Passive avoidance training is correlated with decreased cell proliferation in the chick hippocampus

One‐trial passive avoidance learning (PAL), where the aversive stimulus is the bitter‐tasting substance methylanthranilate (MeA), affects neuronal and synaptic plasticity in learning‐related areas of day‐old domestic chicks ( Gallus domesticus ). Here, cell proliferation was examined in the chick forebrain by using 5‐bromo‐2‐deoxyuridine (BrdU) at 24 h and 9 days after PAL. At 24 h post‐BrdU injection, there was a significant reduction in labelling in MeA‐trained chicks in both the dorsal hippocampus and area parahippocampalis, in comparison to controls. Moreover, double‐immunofluorescence labelling for BrdU and the nuclear neuronal marker (NeuN) showed a reduction of neuronal cells in the dorsal hippocampus of the MeA‐trained group compared with controls (35 and 49%, respectively). There was no difference in BrdU labelling in hippocampal regions between trained and control groups of chicks at 9 days post‐BrdU injection; however, the number of BrdU‐labelled cells was considerably lower than at 24 h post‐BrdU injection, possibly due to migration of cells within the telencephalon rather than cell loss as apoptotic analyses at 24 h and 9 days post‐BrdU injection did not demonstrate differences in cell death between treatment groups. Cortisol levels increased in the chick hippocampus of MeA‐trained birds 20 min after PAL, suggesting the possibility of a stress‐related mechanism of cell proliferation reduction in the hippocampus. In contrast to hippocampal areas, the olfactory bulb, an area strongly stimulated by the strong‐smelling MeA, showed increased cell genesis in comparison to controls at both 24 h and 9 days post‐training.