DENDRITIC SPINE DENSITY IN THE LOBUS PAROLFACTORIUS OF THE DOMESTIC CHICK IS INCREASED 24-H AFTER ONE-TRIAL PASSIVE-AVOIDANCE TRAINING

One to three day old chicks spontaneously peck at small objects. When presented with a chrome bead coated with the bitter tasting substance methyl anthranilate (MeA), chicks peck once, display a characteristic disgust response and subsequently avoid a similar bead. Chicks that ar e trained on a water coated bead continue to peck a similar bead on re trial. Twenty four hours after training on this one-trial passive avoi dance paradigm, chicks were tested for retention. The brains of chicks displaying the correct behavioural response (> 90%) were removed and the lobus parolfactorius from each hemisphere was dissected from the b rain and impregnated using a rapid Golgi technique. Analysis of large multipolar neurones by centrifugal dendritic branch order showed that there were significantly more spines on all orders examined in the lef t hemispheres of MeA-trained chicks compared to water-trained control chicks. Significantly higher spine densities were also found on 4th an d 5th order branches of neurones in the right lobus parolfactorius of MeA-trained chicks compared to water-trained chicks. No significant di fference in dendritic length was observed. These results suggest that substantial plasticity occurs in post-synaptic structures in the lobus parolfactorius following passive avoidance training. It is suggested that this plasticity is related to processes involved in long term inf ormation storage.