Therapeutic time window is a key element of any drug to treat TBI. Patients with moderate to severe TBI can be treated hours after injury;; those with mild TBI may delay treatment for days until their symptoms do not abate. Few drugs have been developed with therapeutic time windows long enough to treat TBI, in part, because little is known about which cellular functions can be targeted by drugs dosed hours to days post-injury (PI). The combination of minocycline (MINO) plus N-acetylcysteine (NAC) retains high potency when first dosed 12h PI (MN12). Published and preliminary data suggest that MN12 prevents neuronal loss and protects dendrites in the hippocampal ipsilateral to the impact site, allows learning of an active place avoidance task that requires both hippocampi;; and restores late long-term potentiation (LTP) to both hippocampi. Preliminary data suggests that a first dose of MINO plus NAC at 72H PI (MN72) is less potent than MN12 yet restores acquisition of Barnes maze, a task that requires only one hippocampus, and late LTP in the hippocampus contralateral to the impact site. MN72 also increases protein synthesis in the contralateral hippocampus. This proposal examines if MN12 and MN72 target dendrites, synapses, spines, and synaptic protein synthesis after closed head injury (CHI) in mice. Proposed studies will examine whether MN12 and MN72 target protein kinase M zeta (PKMz), which is essential for late LTP and retention of hippocampal-dependent tasks. Studies will also examine whether MINO plus NAC remains potent when dosed later than 72H PI. These data support 3 specific aims (SA) that test a central hypothesis that: Dosing of MINO plus NAC at clinically relevant therapeutic time windows limits gray matter injury and improves cognition and memory. SA1: Where does a first dose of MINO plus NAC at 12 or 72h after CHI repair dendrites, spines and synapses? The working hypothesis of SA1 is that MN12 acts bilaterally to prevent injury and induce repair while MN72 acts only on the contralateral hippocampus. SA1 will also assay neuroinflammation, oxidative stress and mitochondrial morphology after MN12 or MN72 treatment. SA2: Does MN12 and MN72 target PKMz expression to restore synaptic plasticity and acquisition of hippocampal-dependent tasks? SA2 will examine a role for PKMz expression by MN12 or MN72 using NTSA, a novel and specific inhibitor of PKMz, or in conditional PKMz mutant mice. SA2 is predicted to show that MN12 or MN72 target PKMz to restore late LTP and long-term memory. SA 3: Does MINO plus NAC limits brain injury and restore function in the subacute (14D PI) or chronic (45D PI) stages of TBI? The utility of MINO plus NAC would be greatly increased if the drugs retained potency when dosed in later phases ...