Spontaneous novelty seeking and amphetamine-induced conditioning and sensitization in adult mice: evidence of dissociation as a function of age at weaning.

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 > 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.