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. Author manuscript; available in PMC: 2021 Sep 1.
Published in final edited form as: Drug Alcohol Depend. 2020 Jul 2;214:108165. doi: 10.1016/j.drugalcdep.2020.108165

The reinforcing effects of ethanol within the prelimbic cortex and ethanol drinking: Involvement of local dopamine D2 receptor-mediated neurotransmission

Eric A Engleman 1, Cynthia M Ingraham 1, Zachary A Rodd 1, James M Murphy 1, William J McBride 1, Zheng-Ming Ding 1,2,*
PMCID: PMC7431019  NIHMSID: NIHMS1613276  PMID: 32688071

Abstract

Previous studies have identified important mesolimbic regions in supporting the reinforcing effects of ethanol. However, the involvement of the medial prefrontal cortex (mPFC), another key region within the mesocorticolimbic system, in ethanol reinforcement has been understudied. The objective of the current study was to examine the role of the prelimbic (PL) cortex sub-region of the mPFC in ethanol reinforcement and drinking. Intracranial self-administration was used to examine the reinforcing effects of ethanol within the PL cortex. Quantitative microdialysis was used to measure basal extracellular DA concentrations and clearance in the PL cortex following chronic ethanol drinking. In addition, the involvement of dopamine (DA) D2 receptors within the PL cortex on the reinforcing effects of ethanol and ethanol drinking was determined. Ethanol was dose-dependent self-administered into the PL cortex, with significantly more infusions elicited by 100–200 mg% ethanol than vehicle. Co-infusion of the D2 receptor antagonist sulpiride significantly reduced ethanol self-administration. Chronic ethanol drinking significantly elevated basal extracellular DA concentrations without altering DA clearance. Microinjection of sulpiride into the PL cortex selectively reduced ethanol, but not saccharine, drinking. These results indicate that the PL cortex supported the reinforcing effects of ethanol, and that ethanol drinking enhanced basal DA neurotransmission within the PL cortex. In addition, D2 receptor antagonism within the PL cortex reduced ethanol self-administration and drinking. Collectively, these findings revealed important DA mechanisms within the PL cortex in mediating ethanol reinforcement and drinking.

Keywords: Dopamine, D2 receptor, ethanol, Intra-cranial self-administration, Medial prefrontal cortex, No-net-flux microdialysis

1. Introduction

Identifying brain regions and mechanisms involved in the reinforcing effect of ethanol and ethanol drinking is critical toward understanding the neurobiology of alcohol use disorders (AUDs). Previous studies using the intracranial self-administration (ICSA) technique reported that ethanol could be self-infused directly into the posterior ventral tegmental area (pVTA) and nucleus accumbens shell (NACsh) (Engleman et al., 2009; Rodd et al., 2004), and that ethanol ICSA into these regions appeared to involve activation of various local receptors (Ding et al., 2009, 2015a, 2015b). The findings suggested these two key limbic regions as important anatomical sites underlying ethanol reinforcement, and revealed important receptor mechanisms involved in these effects. However, it remains unknown whether other key mesocorticolimbic region(s) within the system, e.g., the medial prefrontal cortex (mPFC), can also mediate the reinforcing effects of ethanol.

Both human and animal research has implicated the mPFC in the effects of drugs of abuse, including ethanol (Abernathy et al., 2010). For example, pharmacological manipulations of the mPFC could regulate alcohol self-administration (Hodge et al., 1996, Samson and Chappell., 2003). The mPFC receives sparse dopamine (DA) innervation from the VTA and local DA neurotransmission can play an important role in the function of mPFC (Lammel et al., 2014). Several studies suggested potential involvement of mPFC DA transmission in ethanol’s effects. For example, alcohol-preferring P rats showed lower basal extracellular DA levels within the mPFC than Wistar rats (Engleman et al., 2006). Ethanol could regulate extracellular DA levels within the mPFC (Ding et al., 2011; Schier et al., 2013). Chronic intermittent ethanol vapor exposure disrupted DA receptor-mediated neurotransmission in the mPFC (Trantham-Davidson et al., 2014).

The mPFC could be another key brain region involved in mediating general motivation and reward (Wise and Rompre, 1989). Early studies showed that rats would self-stimulate the mPFC with electrical currents (Phillips and Fibiger, 1978; Routtenberg and Sloan, 1972), and self-administer cocaine into the mPFC (Goeders and Smith, 1983). DA transmission appeared to be critical to these effects (Goeders and Smith, 1986; Phillips and Fibiger, 1978). For example, inhibition of local DA D2 receptors with sulpiride attenuated cocaine self-administration into the mPFC (Goeders and Smith, 1983). However, the reinforcing effects of ethanol within the mPFC and potential involvement of DA neurotransmission in its effects have not been studied.

DA neurotransmission has been shown to be essential to mediating the effects of ethanol and ethanol drinking (Koob and Volkow, 2010). Acute ethanol increased DA release in corticolimbic regions including the mPFC (Ding et al., 2011; Schier et al., 2013). Chronic ethanol drinking by alcohol-preferring P rats increased basal extracellular DA concentrations in the NACsh, accompanied by ethanol downregulation of D2 auto-receptor function within the NACsh (Thielen et al., 2004). Such effects have not been examined in the mPFC. In addition, a number of studies indicated that manipulations of DA receptors within the mesolimbic system, including NAC and VTA, could regulate ethanol self-administration and drinking (Gonzales et al., 2004). On the other hand, the involvement of DA receptors within the mPFC in ethanol drinking has not been adequately explored.

The current study used a combination of behavioral (ICSA and voluntary drinking) and neurochemical (quantitative microdialysis) paradigms to test the hypothesis that DA neurotransmission within the mPFC would play an important role in the reinforcing effects of ethanol within the mPFC and in ethanol drinking. The mPFC is composed of several key subregions, including prelimbic (PL) and infralimbic (IL) cortices that form different circuits and serve different functions (Vertes, 2004). The PL cortex sub-region of the mPFC was examined in the current study due to its prominent role in supporting drug-seeking behavior, especially cocaine (Peters et al., 2009; Peters et al., 2008).

