The Ups and Downs of Firing Rate Homeostasis

gen et al., 2013). Once this homeostatic adjustment of firing rate has occurred, increases in activity levels beyond baseline can be induced by simply reopening the eye (eye reopening, ER) (Toyoizumi et al., 2014). When the authors use this ER paradigm, they observe an increase in activity, where cellular firing rates double in the affected hemisphere after 2 days (Figure 1, phase III). Firing rates in the control hemisphere remain stable throughout. The ER paradigm thereby allows the authors to explore the homeostatic effects resulting from a sustained increase in activity. They find that 4 days after ER, activity levels have decreased and returned to baseline (Figure 1, phase IV). These data demonstrate downward firing rate homeostasis for the first time in vivo. Together with past work (Hengen et al., 2013, 2016; Keck et al., 2017), these results indicate that firing rate homeostasis is bidirectional in the rodent cortex, a One of the key features of the brain is its extensive capacity for plasticity, which facilitates adaptation and learning. But if this plasticity is left unchecked, it can result in extremely high or low neural activity. Homeostatic mechanisms are proposed to balance activity levels and help maintain stable firing rates. A key aspect of proposed homeostatic mechanisms is that they are bidirectional and can adjust to compensate for activity levels that are either too high or too low. To date, firing rate homeostasis in vivo has only been demonstrated to occur when activity levels have decreased (Hengen et al., 2013, 2016; Keck et al., 2017). Although homeostatic mechanisms have been shown to occur in response to an increase in activity or overstimulation (Lee and Kirkwood, 2019), it is not clear whether these mechanisms are associated with a decrease in firing Torrado Pacheco et al. demonstrate that downward firing rate homeostasis occurs when cellular activity levels increase beyond baseline, but only during sleep-dense periods. In contrast, Hebbian-facilitated changes in firing rate occur independently of sleep and wake states.

Automated summary

Research demonstrates that downward firing rate homeostasis occurs in vivo only when cellular activity levels exceed baseline, and only during periods of dense sleep.