Ups and downs in the hippocampus: The influence of oscillatory sleep states on “neuroplasticity” at different time scales

Spontaneous state-dependent oscillatory dynamics in the brain are relevant to both ongoing and prospective neural and behavioural function. In the hippocampus, the presence of different patterns of coordinated network activity during offline (sleep) states has the potential to modify neural connections and thus influence memory storage. In addition to theta (3–12 Hz) and ripple (100–300 Hz) activity, the hippocampus also demonstrates a slow oscillation (SO: ≤1 Hz) during sleep which prominently organizes hippocampal cellular activity and dynamically coordinates ensembles across the entire forebrain into temporal frames of activity (UP) and inactivity (DOWN). The SO also significantly modulates hippocampal excitatory synaptic transmission on both a short (within cycle) and medium (across state) time scale and through its dynamic coordination with neocortical areas has the potential to function as a platform for long-term bidirectional synaptic plasticity. Recent evidence suggests that it is of direct benefit for the consolidation of hippocampal-dependent memories and thus, further investigation of its mechanisms in modulating short-to-long-term neural plasticity is certainly warranted.