Early life tactile stimulation changes adult rat responsiveness to amphetamine.

Previously, we have found that total maternal deprivation of rat pups, achieved through artificial rearing, leads to a number of behavioral and neurophysiological changes, suggesting a change in the dopamine system. The purpose of this study was to further investigate possible changes in the functioning of dopamine systems, associated with artificial rearing, by examining the locomotor stimulant effects of the dopamine releaser amphetamine and the dopamine reuptake inhibitor methylphenidate. Rats were mother-reared or artificially reared. Some of those artificially reared rats were provided with either a maximum level (artificially reared maximal stimulation) or a minimal level of maternal licking-like tactile stimulation (artificially reared minimal stimulation). In adulthood, rats' locomotion was measured after an injection of d-amphetamine (0, 0.25, 0.5 and 1.0 mg/kg) or methylphenidate (0, 2, 5 and 10 mg/kg). Locomotor activity in response to a novel environment was enhanced in artificially reared rats, although this effect habituated over three daily 1-h sessions. Both amphetamine and methylphenidate dose dependently increased locomotor activity. The effect of amphetamine, but not methylphenidate was greatly enhanced in artificially reared minimally stimulated rats. The enhancement of the effect of amphetamine by artificial rearing was not apparent in artificially reared maximal stimulation rats.