2. Materials and Methods

2.1. Animals

Experimentally-naïve adult female Wistar rats (Envigo, Inc., Indianapolis, IN) and alcohol-preferring P rats (Indiana University) with the starting body weight at 250–300g were used in the present study. Rats were kept on a reversed 12hr light-dark cycle (light off at 7:00am). Food and water were available ad libitum except during ICSA and microdialysis experiments. Rats were initially housed in pair during an acclimation period (~ 7 days), then were housed individually. Female rats were used because these rats maintain their head size better than male rats for more accurate stereotaxic placement (Ding et al., 2015b; Engleman et al., 2009). Estrous cycle was not monitored. Previous studies suggested that the estrous cycle did not appear to have a significant effect on the ICSA behavior (Ding et al., 2009, 2015a, 2015b; Engleman et al., 2009). A recent study indicated that estrous cycle did not significantly affect alcohol drinking in rats (Priddy et al., 2017). Experiments were performed during the dark phase. Protocols used were approved by the Institutional Animal Care and Use Committee of Indiana University School of Medicine. All experiments were performed in accordance with the principles outlined in the Guide for the Care and Use of Laboratory Animals (National Research Council, 2011).

2.2. Chemical agents

Sodium chloride, potassium chloride, magnesium chloride, potassium phosphate monobasic, sodium phosphate, magnesium phosphate, sodium bicarbonate, calcium chloride, d-glucose, ascorbate, ethylenediaminetetraacetic acid (EDTA), sodium octylsulfate, acetonitrile, dopamine hydrochloride, and the dopamine D2 receptor antagonist (−)-sulpiride were purchased from Sigma-Aldrich (St. Louis, MO). Ethanol (190 proof) was obtained from McCormick Distilling, Weston, MO. All chemicals were dissolved in the distilled water to desired concentrations.

2.3. Experiment 1. ICSA of ethanol into the PL cortex in female Wistar rats

Following acclimation, rats were surgically implanted with one 22-gauge cannula (I.D. x O.D. = 0.39mm x 0.71mm; Plastics One Inc., Roanoke, VA) aimed at the right PL cortex (AP +3.0mm, ML +0.7mm, DV −3.0mm, with the incisor bar set at −3.3mm) according to the Rat Brain in Stereotaxic Coordinates by Paxinos and Watson (Paxinos and Watson, 1998). Stylets were inserted into cannulae when no experiments were being conducted. Rats were allowed to recover from surgery for 5–7 days prior to tests, during which rats were habituated and handled on a daily basis. During ICSA experiments, rats were divided into six groups (n = 5–8/group), with each group receiving either artificial cerebrospinal fluid (aCSF: 120mM NaCl, 4.8mM KCl, 1.2mM KH2PO4, 1.2mM MgSO4, 25mM NaHCO3, 2.5mM CaCl, 10mM d-glucose, pH 7.2–7.4) or one concentration of Ethanol (50, 100, 150, 200, and 400 mg% in aCSF) for self-administration into the PL cortex.

ICSA tests followed general procedures previously described (Engleman et al., 2009). Briefly, rats were placed into standard Coulbourn operant conditioning chambers equipped with two levers, one active and one inactive. The active lever was connected to an isolated pulse stimulator (Model 2100) from A-M systems, Inc. (Varlsborg, WA) that was controlled by an operant conditioning control system (Coulbourn Instruments, Allentown, Pennsylvania, USA). The A-M pulse stimulator was connected to two electrodes that were immersed in a solution-filled cylinder container equipped with a 28-gauge injection cannula (I.D. x O.D. = 0.18mm x 0.36mm; Plastics One Inc., Roanoke, VA) which extends 1mm beyond the guide cannula. Each response on the active lever (FR1 schedule of reinforcement) activated the pulse stimulator that produced a 5s infusion current between the electrodes, resulting in an infusion of 100-nl solution into the PL cortex. Each infusion was followed by a 5s timeout period. During both the infusion and timeout periods, responses on the active lever were recorded but did not produce further infusions. The responses on the inactive lever were recorded but did not result in any infusions. The assignment of active and inactive lever was counterbalanced among rats. Ethanol at the concentration of 150 mg% was tested for 7 sessions, as previously described (Engleman et al., 2009), i.e., ethanol for the first 4 sessions, aCSF for sessions 5 and 6, and ethanol alone again during session 7. This procedure allows the examination of acquisition, extinction and reacquisition of the self-administration behavior. On the other hand, aCSF and ethanol at other concentrations (50, 100, 200, and 400 mg%) were only available for self-administration for a total of 4 sessions to allow examining only acquisition of the self-administration behavior. Sessions were 4hr long and were conducted with 48–72hr between sessions.

2.4. Experiment 2. Involvement of local DA D2 receptors in ethanol ICSA into the PL cortex of female Wistar rats

DA D2 receptors were examined because previous findings indicate that these receptors within the prelimbic cortex were involved in mediating oral ethanol self-administration (Hodge et al., 1996). Our studies demonstrated that ethanol drinking could alter D2 receptor function within the mesolimbic system (Ding et al., 2016; Thielen et al., 2004). In addition, a recent study showed that prelimbic D2 receptors were preferentially responsive to ethanol’s effects compared with D1 receptors (Trantham-Davidson et al., 2014). Briefly, separated groups of rats started with ICSA of 150 mg% ethanol into the PL cortex during the first 4 sessions (acquisition). For sessions 5 and 6, rats received 150 mg% ethanol in combination of the DA D2 receptor antagonist sulpiride (0, 10, or 100μM; n = 8–9/group) for self-administration (co-infusion). Only 150 mg% ethanol was available again during session 7 (re-acquisition). This co-infusion procedure has been successfully conducted in previous studies examining potential receptor mechanisms involved in ethanol ICSA into the pVTA and NACsh (Ding et al., 2009, 2015a, 2015b). The concentrations of sulpiride were determined based on our previous findings that sulpiride within this concentration range was effective in reducing ethanol ICSA into the pVTA (Ding et al., 2015b).

2.5. Experiment 3. Effects of chronic ethanol drinking on basal extracellular DA levels and clearance within the PL cortex of female P rats

Following acclimation, alcohol-naïve female P rats were randomly divided into two groups (n = 6–7/group). A ‘Water’ group received water as its only fluid throughout the study. An ‘Ethanol’ group received free access to water, 15 and 30% (v/v) ethanol in a three-bottle choice paradigm for 8 weeks. The positions of ethanol and water bottles were randomly altered every week, and fluid intake was recorded to the nearest 0.1g by weighing water and ethanol bottles three times a week (Monday, Wednesday and Friday). Ethanol fluid intake measures were converted into grams of ethanol per kilogram of body weight per day (g/kg/day). Body weights of rats were recorded at the same time when ethanol and water bottles were weighed.

