Cell cycle inhibition reduces inflammatory responses, neuronal loss, and cognitive deficits induced by hypobaria exposure following traumatic brain injury.

BACKGROUND: Traumatic brain injury (TBI) patients in military settings can be exposed to prolonged periods of hypobaria (HB) during aeromedical evacuation. Hypobaric exposure, even with supplemental oxygen to prevent hypoxia, worsens outcome after experimental TBI, in part by increasing neuroinflammation. Cell cycle activation (CCA) after TBI has been implicated as a mechanism contributing to both post-traumatic cell death and neuroinflammation. Here, we examined whether hypobaric exposure in rats subjected to TBI increases CCA and microglial activation in the brain, as compared to TBI alone, and to evaluate the ability of a cyclin-dependent kinase (CDK) inhibitor (CR8) to reduce such changes and improve behavioral outcomes. METHODS: Adult male Sprague Dawley rats were subjected to fluid percussion-induced injury, and HB exposure was performed at 6 h after TBI. Western blot and immunohistochemistry (IHC) were used to assess cell cycle-related protein expression and inflammation at 1 and 30 days after injury. CR8 was administered intraperitoneally at 3 h post-injury; chronic functional recovery and histological changes were assessed. RESULTS: Post-traumatic hypobaric exposure increased upregulation of cell cycle-related proteins (cyclin D1, proliferating cell nuclear antigen, and CDK4) and microglial/macrophage activation in the ipsilateral cortex at day 1 post-injury as compared to TBI alone. Increased immunoreactivity of cell cycle proteins, as well as numbers of Iba-1+ and GFAP+ cells in both the ipsilateral cortex and hippocampus were found at day 30 post-injury. TBI/HB significantly increased the numbers of NADPH oxidase 2 (gp91phox) enzyme-expressing cells that were co-localized with Iba-1+. Each of these changes was significantly reduced by the administration of CR8. Unbiased stereological assessment showed significantly decreased numbers of microglia displaying the highly activated phenotype in the ipsilateral cortex of TBI/HB/CR8 rats compared with TBI/HB/Veh rats. Moreover, treatment with this CDK inhibitor also significantly improved spatial and retention memory and reduced lesion volume and hippocampal neuronal cell loss. CONCLUSIONS: HB exposure following TBI increases CCA, neuroinflammation, and associated neuronal cell loss. These changes and post-traumatic cognitive deficits are reduced by CDK inhibition; such drugs may therefore serve to protect TBI patients requiring aeromedical evacuation.

Spinal cord injury causes brain inflammation associated with cognitive and affective changes: role of cell cycle pathways.

Experimental spinal cord injury (SCI) causes chronic neuropathic pain associated with inflammatory changes in thalamic pain regulatory sites. Our recent studies examining chronic pain mechanisms after rodent SCI showed chronic inflammatory changes not only in thalamus, but also in other regions including hippocampus and cerebral cortex. Because changes appeared similar to those in our rodent TBI models that are associated with neurodegeneration and neurobehavioral dysfunction, we examined effects of mouse SCI on cognition, depressive-like behavior, and brain inflammation. SCI caused spatial and retention memory impairment and depressive-like behavior, as evidenced by poor performance in the Morris water maze, Y-maze, novel objective recognition, step-down passive avoidance, tail suspension, and sucrose preference tests. SCI caused chronic microglial activation in the hippocampus and cerebral cortex, where microglia with hypertrophic morphologies and M1 phenotype predominated. Stereological analyses showed significant neuronal loss in the hippocampus at 12 weeks but not 8 d after injury. Increased cell-cycle-related gene (cyclins A1, A2, D1, E2F1, and PCNA) and protein (cyclin D1 and CDK4) expression were found chronically in hippocampus and cerebral cortex. Systemic administration of the selective cyclin-dependent kinase inhibitor CR8 after SCI significantly reduced cell cycle gene and protein expression, microglial activation and neurodegeneration in the brain, cognitive decline, and depression. These studies indicate that SCI can initiate a chronic brain neurodegenerative response, likely related to delayed, sustained induction of M1-type microglia and related cell cycle activation, which result in cognitive deficits and physiological depression.

