Proper cognitive development and brain function relies on synaptic plasticity -the ability of 44 synapses to strengthen or weaken in response to sensory or neuromodulatory inputs. Synaptic 45 scaling is one mechanism of plasticity, which buffers potentially destabilizing patterns of network 46 activity (Abbott and Nelson, 2000; Miller and MacKay, 1994; Turrigiano, 2008) . In response to a sustained increase in neuronal firing rate, neurons decrease, or "scale-down", the receptivity of 48 the neuron to excitatory neurotransmitters. Conversely, global decreases in firing rate causes 49 neurons to "scale-up" and increase synaptic efficacy. Synaptic scaling is thought to 50 accommodate other forms of plasticity, such as long-term potentiation (LTP), that impose long-51 lasting increase of individual synaptic efficacy, which if left uncompensated, would result in 52 circuits that are overly active (Turrigiano, 2017) . Synaptic scaling appears to play important 
To understand how homeostatic plasticity contributes to normal and pathological brain 59 development, identifying the molecular mechanisms underlying synaptic scaling is an important 60 first step.
Long-lasting forms of synaptic plasticity, including synaptic scaling, require de novo synthesis of 63 RNAs, which in turn produce the proteins that directly modulate synaptic efficacy (Benito and 
The labeled RNAs were isolated by immunoprecipitation using an anti-BrU antibody and 141 subjected to next-generation sequencing (Fig. 1A ).
We first validated the results of Bru-seq by examining transcription of known activity-dependent 144 genes individually. As shown in Figure 1B (Table S1) 1C ). BIC increased transcription of 2,908 genes, whereas TTX did so for 1,820 genes. The 156 magnitude of transcriptional induction is higher in BIC treatment compared to TTX (Fig. 1D ).
Meanwhile, a similar number of genes were transcriptionally suppressed upon BIC (2,842) and 158 TTX (2,307) treatments. A relatively small fraction of genes, 24% (1,798 of 7,592 activity-159 response genes), displayed reciprocal changes between BIC and TTX treatments, e.g.
upregulation by BIC and downregulation by TTX (Fig.1E ). An even smaller fraction, 6% 161 (487/7,592) of activity-response genes altered their transcription levels in the same direction 7 after BIC and TTX treatments. The remaining 70% of genes (5,307/7,592) responded to BIC or 163 TTX uniquely. Supplemental Table 1 lists genes that displayed greater than 2-fold changes in 164 transcription upon network-activity shifts.
We next analyzed two published mRNA-seq datasets, which profiled the steady-state 
Conventional mRNA-seq may not be the ideal approach to detect downregulation of 177 transcription, because after transcription ceases, synthesized RNAs persist for certain periods 178 of time. To test whether Bru-seq can detect transcriptional downregulation sensitively, we 179 compared the induction and suppression of known immediate-early genes in our Bru-seq and 180 published mRNA-seq datasets. We found significantly larger suppression of Fos, Arc, Bdnf, and 181 Npas4 (4-to 16-fold) by TTX treatment in Bru-seq compared to conventional mRNA-seq, in 182 which downregulation was less than 2-fold ( Fig. 1F ). In the Bru-seq data, the four genes showed 183 smaller magnitudes of upregulation in response to BIC, likely because the early transcriptional 184 induction is largely complete 4 hours after BIC treatment (Fig. S1C) . These data highlight an 185 advantage of the Bru-seq approach to probe mechanisms underlying highly-dynamic activity-186 dependent transcription.