Then, rats were surgically implanted with one 18-gauge guide cannula (I.D. x O.D. = 0.82mm x 1.27mm; Plastics One, Inc., Roanoke, VA, USA) aimed above the PL cortex (A/P +3.0mm, L +0.7mm, D/V –2.0mm) under 2% isoflurane inhalation. After two days of recovery, animals were habituated to the microdialysis chambers for two hours each day (11:00am – 13:00pm) on post-surgery days 3 and 4. On day 5, animals were briefly anesthetized and loop-style microdialysis probes with a 2mm-long active membrane (I.D. x O.D = 200μm x 216μm, molecular weight cut-off: 13,000, Spectrum Laboratories, Inc., Rancho Dominguez, CA, USA) were inserted with a 3.0mm extension beyond the guide cannula. After probe insertion, rats were placed into microdialysis chambers for an additional two-hour habituation period.

On day 6, rats were placed in microdialysis chambers and microdialysis probes were perfused at a flow rate of 0.5 μl/min with aCSF (140mM NaCl, 3mM KCl, 2.5mM CaCl2, 1mM MgCl2, 2mM Na2PO4, 0.2mM ascorbate), following procedure previously described to keep consistency with our previous studies (Engleman et al., 2006; Thielen et al., 2004). After a two-hour equilibration period, three baseline samples were collected every 20 min in microfuge tubes containing 5μl of 0.1N perchloric acid. Then, probes were perfused with one of three DA concentrations (5, 10, 20nM in aCSF), as described previously (Engleman et al., 2006). After 20min of perfusion with the new concentration of DA, four 20min samples were collected at each concentration of DA. Dialysate samples were collected 20min after switching perfusates to allow equilibration of the perfusate. At the end of this perfusion period, rats were switched to aCSF containing a different concentration of DA, as described above. The order of perfusion of the DA concentrations was randomized. After rats were perfused with all three DA concentrations, aCSF alone was perfused again and an additional three 20min samples were collected. Samples were immediately frozen on dry ice and stored at −70°C until assayed for DA content.

Samples were analyzed for DA content using HPLC/EC, as previously described (Ding et al., 2016; Engleman et al., 2006). Samples were loaded into a 10μl sample loop and injected onto an analytical column (BDS Hypersil C18, 3μm, 2×150mm, Keystone Scientific, Bellefonte, PA) with a mobile phase consisting of sodium phosphate 9.0 g/l, EDTA 190 mg/l, sodium octyl-sulfate 350 mg/l, and 10% acetonitrile at pH 3.0. Two 3mm dual glassy carbon electrodes (Bioanalytical Systems, Inc. West Lafayette, IN) were used in series at potentials of +720 and +100mV. DA was detected at the second electrode at a sensitivity setting of 0.5 nA/V. Output from the detector was analyzed with a computer program (ChromPerfect, Justice Innovations, Inc., Palo Alto, CA) and levels were determined by comparison with a standard curve constructed from analysis of the DA solutions used in the experiment. The lower sensitivity limit for DA was approximately 0.1nM.

2.6. Experiment 4. Involvement of PL D2 receptors in ethanol or saccharine drinking in female P rats

Alcohol-naïve P rats received free access to water and 15% ethanol (v/v) in a two-bottle choice paradigm for 1 week followed by daily 1hr access for 4 weeks. A separate group of rats received daily 1hr access to water and 0.0125% saccharine with a two-bottle choice paradigm for 4 weeks. Then, rats were implanted with one 22-gauge guide cannula (I.D. x O.D. = 0.39mm x 0.71mm; Plastics One Inc., Roanoke, VA) aimed 1mm above the PL cortex (AP +3.0mm, ML +0.7mm, DV −3.0mm, with the incisor bar set at −3.3mm). Following at least 3 days of recovery, rats were returned to daily 1hr drinking to regain the baseline levels of ethanol or saccharine drinking. Rats were acclimated to the microinjection procedure by taking them out of their home cages, removing and reinserting the dummy cannulae, and placing them in the microinjection chamber for one minute, after which they were returned to their home cages, once a day for at least 3 days. On the day the microinjection experiment, ethanol rats were divided into 4 groups (n = 9–10/group) based on similar basal levels of ethanol intake and each group received microinjection of aCSF or one concentration of the D2 receptor antagonist sulpiride (1.0 [3mM], 2.0 [6mM], or 4.0μg [12mM] in 0.5μL) with a 28-gauge microinjector (I.D. x O.D. = 0.18mm x 0.36mm; Plastics One Inc., Roanoke, VA) extending 1mm beyond the guide cannula into the PL cortex over a 30s period. The saccharine rats were divided into 2 groups (n = 5–7/group) based on similar basal levels of saccharine intake with each group receiving microinjection of either aCSF or 4.0μg of sulpiride (0.5μL) into the PL cortex. The microinjector remained in place for an additional 30s before being removed. Rats were then returned to their home cages for their 1hr access period. Ethanol or saccharine intake was measured.

2.7. Histology

At the end of experiments, rats were euthanized with CO2 overdose and bromophenol blue was injected into the target region or perfused through microdialysis probes, and placements of injection sites and probes were verified following procedures previously described (Ding et al., 2009b).

2.8. Statistical analysis

Data were expressed as Mean ± SEM. For ICSA experiments, numbers of infusions during acquisition sessions 1 through 4 were averaged and analyzed with one-way ANOVA followed by LSD post-hoc analysis. Data on lever responses were analyzed with mixed ANOVA with repeated measures on session. Significant main effects were followed by LSD pairwise multiple comparisons. Effects of extinction or drug treatment were determined by comparing responses on the active lever during sessions 5 and 6 to average baseline responses across 4 acquisition sessions.

