Passive avoidance training enhances cell proliferation in 1-day-old chicks

One‐day‐old domestic chicks were injected i.p. with bromodeoxyuridine (BrdU) before training on a one‐trial passive avoidance task where the aversive experience was a bead coated with a bitter tasting substance, methyl anthranilate (MeA). Animals were tested 24 h later; those avoiding (if MeA‐trained) or pecking if water (W)‐trained (which they peck appetitively), along with a group of untrained naïve chicks, were used to determine cell proliferation either 24 h or 9 days post BrdU injection. In all three groups, BrdU positive cells were identified sparsely throughout the forebrain but labelling was pronounced around ventricular zone (VZ) surfaces at both 24 h and 9 days post‐BrdU‐injection. Double immunolabelling with neuronal specific antibodies, to either NeuN, or β‐tubulin III, confirmed that most BrdU labelled cells appeared to be neurons. Unbiased stereological analysis of labelled cells in selected forebrain areas 24 h post BrdU injection showed a significant MeA‐training induced increase in labelled cells in both the dorsal VZ surface bordering the intermediate and medial hyperstriatum ventrale (IMHV) and the tuberculum olfactorium (TO). By 9 days post‐BrdU‐injection, there was a significantly greater number of BrdU labelled cells in MeA‐trained birds within the IMHV, lobus parolfactorius (LPO) and TO. These results demonstrate that avoidance training in 1‐day‐old chicks has a marked effect on cell proliferation, in the LPO and IMHV, regions of the chick previously identified as a key loci of memory formation, and in a second region (TO), which has olfactory functions, but has not been previously investigated in relation to avoidance learning.