Depression Seen Through an Animal Model

An Expanded Hypothesis to Further Explain: Stress-Induced Behavioral Depression

Stress-Induced Behavioral Depression: An Animal Model for the Study of Depression

When laboratory rodents are exposed to highly stressful events they cannot control, they exhibit behavioral and physiological changes characteristic of clinical depression. First, etiological similarities can be noted between this animal model and human depression. Stressful events, which precipitate the depression-like behaviors seen in rodents, likewise precede the onset of some clinical depressions in humans (e.g., Leff, Roatch, & Bunney, 1970; Lloyd, 1980; Frank & Stewart, 1983; Gold, Goodwin, & Chrousos, 1988). Moreover, the occurrence of depression-like behavioral and physiological changes in the animals has been shown to depend on the uncontrollable nature of the stressful events, since exposure of the animals to similar but controllable events does not produce the relevant behavioral changes (Corum & Thurmond, 1977; Redmond, Mass, Dekirmanjian, & Schlemmer, 1973; Seligman & Maier, 1967; Sutton, Coover, & Lints, 1981; Weiss, 1968; Weiss et al., 1982). Paralleling this, depressed persons

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often report feeling unable to cope or control events (Seligman, 1974; Seligman & Beagley, 1975).

Second, several of the principal symptoms that characterize clinical depression are seen in stressed animals. Prominent symptoms are decreased motor activity (Anisman et al., 1978, 1979; Overmier, 1968; Overmier & Seligman, 1967; Seligman & Maier, 1967; Sutton et al., 1981; Weiss & Glazer, 1975; Weiss, Glazer, Pohorecky, Brick, & Miller, 1975; Weiss et al., 1981) and decreased eating and drinking and weight loss/lack of weight gain (Brady, Thornton, & deFisher, 1962; Pare, 1964, 1965; Ritter, Pelzer, & Ritter, 1978; Weiss, 1968). Stressed animals also show decreased grooming (Redmond et al., 1973; Stone, 1978; Weiss et al., 1981), decreased competitive behavior (Corum & Thurmond, 1977; Peters and Finch, 1961; Redmond et al., 1973), increased errors in a choice/discrimination task (Jackson, Alexander, & Maier, 1980; Minor, Jackson, & Maier, 1984), and decreased responding for “rewarding brain stimulation” (Zacharko, Bowers, Kokkinidis, & Anisman, 1983). In addition, these animals show sleep disturbance (reduced sleep) characterized particularly by “early morning awakening” (Weiss, Simson, Ambrose, Webster, & Hoffman, 1985). These symptoms closely correspond to those typically used for the diagnosis of depression as listed in the Diagnostic and statistical manual of the mental disorders (American Psychiatric Association, 1980, 1987, 1994; DSM-III, DSM-III-R, and DSM IV). Third, effective treatments for relieving depression—electroconvulsive shock and drug therapy—can eliminate stress-induced behavioral deficits in animals and/or prevent their occurrence (Dorworth & Overmier, 1977; Glazer et al., 1975; Leshner, Remler, Biegon, & Samuel, 1979; Petty & Sherman, 1979; Sherman, Allers, Petty, & Henn, 1979). In summary, exposure of animals to highly stressful, uncontrollable events produces a model of depression that resembles clinical depression in humans with respect to aspects of etiology, symptomatology, and responsiveness to treatment.