Influence of the N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovaleric acid on auditory filial imprinting in the domestic chick.

Newly hatched domestic chicks were injected uni- or bilaterally into the imprinting relevant forebrain region mediostral neostriatum/ hyperstriatum ventrale (MNH) with different concentrations (1, 3, 12.5, 50 nmol) of the competitive NMDA antagonist DL-5-amino-5- phosphonovaleric acid (APV) prior to exposure to the imprinting stimulus (400-Hz tone pulses). In both APV-injected experimental groups there was a dose-dependent decrease of imprinting rates compared to that seen in controls, suggesting an APV-induced blockade of the learning process. In order to find out how the injected APV interferes with the stimulus-evoked enhanced metabolic activity in imprinting-relevant brain regions and to gain information about the spatial extent of its physiological effect, 2-fluorodeoxyglucose (2-FDG) experiments were performed. Chronically APV-treated chicks which failed in the imprinting tests showed similarly low 2-FDG uptake in imprinting relevant areas as naive untreated controls, further suggesting that no learning process has occurred in these animals. In imprinted chicks acute APV injections into the MNH led to a suppression of the stimulus-evoked 2-FDG uptake in this region. This pharmacologically induced reduction extended beyond the MNH region to adjacent regions. Furthermore, unilateral injections into the MNH always resulted in a bilateral reduction of 2-FDG uptake in this area. The results suggest that NMDA-mediated mechanisms in restricted telecephalic areas such as the MNH play a crucial role in the process of auditory filial imprinting.