Effects of 4-pyridine aldoxime on nerve agent-inhibited acetylcholinesterase activity in guinea pigs.

Methoxime (MMB-4) is a leading candidate oxime acetylcholinesterase (AChE) reactivator to replace pralidoxime (2-PAM) for therapeutic treatment of nerve agent intoxication. 4-Pyridine aldoxime (4-PA) is a synthetic starting material, a breakdown product, and a probable metabolite of MMB-4. There is a possibility that 4-PA may adversely interact with the nerve agent, thereby affecting nerve agent toxicity and biological AChE activity. This study evaluated the effects of 4-PA on sarin (GB)-, cyclosarin (GF)-, and VX-induced toxicity and AChE activity in blood, brain, and peripheral tissues of guinea pigs. Animals were pretreated with atropine methyl nitrate (1.0 mg/kg, im) 15 min prior to subcutaneous administration with 1.0 x LD(50) of GB, GF, or VX and then treated 15 min after the administration of nerve agents with 4-PA (3.5, 7.0, or 14.0 mg/kg, im). The dose-response effects of 4-PA alone were also examined. Toxic signs and lethality were monitored, blood and tissues were collected, and AChE activities were determined at 60 min after nerve agent administration. Under the condition of this study, all animals exposed to nerve agents exhibited some degree of toxic signs such as salivation, lacrimation, rhinorrhea, and convulsions. 4-PA at the three doses tested neither induced toxic signs nor altered the toxicity of GB, GF, or VX at the 1.0 x LD(50) exposure dose. Additionally, it did not modify the AChE activity in blood, brain, and peripheral tissues by itself or affect the AChE activity inhibited by a 1.0 x LD(50) dose of these three nerve agents in guinea pigs.

Evaluation of nine oximes on in vivo reactivation of blood, brain, and tissue cholinesterase activity inhibited by organophosphorus nerve agents at lethal dose.

The capability of several oximes (HI-6, HLö7, MMB-4, TMB-4, carboxime, ICD 585, ICD 692, ICD 3805, and 2-PAM) to reactivate in vivo AChE inhibited by the nerve agents sarin, cyclosarin, VX, or VR in blood, brain regions, and peripheral tissues in guinea pigs was examined and compared. Animals were injected subcutaneously with 1.0 LD(50) of sarin, cyclosarin, VR, or VX, and treated intramuscularly 5 min later with one of these compounds. Toxic signs and lethality were monitored, and tissue AChE activities were determined at 60 min after nerve agent. The animals exposed to sarin or cyclosarin, alone or with non-oxime treatment, some died within 60 min; however, when treated with an oxime, no animal died. For VR or VX, all animals survived for 60 min after exposure, with or without non-oxime or oxime therapy. These nerve agents caused differential degrees of inhibition: in whole blood sarin = cyclosarin > VR = VX; in brain regions sarin > cyclosarin > VX > VR; and in peripheral tissues sarin > VX > cyclosarin > VR. These oximes exhibited differential potency in reactivating nerve agent-inhibited AChE in various peripheral tissues, but not AChE activity in the brain regions. There was no difference in the AChE reactivating potency between the dichloride and dimethanesulfonate salts of HI-6. AChE inhibited by sarin was the most and cyclosarin the least susceptible to oxime reactivation. Overall, MMB-4 appeared to be, among all oximes tested, the most effective in vivo AChE reactivator against the broadest spectrum of nerve agents.

In vivo oxime administration does not influence Ellman acetylcholinesterase assay results.

