Glial cell line-derived neurotrophic factor reverses alcohol-induced allostasis of the mesolimbic dopaminergic system: implications for alcohol reward and seeking

Abstract

We previously showed that infusion of glial cell line-derived neurotrophic factor (GDNF) into the ventral tegmental area (VTA) rapidly reduces alcohol intake and relapse (Carnicella et al., 2008, 2009a), and increases dopamine (DA) levels in the nucleus accumbens (NAc) of alcohol-naive rats (Wang et al., 2010). Withdrawal from excessive alcohol intake is associated with a reduction in NAc DA levels, whereas drug-induced increases in NAc DA levels are associated with reward. We therefore tested whether GDNF in the VTA reverses alcohol withdrawal-associated DA deficiency and/or possesses rewarding properties. Rats were trained for 7 weeks to consume high levels of alcohol (5.47 ± 0.37 g/kg/24 h) in intermittent access to 20% alcohol in a two-bottle choice procedure. Using in vivo microdialysis, we show that 24 h withdrawal from alcohol causes a substantial reduction in NAc DA overflow, which was reversed by intra-VTA GDNF infusion. Using conditioned place preference (CPP) paradigm, we observed that GDNF on its own does not induce CPP, suggesting that the growth factor is not rewarding. However, GDNF blocked acquisition and expression of alcohol-CPP. In addition, GDNF induced a downward shift in the dose-response curve for operant self-administration of alcohol, further suggesting that GDNF suppresses, rather than substitutes for, the reinforcing effects of alcohol. Our findings suggest that GDNF reduces alcohol-drinking behaviors by reversing an alcohol-induced allostatic DA deficiency in the mesolimbic system. In addition, as it lacks abuse liability, the study further highlights GDNF as a promising target for treatment of alcohol use/abuse disorders.

Figure 1.

Intra-VTA infusion of GDNF reverses alcohol withdrawal-associated NAc DA deficiency. Rats were trained to achieve a stable level of alcohol intake using the intermittent access to 20% alcohol in two-bottle choice procedure. For the alcohol group (red), microdialysis was performed immediately after the last session of 24 h alcohol access, whereas for the withdrawal group (wdrw) (green and black), microdialysis was performed 24 h after the last drinking session was terminated. An alcohol-naive control group (water) (blue) was included as a control. Samples were collected every 15 min. After collecting four samples, the wdrw group was infused with GDNF (10 μg/μl/side; black) or vehicle (green) into the VTA and returned to the microdialysis chambers for an additional 90 min. A, Schematic representation of the microdialysis probe placement in coronal sections (Paxinos and Watson, 1998). The locations of the dialysis membrane are represented by vertical bars. The numbers indicate the distance anterior to bregma (in millimeters). B–D, NAc DA levels are presented as mean percentage (±SEM) of the baseline of water controls. B, DA levels in the NAc throughout 10 fractions of 15 min in the water, alcohol, wdrw+veh, and wdrw+GDNF groups. C, Average NAc DA levels during fractions 1–4 in the water, alcohol, and wdrw groups. **p < 0.01. D, Average NAc DA levels during fractions 5–10 (following intra-VTA GDNF or vehicle infusion to wdrw groups) in the water, alcohol, wdrw+veh, and wdrw+GDNF groups. *p < 0.05. n = 4–6.

Figure 2.

Intra-VTA infusion of GDNF does not induce CPP and does not alter locomotor activity in rats. A, Design and schedule of the GDNF place preference experiment: rats of the GDNF condition were infused into the VTA with GDNF (10 μg · μl⁻¹ · side⁻¹) and vehicle (PBS) 10 min before the 30 min conditioning paired and unpaired sessions, respectively. Rats of the vehicle condition received vehicle only. B, Place preference for GDNF is expressed as the ratio ± SEM of the time spent in the GDNF-paired and unpaired compartment. C, D, Horizontal locomotor activity measured as mean ± SEM of the number of beam breaks during the entire conditioning sessions (C) or by blocks of 5 min (D). n = 6–8.

Figure 3.