For microdialysis data, the extracellular concentration of DA and the DA “extraction fraction” (i.e., in vivo recovery) were obtained by the point of no-net flux technique. A difference score of the concentration of DA perfused through the probe minus the concentration of DA recovered out of the probe (DAin – DAout) was calculated for each sample. These differences between inflow and outflow concentrations of DA were plotted against their inflow concentrations and multiple linear regression analysis was used to compare the point of no-net flux (x-intercept, the extracellular DA concentration) and the DA extraction fraction (slope) of the different experimental groups.

For microinjection experiments, data were analyzed with either one-way ANOVA or t-test. Significance level was set at p < 0.05.

3. Results

3.1. ICSA of ethanol into the PL cortex

Figure 1A depicts representative, non-overlapping placements of injection sites within the PL cortex. Rats with injection sites outside of the PL cortex were not included for analysis (n = 3). The injection sites were distributed within both dorsal and ventral portions of the PL cortex. There were no apparent differences in self-infusions between the dorsal and ventral portions.

Figure 1.

Figure 1.

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Representative placements of injection sites within the PL cortex (A) and average numbers of infusions during the 4 acquisition sessions (B) of ethanol self-administration in female Wistar rats (n = 5–8/group). Overlapping placements were not displayed for clarity purpose. * p < 0.05, significantly greater than the ‘aCSF’ and ‘400 mg% ethanol’ groups; # p < 0.05, significantly greater than the ‘50 mg% ethanol’ group.

Figure 1B shows the average numbers of infusions of aCSF or ethanol at different concentrations into the PL cortex. One-way ANOVA revealed a significant main effect (F5, 33 = 5.25, p = 0.001). Post-hoc analysis indicated that rats produced robust ethanol self-administration into the PL cortex at concentrations between 100–200 mg%, receiving significantly greater numbers of infusions (100 mg%: 23 ± 3/session, p = 0.03; 150 mg%: 26 ± 4/session, p = 0.008; and 200 mg%: 25 ± 3/session, p = 0.007) than rats on aCSF (13 ± 1/session). These rats also elicited greater numbers of infusions than rats on 400 mg% ethanol (8 ± 2/session, ps < 0.01). In addition, rats on 150 mg% and 200 mg% ethanol produced more infusions than rats on 50 mg% ethanol (15 ± 3/session, ps < 0.05). These results indicate that ethanol induced dose-dependent self-administration into the PL cortex.

Figure 2 shows lever responses on both active and inactive levers during self-administration of aCSF or various concentrations of ethanol into the PL cortex. Data during the first 4 acquisition sessions were analyzed with mixed ANOVA with repeated measures on session. The analysis revealed significant effects of ‘concentration’ (F5, 33 = 5.3, p = 0.001), ‘response’ (F1, 33 = 100.0, p <0.001), and the ‘concentration’ x ‘response’ interaction (F5, 33 = 5.8, p = 0.001). Further analysis with pairwise comparisons indicated that rats produced significantly more responses on the active lever than the inactive lever when 100, 150 or 200 mg% ethanol (ps < 0.01), but not aCSF, 50 or 400 mg% ethanol (ps > 0.05), was available for self-administration. In addition, rats significantly reduced active responses when ethanol was replaced by aCSF during session 6 (p < 0.001), but returned responses to baseline levels during session 7 when ethanol was available again (p > 0.05).

Figure 2.

Figure 2.

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Average responses on both active and inactive levers during each session of ethanol self-administration into the PL cortex of female Wistar rats (n = 5–8/group). * p < 0.05, significant difference between active and inactive lever during acquisition. # p < 0.05, significantly different from the average responses on the active lever over the 4 acquisition sessions

3.2. The involvement of D2 receptors in ethanol ICSA into the PL cortex

Figure 3 depicts the effects of co-infusion of the D2 receptor antagonist sulpiride on ethanol self-administration into the PL cortex. Mixed ANOVA with repeated measures on session revealed significant effect of ‘session’ x ‘treatment’ x ‘response’ interaction (F12, 138 = 2.9, p = 0.001). Rats gradually developed lever discrimination during acquisition, with all rats showing greater responses on active lever toward the end of acquisition (sessions 3–4, ps < 0.05). Further analysis with pairwise comparisons indicated that sulpiride at 100μM gradually decreased responses on active lever compared to average baseline responses across 4 acquisition sessions, with significant reduction during session 6 (p < 0.05). Sulpiride co-infusion also eliminated lever discrimination between active and inactive lever during session 6 (p > 0.05). Removal of sulpiride from ethanol returned active responses to baseline levels (Fig. 3, bottom panel). On the other hand, co-infusion of sulpiride at 10 μM did not significantly alter responses on active lever, nor did it alter lever discrimination between active and inactive lever (ps > 0.05; Fig. 3, middle panel).

Figure 3.

Figure 3.

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Effects of co-infusion of the D2 receptor antagonist sulpiride (0, 10 or 100 μM) on ICSA of 150 mg% ethanol into the PL cortex in female Wistar rats (n = 8–9/group). * p < 0.05, significantly greater than responses on the inactive lever. # p < 0.05, significantly lower than average responses on the active lever during acquisition sessions 1–4.

3.3. Effects of chronic ethanol drinking on basal extracellular DA transmission within the PL cortex

P rats in the ‘Ethanol’ group consumed, in average, 7.1 ± 1.0 g/kg/day ethanol prior to surgery. Fig. 4A shows representative placements of probes within the PL cortex. Rats with less than 70% of active membrane within the PL cortex were excluded from the analysis (n = 3).

Figure 4.

Figure 4.

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Effects of chronic ethanol drinking on basal extracellular dopamine levels and clearance within the PL cortex in female P rats (n = 6–7/group). A) Representative placements of microdialysis probes within the PL cortex. Overlapping placements are not presented for clarity purpose. B) Linear regression plots of dopamine; C) Basal extracellular dopamine concentrations determined from points of no-net-flux; D) Ed values of dopamine determined from the slope of the plots for the linear regression analysis. * p < 0.05, significantly different from the Water group.

Extracellular DA concentrations were shown in Fig. 4C and the extraction fractions (Eds) were shown in Fig. 4D. Overall ANOVA revealed a significant effect of treatment (F1, 190 = 23.0, p < 0.001) on basal extracellular DA concentrations, but no significant effect of treatment on Eds (F1, 189 = 3.9, p > 0.05).

3.4. Involvement of D2 receptors within the PL cortex in ethanol or saccharine drinking

Fig. 5A shows representative placements of injection sites within the PL cortex. Rats with injection sites outside of the PL cortex were not included for analysis.