Organophosphorus compounds (OPs) are potent inhibitors of acetylcholinesterase (AChE). Treatment for OP poisoning is by administration of atropine sulfate, an oxime, and diazepam. Oximes such as 2-PAM are used to reactivate OP-inhibited AChE so as to restore normal enzymatic function and serve as a true antidote. There are reports of non-enzymatic hydrolysis by oximes of acetylthiocholine in in vitro preparations in the widely used Ellman assay for AChE activity, which may confound the interpretation of AChE activity by producing elevated results. The purpose of this experiment was to determine if there is appreciable interference by therapeutic levels of oximes on the results of the Ellman assay in assessing AChE reactivation by oxime compounds in vivo. When therapeutic doses of oximes (2-PAM, HI-6, MMB-4, or MINA) were administered intramuscularly to guinea pigs and samples collected 60 min later, there was no statistical difference between oxime and saline control groups in measured AChE activity in various tissue samples, including blood. With appropriate dilution of samples prior to spectrophotometric assay, the Ellman assay is an acceptable method to measure in vivo oxime reactivation of inhibited AChE. Inclusion of an oxime control group to insure that this particular type of interference is not causing false readings in the assay is a prudent step.

Anticonvulsant efficacy of antihistamine cyproheptadine in rats exposed to the chemical warfare nerve agent soman.

Organophosphate compounds, such as soman and sarin, are highly toxic chemical warfare nerve agents that cause a build-up of acetylcholine in synapses and neuromuscular junctions. Current therapies aim to prevent seizures and protect against brain injury following exposure. The present study was designed to evaluate the effectiveness of the antihistamine cyproheptadine in improving survival and controlling seizures in rats exposed to soman. Rats were pretreated with the oxime reactivator HI-6 (125mg/kg, ip) 30min prior to soman exposure (225µg/kg, sc) and then treated with atropine methylnitrate (AMN, 2.0mg/kg, im) 1min after soman. Cyproheptadine (10, 13, 16 or 20mg/kg, ip) was given at one of three time points: 1min after soman intoxication, at the onset of soman-induced seizures or 5min after seizure onset. Control animals were exposed to soman and given an equivalent volume of sterile water instead of cyproheptadine. The incidence of seizures, mortality, neuron counts, neuropathology and apoptosis in specific regions of the brain were evaluated. In animals given HI-6 and AMN the incidence of soman-induced seizure and mortality rate within the first 24h were 100%. When cyproheptadine was given at a dose of 13 or 20mg/kg 1min after soman exposure, the incidence of seizures was reduced from 100% to 13% and 30%, respectively. In addition, cyproheptadine given at 1min after soman exposure increased the survival rate to 100% regardless of dose. When cyproheptadine was administered at seizure onset, seizures were terminated in 100% of the animals at doses above 10mg/kg. The survival rate with cyproheptadine treatment at the onset of seizure was ≥83%. Seizures terminated in ≥75% of the animals that received cyproheptadine 5min after soman-induced seizure onset. When given at 5min after seizure onset the survival rate was 100% at all tested doses of cyproheptadine. The neuropathology scores and the number of TUNEL positive cells in the brain regions examined decreased at all time points and cyproheptadine doses tested. These observations indicate that cyproheptadine treatment can effectively control seizures, improve survival, reduce seizure duration and reduce the number of dying cells in the brain following soman exposure.

Simulated Aeromedical Evacuation Exacerbates Experimental Brain Injury.

