arrows denote significance (p < 0.05) vs. C57 mice.

In the second 15-minute interval, there was a significant interaction between alcohol dose and mouse strain on the θ0 [F(4,99) = 5.2; p = 0.001; Fig. 3A left center panel], and MAX response rate [F(4,99) = 6.7; p < 0.001; Fig. 3B left center panel]. The 1.7 g/kg dose lowered θ0 in DBA mice and θ0 was lower in these mice than in the C57 mice after the 0.6 to 1.7 g/kg doses. In DBA mice, the 1.0 and 1.7 g/kg doses increased MAX, which was higher than in the C57 mice treated with these doses.

In the third 15-minute interval, there was a significant interaction between alcohol dose and mouse strain on the θ0 [F(4,99) = 3.2; p = 0.02; Fig. 3A right center panel] and MAX response rate [F(4,99) = 6.8; p < 0.001; Fig. 3B right center panel]. In DBA mice, the 1.0 and 1.7 g/kg doses increased MAX response rate, but did not affect θ0. However, θ0 was significantly lower in the DBA mice than in the C57 mice after the 0.3, 1.0, and 1.7 g/kg doses.

In the fourth 15-minute interval, there was a significant interaction between alcohol dose and mouse strain on the MAX response rate [F(4,99) = 3.4; p < 0.01; Fig. 3B, fourth panel], but not θ0. The 1.7 g/kg dose increased MAX in the DBA mice and MAX was higher in DBA mice than in C57 mice.

Blood Alcohol Concentration After 0.6 or 1.7 g/kg

Blood alcohol concentrations were similar in both strains at all time points after the 0.6 g/kg dose (Fig. 4). There were no interactions between mouse strain and time of measurement. BAC rose rapidly to between 40 and 45 mg/dl within the first 15 minutes and declined almost entirely by the end of the hour. After the 1.7 g/kg dose, BAC rose to between 140 and 145 mg/dl in both strains. This peak occurred 15 minutes after injection in the DBA mice and 30 minutes after injection in the C57 mice. There was a significant interaction between time after injection and mouse strain [F(3,54) = 4.0; p = 0.012; Fig. 4]. DBA mice had lower a BAC 60 minutes after injection (p = 0.05).

Fig. 4.

Fig. 4

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Blood alcohol concentrations after oral injection of 0.6 or 1.7 g/kg alcohol in C57Bl6/J and DBA2/J mice. The mean (± 1 SEM, vertical lines) blood alcohol concentrations (mg/dl, y-axis) were determined from tail blood samples taken 5, 15, 30, and 60 minutes after alcohol administration in C57 (filled black circles; n = 5, 0.6 g/kg dose; n = 10, 1.7 g/kg dose) and DBA (filled gray triangles; n = 5, 0.6 g/kg dose; n = 10, 1.7 g/kg dose) mice. Asterisks denote significance (p = 0.05) vs. C57 mice.

Cocaine Dose-Effects

The effects of cocaine on ICSS responding depended on drug dose, mouse strain, and time interval. As with alcohol, the four 15-minute time intervals were analyzed separately. In the first 15-minute interval, there was a significant interaction between dose and strain on the θ0 [F(4,84) = 3.6; p = 0.01; Fig. 5A far left panel] and MAX response rate [F(4,84) = 2.7; p = 0.04; Fig. 5B far left panel]. In the C57 mice, the 1.0 mg/kg dose raised θ0 while the 3.0 to 30.0 mg/kg doses lowered θ0 in both C57 and DBA mice. θ0 was lower in the DBA mice than in the C57 mice after the 1.0 to 10.0 mg/kg doses. The 10.0 and 30.0 mg/kg doses increased MAX in the DBA mice more than in the C57 mice.

Fig. 5.

Fig. 5

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Dose–response relationship for cocaine on BSR thresholds and maximum response rates in C57Bl/6J and DBA2/J mice. BSR thresholds (θ0, Figure A, y-axis) and maximum response rates (MAX, Figure B, y-axis) were measured before and after i.p. injection and are expressed as mean (± 1 SEM, vertical lines) percent change from the preinjection baseline for C57 (n = 9, circles) and DBA (n = 6, triangles) mice given saline (V, open symbols) or cocaine (1.0, 3.0, 10, 30 mg/kg, filled symbols). Each panel (left to right) represents the effects on BSR thresholds and MAX rates during the four 15-minute intervals following the injection. Asterisks denote significance (p < 0.05) vs. vehicle. Arrows denote significance (p < 0.05) vs. C57 mice.

