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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 beta-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.

Mechanisms of expression of long-term synaptic plasticity are believed to involve morphological changes of the activated synapses and remodelling of connectivity. Here, we investigated changes in synaptic and neuronal parameters in the dentate gyrus 24 h after induction of long-term potentiation (LTP) and long-term depression (LTD) in awake rats. In dentate granule cells, tetanization of the medial or lateral perforant paths induces LTP in specific synaptic bands along the dendrites in the middle and outer molecular layers, respectively, and tetanization of the lateral path induces robust LTD heterosynaptically in the middle molecular layer. This functional segregation allowed us to assess morphological changes associated with LTP and LTD in each pathway in the same population of neurons. Electron microscopy and unbiased counting methods were used to estimate neuronal density, axospinous, axodendritic and perforated synapse density, multiple synapse bouton density and postsynaptic density (PSD) area. Whereas there was no change in neuronal density, PSD area and multiple synapse boutons 24 h after either LTP or LTD, there was a noninput-specific increase in unperforated axospinous synapses after both LTP and LTD. However, we found that LTP of the medial, but not lateral, perforant path is associated with a specific increase in perforated axospinous synapses in the potentiated area. We also show that heterosynaptic LTD is associated with an input-specific increase in axodendritic synapse density. These results suggest that each perforant pathway may differ with respect to the nature of LTP-induced long-term changes and show that morphologically LTD is not simply the converse of LTP.