Nuclear histone deacetylase (HDAC) activity in the retina is increased following optic nerve crush (ONC). The data shown was adjusted for protein input and normalized to day 0 control samples. The mean (± SE) HDAC activity is shown (n = 3-5 retinas per data point). (A) No increase in HDAC activity was observed in control eyes (OD) post ONC. There was a significant increase in nuclear HDAC activity in the crush samples (OS) at both 5 and 7 days post ONC (*P = 0.0014 for day 5 vs. day 0 and **P = 0.0006 for day 7 vs. day 0). HDAC activity in this assay was completely inhibited by 500 nM of trichostatin A (TSA). (B) HDAC activity in nuclear extracts from retinas of mice 5 days post ONC was titrated with TSA (n = 3 samples at each concentration). Both control and experimental samples displayed similar inhibition kinetics with an IC50 value ranging from 15 -- 22 nM TSA, comparable to the reported IC50 values of 1 -- 10 nM for class I HDACs . (C) HDAC activity in the same samples as (B) was titrated with the HDAC2 and HDAC3 selective inhibitor apicidin . Apicidin results in an 80% reduction in nuclear HDAC activity, yielding an IC50 ranging from 1.88 to 2.55 nM, consistent with the reported HDAC3 IC50 for apicidin . These data suggest that the majority of nuclear HDAC activity in the retina is provided by class I HDACs, principally HDAC2 and HDAC3. AFU, arbitrary fluorescence units.