There were no significant overall interactions on θ0 or MAX in the second, third, or fourth time intervals after cocaine injection. There were main effects of both strain and dose, but these were not analyzed further due to potential strain and dose interactions that likely contributed significance to the main effects.

DISCUSSION

C57Bl6/J and DBA2/J mice are 2 strains commonly used in drug abuse research because of their differential responses to alcohol, psychomotor stimulants, and opiates. The current experiments used these strains to study the effects of alcohol and cocaine on brain stimulation-reward (BSR), the positive reinforcer delivered by the operant behavior of intracranial self-stimulation (ICSS). Baseline reward thresholds were similar between C57 and DBA mice, despite a small difference in their maximal rate of responding. Alcohol and cocaine, given separately, dose-dependently lowered thresholds in both strains. However, the strains differed in their sensitivity to the effects of alcohol and cocaine on threshold and maximum response rates. An alcohol dose of 0.6 g/kg lowered thresholds to a similar degree in both the C57 and DBA mice, but higher doses that lowered thresholds even further in DBA mice were ineffective in C57 mice. The results provide evidence for the reward-potentiating effects of alcohol in mice. They also demonstrate a novel application of a well-characterized behavioral method to use mice to study how alcohol alters the function of the mesolimbic brain reward system.

Intracranial self-stimulation has been studied extensively since the first experiments by Olds and Milner (1954) and is appreciated as a behavioral method for measuring the function of the limbic motor system involved with motivation, action, and reward. Dopamine is necessary but not sufficient for BSR which is also modulated by several neurotransmitters including glutamate, acetylcholine, serotonin, GABA, and endogenous opiates (Wise, 1996). Quantifying ICSS responses has greatly improved from measuring simple response rates to more precise calculation of BSR thresholds (Miliaressis et al., 1986). The present experiment used the “rate-frequency” or “curve-shift” method that calculates threshold by measuring responding for varying stimulation frequencies (Carlezon and Chartoff, 2007; Miliaressis et al., 1986). This method has previously been shown to be reliable in outbred Swiss-Webster and inbred C57 mice for the detection of the reward-potentiating effects of cocaine and DA receptor agonists (Gilliss et al., 2002; Malanga et al., 2008). DBA mice can also respond for BSR in a frequency dependent manner and the BSR threshold is similar between the DBA and C57 strains as was observed with FVB and 129S6 mice (Fish and Malanga, unpublished data). It appears that DBA mice may have a more restricted anatomy that supports BSR based on the behavioral differences observed with more rostral electrode placements in the lateral hypothalamus. This may possibly relate to the distribution of the dopamine system in the DBA strain (D’Este et al., 2007) but future anatomical and physiological mapping studies will confirm this, perhaps through the use of a bipolar ICSS electrode which offers a more localized injection of current. Nonetheless, it seems unlikely that electrode placement accounts for the C57 and DBA strain difference in response to alcohol and cocaine because C57 mice with rostral or caudal placements had similar dose sensitivities.

Alcohol lowered BSR thresholds in both strains, providing evidence that acute alcohol intoxication can be rewarding in mice. This interpretation is based on a model of reward perception in which the effects of alcohol and electrical brain stimulation on mesocorticolimbic activity sum to increase the total amount of positive reinforcement experienced as the mice respond for a given stimulation frequency. This finding replicates earlier studies in rats, where alcohol administration directly lowered BSR thresholds (Eiler et al., 2007; Lewis and June, 1994; Moolten and Kornetsky, 1990) and is consistent with chronic exposure studies that find elevated BSR thresholds upon the removal of alcohol from dependent rats (Rylkova et al., 2008; Schulteis et al., 1995). In the current study, 0.6 g/kg alcohol lowered BSR threshold by approximately 20% in both C57 and DBA mice, an effect that occurred in the first 15 minutes after administration and was not significant in either strain at later time points. Notably, this time course followed that of rising blood alcohol concentrations. In humans, reports of euphoria and locomotor stimulation are greater during the rising—rather than in the falling—phases of blood alcohol (Holdstock et al., 2000; Lukas and Mendelson, 1988; Smith et al., 1975). The BAC peaked between 40 and 45 mg/dl, corresponding to the levels achieved by C57 mice during standard 1-hour self-administration sessions (Faccidomo et al., 2009; Fish and Malanga, unpublished data; Middaugh et al., 2003). This suggests that brain reward thresholds may be lowered during the initial phases of a drinking session. The potentiation of reward may increase the magnitude of reinforcement obtained by subsequent drinking behavior, thereby driving continued alcohol consumption in a manner resembling feed-forward facilitation. Confirmation of this hypothesis awaits studies combining the ICSS and self-administration methods.

