Figure 4.

Diagram illustrating an extension of Solomon and Corbit's opponent-process model of motivation to outline the conceptual framework of the allostatic hypothesis. Both panels represent the affective response to the presentation of a drug. (Top) This diagram represents the initial experience of a drug with no prior drug history. The a-process represents a positive hedonic or positive mood state, and the b-process represents a negative hedonic or negative mood state. The affective stimulus (state) has been argued to be the sum of both an a-process and a b-process. An individual experiencing a positive hedonic mood state from a drug of abuse with sufficient time between re-administering the drug is hypothesized to retain the a-process. In other words, an appropriate counteradaptive opponent-process (b-process) that balances the activational process (a-process) does not lead to an allostatic state. (Bottom) Changes in the affective stimulus (state) in an individual with repeated frequent drug use that may represent a transition to an allostatic state in the brain reward systems and, by extrapolation, a transition to addiction. Notice that the apparent b-process never returns to the original homeostatic level before drug-taking is reinitiated, thus creating a greater and greater allostatic state in the brain reward system. In other words, the counteradaptive opponent process (b-process) does not balance the activational process (a-process) but in fact shows a residual hysteresis. Although these changes are exaggerated and condensed over time in the present conceptualization, the hypothesis here is that even during post-detoxification (a period of “protracted abstinence”) the reward system is still bearing allostatic changes. In the nondependent state, reward experiences are normal, and the brain stress systems are not greatly engaged. During the transition to the state known as addiction, the brain reward system is in a major underactivated state while the brain stress system is highly activated. CRF, corticotropin-releasing factor; GABA, γ-aminobutyric acid; NPY, neuropeptide Y. The following definitions apply: allostasis, the process of achieving stability through change; allostatic state, a state of chronic deviation of the regulatory system from its normal (homeostatic) operating level; allostatic load, the cost to the brain and body of the deviation, accumulating over time, and reflecting in many cases pathological states and accumulation of damage. (Modified with permission from [28].