Discriminative taste aversion learning: a learning task for older chickens

Quantitative analysis of age and life-history stage related changes in DCX expression in the male Japanese quail (Cortunix japonica) telencephalon

Most avian neurogenesis studies focused on the song control system and little attention has been given to non-song birds such as the Japanese quail. However, the only few neurogenesis studies in quails mainly focused on the sex steroid sensitive areas of the brain such as the medial preoptic and lateral septal nuclei. Despite the important role the quail telencephalon plays in filial imprinting and passive avoidance learning, neurogenesis in this structure has been completely overlooked. The aim of this study was therefore to quantitatively determine how DCX expression in the Japanese quail telencephalon changes with post hatching age (3-12 weeks) and life history stage. In this study, DCX was used as a proxy for neuronal incorporation. Bipolar and multipolar DCX immunoreactive cells were observed in the entire telencephalon except for the entopallium and arcopallium. In addition, DCX expression in all the eight telencephalic areas quantified was strongly negatively correlated with post-hatching age. Furthermore, numbers of bipolar and multipolar DCX immunoreactive cells were higher in the juvenile compared to subadult and adult quails. In conclusion, neuronal incorporation in the quail telencephalon is widespread but it declines with post hatching age. In addition, the most dramatic decline in neuronal incorporation in the telencephalic areas quantified takes place just after the birds have attained sexual maturity.

Biochemical, behavioural and electrophysiological investigations of brain maturation in chickens

It is convenient to divide the development of synaptic networks into two phases: synapse formation during which synaptic contacts are established, and a subsequent maturation phase during which synaptic circuits are fine tuned and the properties of individual synapses are modified. Understanding the complex factors that control the protracted maturation process in humans is likely to be important for understanding a variety of neurological and psychiatric disorders. Chickens provide an ideal experimental model in which maturation specific changes can be identified and the mechanisms controlling them can be elucidated because the maturation phase is protracted and temporally separated from the formation phase. This paper reviews the knowledge about the biological mechanisms involved in the maturation phase of brain development in chickens and presents some new data. Studies of synaptic physiology suggest that maturation may alter the basal set point for stimulus induced synaptic plasticity. Biochemical and pharmacological studies of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors (mGluRs) revealed major changes in receptor regulation and the intracellular signalling pathways linked to receptor activation. Not surprisingly, therefore, when immature or mature chickens learn the same behavioural task the learning induced molecular events at the synapse are different. Changes in the features of auditory event related potentials and the basal EEG provide non-invasive techniques for monitoring maturation changes in chicken brain but prepulse inhibition (PPI) is too small and variable in chickens to be useful. Experimentally induced mild late-onset hypothyroidism retards some aspects of brain maturation and may help identify some of the mechanisms controlling maturation.

Molecular changes in the intermediate medial mesopallium after a one trial avoidance learning in immature and mature chickens

Because brain maturation in chickens is protracted and occurs well after the major developmental period of synaptogenesis, chicken forebrain is suitable to investigate whether the molecular mechanisms underlying memory consolidation are different in immature and mature animals. We have used antibodies and western blotting to analyze subcellular fractions from the intermediate medial mesopallium region of 14-day and 8-week chicken forebrain prepared 0, 45, and 120 min after learning a discriminative taste avoidance task. At both ages learning induced changes in the phosphorylation of the glutamate receptor subunit 1 at Ser831, the levels of calcium-calmodulin stimulated/dependent protein kinase II and the phosphorylation of calcium-calmodulin stimulated/dependent protein kinase II at Thr286 were observed only in the fraction enriched in post-synaptic densities. The changes were of the same type at the two ages but occurred faster in mature animals. The changes in extracellular signal regulated kinase and phosphorylated-extracellular signal regulated kinase were more complex with different subcellular fractions showing different patterns of change at the two ages. These results imply that the molecular changes induced by learning a behavioral task are faster in mature than immature brain and may involve a different balance of intracellular signaling pathways.