Pharmaco-ontogeny of reward: enhancement of self-stimulation by D-amphetamine and cocaine in 3- and 10-day-old rats.

Electrodes were implanted in various forebrain sites in 3- and 10-day-old rat pups that were allowed to nudge a pole for electrical stimulation of those areas. After 5 h pups were injected with amphetamine, cocaine or saline and the change in responding noted. Both stimulants, at relatively high doses, enhanced responding on the pole that produced stimulation to a greater extent than responding on a non-active pole. At lower doses, responding on both the active and inactive poles were enhanced equally. These effects lasted 2-3 h. Histological analyses showed that the most sensitive loci that were consistently enhanced by these stimulants included the medial forebrain bundle, nucleus accumbens, anterior olfactory nucleus and olfactory tubercle. The results suggest that the biological mechanisms by which these drugs enhance self-stimulation in adult animals are present in neonates.