Extinction-induced immobility in the water maze and its neurochemical concomitants in aged and adult rats: a possible model for depression?

Extinction of escape behavior in the water maze due to the removal of the platform, was hypothesized to induce a negative state, including the development of immobility, which is held to reflect a state of "despair" when measured in the forced swimming test. 27 aged and 8 adult animals (26 and 3 months old Wistar rats, respectively) were tested in the water maze during nine days with a platform hidden, followed by 7 days of extinction trials with the platform absent. As expected, both age groups developed immobility over the extinction trials, with the aged showing more than the adults. To examine whether the age difference in immobility was related to performance differences during acquisition, the aged were subdivided into superior, intermediate and inferior learners (n = 9 per group) on the basis of overall times to platform during acquisition, and compared with each other and the adults. Results showed that the aged inferior learners displayed the highest levels of immobility among the aged. Immobility scores were then correlated with post-mortem neurotransmitter contents in the hippocampus and ventral striatum. In the ventral striatum, levels of immobility were correlated with levels of acetylcholine, dopamine and the metabolite dihydroxyphenylacetic acid in the aged, and with norepinephrine in the adults. The data support the hypothesis that multiple extinction trials in the water maze result in immobility that may indicate "behavioral despair," and that striatal neurotransmitter systems correlate with the degree of its expression. The concept of extinction-induced despair is held to provide the promise of a conceptual and empirical model of human depression that is the consequence of loss of reinforcers.