Figure 5.

Figure 5.

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Effects of microinjection of the D2 receptor antagonist sulpiride into the PL cortex on ethanol or saccharine drinking in female P rats. A) Representative placements of injection sites within the PL cortex; B) Effects of sulpiride (in 0.5μl) on ethanol drinking (n = 9–10/group). * p < 0.05, significantly reduced compared to the ‘aCSF’ group; C) Effects of sulpiride (in 0.5μl) on saccharine drinking (n = 5–7/group).

In ethanol-drinking groups, baseline ethanol intake was 1.3 ± 0.1, 1.2± 0.1, 1.3 ± 0.2, 1.3 ± 0.1 g/kg/h for the aCSF, 1 μg, 2 μg, and 4 μg sulpiride treatment groups, respectively. One-way ANOVA did not detect significant difference in baseline ethanol intake among groups (F3, 38 = 0.23, p = 0.85). Fig. 5B shows the effects of sulpiride on ethanol drinking. One-way ANOVA revealed significant effect of treatment (F3, 38 = 3.1, p < 0.05). Post-hoc Dunnetts analysis indicated that microinjection of sulpiride at 4.0, but not 1.0 or 2.0 μg, significantly reduced ethanol drinking in P rats (by 45%, p < 0.05).

In saccharine-drinking rats, baseline saccharine intake was 3.0 ± 0.7 and 3.6 ± 0.9 mg/kg/h for the aCSF and 4 μg sulpiride treatment groups, respectively. Student t-test did not detect significant difference in baseline saccharine intake between groups (t =0.66, p = 0.53). Fig. 5C shows the effects of sulpiride on saccharine drinking. Student t-test did not reveal a significant effect of treatment, (t = 0.18, p = 0.86). Microinjection of sulpiride or aCSF similarly produced a trend to reduce saccharine drinking in P rats (t =1,5 and 2.4, respectively; p > 0.05).

4. Discussion

The major findings of the current study are a) ethanol could be self-administered into the PL cortex sub-region of the mPFC, b) antagonism of local D2 receptors reduced ethanol self-administration, c) chronic ethanol drinking increased basal extracellular DA concentrations within the PL cortex, and d) inhibition of D2 receptor function within the PL cortex attenuated ethanol drinking. These results suggest that the PL cortex is one brain region that supports the reinforcing effects of ethanol, which may play an important role in the initiation or maintenance of ethanol drinking. In addition, DA neurotransmission within the PL cortex is critical for the reinforcing and rewarding effects of ethanol since DA neurotransmission was altered by ethanol drinking and inhibition of DA neurotransmission mediated by D2 receptors within this region reduced both ethanol self-administration and drinking.

Ethanol appeared to support self-administration into the PL cortex, suggesting that ethanol is reinforcing within the PL cortex (Fig. 1). This finding is consistent with the concept that the mPFC is a general reward zone that can mediate local self-stimulation with electrical currents and self-administration of cocaine (Goeders and Smith 1983, Wise and Rompre 1989). Ethanol at doses between 100 and 200 mg% elicited more infusions than the vehicle (Fig. 1), and induced lever discrimination with significantly greater responses on the active lever than the inactive lever during acquisition (Fig. 2). In addition, rats reduced responses on the active lever when ethanol (150 mg%) was replaced by vehicle, and resumed robust active responses when ethanol (150 mg%) was re-introduced (Fig. 2C). This further suggests that the self-administration behavior was reinforced by ethanol. On the other hand, ethanol at doses lower or higher than this range did not support self-administration or lever discrimination (Figs. 1 and 2). These results indicate that the self-administration of ethanol into the PL cortex is dose-dependent. This typical inverted U-shaped curve of self-administration is similar to previous findings showing ethanol self-administration into the pVTA and NACsh (Engleman et al., 2009; Rodd et al., 2004), implicating the PL cortex as another important brain region, in addition to the pVTA and NACsh, in mediating the reinforcing effects of ethanol. These results suggest that these regions may play a critical role in mediating the initiation or maintenance of ethanol drinking.

The D2 receptor antagonist sulpiride dose-dependently reduced active responses for ethanol self-administration into the PL cortex and abolished lever discrimination (Fig. 3), suggesting that D2 receptors-mediated DA neurotransmission is required for the reinforcing effects of ethanol within the PL cortex. The effects of sulpiride was not likely due to locomotor impairment because previous studies have shown that microinjection of sulpiride into the mPFC did not alter locomotor activity at concentrations up to 3mM (Beyer and Steketee, 2001; Steketee and Walsh, 2005). In addition, local DA transmission within the mPFC also appeared to be critical to the reinforcing effects of cocaine and electrical self-stimulation within the mPFC (Goeders et al., 1986; Phillips and Fibiger, 1978). These results suggest that DA neurotransmission within the PL cortex can contribute to general reinforcement and the reinforcing effects of various drugs of abuse.

Several mechanisms might contribute to the involvement of local DA neurotransmission in the reinforcing effects of ethanol within the PL cortex. First, it is possible that the delivery of ethanol into the PL may increase extracellular DA levels to enhance DA modulation of mPFC pyramidal neurons. This notion is consistent with findings showing ethanol increased extracellular DA levels within the mPFC following systemic administration (Doherty et al., 2016; Schier et al., 2013). Another possibility is that ethanol in the PL cortex may influence D2 receptor function. Ethanol has not been shown to directly interact with D2 receptors. However, ethanol was shown to impair the desensitization of D2 receptors on DA neurons within the VTA through unknown indirect pathways, which might, in theory, result in prolongation and enhancement of D2 receptor-mediated DA transmission (Nimitvilai et al., 2012; You et al., 2018). Therefore, these findings suggest that sulpiride might block ethanol’s effects on both extracellular DA overflow and/or on D2 receptor function, leading to attenuation of the ethanol self-administration behavior as observed in the current study (Fig. 3).

