Memory Formation in Day‐old Chicks Requires NMDA but not Non‐NMDA Glutamate Receptors

The non‐competitive N ‐methyl‐D‐aspartate (NMDA) antagonist MK‐801, injected intraperitonealy at 0.1 mg/kg, at times between 1 h before and 5 min after training chicks on a one‐trial passive avoidance task, resulted in amnesia for the task on test 3 or 24 h subsequently. No amnesia was apparent at 24 h if chicks were injected between 1 and 6 h after training. Amnesia did not develop immediately; it was not apparent 30 min after training in chicks injected 5 min after training. At this dose of MK‐801 no other effects on motor or pecking behaviour of the birds were observed. Bilateral or unilateral intracerebral injections of 1.5 nM MK‐801 5 min after training produced a similar amnesia at 3 h to that of intraperitonealy injected MK‐801; no hemispheric differences were observed, presumably because of the ready diffusion of the MK‐801. By contrast, intracerebral injections of the non‐NMDA glutamate antagonists 6‐cyano‐7‐nitroquinoxaline‐2, 3‐dione (CNQX), 6,7‐dinitroquinoxaline‐2,3‐dione (DNQX) and 6,7‐nitro‐7‐sulphamoyl‐benzoquinoxaline‐2,3‐dione (NBQX) (0.066 μM) 5 min after training, despite producing severe if transient behavioural disturbances, were without effect on retention for the avoidance response in chicks tested 3 h subsequently. We interpret these results as pointing to a requirement for NMDA, but not kainate or quisqualate, receptor activation as an early enabling event in the biochemical cascade required for long‐term memory formation for passive avoidance in the chick.