SHORT-TERM CHANGES IN THE NUMERICAL DENSITY OF SYNAPSES IN THE INTERMEDIATE AND MEDIAL HYPERSTRIATUM-VENTRALE FOLLOWING ONE-TRIAL PASSIVE-AVOIDANCE TRAINING IN THE CHICK

Previous ultrastructural studies using stereological counting techniqu es, based on assumptions regarding shape, size, and orientation of syn apses, have suggested synaptic remodeling occurred at least 24 hr afte r one-trial passive avoidance training in day-old chicks. The present study estimates the mean synaptic density (Nv(v syn)) in a region of t he chick forebrain known to be involved in memory formation, the inter mediate and medial hyperstriatum ventrale (IMHV), 1 and 24 hr followin g one-trial passive avoidance training. A stereological technique, the ''disector,'' that makes no assumptions about size, shape, and orient ation of synapses was used in the synaptic analyses. The density of ax ospinous synapses increased by approximately 77% at 1 hr posttraining in the right IMHV of chicks (M-trained) that learned to avoid a bitter -tasting bead, compared to those (W-trained controls) that peck a wate r-coated bead. A measure of the postsynaptic density size, the mean pr ojected height of synapses (H), was 57% smaller 1 hr posttraining in t he right IMHV of M-trained chicks. These differences were not found at 24 hr posttraining. We suggest that structural modification of synaps es may be a key part of the processes involved in short-term memory fo rmation.