Although the effects of the 0.6 g/kg dose of alcohol were similar in C57 and DBA mice, higher alcohol doses produced significantly different effects on BSR thresholds in the 2 strains. The 1.0 and 1.7 g/kg doses were ineffective or tended to increase BSR thresholds in C57 mice, whereas these doses lowered BSR thresholds further in the DBA mice. BAC measured after the 1.7 g/kg dose revealed similar content in the rising phase, but a tendency for DBA mice to have lower BAC 60-minutes after alcohol injection; this difference is inconsistent with previous strain comparisons that reported both no difference and faster metabolism in C57 mice (e.g., Crabbe et al., 1982; Rodgers et al., 1963; Sheppard et al., 1970). These falling phase differences are unlikely to account for the differential sensitivity to the effects of the 1.7 g/kg dose on BSR threshold. Similar differences between C57 and DBA mice have been noted on a variety of behaviors, including acute locomotor activity (Crabbe, 1986) and conditioned place preference (Cunningham et al., 1992). In all these tests, DBA mice show a response that suggests they are more sensitive to the rewarding effects of alcohol. The present data extend these strain differences to brain stimulation-reward. Unfortunately, a strong taste aversion limits the DBA mouse’s intake of alcohol and complicates strain comparisons on the relationship between reward and alcohol drinking. A further limitation of the current study is that alcohol administration occurs acutely, to an individual with limited prior alcohol experience. Thus, the current results do not account for effects such as expectancies or changes in baseline reward thresholds that are acquired after chronic exposure and dependence (Koob, 2003).

One concern is that strain differences in the effects of alcohol on locomotor activity may influence operant performance and thereby alter the effects of alcohol on BSR thresholds as determined by a rate-dependent measure. The curve-shift method allows for a measure of locomotor activity, the maximum (MAX) rate of responding. While this is an indirect measure of motor activity, it does address whether or not a drug affects the capacity to respond for BSR. It should be noted that the primary effect of alcohol on fixed ratio responding in mice is to decrease high response rates, even at doses that stimulate locomotor behavior (Balster et al., 1992, 1993; Elmer and George, 1995). For BSR in DBA mice, the highest alcohol doses increased responding by about 20%, to the baseline level of the C57 mice. Thus, there were no differences between the C57 and DBA mice on absolute response. There was also a temporal dissociation between the effects of 1.7 g/kg alcohol on BSR thresholds and responding; the increase in MAX persisted longer than the lowering of BSR threshold. Most important, even at doses that stimulate locomotor activity, responding for BSR maintained stimulus control. The mice did not respond continuously during the rate-frequency interval; they responded only for the higher frequencies, with a shift toward greater responses at the lower end of the frequency range. No behavioral method is currently capable of entirely distinguishing between drug effects on motor activity and those on brain reward. Particularly in animal models, locomotor stimulation may be a component of reward (Wise and Bozarth, 1984) and whether increases in activity and positive reinforcement are distinct drug effects remain an open question.

The neural mechanisms through which alcohol lowers BSR thresholds likely involve dopamine. Dopamine is an important component of alcohol reinforcement (for review see Weiss and Porrino, 2002) and is essential for BSR (Wise, 1980). Importantly, the dopamine system is rapidly activated after alcohol administration (Gilman et al., 2008; Williams-Hemby and Porrino, 1997), which increases the firing of DA cell bodies in the VTA (Brodie et al., 1990; Okamoto et al., 2006; McDaid et al., 2008) and regulates of the amount of dopamine released in forebrain targets (Budygin et al., 2001). In the nucleus accumbens, there are prolonged elevations in extracellular DA concentrations that depend on the alcohol dose (Imperato and Di Chiara, 1986). In both DBA and C57 mice, ventral striatal DA concentrations have been shown to peak 15 minutes after injection with alcohol and decline over the course of an hour (Ramachandra et al., 2007; Tang et al., 2003). It is noteworthy that the rise in blood alcohol levels and lowering of BSR thresholds share a similar time course to the rise in extracellular DA concentrations previously reported. As blood alcohol levels and extracellular DA concentrations decline, BSR thresholds return to baseline or become elevated, particularly in C57 mice. During the falling phase of the BAC, it is quite likely that positive modulation of the GABAA receptor complex (Nie et al., 2000) and/or antagonism of NMDA receptors (Nie et al., 1994) in the nucleus accumbens attenuate the effects of alcohol on BSR thresholds. Microinjection of GABAA receptor positive modulators and NMDA receptor antagonists block the reinforcing and place conditioning effects of alcohol (Gremel and Cunningham, 2008; Hodge et al., 1995; Rassnick et al., 1992) perhaps by directly affecting reward.

