Neural and synaptic plasticity in the chick brain after passive avoidance learning

Abstract

The avian hippocampus (Hp) is believed to be homologous to mammalian hippocampus both from a developmental and an anatomical view-point. In one-day old domestic chicks (Gallus domesticus), studies have demonstrated that it may play a key role in the acquisition of one trial passive avoidance learning (PAL) where the aversive experience is exposure to a bitter tasting substance, methyl anthranilate. In the present study, following PAL the numerical density of asymmetric axospinous synapses decreased by 36% in the dorsal Hp of the right hemisphere of MeA trained compared to control birds, 6 hours after training. In contrast, 24h post PAL there is a 33% decrease in numerical density of asymmetric axodendritic synapses in the MeA trained group in relation to the same area of control chicks. Cell proliferation studies using the thymidine analogue, bromodeoxyuridine, (BrdU) demonstrated a 47% reduction in cell proliferation in the dorsal Hp of the MeA trained group 24h later in comparison with controls, which disappears after 9 days. In nucleus taeniae of amygdala and the arcopallium dorsale and intermediale, there are no differences between birds 24h or 9 days post BrdU injection but in olfactory bulb of MeA trained chicks cell labelling increases by 95% and 71.4% respectively, compared to control and water-trained birds, 24 h after PAL. The increase is more dramatic 9 days post PAL, when the MeA-trained group shows a 259% and 314% increase respectively in relation to control and water-trained animals. Following PAL apoptotic studies in the Hp 24h, 5 and 9 days post BrdU injection demonstrated that there were no differences in cell death between the different groups. Radioimmunoassay measurements of cortisol in chick forebrain tissue demonstrated longer term increase in levels of steroid in the chick Hp compared to arcopallium and striatum mediale 20 minutes after training, indicating that PAL is a stressful experience which may explain synaptic density and cell proliferation reduction observed after PAL