Corticosteroid-dependent plasticity mediates compulsive alcohol drinking in rats

Abstract

Alcoholism is characterized by a compulsion to seek and ingest alcohol, loss of control over intake, and the emergence of a negative emotional state during abstinence. We hypothesized that sustained activation of neuroendocrine stress systems (e.g., corticosteroid release via the hypothalamic-pituitary-adrenal axis) by alcohol intoxication and withdrawal and consequent alterations in glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation drive compulsive alcohol drinking. Our results showed that rats exposed to alcohol vapor to the point of dependence displayed increased alcohol intake, compulsive drinking measured by progressive-ratio responding, and persistent alcohol consumption despite punishment, assessed by adding quinine to the alcohol solution, compared with control rats that were not exposed to alcohol vapor. No group differences were observed in the self-administration of saccharin-sweetened water. Acute alcohol withdrawal was accompanied by downregulated GR mRNA in various stress/reward-related brain regions [i.e., prefrontal cortex, nucleus accumbens (NAc), and bed nucleus of the stria terminalis (BNST)], whereas protracted alcohol abstinence was accompanied by upregulated GR mRNA in the NAc core, ventral BNST, and central nucleus of the amygdala. No significant alterations in MR mRNA levels were found. Chronic GR antagonism with mifepristone (RU38486) prevented the escalation of alcohol intake and compulsive responding induced by chronic, intermittent alcohol vapor exposure. Chronic treatment with mifepristone also blocked escalated alcohol drinking and compulsive responding during protracted abstinence. Thus, the GR system appears to be involved in the development of alcohol dependence and may represent a potential pharmacological target for the treatment of alcoholism.

Figure 1.

Specific increase in alcohol intake and compulsive drinking in alcohol vapor-exposed rats during acute alcohol withdrawal. a, Number of lever presses for alcohol before (pre-vapor) and during alcohol vapor exposure on a fixed-ratio 1 (FR1) schedule of reinforcement (i.e., every active lever press was reinforced with 0.1 ml of 10% alcohol, w/v). b, Number of alcohol reinforcers earned and last ratio achieved in a progressive-ratio test. c, Compulsive-like drinking (i.e., persistent alcohol drinking despite the aversive bitter taste of quinine added the alcohol solution). The data represent the percentage change from baseline (i.e., lever presses for alcohol alone before adulteration with quinine). d, Number of lever presses for a saccharin (0.004%, w/v) solution (FR1). The data represent mean and SE. *p < 0.05, significant difference between dependent and nondependent. n = 16–18 per group.

Figure 2.

Acute alcohol withdrawal (24 h after the vapor was turned off) was accompanied by GR downregulation in stress/reward-related brain areas. Dependent rats showed lower GR mRNA levels in the (a) PFC, (b) NAc, and (c) BNST but not (d) amygdala or (e) hippocampus compared with nondependent rats. Insets illustrate the approximate location of brain punches. Data represent mean and SE. *p < 0.05, significant difference between dependent and nondependent. n = 7–17 per group.

Figure 3.

Protracted alcohol abstinence (3 weeks after the vapor was turned off) was accompanied by GR upregulation in stress/reward-related brain areas. Dependent rats showed higher GR mRNA levels in the NAc shell, ventral BNST, and central nucleus of the amygdala (CeA) but not PFC, NAc core, dorsolateral (DL) BNST, or basolateral amygdala (BLA) compared with nondependent rats. The data represent mean and SE. *p < 0.05, significant difference between dependent and nondependent. n = 5–8 per group.

Figure 4.

Chronic GR blockade by mifepristone prevented the escalation of alcohol intake and motivation for alcohol in vapor-exposed animals during acute alcohol withdrawal. a, Timeline of the experiment. Dependent and nondependent rats were implanted with pellets for the chronic release of the GR antagonist mifepristone (150 mg for 21 d) or placebo before exposure to alcohol vapor. Mifepristone-treated vapor-exposed rats did not exhibit an escalation of alcohol intake (b) or increased PR responding (c) compared with placebo-treated vapor-exposed rats. Mifepristone did not influence alcohol intake in nondependent rats. The data represent mean and SE. *p < 0.05, significant difference from mifepristone-treated vapor exposed rats; ⁺p < 0.05, significant difference from placebo-treated nondependent rats. n = 9–10 per group.

Figure 5.

Chronic GR blockade by mifepristone decreased escalated alcohol intake in vapor-exposed animals during protracted alcohol abstinence. a, Timeline of the experiment. The rats were made dependent on alcohol by exposure to chronic, intermittent vapor exposure and then removed from the vapor chambers. One week later, dependent and nondependent rats were implanted with pellets for the chronic release of the GR antagonist mifepristone (150 mg for 21 d) or placebo. Behavioral testing began 1 week after pellet implantation (3 weeks of protracted withdrawal). Mifepristone-treated vapor-exposed rats did not exhibit escalated alcohol intake (b) or increased PR responding (c) compared with placebo-treated vapor-exposed rats. Mifepristone did not influence alcohol intake in nondependent rats. The data represent mean and SE. *p < 0.05, significant different from mifepristone-treated dependent rats and placebo-treated nondependent rats. n = 5–7 per group.

Figure 6.

Hypothalamic-pituitary-adrenal axis, extrahypothalamic GR levels, and brain stress/reward function hypothesized to be recruited at different stages of the addiction cycle as addiction moves from positive reinforcement to negative reinforcement. Left, In nondependent subjects, alcohol activates the HPA axis to release CORT from the adrenal gland (Ellis, 1966; Lee and Rivier, 1997; Richardson et al., 2008), and CORT facilitates the reinforcing effects of alcohol (Fahlke et al., 1995, 1996) via positive reinforcement. High CORT levels decrease HPA axis activity via negative feedback. Middle, Repeated cycles of alcohol intoxication/withdrawal induce overactivation of the HPA axis that disrupt HPA axis function (i.e., blunted activity; Richardson et al., 2008) and downregulate GRs levels in stress/reward-related brain regions. These long-lasting changes can “sensitize” extrahypothalamic stress systems (e.g., CRF) involved in the behavioral response to stressors and further drive escalated and compulsive drug intake (Makino et al., 2002) via negative reinforcement (Edwards and Koob, 2010). Right, During protracted abstinence, GR levels are upregulated in stress/reward-related brain regions, suggesting receptor adaptation when alcohol vapor exposure ceases. Although peak HPA axis activation is blunted during protracted abstinence in alcohol dependence (Adinoff et al., 1990; Zorrilla et al., 2001), alcohol dependence-mediated dysregulation of GRs remains. A sensitized GR system would be expected to increase the “gain” for a neuronal response to CORT, sustaining the escalation of alcohol intake even in the absence of peak levels of released CORT. There are several steps between GR binding, mRNA expression, and function. Pinpointing the single molecular mechanism that underlies escalated alcohol intake is difficult at this time. The bidirectional regulation of GRs at different withdrawal time-points suggests that GR expression is dynamically regulated in the alcohol-dependent and postdependent brain and mediates escalated alcohol drinking. Amyg, amygdala; Hippo, hippocampus.