Further evidence for the role of dopamine in the strain difference in the acute effects of alcohol on BSR threshold comes from the administration of cocaine. Cocaine lowers thresholds robustly in both strains, but the dose-effect curve is shifted to the left in the DBA mice, compared to the C57 mice. The increased sensitivity of DBA mice to the effects of cocaine on BSR threshold is consistent with prior measures on brain reward (Cazala, 1976) and reinforcement (van der Veen et al., 2007). Brain levels of cocaine were shown to be similar between the 2 strains after i.p. injection (Rocha et al., 1998), eliminating metabolism as a cause of the shift in sensitivity. More likely, the leftward shift in the cocaine dose–response curve may be related to strain differences in the dopamine transporter (D’Este et al., 2007; Janowsky et al., 2001). As another example for dissociation between brain reward thresholds and locomotor stimulation, C57 and DBA mice tend to have similar acute locomotor stimulating effects of cocaine (Janowsky et al., 2001; Orsini et al., 2005; Phillips et al., 1994) but different sensitivity to effects of cocaine on brain reward.

The use of alcohol, like other psychoactive drug use, is motivated by a desire to feel different, most often to feel better. Enhanced positive mood, elation, and euphoria are all quantifiable features of mild alcohol intoxication in humans. The current findings using ICSS in mice reveal that mild alcohol intoxication potentiates the mechanisms of brain reward. The ability to measure changes in brain reward repeatedly and across different drugs and drug doses is a major advantage of the ICSS method. Future studies may be designed to investigate the pharmacological mechanisms for the effects of alcohol on brain reward, the consequences of repeated or chronic alcohol exposure, or experiences that may alter brain reward sensitivity.

Acknowledgments

This research was supported by grants DA 015429 to CJM and AA007573 to the Bowles Center of Alcohol Studies.