DA projection to the mPFC originates from the VTA. Recent studies suggested that VTA DA neurons are highly heterogeneous, exhibiting differential anatomical, electrophysiological, and molecular properties along the latero-medial axis, with mesocortical DA neurons selectively located in the medial VTA (Lammel et al., 2014). Known inputs to mesocortical DA neurons include projections from the mPFC (Carr and Sesack, 2000), laterodorsal tegmentum (Omelchenko and Sesack, 2005), and lateral habenula (Lammel et al., 2012). It is noted that there is evidence implicating the mPFC DA neurotransmission in mediating aversion in addition to reward (Weele et al., 2019). For example, optogenetic stimulation of VTA mesocortical DA neurons receiving projections from lateral habenula elicited conditioned place aversion in mice, suggesting mesocortical DA transmission in encoding aversion (Lammel et al., 2012). These findings revealed a complex picture of mPFC DA involvement in mediating reward and aversion, and warrant further research.

Voluntary ethanol drinking increased basal extracellular DA neurotransmission within the PL cortex following approximately one week of abstinence (Fig. 4). In addition, DA clearance did not seem to be altered, suggesting that the greater extracellular DA levels may have originated from increased pre-synaptic DA release, but not changes in DA uptake, following chronic ethanol exposure and a short period of abstinence. This finding is consistent with a previous study showing that chronic ethanol exposure through liquid diet in Fischer 344 rats increased tissue content of DA within the frontal cortex (Pellegrino and Druse, 1992). Together, these results suggest that ethanol-enhanced DA neurotransmission within the mPFC may contribute to maintenance of ethanol drinking. On the other hand, a previous study demonstrated that alcohol drinking significantly reduced baseline extracellular DA levels within the mPFC (Doherty et al., 2016). Several methodological differences may contribute to this apparent discrepancy, including but not limited to drinking period (8 weeks vs 1 week), abstinence period (~ 1 week vs ~ 1 day), quantitative NNF microdialysis vs conventional microdialysis, female P rats vs male Long-Evans rats, between our and the previous study, respectively.

This finding is consistent with studies demonstrating significantly increased basal extracellular DA concentrations within the NACsh in rats following chronic ethanol exposure (Badanich et al., 2007; Franklin et al., 2009; Smith and Weiss., 1999; Thielen et al., 2004). Previous studies indicated that chronic ethanol drinking could increase the number of spontaneously active DA neurons (Morzorati et al., 2010), and reduce D2 auto-receptor function within the VTA (Ding et al., 2016), leading to a hyperdopaminergic state within the NAC and mPFC that contributes to ethanol drinking. However, there is also evidence suggesting an ethanol-induced hypodopaminergic state. For example, ethanol dependence was shown to decrease basal extracellular DA levels within the NAC (Diana et al., 1993; Weiss et al., 1996), and reduce basal VTA DA neuron activity (Diana et al., 1993; Shen, 2003). Although it is difficult to reconcile these findings, the differences may originate from factors such as strain differences, ethanol exposure procedures (dependence- vs non-dependence-induced), and conventional vs quantitative microdialysis among these studies. It should be noted that based on the findings of this experiment, a concentration range of 1 to 5 nM would appear to be optimal to more accurately assess effects of chronic EtOH administration and should be used in future studies.

In vivo microdialysis is a technique that samples extracellular levels of neurotransmitters and other analytes in the brain. To accomplish this, a microdialysis probe is inserted into the targeted area which induces disruption and cellular damage. Thus, it should be noted that although the no-net flux microdilaysis technique has been used for many years to study dopamine levels (Parsons and Justice 1992; Smith and Weiss, 1999; Engleman et al. 2006; Franklin et al., 2009), it is conducted within a disrupted system which could affect the results.

The up-regulation of DA neurotransmission within the PL cortex may play a critical in ethanol drinking. Indeed, inhibition of DA neurotransmission by blocking D2 receptors with sulpiride microinjection into the PL cortex was shown to dose-dependently decrease ethanol drinking in P rats (Fig. 5). In addition, the same manipulation did not appear to alter saccharine drinking, suggesting that the reduction of ethanol drinking may not be due to non-specific effects of sulpiride on general reward or locomotor activity. These results are consistent with previous neurochemical data indicating that ethanol can increase extracellular DA levels within the mPFC (Ding et al., 2011; Doherty et al., 2016; Schier et al., 2013). In addition, our finding agrees with reports that infusion of the D2 receptor antagonist raclopride into the mPFC reduced operant responding for oral self-administration of ethanol in rats (Hodge et al., 1996; Samson and Chappell, 2003).

One weakness of the current study is that only female rats were examined. Both clinical and animal research suggest sex differences in effects of ethanol, with females consuming more alcohol and/or progressing faster than males (Becker and Koob, 2016; Bell et al., 2011; Priddy et al., 2017). Therefore, it is possible that the reinforcing and DA-enhancing effects of ethanol within the PL cortex may also be different between male and female rats, with potentially more robust effects in female than male rats.

In conclusion, the results of the current study indicate an important role of the PL cortex in mediating the reinforcing effects of ethanol and ethanol drinking. This provides additional evidence to the growing recognition of the significance of this region in the development of ethanol drinking and dependence (Faccidomo et al., 2015; Seif et al., 2013; Warnault et al., 2016). Furthermore, activation and neuroadaptations of local DA neurotransmission, especially that mediated by D2 receptors, appears to be critical to these effects. This conclusion is consistent with the proposed role of the mesocortical DA system in mediating motivation and reinforcement processes involved in the development of substance use disorder (Koob and Volkow, 2010). This increased understanding of the DA neurotransmission within the PL cortex in mediating ethanol reinforcement and reward may provide a target for future development of potential therapeutic strategy combating AUDs.

Highlights.

  • Ethanol can be self-infused into the prelimbic cortex

  • Inhibition of local D2 receptors reduces ethanol self-infusion

  • Ethanol drinking increases prelimbic basal extracellular dopamine levels

  • Inhibition of prelimbic D2 receptors attenuates ethanol drinking

Acknowledgments

Author Disclosure

This study was supported by research grants AA012262 (WJM), AA010717 (JMM), AA020396 (EAE), DA044242 (ZMD), P60 007611 (Indiana Alcohol Research Center). The authors declare no conflict of interest. The content of this manuscript is solely the responsibility of the authors and does not necessarily represent the official views of the NIAAA, NIDA or NIH.

Footnotes

Conflict of Interest Statement

The authors declare no conflict of interest.

