SPATIAL RE-ARRANGEMENT OF THE VESICLE APPARATUS IN FOREBRAIN SYNAPSES OF CHICKS 30 MIN AFTER PASSIVE-AVOIDANCE TRAINING

A quantitative ultrastructural study of synapses was carried out in the forebrain IMHV (intermediate and medial hyperstriatum ventrale) of 1-day-old chicks 30 min after training to avoid pecking at a bead coated with methyl anthranilate. In 10 birds (5 control and 5 trained), the length, curvature and the number of the synaptic active zone profiles were measured, and the active zone profile length was observed to increase in trained chicks. The spatial arrangement of synaptic vesicles with respect to the active zone was examined using a statistical stereological approach. This showed that vesicles are not located uniformly but accumulate in two spatial pools which appear to rearrange following training, with a greater number of vesicles near the active zone. These data may reflect subtle changes in the functional efficacy of synapses in the IMHV in the initial phases of memory formation.