References

  1. Balster RL, Wiley JL, Tokarz ME, Knisely JS. Effects of ethanol and toluene on fixed-ratio performance in short sleep and long sleep mice. Drug Alcohol Depend. 1992;31(1):65–75. doi: 10.1016/0376-8716(92)90010-a. [DOI] [PubMed] [Google Scholar]
  2. Balster RL, Wiley JL, Tokarz ME, Tabakoff B. Effects of ethanol and nmda antagonists on operant behavior in ethanol withdrawal seizure-prone and-resistant mice. Behav Pharmacol. 1993;4(2):107–113. [PubMed] [Google Scholar]
  3. Bauco P, Wise RA. Potentiation of lateral hypothalamic and midline mesencephalic brain stimulation reinforcement by nicotine: examination of repeated treatment. J Pharmacol Exp Ther. 1994;271(1):294–301. [PubMed] [Google Scholar]
  4. Belknap JK, Belknap ND, Berg JH, Coleman R. Preabsorptive vs. Postabsorptive control of ethanol intake in c57bl/6j and dba/2j mice. Behav Genet. 1977;7(6):413–425. doi: 10.1007/BF01066776. [DOI] [PubMed] [Google Scholar]
  5. Brodie MS, Appel SB. Dopaminergic neurons in the ventral tegmental area of c57bl/6j and dba/2j mice differ in sensitivity to ethanol excitation. Alcohol Clin Exp Res. 2000;24(7):1120–1124. [PubMed] [Google Scholar]
  6. Brodie MS, Shefner SA, Dunwiddie TV. Ethanol increases the firing rate of dopamine neurons of the rat ventral tegmental area in vitro. Brain Res. 1990;508(1):65–69. doi: 10.1016/0006-8993(90)91118-z. [DOI] [PubMed] [Google Scholar]
  7. Budygin EA, Phillips PE, Robinson DL, Kennedy AP, Gainetdinov RR, Wightman RM. Effect of acute ethanol on striatal dopamine neuro-transmission in ambulatory rats. J Pharmacol Exp Ther. 2001;297(1):27–34. [PubMed] [Google Scholar]
  8. Carlezon WA, Jr, Chartoff EH. Intracranial self-stimulation (ICSS) in rodents to study the neurobiology of motivation. Nat Protoc. 2007;2(11):2987–2995. doi: 10.1038/nprot.2007.441. [DOI] [PubMed] [Google Scholar]
  9. Carlson RH, Lydic R. The effects of ethanol upon threshold and response rate for self-stimulation. Psychopharmacology (Berl) 1976;50(1):61–64. doi: 10.1007/BF00634156. [DOI] [PubMed] [Google Scholar]
  10. Cazala P. Effects of d- and l-amphetamine on dorsal and ventral hypothalamic self-stimulation in three inbred strains of mice. Pharmacol Bio-chem Behav. 1976;5(5):505–510. doi: 10.1016/0091-3057(76)90259-8. [DOI] [PubMed] [Google Scholar]
  11. Crabbe JC. Genetic differences in locomotor activation in mice. Pharmacol Biochem Behav. 1986;25(1):289–292. doi: 10.1016/0091-3057(86)90267-4. [DOI] [PubMed] [Google Scholar]
  12. Crabbe JC, Jr, Johnson NA, Gray DK, Kosobud A, Young ER. Biphasic effects of ethanol on open-field activity: sensitivity and tolerance in c57bl/6n and dba/2n mice. J Comp Physiol Psychol. 1982;96(3):440–451. doi: 10.1037/h0077898. [DOI] [PubMed] [Google Scholar]
  13. Cunningham CL, Niehus DR, Malott DH, Prather LK. Genetic differences in the rewarding and activating effects of morphine and ethanol. Psychopharmacology (Berl) 1992;107(2–3):385–393. doi: 10.1007/BF02245166. [DOI] [PubMed] [Google Scholar]
  14. D’Este L, Casini A, Puglisi-Allegra S, Cabib S, Renda TG. Comparative immunohistochemical study of the dopaminergic systems in two inbred mouse strains (c57bl/6j and dba/2j) J Chem Neuroanat. 2007;33(2):67–74. doi: 10.1016/j.jchemneu.2006.12.005. [DOI] [PubMed] [Google Scholar]
  15. Eiler WJ, II, Hardy L, III, Goergen J, Seyoum R, Mensah-Zoe B, June HL. Responding for brain stimulation reward in the bed nucleus of the stria terminalis in alcohol-preferring rats following alcohol and amphetamine pretreatments. Synapse. 2007;61(11):912–924. doi: 10.1002/syn.20437. [DOI] [PubMed] [Google Scholar]
  16. Elmer GI, George FR. Genetic differences in the operant rate-depressant effects of ethanol between four inbred mouse strains. Behav Pharmacol. 1995;6:794–800. [PubMed] [Google Scholar]
  17. Esposito R, Kornetsky C. Morphine lowering of self-stimulation thresholds: lack of tolerance with long-term administration. Science. 1977;195(4274):189–191. doi: 10.1126/science.831268. [DOI] [PubMed] [Google Scholar]