Aeromedical evacuation, an important component in the care of many patients with traumatic brain injury (TBI), particularly in war zones, exposes them to prolonged periods of hypobaria. The effects of such exposure on pathophysiological changes and outcome after TBI are largely unexplored. The objective of this study was to investigate whether prolonged hypobaria in rats subjected to TBI alters behavioral and histological outcomes. Adult male Sprague-Dawley rats underwent fluid percussion induced injury at 1.5-1.9 atmospheres of pressure. The effects of hypobaric exposure (6 h duration; equivalent to 0.75 atmospheres) at 6, 24, and 72 h, or 7 days after TBI were evaluated with regard to sensorimotor, cognitive, and histological changes. Additional groups were evaluated to determine the effects of two hypobaric exposures after TBI, representing primary simulated aeromedical evacuation (6 h duration at 24 h after injury) and secondary evacuation (10 h duration at 72 h after injury), as well as the effects of 100% inspired oxygen concentrations during simulated evacuation. Hypobaric exposure up to 7 days after injury significantly worsened cognitive deficits, hippocampal neuronal loss, and microglial/astrocyte activation in comparison with injured controls not exposed to hypobaria. Hyperoxia during hypobaric exposure or two exposures to prolonged hypobaric conditions further exacerbated spatial memory deficits. These findings indicate that exposure to prolonged hypobaria up to 7 days after TBI, even while maintaining physiological oxygen concentration, worsens long-term cognitive function and neuroinflammation. Multiple exposures or use of 100% oxygen further exacerbates these pathophysiological effects.

Neuropharmacological specificity of brain structures involved in soman-induced seizures.

Pharmacological control of seizure activity following nerve agent exposure is critical in reducing neuropathology and improving survival in casualties. Three classes of drugs, anticholinergics, benzodiazepines and excitatory amino acid (EAA) antagonists, have been shown to be effective at moderating nerve agent-induced seizures. However, little is known about which brain structures are involved in producing the anticonvulsant response. This study evaluated drugs from each class, injected directly into one of three specific brain structures, the perirhinal cortex, the entorhinal cortex, or the mediodorsal thalamus, for their ability to modulate seizures induced by the nerve agent soman. The drugs evaluated were the anticholinergic scopolamine, the benzodiazepine midazolam, and the EAA antagonist MK-801. For each drug treatment in each brain area, anticonvulsant ED50 values were calculated using an up-down dosing procedure over successive animals. There was no statistical difference in the anticonvulsant ED50 values for scopolamine and MK-801 in the perirhinal and entorhinal cortices. MK-801 pretreatment in the mediodorsal thalamus had a significantly lower anticonvulsant ED50 value than any other treatment/injection site combination. Midazolam required significantly higher doses than scopolamine and MK-801 in the perirhinal and entorhinal cortices to produce an anticonvulsant response and was ineffective in the mediodorsal thalamus. These findings support the contention that specific neuroanatomical pathways are activated during nerve agentinduced seizures and that the discrete brain structures involved have unique pharmacological thresholds for producing an anticonvulsant response. This study is also the first to show the involvement of the mediodorsal thalamus in the control of nerve agent-induced seizures.

Protection against sarin-induced seizures in rats by direct brain microinjection of scopolamine, midazolam or MK-801.

Control of seizure activity is critical to survival and neuroprotection following nerve agent exposure. Extensive research has shown that three classes of drugs, muscarinic antagonists, benzodiazepines, and N-methyl-D: -aspartate antagonists, are capable of moderating these seizures. This study began to map the neural areas in rat brain that respond to these three drug classes resulting in anticonvulsant effects. Drugs of each class (scopolamine, midazolam, MK-801) were evaluated for their ability to prevent sarin-induced seizures when injected into specific brain areas (lateral ventricle, anterior piriform cortex, basolateral amygdala, area tempestas). Animals were pretreated by microinjection with saline or a dose of drug from one of the three classes 30 min prior to receiving 150 microg/kg sarin, subcutaneously, followed by 2.0 mg/kg atropine methylnitrate, intramuscularly. Animals were then returned to their cages, where electroencephalographic activity was monitored for seizures. Anticonvulsant effective doses (ED(50)) were determined using an up-down dosing procedure over successive animals. Scopolamine provided anticonvulsant effects in each area tested, while midazolam was effective in each area except the lateral ventricle. MK-801 was only effective at preventing seizures when injected into the basolateral amygdala or area tempestas. The results show a unique neuroanatomical and pharmacological specificity for control of nerve agent-induced seizures.

Treatment with tertiary oximes prevents seizures and improves survival following sarin intoxication.

