Morphological changes associated with stages of memory formation in the chick following passive avoidance training.

Memory formation following learning is presumed to result from modification in the efficacy of neural circuitry, either through strengthening of pre-existing synapses, or formation of new contacts. An ideal paradigm to investigate memory formation is one-trial passive avoidance training of day-old chicks, in which the birds learn to avoid pecking a bead coated with an aversive substance, methylanthranilate. Following training, a sequence of biochemical, electro-physiological, pharmacological and morphological events takes place within two loci in the forebrain, the intermediate and medial hyperstriatum ventrale (IMHV), and part of the paleostriatal complex, the lobus parolfactorius (LPO). Our data reviewed here suggest that the initial acquisition of memory involves population changes in the fine spatial organization of synaptic vesicles and active zones in synapses in the IMHV whereas longer-term changes are more prominent in the LPO and involve, primarily, a bilateral increase in the density of synapses and dendritic spines. The short-term synaptic changes are as dynamic as the molecular changes which have hitherto been considered the preserve of short-term correlates of memory formation.