Retarded acquisition and reduced expression of conditioned locomotor activity in adult rats following repeated early maternal separation: effects of prefeeding, d-amphetamine, dopamine antagonists and clonidine.

Adult hooded rats exposed to a repeated maternal separation procedure during the neonatal period showed a blunted expression of locomotor hyperactivity conditioned to the presentation of the daily food ration. We have demonstrated that the expression of food-conditioned anticipatory hyperactivity is sensitive to the response-enhancing effects of systemic d-amphetamine (0.5; 1.0 mg/kg) and to the response-attenuating effects of the selective dopamine D2 antagonist sulpiride (8; 20 mg/kg), the selective dopamine D1 antagonist SCH 23390 (0.01; 0.022 mg/kg) and the mixed alpha 1/alpha 2 adrenoceptor agonist clonidine (5; 15 micrograms/kg) in a dose dependent manner. Animals from the early separation groups showed a reduced enhancement of activity in response to 0.5 mg/kg d-amphetamine and a greater attenuation of activity in response to 8 mg/kg sulpiride and 5 micrograms/kg clonidine. Female separated rats also exhibited an attenuated locomotor response to the unconditioned stimulant effects of 0.5 mg/kg systemic d-amphetamine. The experiments confirm that early maternal separation attenuates the response to conditioned appetitive cues in adult rats and implicate altered dopaminergic and noradrenergic function in the changes. It is possible that early maternal separation in the rat may offer a useful preparation for investigation of the neural substrates mediating affective development and affective psychopathology.