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Individual differences in coping with novelty and in the response to psychoactive drugs have been related to early life events, such as the age of weaning. Outbred CD-1 mice underwent a precocious (postnatal day (pnd) 15, Wean-15 group), regular (pnd 21, Wean-21 group), or delayed (pnd 27, Wean-27 group) weaning, and were tested as adults (pnd &gt; 60). In Experiment 1, animals underwent a treatment history with d-amphetamine (AMPH 0, 1, or 5 mg/kg once/day for three days) in a familiar environment. On testing day, mice were challenged with SAL or a standard 1 mg/kg AMPH dose (to evaluate acute drug effects and sensitization), and placed in the familiar environment. As expected, regular Wean-21 animals showed an AMPH-induced hyperactivity and a profile of conditioned locomotion, whereas the same dosage failed to induce any change in Wean-15 and Wean-27 groups. Levels of spontaneous novelty seeking were particularly elevated for Wean-27 mice, when compared with the other weaning groups. In Experiment 2, pairing of AMPH administration (0, 1, 2, 3.3, or 5 mg/kg once/day for three days) with a distinct environment produced a classical conditioned place preference. The magnitude of the preference profile was significantly more marked for Wean-15 mice, when compared with the other two weaning groups. Both experiments also provided evidence that the development of sensitization was particularly evident in Wean-27 mice. In summary, delayed weaning was associated in adult mice with both elevated levels of novelty seeking and increased sensitization to drug effects. Conversely, animals weaned precociously were much more responsive to AMPH-induced incentive conditioning. These results appear relevant to the issue of early experiences as possible risk factors for a number of psychiatric disorders in humans, including the abuse of drugs.

neuropsychopharmacology

Spontaneous Novelty Seeking and Amphetamine-induced Conditioning and Sensitization in Adult Mice Evidence of Dissociation as a Function of Age at Weaning