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REFERENCES

  1. Abernathy K, Chandler LJ, Woodward JJ, 2010. Alcohol and the prefrontal cortex. Int. Rev. Neurobiol 91, 289–320. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Badanich KA, Maldonado AM, Kirstein CL, 2007. Chronic ethanol exposure during adolescence increases basal dopamine in the nucleus accumbens septi druing adulthood. Alcohol. Clin. Exp. Res 31, 895–900. [DOI] [PubMed] [Google Scholar]
  3. Becker JB, Koob GF, 2016. Sex Differences in Animal Models: Focus on Addiction. Pharmacol. Rev 68, 242–63. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Bell RL, Rodd ZA, Smith RJ, Toalston JE, Franklin KM, McBride WJ, 2011. Modeling binge-like ethanol drinking by peri-adolescent and adult P rats. Pharmacol. Biochem. Behav 100, 90–97. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Carr DB, Sesack SR, 2000. Projections from the rat prefrontal cortex to the ventral tegmental area: target specificity in the synaptic associations with mesoaccumbens and mesocortical neurons. J. Neurosci 20, 3864–73. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Diana M, Pistis M, Carbon S, Gessa GL, Rossetti ZL, 1993. Profound decrement of mesolimbic dopaminergic neuronal activity during ethanol withdrawal syndrome in rats: electrophysiological and biochemical evidence. Proc. Natl. Acad. Sci. USA 90, 7966–69. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Ding ZM, Ingraham CM, Rodd ZA, McBride WJ, 2015a. The reinforcing effects of ethanol within the nucleus accumbens shell involve activation of local GABA and serotonin receptors. J. Psychopharmacol 29, 725–33. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Ding ZM, Ingraham CM, Rodd ZA, McBride WJ 2015b. The reinforcing effects of ethanol within the posterior ventral tegmental area depend on dopamine neurotransmission to forebrain cortico-limbic systems. Addict. Biol 20, 458–68. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Ding ZM, Ingraham CM, Rodd ZA, McBride WJ, 2016. Alcohol drinking increases the dopamine-stimulating effects of ethanol and reduces D2 auto-receptor and group II metabotropic glutamate receptor function within the posterior ventral tegmental area of alcohol preferring (P) rats. Neuropharmacology. 109, 41–48. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Ding ZM, Oster SM, Hall SR, Engleman EA, Hauser SR, McBride WJ, Rodd ZA, 2011. The stimulating effects of ethanol on ventral tegmental area dopamine neurons projecting to the ventral pallidum and medial prefrontal cortex in female Wistar rats: regional difference and involvement of serotonin-3 receptors. Psychopharmacology. 216, 245–55. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Ding ZM, Toalston JE, Oster SM, McBride WJ, Rodd ZA, 2009. Involvement of local serotonin-2A but not serotonin-1B receptors in the reinforcing effects of ethanol within the posterior ventral tegmental area of female Wistar rats. Psychopharmacology. 204, 381–90. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Doherty JM, Schier CJ,, Vena AA, Dilly GA, Gonzales RA, 2016. Medial Prefrontal Cortical Dopamine Responses During Operant Self-Administration of Sweetened Ethanol. Alcohol. Clin. Exp. Res 40, 1662–70. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Engleman EA, Ding ZM, Oster SM, Toalston JE, Bell RL, Murphy JM, McBride WJ, Rodd ZA, 2009. Ethanol is self-administered into the nucleus accumbens shell, but not the core: evidence of genetic sensitivity. Alcohol. Clin. Exp. Res 33, 2162–71. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Engleman EA, Ingraham CM, McBride WJ, Lumeng L, Murphy JM, 2006. Extracellular dopamine levels are lower in the medial prefrontal cortex of alcohol-preferring rats compared to Wistar rats. Alcohol. 38, 5–12. [DOI] [PubMed] [Google Scholar]
  15. Faccidomo S, Salling MC, Galunas C, Hodge CW, 2015. Operant ethanol self-administration increases extracellular-signal regulated protein kinase (ERK) phosphorylation in reward-related brain regions: selective regulation of positive reinforcement in the prefrontal cortex of C57BL/6J mice. Psychopharmacology. 232, 3417–30. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Franklin KM, Engleman EA, Ingraham CM, McClaren JA, Keith CM, McBride WJ, Murphy JM, 2009. A single, moderate ethanol exposure alters extracellular dopamine levels and dopamine D2 receptor function in the nucleus accumbens of Wistar rats. Alcohol. Clin. Exp. Res 33, 1721–30. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Goeders NE, Dworkin SI, Smith JE, 1986. Neuropharmacological assessment of cocaine self-administration into the medial prefrontal cortex. Pharmacol. Biochem. Behav 24, 1429–40. [DOI] [PubMed] [Google Scholar]
  18. Goeders NE, Smith JE, 1983. Cortical dopaminergic involvement in cocaine reinforcement. Science. 221, 773–5. [DOI] [PubMed] [Google Scholar]
  19. Goeders NE, Smith JE, 1986. Reinforcing properties of cocaine in the medial prefrontal cortex: primary action on presynaptic dopaminergic terminals. Pharmacol. Biochem. Behav 25, 191–99. [DOI] [PubMed] [Google Scholar]
  20. Gonzales RA, Job MO, Doyon WM, 2004. The role of mesolimbic dopamine in the development and maintenance of ethanol reinforcement. Pharmacol. Ther 103, 121–46. [DOI] [PubMed] [Google Scholar]
  21. Hodge CW, Chappell AM, Samson HH, 1996. Dopamine receptors in the medial prefrontal cortex influence ethanol and sucrose-reinforced responding. Alcohol. Clin. Exp. Res 20, 1631–38. [DOI] [PubMed] [Google Scholar]
