Animal Models of the Binge/Intoxication Stage of the Addiction Cycle
Researchers have discovered other ways to assess the rewarding or reinforcing effects of various drugs of abuse. One widely used method is called brain stimulation reward (or intracranial self-stimulation [ICSS]). Animals will perform a variety of tasks to self-administer short electrical pulses (approximately 250 ms) directly into many areas of the brain (Olds and Milner, 1954). The brain site that supports the highest rates of self-stimulation is the medial forebrain bundle, which courses bidirectionally from the midbrain to basal forebrain. Several seminal studies found that ICSS directly activates many of the same neuronal circuits that are activated by conventional reinforcers, such as food, water, sex, and drugs, leading many to hypothesize that ICSS studies can reveal the brain systems involved in motivated behavior. The acute administration of drugs of abuse decreases ICSS thresholds, in which less electrical stimulation is needed for the animal to perceive the stimulation as rewarding. Conversely, withdrawal from drugs of abuse increases reward thresholds, meaning that more stimulation is needed to perceive the stimulation as rewarding. There is good correspondence between the ability of drugs to decrease ICSS thresholds and their abuse potential (Kornetsky and Esposito, 1979; Kornetsky and Bain, 1990). Two ICSS procedures that have been used extensively to measure the changes in reward threshold are the rate-frequency curve shift procedure (Campbell et al., 1985) and the discrete-trial, current-intensity procedure (Kornetsky and Esposito, 1979; Markou and Koob, 1992; Figures 3.4–3.6).
FIGURE 3.3 Different degrees of alcohol exposure in individual rats that lever-press to obtain a 10% alcohol solution in a free choice operant task, following training with the saccharin fade-out procedure. The data were derived from Rassnick S, Pulvirenti L, Koob GF. SDZ 205,152, a novel dopamine receptor agonist, reduces oral ethanol self-administration in rats. Alcohol, 1993, (10), 127–132.