Homeostatic plasticity is hypothesized to bidirectionally regulate neuronal activity around a stable set point to compensate for learning-related plasticity, but to date only upward firing rate homeostasis (FRH) has been demonstrated in vivo. We combined chronic electrophysiology in freely behaving animals with an eyereopening paradigm to enhance firing in primary visual cortex (V1) and found that neurons bidirectionally regulate firing rates around an individual set point. Downward FRH did not require N-methyl-D-aspartate receptor (NMDAR) signaling and was associated with homeostatic scaling down of synaptic strengths. Like upward FRH, downward FRH was gated by arousal state but in the opposite direction: it occurred during sleep, not during wake. In contrast, firing rate depression associated with Hebbian plasticity happened independently of sleep and wake. Thus, sleep and wake states temporally segregate upward and downward FRH, which might prevent interference or provide unopposed homeostatic compensation when it is needed most.

Homeostatic plasticity is hypothesized to regulate neuronal activity around a stable set point to compensate for learning-related plasticity. This regulation is predicted to be bidirectional but only upward firing rate homeostasis (FRH) has been observed in vivo. We combined chronic electrophysiology in freely behaving animals with a protocol that induces robust plasticity in primary visual cortex (V1) to induce downward FRH and show that FRs are bidirectionally regulated around a cell-autonomous set point. Downward FRH did not require Nmethyl-D-aspartate receptor (NMDAR) function and was associated with homeostatic scaling down of synaptic strengths. Like upward FRH, downward FRH was gated by vigilance state, but in the opposite direction: it occurred during sleep and not during wake. In contrast, FR changes associated with Hebbian plasticity happened independently of sleep and wake. Thus, we find that sleep's impact on neuronal plasticity depends on the particular mechanisms of synaptic plasticity that are engaged.

Individual excitatory neurons in visual cortex (V1) display remarkably stable mean firing rates over many days, even though these rates can differ by several orders of magnitude between neurons. When perturbed, each neuron's firing rate is slowly regulated back to its pre-perturbation level, demonstrating that neurons maintain their mean firing rate around an individual firing rate set point (FRSP). To better understand the mechanisms that neurons within a single cell type use to maintain different FRSPs in vivo, we implemented a novel method of activity labeling that uses CaMPARI2, a fluorescent protein that undergoes Ca 2+ -and UV-dependent green-to-red photoconversion, to permanently label neurons in freely behaving mice based on their firing rates. We found that immediate early gene (IEG) expression was correlated with CaMPARI2 red/green ratio following an activity stimulation paradigm, and that neurons with greater photoconversion in vivo tended to have a higher firing rate ex vivo. In layer 4 (L4) pyramidal neurons in mouse monocular V1, which comprise a single transcriptional cell type, we found that high activity neurons had a left-shifted F-I curve, lower rheobase current, and decreased spike adaptation index relative to low activity neurons, demonstrating increased intrinsic excitability. Surprisingly, we found no difference in total excitatory or inhibitory synaptic current or in E/I ratio between high and low activity neurons. Thus, within a single cell type differences in intrinsic excitability and spike frequency adaptation can contribute to divergent activity set points. These results reveal that E/I ratio plays only a minor role in determining the firing rate set point of L4 pyramidal neurons, while intrinsic excitability is an important factor..

Bidirectional homeostatic plasticity allows neurons and circuits to maintain stable firing in the face of developmental or learning-induced perturbations. In primary visual cortex (V1), upward firing rate homeostasis (FRH) only occurs during active wake (AW) and downward during sleep, but how this behavioral state-dependent gating is accomplished is unknown. Here we focus on how AW enables upward FRH in V1 of juvenile Long Evans rats. A major difference between quiet wake (QW) when upward FRH is absent, and AW when it is present, is increased cholinergic (ACh) tone; we therefore chemogenetically inhibited V1-projecting basal forebrain cholinergic (BF ACh) neurons while inducing upward FRH using visual deprivation, and found that upward FRH was completely abolished. Next, we examined the impact on synaptic scaling and intrinsic excitability, two important cellular targets of homeostatic regulation. BF ACh inhibition impaired synaptic scaling up, and dramatically decreased the intrinsic excitability of activity-deprived V1 pyramidal neurons, consistent with the block of upward FRH. Interestingly, knock down of the highly abundant M1 ACh receptor in V1 failed to phenocopy the effects of decreased BF ACh activity on intrinsic excitability, suggesting either that BF ACh activity acts through a different receptor within V1, or acts indirectly via other brain regions or cell types. Together, our results show that BF ACh modulation is a key enabler of upward homeostatic plasticity, and more broadly suggest that neuromodulatory tone is a critical factor that segregates upward and downward homeostatic plasticity into distinct behavioral states.