  22. Koob GF, Volkow ND, 2010. Neurocircuitry of addiction. Neuropsychopharmacology. 35, 217–38. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Lammel S, Lim BK, Malenka RC, 2014. Reward and aversion in a heterogeneous midbrain dopamine system. Neuropharmacology. 76, 351–59. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Lammel S, Lim BK, Ran C, Huang KW, Betley MJ, Tye KM, Deisseroth K, Malenka RC, 2012. Input-specific control of reward and aversion in the ventral tegmental area. Nature. 491, 212–7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Morzorati SL, Marunde RL, Downey D, 2010. Limited access to ethanol increases the number of spontaneously active dopamine neurons in the posterior ventral tegmental area of nondependent P rats. Alcohol. 44, 257–64. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. National Research Council., 2011. Guide for the care and use of laboratory animals. Washington, D.C.: National Academies Press. [Google Scholar]
  27. Nimitvilai S, Arora DS, McElvain MA, Brodie MS, 2012. Ethanol blocks the reversal of prolonged dopamine inhibition of dopaminergic neurons of the ventral tegmental area. Alcohol. Clin. Exp. Res 36, 1913–21. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Omelchenko N, Sesack SR, 2005. Laterodorsal tegmental projections to identified cell populations in the rat ventral tegmental area. J. Comp. Neurol 483, 217–35. [DOI] [PubMed] [Google Scholar]
  29. Parsons LH, Justice JB, 1992. Extracellular concentration and in vivo recovery of dopamine in the nucleus accumbens using microdialysis. J. Neurochem 58, 212–8. [DOI] [PubMed] [Google Scholar]
  30. Paxinos G, Watson C, 1998. The rat brain in stereotaxic coordinates. New York: Academic Press. [DOI] [PubMed] [Google Scholar]
  31. Pellegrino SM, Druse MJ, 1992. The effects of chronic ethanol consumption on the mesolimbic and nigrostriatal dopamine systems. Alcohol. Clin. Exp. Res 16, 275–80. [DOI] [PubMed] [Google Scholar]
  32. Peters J, Kalivas PW, Quirk GJ, 2009. Extinction circuits for fear and addiction overlap in prefrontal cortex. Learn. Mem 16, 279–88. [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. Peters J, LaLumiere RT, Kalivas PW, 2008. Infralimbic prefrontal cortex is responsible for inhibiting cocaine seeking in extinguished rats. J. Neurosci 28, 6046–53. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Phillips AG, Fibiger HC, 1978. The role of dopamine in maintaining intracranial self-stimulation in the ventral tegmentum, nucleus accumbens, and medial prefrontal cortex. Can.J. Psycho 32, 58–66. [DOI] [PubMed] [Google Scholar]
  35. Priddy BM, Carmack SA, Thomas LC, Vendruscolo JC, Koob GF, Vendruscolo LF, 2017. Sex, strain, and estrous cycle influences on alcohol drinking in rats. Pharmacol. Biochem. Behav 152, 61–67. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Rodd ZA, Melendez RI, Bell RL, Kuc KA, Zhang Y, Murphy JM, McBride WJ, 2004. Intracranial self-administration of ethanol within the ventral tegmental area of male Wistar rats: evidence for involvement of dopamine neurons. J. Neurosci 24, 1050–57. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Routtenberg A, Sloan M, 1972. Self-stimulation in the frontal cortex of Rattus norvegicus. Behav. Biol 7, 567–72. [DOI] [PubMed] [Google Scholar]
  38. Samson HH, Chappell A, 2003. Dopaminergic involvement in medial prefrontal cortex and core of the nucleus accumbens in the regulation of ethanol self-administration: a dual-site microinjection study in the rat. Physiol. Behav 79, 581–90. [DOI] [PubMed] [Google Scholar]
  39. Schier CJ, Dilly GA, Gonzales RA, 2013. Intravenous ethanol increases extracellular dopamine in the medial prefrontal cortex of the long-evans rat. Alcohol. Clin. Exp. Res 37, 740–47. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Seif T, Chang SJ, Simms JA, Gibb SL, Dadgar J, Chen BT, Harvey BK, Ron D, Messing RO, Bonci A, Hopf FW, 2013. Cortical activation of accumbens hyperpolarization-active NMDARs mediates aversion-resistant alcohol intake. Nat. Neurosci 16, 1094–100. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Shen RY, 2003. Ethanol withdrawal reduces the number of spontaneously active ventral tegmental area dopamine neurons in conscious animals. J. Pharmacol. Exp. Ther 307, 566–72. [DOI] [PubMed] [Google Scholar]
  42. Smith AD, Weiss F, 1999. Ethanol exposure differentially alters central monoamine neurotransmission in alcohol-preferring versus -nonpreferring rats. J. Pharmacol. Exp. Ther 288, 1223–28. [PubMed] [Google Scholar]
  43. Thielen RJ, Engleman EA, Rodd ZA, Murphy JM, Lumeng L, Li T-K., McBride, W.J., 2004. Ethanol drinking and deprivation alter dopaminergic and serotonergic function in the nucleus accumbens of alcohol-preferring rats. J. Pharmacol. Exp. Ther 309, 216–25. [DOI] [PubMed] [Google Scholar]
  44. Trantham-Davidson H, Burnett EJ, Gass JT, Lopez MF, Mulholland PJ, Centanni SW, Floresco SB, Chandler LJ, 2014. Chronic alcohol disrupts dopamine receptor activity and the cognitive function of the medial prefrontal cortex. J. Neurosci 34, 3706–0718. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Vertes RP, 2004. Differential projections of the infralimbic and prelimbic cortex in the rat. Synapse. 51, 32–58 [DOI] [PubMed] [Google Scholar]
  46. Warnault V, Darcq E, Morisot N, Phamluong K, Wilbrecht L, Massa SM, Longo FM, Ron D, 2016. The BDNF valine 68 to methionine polymorphism increases compulsive alcohol drinking in mice that is reversed by tropomyosin receptor kinase B activation. Biol. Psychiatry 79: 463–73 [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Weele CMV, Siciliano CA, Tye KM, 2019. Dopamine tunes prefrontal outputs to orchestrate aversive processing. Brain. Res 1713, 16–31. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Weiss F, Parsons LH, Schulteis G, Hyytia P, Lorang MT, Bloom FE, Koob GF, 1996. Ethanol self-administration restores withdrawal-associated deficiencies in accumbal dopamine and 5-hydroxytryptamine release in dependent rats. J. Neurosci 16, 3474–85. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Wise RA, Rompre PP, 1989. Brain dopamine and reward. Ann. Rev. Psychol 40, 191–225. [DOI] [PubMed] [Google Scholar]
  50. You C, Vandegrift B, Brodie MS, 2018. Ethanol actions on the ventral tegmental area: novel potential targets on reward pathway neurons. Psychopharmacology. 235, 1711–26. [DOI] [PMC free article] [PubMed] [Google Scholar]

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