Repeated high frequency tetanic stimulation saturates long-term potentiation (LTP), a cellular basis of learning and memory, in the hippocampus, which in turn impairs hippocampus-dependent learning. This impairment recovers in parallel with the LTP decay. However, which factor regulates the recovery of learning capacity remains to be elucidated. Neurogenesis in the brain of adult mammals occurs throughout life. Previous studies showed that newborn neurons in the adult brain are functionally integrated into the existing circuit and involved in the decay process of hippocampus-dependency of memories (1). We hypothesized that the adult hippocampal neurogenesis plays a role in the recovery process of hippocampal learning capacity. Repeated maximum electroconvulsive shock (MECS) saturated the hippocampal learning capacity in rats. When animals were trained in the contextual fear conditioning, a hippocampus-dependent learning task, 2 days after the last MECS, they failed to learn. On the other hand, the repeated MECS had no effect on tone-fear conditioning, an amygdala-dependent learning. The saturated learning capacity completely recovered within 7 days after the last MECS treatment. Inhibiting neurogenesis by X-ray irradiation, which reduces the proliferation and survival of the newborn cells in the hippocampus, delayed the recovery from the saturation level. These results indicate that repeated MECS leads to the saturation of hippocampal learning capacity, and the hippocampal learning capacity gradually recovers from the saturation level. Our results further suggest that the adult hippocampal neurogenesis promotes the recovery from the saturation level. This implies an involvement of adult neurogenesis in renewal of the hippocampal memory circuit.
(1) Kitamura et al., Cell, 139, 814-827, 2009.