Filial imprinting in domestic chicks; cytoplasmic polyadenylation element binding protein 3, predispositions and learning.

Visual imprinting is a learning process, whereby young animals come to prefer a visual stimulus after exposure to it (training). Available evidence indicates that the intermediate medial mesopallium (IMM) in the domestic chick forebrain is a site of memory formation during visual imprinting. We have found previously that cytoplasmic polyadenylation element binding protein 3 in the P2 plasma membrane-mitochondrial fraction (CPEB3-P2) is upregulated in a learning-dependent way in the left IMM 24 h after training. CPEB3 has two forms, soluble and aggregated. Soluble CPEB3 represses translation; the aggregated form (CPEB3-AF) is amyloid-like and can promote translation. Our previous study did not show which of these two forms is increased after imprinting. We have now resolved this matter by measuring, 24 h after training, CPEB3-P2 and CPEB3-AF in the IMM and a control brain region, the posterior pole of nidopallium (PPN). The methods include imprinting training with a visual stimulus, behavioral measurement of preference, preparation of aggregated CPEB3, western immunoblotting, quantitation of proteins, statistical linear modeling. Only in the left IMM were the level of CPEB3-AF and learning strength correlated, increased CPEB3-AF level reflecting a predisposition to learn readily. CPEB3-P2 level also increased with learning strength in the left IMM, but as a result of learning. No correlations were detected in the right IMM or PPN. We propose two separate systems, both modulating synaptic strength through control of local translation. They are represented by CPEB3-AF (associated with a predisposition to learn) and soluble CPEB3 (associated with learning itself).