We next examined the cell-type specificity of activity-dependent genes in our datasets. Recent CD11b, and Olig2, we estimated that our cultures consist of 41% excitatory neurons, 11% 192 inhibitory neurons, 33% astrocytes, 15% of cells within the oligodendrocyte lineage, and no 193 microglia ( Fig. S2A-B) . Indeed, several non-neuronal genes are represented in our dataset,
including Thbs1, a known synaptic regulator specifically expressed in astrocytes (Risher and 195 8 Eroglu, 2012) . Gene ontology analysis of the Bru-seq data detected enrichment of biological 196 processes specific for both neurons and non-neuronal cell types (Table S2) . For example, 197 regulation of axon diameter (p adj = 2.6 x 10 −5 ), axonal transport of mitochondrion (p adj =3.5 x 198 10 −4 ), and glial cell proliferation (p adj : 7.8 x 10 −3 ) represent genes that are transcriptionally 199 induced upon BIC treatment, while transcription of genes involved in astrocyte activation are 
The superior sensitivity of Bru-seq over conventional RNA-seq prompted us to assess the role 210 of RAI1 in activity-dependent gene expression. We first determined RAI1 expression in our 211 culture systems. Publically-available databases indicate ubiquitous Rai1 expression in a broad 212 array of cell types in the brain (Fig. S3 ). Previous studies have demonstrated that Rai1 mRNA 213 expression rises from E13.5 to peak at P7, and its expression continues throughout adulthood were found at promoters of ~80% genes that are expressed at detectable levels, regardless of 227 their activity-dependent transcriptional changes (Fig. S6A) . We did not find any statistically-9 significant enrichment or depletion of RAI1 occupancy of TTX-or BIC-response genes ( Fig.   229 S6A). Thus, these data indicate that neuronal activity does not influence RAI1's expression level 230 or subcellular localization and that steady-state chromatin occupancy by RAI1 is not selective 231 between BIC or TTX-response genes.
To directly test RAI1's role in activity-dependent transcription, we went on to perform Rai1 were assessed by Bru-seq as described above.
We initially sought to establish if Rai1-KD alone was sufficient to alter nascent transcription in Having uncovered that RAI1 is essential to suppress the TTX-associated transcriptional 293 program under baseline activity conditions (Fig. 2) , we next tested if Rai1-KD has any impact on 294 transcriptional induction and suppression upon TTX and BIC treatments. By calculating fold-295 changes of transcription, we found that Rai1-KD led to a significant impairment of transcriptional 296 response to TTX, while transcriptional response to BIC was slightly weakened only for Table S3 ).
We then sought to determine if the strongly-impaired transcriptional response to TTX ( 
Taken together, the Bru-seq results led us to conclude that 1) RAI1-deficiency shifts 318 transcriptional profiles towards an activity-suppressed state in the resting network (Fig. 2) , and 319 2) RAI1 is selectively required for the transcriptional response driven by network activity 320 suppression (Fig. 3 ).
We sought to explore biological implications for such small but pervasive deficits in 322 transcriptional response to TTX. We utilized RNA-Enrich, a gene ontology algorithm, in which 323 the entire output of DESeq2 is analyzed, such that the program takes into account statistically- 
Our Bru-seq method and analyses have provided novel insights into activity-dependent 364 transcription. We found widespread transcriptional responses to network activity shifts and its 365 high sensitivity in detecting transcriptional downregulation. Furthermore, our data indicate that 366 most dynamically regulated genes altered by hyperactivity or suppression are unique, not 367 reciprocal. (Fig. 1) . This is particularly interesting given that gene expression studies have 368 tended to focus on bidirectional regulation of target genes (e.g. Arc, Fos, Homer1, Bdnf) 369 (Okuno, 2011) . Our results agree with a nascent proteome study on rat hippocampal neurons, in 
Our data indicate that RAI1 is as a chromatin regulator that is selectively required for the 379 transcription program of activity-suppression. Loss of RAI1 leads to misregulation of TTX-380 response genes, while leaving the uniquely BIC-genes unaffected (Fig. 2) . In addition to its 381 exclusive impact on TTX-response genes, an intriguing feature of RAI1 is its state-dependent 
(n = 6-6) treated neurons. All bar graphs represent mean ± SEM, and comparisons between sh-523 Ctrl and sh-Rai1 were made with unpaired Student's t-tests. *p < 0.05, **p < 0.01, ***p < 0.001. 
The cortices and hippocampi from E18.5 mouse pups were pooled into biological replicates with 614 identical female-male ratios. Sex of the pups was determined by PCR using primers for the ZFY 615 gene (Table S5 ). Primary culture of neurons was carried out as previously described (Iwase et (Table S5) (Table S5 ). Adaptor duplexes with 5-or 6-base pair random 647 nucleotide overhangs were ligated to the 3' end of the cDNA (Table S5) 
Immunoreactivity was quantified semi-automatedly using a custom ImageJ script after Fisher's LSD test (Fig. 6) . Transcription of activity-dependent genes after drug treatments