Behavioural trait of reactivity to novelty is related to hippocampal neurogenesis

The hippocampal formation is one of the brain areas where neurogenesis pers ists during adulthood, with new neurons being continuously added to the pop ulation of dentate granule cells. However, the functional implications of t his neurogenesis are unknown. On the other hand, the hippocampal formation is particularly concerned with the detection of novelty, and there are indi cations that dentate granule cells play a significant role in this function . Recently, the existence of inter-individual differences in behavioural re activity to novelty has been evidenced, related to differences in the react ivity of the hypothalamic-pituitary-adrenal axis (HPA). Rats that are highl y reactive to novelty (HR) exhibit a prolonged corticosterone secretion in response to novelty and to stress when compared with low reactive rats (LR) . Taking advantage of the existence of these inter-individual differences, we investigated whether neurogenesis in the dentate gyrus is correlated wit h the behavioural trait of reactivity to novelty. Rats were first selected according to their locomotor reactivity to a novel environment. Two weeks l ater, cell proliferation, evaluated by the incorporation of 5-bromo-2'-deox yuridine (BrdU) in progenitors, was studied by immunohistochemistry. We fou nd that cell proliferation in the dentate gyrus was negatively correlated w ith locomotor reactivity to novelty. Indeed, cell proliferation in LR rats was twice that observed in HR rats. In contrast, survival of nascent neuron s was not influenced by the behavioural trait of reactivity to novelty. Usi ng an unbiased stereology, we show that LR rats had more cells within the g ranule cell layer of the dentate gyrus than did HR rats. These results demo nstrate the existence of inter-individual differences in neurogenesis and t otal granule cell number within the dentate gyrus. These differences in hip pocampal plasticity can be predicted by the behavioural trait of reactivity to novelty.