  18. Faccidomo S, Besheer J, Stanford PC, Hodge CW. Increased operant responding for ethanol in male c57bl/6j mice: specific regulation by the erk(1/2), but not jnk, map kinase pathway. Psychopharmacology (Berl) 2009;204:135–147. doi: 10.1007/s00213-008-1444-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Ghosh TK, Copeland RL, Jr, Alex PK, Pradhan SN. Behavioral effects of ethanol inhalation in rats. Pharmacol Biochem Behav. 1991;38(4):699–704. doi: 10.1016/0091-3057(91)90229-u. [DOI] [PubMed] [Google Scholar]
  20. Gilliss B, Malanga CJ, Pieper JO, Carlezon WA., Jr Cocaine and skf-82958 potentiate brain stimulation reward in swiss-webster mice. Psychopharmacology (Berl) 2002;163(2):238–248. doi: 10.1007/s00213-002-1153-8. [DOI] [PubMed] [Google Scholar]
  21. Gilman JM, Ramchandani VA, Davis MB, Bjork JM, Hommer DW. Why we like to drink: a functional magnetic resonance imaging study of the rewarding and anxiolytic effects of alcohol. J Neurosci. 2008;28(18):4583–4591. doi: 10.1523/JNEUROSCI.0086-08.2008. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Grahame NJ, Cunningham CL. Intravenous ethanol self-administration in c57bl/6j and dba/2j mice. Alcohol Clin Exp Res. 1997;21(1):56–62. [PubMed] [Google Scholar]
  23. Gremel CM, Cunningham CL. Roles of the nucleus accumbens and amygdala in the acquisition and expression of ethanol-conditioned behavior in mice. J Neurosci. 2008;28(5):1076–1084. doi: 10.1523/JNEUROSCI.4520-07.2008. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Hodge CW, Chappelle AM, Samson HH. Gabaergic transmission in the nucleus accumbens is involved in the termination of ethanol self-administration in rats. Alcohol Clin Exp Res. 1995;19(6):1486–1493. doi: 10.1111/j.1530-0277.1995.tb01012.x. [DOI] [PubMed] [Google Scholar]
  25. Holdstock L, King AC, de Wit H. Subjective and objective responses to ethanol in moderate/heavy and light social drinkers. Alcohol Clin Exp Res. 2000;24(6):789–794. [PubMed] [Google Scholar]
  26. Imperato A, Di Chiara G. Preferential stimulation of dopamine release in the nucleus accumbens of freely moving rats by ethanol. J Pharmacol Exp Ther. 1986;239(1):219–228. [PubMed] [Google Scholar]
  27. Janowsky A, Mah C, Johnson RA, Cunningham CL, Phillips TJ, Crabbe JC, Eshleman AJ, Belknap JK. Mapping genes that regulate density of dopamine transporters and correlated behaviors in recombinant inbred mice. J Pharmacol Exp Ther. 2001;298(2):634–643. [PubMed] [Google Scholar]
  28. Johnson PM, Hollander JA, Kenny PJ. Decreased brain reward function during nicotine withdrawal in c57bl6 mice: evidence from intracranial self-stimulation (ICSS) studies. Pharmacol Biochem Behav. 2008;90(3):409–415. doi: 10.1016/j.pbb.2008.03.024. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Kapasova Z, Szumlinski KK. Strain differences in alcohol-induced neurochemical plasticity: a role for accumbens glutamate in alcohol intake. Alcohol Clin Exp Res. 2008;32(4):617–631. doi: 10.1111/j.1530-0277.2008.00620.x. [DOI] [PubMed] [Google Scholar]
  30. Kenny PJ, Chartoff E, Roberto M, Carlezon WA, Jr, Markou A. Nmda receptors regulate nicotine-enhanced brain reward function and intravenous nicotine self-administration: role of the ventral tegmental area and central nucleus of the amygdala. Neuropsychopharmacology. 2009;34(2):266–281. doi: 10.1038/npp.2008.58. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Kenny PJ, Chen SA, Kitamura O, Markou A, Koob GF. Conditioned withdrawal drives heroin consumption and decreases reward sensitivity. J Neurosci. 2006;26(22):5894–5900. doi: 10.1523/JNEUROSCI.0740-06.2006. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. King AC, Houle T, de Wit H, Holdstock L, Schuster A. Biphasic alcohol response differs in heavy versus light drinkers. Alcohol Clin Exp Res. 2002;26(6):827–835. [PubMed] [Google Scholar]
  33. Koob GF. Alcoholism: allostasis and beyond Alcohol. Clin Exp Res. 2003;27(2):232–243. doi: 10.1097/01.ALC.0000057122.36127.C2. [DOI] [PubMed] [Google Scholar]
  34. Kornetsky C, Bain G. Brain-stimulation reward: a model for the study of the rewarding effects of abused drugs NIDA. Res Monogr. 1992;124:73–93. [PubMed] [Google Scholar]
  35. Lewis MJ, June HL. Synergistic effects of ethanol and cocaine on brain stimulation reward. J Exp Anal Behav. 1994;61(2):223–229. doi: 10.1901/jeab.1994.61-223. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Lukas S, Mendelson J. Behavioral concomitants of ethanol and drug reinforcement NIDA. Res Monogr. 1988;81:422–427. [PubMed] [Google Scholar]
