INVOLVEMENT OF AMPA RECEPTORS IN MAINTENANCE OF MEMORY FOR A PASSIVE-AVOIDANCE TASK IN DAY-OLD DOMESTIC CHICKS (GALLUS-DOMESTICUS)

Day-old chicks (Gallus domesticus) were trained on a one-trial passive avoidance task where the aversive stimulus was an unpleasant tasting substance, methyl anthranilate. Chicks were killed 6.5 h after trainin g. The kinetic parameters of [H-3] lpha-amino-3-hydroxy-5-methyl-4-iso xazolepropionic acid ([H-3]AMPA) binding were determined using quantit ative receptor autoradiography and Scatchard analyses in 15 discrete f orebrain regions of trained and control (water-trained) chicks, reveal ing two components of binding in each. KD values showed some regional variation, but were 22.2 +/- 1.1 nmol 1(-1) for the high-affinity comp onent and 685 +/- 25 nmol 1(-1) for the low-affinity component of bind ing to whole forebrain sections from control chicks. Analyses also rev ealed that Hill coefficients were significantly less than 1 in all reg ions measured. A significant decrease in KD for the low-affinity compo nent occurred bilaterally in the intermediate and medial hyperstriatum ventrale (IMHV; left, 34.8%; right, 33.3%), a region that has previou sly been shown to be implicated in the processes of memory formation, following passive avoidance training. A significant decrease in KD for the high-affinity component occurred in the right palaeostriatum augm entatum (19.5%). Significant decreases in B-max accompanied the KD alt erations in both cases. Additionally, bilateral intracerebral injectio ns (administered 4.5-5.5 h after training) into the IMHV of 500 nmol 1 (-1) 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective antagoni st of non-NM DA glutamate receptors (particularly AMPA receptors), res ulted in amnesia for one-trial passive avoidance training in day-old c hicks tested 6.5 h after training. Unilateral injections administered 5.5 h after training had a significant but smaller effect. Injections administered before training or 5 min after training had no effect on learning or retention of memory for the task. These results show that the affinity of AMPA receptors increases at a time point after passive avoidance training at which their inhibition by CNQX blocks maintenan ce of long-term memory for this task.