The effects of protein synthesis inhibition on structural changes associated with learning in the chick.

The effect of the protein synthesis inhibitor anisomycin on the structural changes associated with passive avoidance learning in the chick was investigated. Chicks were trained when they were 24 h old by allowing them to peck at a shiny bead coated with either water or the aversive-tasting substance methylanthranilate (MeA). Chicks which peck the MeA-coated bead will on subsequent testing avoid pecking a similar, but water-coated bead. Behavioural testing was carried out 12 h after training and immediately afterwards the chicks were killed and their brains prepared for electron microscopy. A specific region of the forebrain, the intermediate and medial part of the hyperstriatum ventrale (IMHV) was investigated. When the IMHV of the MeA trained chicks was compared with that of water-trained controls structural changes of the synapse were detected. These changes involved a significant increase in the mean length of the postsynaptic density (LPSD) of symmetrical synapses in the left IMHV. Chicks injected with 0.8 mg of anisomycin 30 min before training with a MeA-coated bead showed aversion for the shiny bead when tested 12 h later. Electron microscopic analysis of the IMHV from these amnestic chicks showed no evidence for the change in LPSD demonstrated in the water-injected controls. These results are discussed in relation to the nature of the memory trace induced by training on a passive avoidance task.