  37. Malanga CJ, Riday TT, Carlezon WA, Jr, Kosofsky BE. Prenatal exposure to cocaine increases the rewarding potency of cocaine and selective dopaminergic agonists in adult mice. Biol Psychiatry. 2008;63(2):214–221. doi: 10.1016/j.biopsych.2007.01.014. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. McClearn GE, Rodgers DA. Differences in alcohol preference among inbred strains of mice Quarterly. J Stud Alcohol. 1959;20(4):691–695. [Google Scholar]
  39. McDaid J, McElvain MA, Brodie MS. Ethanol effects on dopaminergic ventral tegmental area neurons during block of ih: involvement of barium-sensitive potassium currents. J Neurophysiol. 2008;100(3):1202–1210. doi: 10.1152/jn.00994.2007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Middaugh LD, Szumlinski KK, Van Patten Y, Marlowe AL, Kalivas PW. Chronic ethanol consumption by c57bl/6 mice promotes tolerance to its interoceptive cues and increases extracellular dopamine, an effect blocked by naltrexone. Alcohol Clin Exp Res. 2003;27(12):1892–1900. doi: 10.1097/01.ALC.0000099264.36220.48. [DOI] [PubMed] [Google Scholar]
  41. Miliaressis E, Rompre PP, Laviolette P, Philippe L, Coulombe D. The curve-shift paradigm in self-stimulation. Physiol Behav. 1986;37(1):85–91. doi: 10.1016/0031-9384(86)90388-4. [DOI] [PubMed] [Google Scholar]
  42. Moolten M, Kornetsky C. Oral self-administration of ethanol and not experimenter-administered ethanol facilitates rewarding electrical brain stimulation. Alcohol. 1990;7(3):221–225. doi: 10.1016/0741-8329(90)90008-z. [DOI] [PubMed] [Google Scholar]
  43. Nie Z, Madamba SG, Siggins GR. Ethanol inhibits glutamatergic neurotransmission in nucleus accumbens neurons by multiple mechanisms. J Pharmacol Exp Ther. 1994;271(3):1566–1573. [PubMed] [Google Scholar]
  44. Nie Z, Madamba SG, Siggins GR. Ethanol enhances gamma-aminobutyric acid responses in a subpopulation of nucleus accumbens neurons: role of metabotropic glutamate receptors. J Pharmacol Exp Ther. 2000;293(2):654–661. [PubMed] [Google Scholar]
  45. Okamoto T, Harnett MT, Morikawa H. Hyperpolarization-activated cation current (ih) is an ethanol target in midbrain dopamine neurons of mice. J Neurophysiol. 2006;95(2):619–626. doi: 10.1152/jn.00682.2005. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Olds J, Milner P. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J Comp Physiol Psychol. 1954;47(6):419–427. doi: 10.1037/h0058775. [DOI] [PubMed] [Google Scholar]
  47. Orsini C, Bonito-Oliva A, Conversi D, Cabib S. Susceptibility to conditioned place preference induced by addictive drugs in mice of the c57bl/6 and dba/2 inbred strains. Psychopharmacology (Berl) 2005;181(2):327–336. doi: 10.1007/s00213-005-2259-6. [DOI] [PubMed] [Google Scholar]
  48. Paxinos GT, Franklin KB. The Mouse Brain in Stereotaxic Coordinates. Academic Press; San Diego: 1996. [Google Scholar]
  49. Phillips TJ, Dickinson S, Burkhart-Kasch S. Behavioral sensitization to drug stimulant effects in c57bl/6j and dba/2j inbred mice. Behav Neurosci. 1994;108(4):789–803. doi: 10.1037//0735-7044.108.4.789. [DOI] [PubMed] [Google Scholar]
  50. Pohorecky LA. Biphasic action of ethanol. Biobehav Rev. 1977;1(4):231–240. [Google Scholar]
  51. Ramachandra V, Phuc S, Franco AC, Gonzales RA. Ethanol preference is inversely correlated with ethanol-induced dopamine release in 2 sub-strains of c57bl/6 mice. Alcohol Clin Exp Res. 2007;31(10):1669–1676. doi: 10.1111/j.1530-0277.2007.00463.x. [DOI] [PubMed] [Google Scholar]
  52. Rassnick S, Pulvirenti L, Koob GF. Oral ethanol self-administration in rats is reduced by the administration of dopamine and glutamate receptor antagonists into the nucleus accumbens. Psychopharmacology (Berl) 1992;109(1–2):92–98. doi: 10.1007/BF02245485. [DOI] [PubMed] [Google Scholar]
  53. Risinger FO, Brown MM, Doan AM, Oakes RA. Mouse strain differences in oral operant ethanol reinforcement under continuous access conditions. Alcohol Clin Exp Res. 1998;22(3):677–684. doi: 10.1111/j.1530-0277.1998.tb04311.x. [DOI] [PubMed] [Google Scholar]
