Increased BDNF and trk-B mRNA expression in cortical and limbic regions following formation of a social recognition memory

Brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine receptor kinase (trk-B ), play important roles in neural plasticity, long-term potentiation and memory formation. Sheep form a selective recognition memory for their lambs within 2 h of birth. Initially, this memory is exclusively based on olfactory cues; however, as it consolidates over a 12-h recognition period it extends to incorporate visual cues. We investigated whether changes in BDNF and trk-B mRNA expression occurred in both olfactory and visual processing systems at 4.5 h postpartum, 2-3 h after the behavioural manifestations of an olfactory recognition memory were found. Animals that formed a recognition memory showed increased BDNF mRNA expression in the inferior part of the temporal cortex, subfield CA1 of the hippocampus, the diagonal band, basolateral amygdala and the anterior cingulate, medial frontal, entorhinal and pyriform cortices. No increases were observed in either the olfactory bulbs or the dentate gyrus. Expression of trk-B mRNA was significantly increased only in the medial temporal, entorhinal and pyriform cortices. These findings demonstrate that by 2-3 h following the initial formation of olfactory recognition memory there are BDNF /trk-B -mediated plasticity changes in brain areas involved in the consolidation of olfactory memory (the pyriform and entorhinal cortices). However, similar changes also occur in areas of the brain involved in visual memory, face and object recognition (the temporal cortex, entorhinal cortex, hippocampal subfield CA1 and basolateral amygdala), and in areas of the brain with integrative and attentional functions (the medial frontal and anterior cingulate cortices and diagonal band). This suggests that reorganization of neural circuits underlying the visual recognition of lambs or the integration of olfactory/visual information is occurring even at this time even though accurate behavioural recognition at this stage can only be made using olfactory cues.