NMDA receptor antagonists extend the sensitive period for imprinting

Filial imprinting in the domestic chick occurs during a sensitive period of development. The exact timing of this period can vary according to the met hods used to measure imprinting. Using our imprinting paradigm, we have sho wn that normal, dark-reared chicks lose the ability to imprint after the se cond day post-hatching. Further, we reported that chicks treated 10 h after hatching with a mixture of the noncompetitive NMDA receptor antagonist ket amine (55 mg/kg) and the alpha(2)-adrenergic receptor agonist xylazine (6 m g/kg) were able to imprint on day 8 after hatching, whereas controls treate d with saline did not imprint. We now show that the effect of the ketamine- xylazine mixture can be mimicked by treating chicks with ketamine alone or with another noncompetitive NMDA receptor antagonist, MK-801 (5 mg/kg). Tre ating chicks with a single dose of ketamine (55 mg/kg) or with a single dos e of xylazine (6 mg/kg) failed to produce the effect on the sensitive perio d. However, prolonging the action of ketamine by treating chicks with two d oses of ketamine (at 10 and 12 h after hatching) did allow imprinting on da y 8. In contrast, prolonging the action of xylazine had no effect on the se nsitive period for imprinting. Chicks treated with MK-801 were also able to imprint on day 8. Thus, we have evidence that the NMDA receptor system is involved in the mechanisms that control the sensitive period for imprinting . (C) 2000 Elsevier Science Inc. All rights reserved.