Molecular Genetic Approaches Using Knockout Models
Many molecular genetic knockouts have been studied. Alcohol-drinking mouse models have been identified using two-bottle choice drinking and operant responding for alcohol (Animal Models). The knockout of several different neurotransmitters, modulators, receptors, and other molecules decreases drinking behavior and the preference for alcohol, contributing to a detailed symphony of brain systems that contribute to the acute reinforcing effects of alcohol. For example, μ opioid receptor knockout mice do not self-administer alcohol. The knockout of cannabinoid CB1 and δ opioid receptors, neuropeptide Y (NPY) and NPY1 receptors, and PKA, among others, increases alcohol drinking. This complex interplay provides clues to the systems in which inhibition may contribute to the reinforcing effects of alcohol (for further reading, see Crabbe et al., 2006).
Figure 6.13 Effects of microinjection of the dopamine antagonist fluphenazine into the nucleus accumbens in rats on responding for alcohol (10% w/v) and water. The data are expressed as the mean percentage of baseline responding and are plotted as a function of fluphenazine dose. Asterisks (**) indicate a significant difference in responding compared with responding after vehicle injection (p < 0.01). These data show that a dopamine D2 receptor antagonist injected into the nucleus accumbens dose-dependently decreased alcohol self-administration, suggesting a key role for dopamine release in the nucleus accumbens in the rewarding effects of alcohol. [Taken with permission from 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, 1992, (109), 92–98.]
Figure 6.14 The effect of γ-aminobutyric acid-A receptor antagonist SR 95531 injections into the central nucleus of the amygdala, bed nucleus of the stria terminalis, and shell of the nucleus accumbens on responding for alcohol and water in rats. The data are expressed as the mean number of responses for alcohol and water during 30 min sessions for each injection site. Notice the change in the abscissa scale for injections into the bed nucleus of the stria terminalis and nucleus accumbens shell. Significant differences from corresponding saline control values: *p < 0.05, **p < 0.01, for ethanol responses; #p < 0.05, for water responses. These data show that a highly potent GABAA receptor antagonist injected into the central nucleus of the amygdala, bed nucleus of the stria terminalis, and nucleus accumbens dose-dependently decreased alcohol self-administration. Notice that the most potent effects were observed with the injections into the central nucleus of the amygdala (2 ng was a significant dose), suggesting a key role for GABA activity in the central nucleus of the amygdala in the rewarding effects of alcohol. [Taken with permission from Hyytia P, Koob GF. GABAA receptor antagonism in the extended amygdala decreases ethanol self-administration in rats. European Journal of Pharmacology, 1995, (283), 151–159.]
Figure 6.15 The effects of intracerebral administration of the opioid receptor antagonist methylnaloxonium into the amygdala and nucleus accumbens on responding for alcohol in rats. The data are expressed as the mean total responses. Asterisks (*) indicate significant differences from saline injections (p < 0.05). These data show that the opioid receptor antagonist methylnaloxonium injected into the central nucleus of the amygdala and nucleus accumbens dose-dependently decreased alcohol self-administration. Notice that the most potent effects were observed with the injections into the central nucleus of the amygdala (250 ng was a significant dose), suggesting a key role for opioid peptide activity in the central nucleus of the amygdala in the rewarding effects of alcohol. [Modified with permission from Heyser CJ, Roberts AJ, Schulteis G, Koob GF. Central administration of an opiate antagonist decreases oral ethanol self-administration in rats. Alcoholism: Clinical and Experimental Research, 1999, (23), 1468–1476.]
Withdrawal/Negative Affect Stage
Metabolic tolerance can account for some of the resistance of the brain to alcohol, as noted above. Early animal studies, however, also showed that rats treated chronically with alcohol had higher brain levels of alcohol than control animals at the time of recovery from overt intoxication, arguing for pharmacodynamic tolerance (see What is Addiction?). In rodents, chronic functional tolerance has been measured by changes in the sedative effects of alcohol, usually reflected by the ability of the animal to perform a motor task, such as run on a treadmill or balance on a rotating rod. Behavioral factors, including learning, play a prominent role in the development of tolerance. Animals and humans can learn to counteract the effects of alcohol through Pavlovian or operant conditioning. For example, rats that were allowed to experience a treadmill task while intoxicated showed much more rapid and complete tolerance to alcohol than rats that were not allowed to experience the task while intoxicated but received alcohol after completion of the task.
Numerous neuropharmacological substrates are involved in behavioral tolerance as a model of the neuroadaptive processes associated with the development of pharmacodynamic tolerance. These include serotonin, glutamate, and vasopressin. Blockade of the activity of any of these systems blocks acute and chronic tolerance. A neural circuit that involves these neurotransmitters and connections between the septum and hippocampus has been proposed to play an essential role in the development and retention of tolerance (for further reading, see Kalant, 1998).