  54. Rocha BA, Odom LA, Barron BA, Ator R, Wild SA, Forster MJ. Differential responsiveness to cocaine in c57bl/6j and dba/2j mice. Psychopharmacology (Berl) 1998;138(1):82–88. doi: 10.1007/s002130050648. [DOI] [PubMed] [Google Scholar]
  55. Rodgers DA, McClearn GE, Bennett EL, Hebert M. Alcohol preference as a function of its caloric ultility in mice. J Comp Physiol Psychol. 1963;56:666–672. doi: 10.1037/h0040350. [DOI] [PubMed] [Google Scholar]
  56. Rylkova D, Shah HP, Small E, Bruijnzeel AW. Deficit in brain reward function and acute and protracted anxiety-like behavior after discontinuation of a chronic alcohol liquid diet in rats. Psychopharmacology (Berl) 2008;203:629–640. doi: 10.1007/s00213-008-1409-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Schaefer GJ, Michael RP. Ethanol and current thresholds for brain self-stimulation in the lateral hypothalamus of the rat. Alcohol. 1987;4(3):209–213. doi: 10.1016/0741-8329(87)90045-0. [DOI] [PubMed] [Google Scholar]
  58. Schulteis G, Markou A, Cole M, Koob GF. Decreased brain reward produced by ethanol withdrawal. Proc Natl Acad Sci U S A. 1995;92(13):5880–5884. doi: 10.1073/pnas.92.13.5880. [DOI] [PMC free article] [PubMed] [Google Scholar]
  59. Shelton KL, Grant KA. Discriminative stimulus effects of ethanol in c57bl/6j and dba/2j inbred mice. Alcohol Clin Exp Res. 2002;26(6):747–757. [PubMed] [Google Scholar]
  60. Sheppard JR, Albersheim P, McClearn G. Aldehyde dehydrogenase and ethanol preference in mice. J Biol Chem. 1970;245:2876–2882. [PubMed] [Google Scholar]
  61. Smith RC, Parker ES, Noble EP. Alcohol and affect in dyadic social interaction. Psychosom Med. 1975;37(1):25–40. doi: 10.1097/00006842-197501000-00004. [DOI] [PubMed] [Google Scholar]
  62. Tang A, George MA, Randall JA, Gonzales RA. Ethanol increases extracellular dopamine concentration in the ventral striatum in c57bl/6 mice. Alcohol Clin Exp Res. 2003;27(7):1083–1089. doi: 10.1097/01.ALC.0000075825.14331.65. [DOI] [PubMed] [Google Scholar]
  63. van der Veen R, Piazza PV, Deroche-Gamonet V. Gene-environment interactions in vulnerability to cocaine intravenous self-administration: a brief social experience affects intake in dba/2j but not in c57bl/6j mice. Psychopharmacology (Berl) 2007;193(2):179–186. doi: 10.1007/s00213-007-0777-0. [DOI] [PubMed] [Google Scholar]
  64. Vrtunski P, Murray R, Wolin LR. The effect of alcohol on intracranially reinforced response. Q J Stud Alcohol. 1973;34(3):718–725. [PubMed] [Google Scholar]
  65. Weiss F, Porrino LJ. Behavioral neurobiology of alcohol addiction: recent advances and challenges. J Neurosci. 2002;22(9):3332–3337. doi: 10.1523/JNEUROSCI.22-09-03332.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  66. Williams AF. Social drinking, anxiety, and depression. J Pers Soc Psychol. 1966;3(6):689–693. doi: 10.1037/h0023299. [DOI] [PubMed] [Google Scholar]
  67. Williams-Hemby L, Porrino LJ. Ii Functional consequences of intragastrically administered ethanol in rats as measured by the 2-[14c]deoxyglucose method: the contribution of dopamine. Alcohol Clin Exp Res. 1997;21(9):1581–1591. [PubMed] [Google Scholar]
  68. Wise RA. Action of drugs of abuse on brain reward systems. Pharmacol Biochem Behav. 1980;13(Suppl 1):213–223. doi: 10.1016/s0091-3057(80)80033-5. [DOI] [PubMed] [Google Scholar]
  69. Wise RA. Neurobiology of addiction. Curr Opin Neurobiol. 1996;6(2):243–251. doi: 10.1016/s0959-4388(96)80079-1. [DOI] [PubMed] [Google Scholar]
  70. Wise RA. Brain reward circuitry: insights from unsensed incentives. Neuron. 2002;36(2):229–240. doi: 10.1016/s0896-6273(02)00965-0. [DOI] [PubMed] [Google Scholar]
  71. Wise RA. Forebrain substrates of reward and motivation. J Comp Neurol. 2005;493(1):115–121. doi: 10.1002/cne.20689. [DOI] [PMC free article] [PubMed] [Google Scholar]
  72. Wise RA, Bozarth MA. Brain reward circuitry: four circuit elements “wired” in apparent series. Brain Res Bull. 1984;12(2):203–208. doi: 10.1016/0361-9230(84)90190-4. [DOI] [PubMed] [Google Scholar]
  73. Wise RA, Munn E. Effects of repeated amphetamine injections on lateral hypothalamic brain stimulation reward and subsequent locomotion. Behav Brain Res. 1993;55(2):195–201. doi: 10.1016/0166-4328(93)90115-7. [DOI] [PubMed] [Google Scholar]

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