Excitotoxic injury has been shown to phosphorylate tyrosine receptor kinase B (TrkB) 59 pathway signaling (1-3). TrkB is activated by its endogenous ligand, the neurotrophin brain- postnatal day 3 (P3) and were allowed to acclimate. Food and water were provided ad libitum. 99 Doses for LM used in vivo and in vitro were determined from previously described in vitro 100 pharmacokinetics (14). Pups of both sexes at P7 and P10 (see Table S1 for sample sizes) were 101 intraperitoneally (IP; Fig. 1A TrkB showed no significant differences between treatment groups (Fig. S3A ). Post-ischemic 343 TrkB activation was analyzed by sex, and no significant sex-dependent differences were found. 344 Total PLCγ1 expression was not significantly modulated by ischemic insult or Post and 345 Pre LM intervention ( Fig. 4D and E) . In contrast, pPLCγ1-Y783 expression was significantly 346 higher both ipsi-and contralateral to ischemic insult, which was rescued both in Post LM and Pre 347 LM groups (Fig. 4D and F ). PLCγ1 and pPLCγ1-Y783 expression levels were also significantly 348 lower ipsilateral to insult (Fig. 4D-F) . The ratio of pPLCγ1-Y783 to total PLCγ1 was not 349 significantly different between treatment groups (Fig. S3B) . 350 TrkB pathway activation is also known to activate downstream ERK1/2 signaling (37).
At P7, Pre LM was the most efficacious treatment paradigm in reducing EEG seizure burdens 352 when quantified as percent seizure suppression (Fig 1E) LM group both ipsi-and contralateral to insult (Fig. S3C) . Post-ischemic TrkB-PLCγ1 pathway activation was not evident in ischemic P10 pups 376 In contrast to P7 pups, no increase in pTrkB-Y816 expression was detected in any 377 treatment group at P10 (Fig. S4A-C) . The Pre LM group showed significantly higher ratios of 378 pTrkB-Y816 to TrkB ipsi-and contralateral to insult (Fig. S4D ). PLCγ1 expression was not 379 different between treated and untreated pups, though Post and Pre LM pups showed ipsilateral 380 downregulation of pPLCγ1-Y783 (Fig. S4E-G) . Ratios of pPLCγ1-Y784 to total PLCγ1 were not 381 affected by LM intervention (Fig. S4H) . No significant KCC2 degradation was detected in the 382 Ligate+PB group (Fig. S4I and J) following ischemia, though Post LM pups showed lower 383 ipsilateral pKCC2-S940 expression (Fig. S4K, L) . In summary, LM intervention showed mild 384 activation of the TrkB pathway, but KCC2 levels were not significantly modulated at P10 when 385 ischemic seizures were responsive to PB. Post and Pre HIOC groups significantly rescued TrkB-pathway activation (Fig. 5D ) and pTrkB-399 Y816 activation bilaterally (Fig. 5E ). Total ERK1/2 expression was not significantly impacted by 400 ischemic insult or any HIOC treatment (Fig. 5F ) similar to LM intervention (Fig. 4H) . In contrast 401 to Pre LM intervention (Fig. 4I) , Post HIOC and Pre HIOC groups did not demonstrate an 402 increase in pERK1/2-T202/Y204 expression (Fig. 5G ). In summary, the differences in LM vs. (Fig. 6A1, A2 and B) . Developmentally, TrkB expression in the cortex of female pups 426 declined significantly from P5 to P21. In contrast, TrkB expression in males did not decline 427 significantly from P5 to P21. Both female and male pups showed significant decline of pTrkB-428 Y816 expression from P5 to P21 (Fig. 6C ). In the cortex, ratios of pTrkB-Y816 to total TrkB 429 were not significantly different by sex or age (Fig. 6D) . 430 Similar to the results found in the cortex, TrkB expression in the hippocampi of female 431 pups declined significantly from P5 to P21, whereas TrkB expression in males did not decline 432 significantly (Fig. 6E1, E2 , and F). pTrkB-Y816 expression also declined significantly in the 433 hippocampi of female pups from P5 to P21 (Fig. 6G) . Ratios of pTrkB-Y816 to total TrkB were 434 not sexually dimorphic and did not change significantly between ages of P5 to P21 in the 435 hippocampus (Fig. 6H ). In deep gray matter, TrkB expression decreased significantly between 436 P5 and P21 in females, whereas TrkB expression remained stable and did not decline 437 significantly in males (Fig. 6I1, I2 , and J). pTrkB-Y816 expression decreased significantly in 438 both females and males from P5 to P21 (Fig. 6I1, I2 , and K). However, ratios of pTrkB-Y816 to 439 total TrkB were not different between sexes sexually dimorphic and did not change significantly 440 between ages of P5 to P21 in the deep gray matter (Fig. 6L) . These results suggest that sexually 441 dimorphic expression levels of TrkB in the cortex may underlie the sexually dimorphic seizure 442 susceptibility and rescue of PB-refractoriness with LM ( Fig. 1 and 2) , as neonatal seizures are 443 cortical. Furthermore, the attenuation of pTrkB-Y816 from P5 to P21 in both males and females 444 suggests that TrkB pathway activation decreases with age. 445 At P5, females showed significantly higher PLCγ1 and pPLCγ1-Y783 expression in the 446 cortex than at P21 (Fig. S5A and B) . In contrast, PLCγ1 and pPLCγ1-Y783 expression in males 447 was not significantly different between age groups. The hippocampus and deep gray matter did 448 not show significant differences between sexes in PLCγ1 or pPLCγ1-Y783 expression during 449 development (Fig. S5D, E, G, and H) . Similarly, ratios of pPLCγ1-Y783 to total PLCγ1 were not 450 significantly different between sexes during development for all examined brain regions (Fig. 451 S5C, F, and I). Males showed a significant increase in KCC2 expression between P5 and P21, and P7 455 and P21, whereas females did not (Fig. S5J and K) . pKCC2-S940 was not found to be 456 significantly dephosphorylated in both sexes ( Fig. S5J and K) . In contrast, only females showed 457 significantly lower ratios of pKCC2-S940 to total KCC2 in the cortex (Fig. S5L ). In the 458 hippocampus, both males and females did not show a significant change in KCC2 expression 459 ( Fig. S5M and N) . pKCC2-S940 expression and ratios of pKCC2-S940 to total KCC2 were not 460 found to be sexually dimorphic between ages (Fig. S5M-O) . In deep gray matter, both females 461 and males showed significantly lower expression of pKCC2-S940 at P21 (Fig. S5P-R) TrkB signaling is known to be sexually dimorphic in a region-specific manner (80-82). 584 In BDNF +/mice, greater levels of pTrkB-Y705 activation were shown in the frontal cortex and 585 striatum of male mice 10-16 weeks of age compared to females. These changes in TrkB 586 phosphorylation state were accompanied by concomitant increases in ERK1/2 587 phosphorylation(80), suggesting a differential signaling mechanism that was sexually dimorphic. 588 The developmental expression of KCC2 is also known to increase with age, and this 589 developmental profile is sex-dependent, with female pups expressing higher levels than male 590 pups (15, 83, 84) Table S1 normalized to housekeeping protein β-actin. Phospho-proteins were also normalized to their 948 respective total protein (see Figure S3 ). * indicate differences between treatment group and naïve 949 controls; # indicated differences between contralateral and ipsilateral hemispheres within groups. 