Intra-VTA infusion of GDNF inhibits acquisition of CPP to alcohol. A, Design and schedule of place conditioning experiment testing the effects of GDNF on acquisition of alcohol-CPP: alcohol (0.5 g/kg, 15% v/v) or saline (Sal) was administered intraperitoneally immediately before the 7 min conditioning paired sessions. Saline was administered before the conditioning unpaired sessions. Rats of the GDNF condition were infused into the VTA with GDNF (10 μg/μl/side) and vehicle (PBS) 10 min before the conditioning paired and unpaired sessions, respectively. Rats of the vehicle condition received vehicle only. B, Place preference for alcohol expressed as the ratio ± SEM of the time spent in the alcohol-paired and unpaired compartment during the entire session. C, D, Place preference to alcohol expressed as the ratio ± SEM of the time spent in the alcohol-paired and unpaired compartment by blocks of 5 min for the alcohol-vehicle (C) or the alcohol-GDNF (D) conditions. n = 6–7. *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 4.

Intra-VTA infusion of GDNF inhibits expression of CPP to alcohol. A, Design and schedule of place conditioning experiment testing the effects of GDNF on expression of alcohol-CPP: alcohol (0.5 g/kg, 15% v/v) or saline (Sal) was administered intraperitoneally immediately before the 7 min conditioning paired sessions. Saline was administered before the conditioning unpaired sessions. GDNF (10 μg/μl/side) or vehicle (PBS) was infused into the VTA 10 min before the test session. B, Place preference for alcohol expressed as the ratio ± SEM of the time spent in the alcohol-paired and unpaired compartment during the entire session. C, D, Place preference to alcohol expressed as the ratio ± SEM of the time spent in the alcohol-paired and unpaired compartment by blocks of 5 min for the alcohol-vehicle (C) or the alcohol-GDNF (D) conditions. n = 7–9. *p < 0.05; **p < 0.01.

Figure 5.

Intra-VTA infusion of GDNF produces a downward shift in the dose–response curve for alcohol. A–E, An illustrative theoretical dose–response curve for drug self-administration and its modulation by pharmacological agents. A, B, Number of operant responses (A) and the resulting drug intake (B) as a function of the dose of the presented drug. C–E, Pharmacological agents (potential therapeutics) can reduce self-administration at a relevant reinforcing dose of the drug of abuse (represented by the open and solid circle for the vehicle and treatment condition, respectively) by shifting the dose–response curve to the right (“antagonist” effect) (C), or to the left (“agonist” effect, that substitutes for the rewarding effects of the drug) (D), or by inducing a downward shift (direct inhibition of the reinforcing effect of the drug) (E). F, G, GDNF (10 μg/μl/side) or vehicle was infused into the VTA 10 min before 30 min operant oral alcohol self-administration sessions of 2.5, 10, 20, and 40% alcohol. F, Mean ± SEM of the number of alcohol deliveries. G, Mean ± SEM of the alcohol intake (grams/kilogram). n = 9. **p < 0.01; ***p < 0.001.

Figure 6.

Diagram illustrating a model of allostatic aberration in DA levels in the NAc during prolonged excessive alcohol consumption and its reversal by GDNF. A, Consumption of alcohol during early drinking sessions causes an elevation in DA levels, and this is followed by a period of temporary DA deficiency during withdrawal until DA levels stabilize back to the homeostatic point. B, Repeated cycles of alcohol use and withdrawal cause a progressive transition to an allostatic state in brain reward system. Thus, the basal levels of NAc DA are gradually reduced and stabilize at a lower allostatic point. C, After a prolonged period of excessive drinking and withdrawal cycles, basal NAc DA levels are lowered compared with controls. This might be associated with a negative hedonic/mood state when alcohol is absent, causing craving and alcohol seeking. Alcohol consumption can increase DA levels to levels similar to those of control rats (Weiss et al., 1996), but the levels lapse again to the allostatic levels. D, A single infusion of GDNF into the VTA of rats during withdrawal causes a reversal of the allostatic deviation in DA levels, as well as suppression of alcohol reward seeking, alcohol intake, and relapse (Carnicella et al., 2008, 2009a). The effects of GDNF on alcohol consumption last at least 48 h (Carnicella and Ron, 2009). This diagram has been adapted from a model first published in the study by Koob and Le Moal (2001).