The capability of the tertiary oximes, monoisonitrosoacetone (MINA) and diacetylmonoxime (DAM), to reactivate acetylcholinesterase (AChE) inhibited by sarin (GB) in the blood, brain, and peripheral tissues of guinea pigs was compared with that of the quaternary oximes 2-PAM, HLö7, and MMB-4. Animals were injected subcutaneously (s.c.) with 1.0 x LD(50) of GB and treated intramuscularly (i.m.) 5 min later with one of these oximes. Sixty minutes after GB exposure, tissues were collected for AChE analysis. At low doses, MINA and DAM produced significant increases in AChE activity in all brain areas examined, but no significant AChE reactivation in peripheral tissues or blood. At higher doses, MINA and DAM increased AChE activity in the brain, peripheral tissues, and blood. In contrast, the quaternary oximes produced significant reactivation in peripheral tissues and blood AChE, but no significant reactivation of brain AChE. In another study, animals were pretreated i.m. with pyridostigmine 30 min prior to s.c. challenge with 2.0 x LD(50) of GB and treated i.m. 1 min later with atropine sulfate (2.0 mg/kg), plus a varied dose of oximes. MINA and DAM prevented or terminated GB-induced seizure activity and protected against GB lethality in a dose-dependent fashion. In contrast, none of the quaternary oximes prevented or stopped GB-induced seizures. Thus, tertiary oximes reactivated AChE in the brain, improved survival, and terminated seizures following GB intoxication.

Reactivation of brain acetylcholinesterase by monoisonitrosoacetone increases the therapeutic efficacy against nerve agents in guinea pigs.

Current oxime therapies do not readily cross the blood-brain barrier to reactivate organophosphorus nerve agent-inhibited cholinesterase (ChE) within the CNS. We investigated the ability of monoisonitrosoacetone (MINA), a tertiary oxime, to reactivate ChE inhibited by the nerve agent sarin (GB), cyclosarin (GF), or VX, in peripheral tissues and brain of guinea pigs and determined whether reactivation in the CNS will enhance protection against the lethal effects of these three agents. In the reactivation experiment, animals were pretreated with atropine methylnitrate (1.0mg/kg, i.m.) 15 min prior to subcutaneous (s.c.) challenge with 1.0 x LD(50) of GB, GF, or VX. Fifteen minutes later animals were treated intramuscularly (i.m.) with MINA (ranging from 22.1 to 139.3mg/kg) or 2-PAM (25.0mg/kg). At 60 min after nerve agent, CNS (brainstem, cerebellum, cortex, hippocampus, midbrain, spinal cord, and striatum) and peripheral (blood, diaphragm, heart, and skeletal muscle) tissues were collected for ChE analysis. MINA reactivated nerve agent-inhibited ChE in the CNS and peripheral tissues in a dose-dependent manner in the following order of potency: GB>GF>VX. In a survival experiment, animals were injected i.m. with atropine sulfate (0.5mg/kg), 2-PAM (25.0mg/kg), or MINA (35.0, 60.0, or 100.0mg/kg) alone or in combination 1 min after challenge with varying s.c. doses of GB, GF, or VX to determine the level of protection. The rank order of MINA's efficacy in guinea pigs against nerve agent lethality was the same as for reactivation of inhibited ChE in the CNS. These data show that MINA is capable of reactivating nerve agent-inhibited ChE and that the extent of ChE reactivation within the CNS strongly relates to its therapeutic efficacy.

In vivo reactivation by oximes of inhibited blood, brain and peripheral tissue cholinesterase activity following exposure to nerve agents in guinea pigs.

This study compared the ability of nine oximes (HI-6, HLö7, MMB-4, TMB-4, carboxime, ICD585, ICD692, ICD3805, and 2-PAM) to reactivate in vivo cholinesterase (ChE) in blood, brain, and peripheral tissues in guinea pigs intoxicated by one of four organophosphorus nerve agents. Two bis-pyridinium compounds without an oxime group, SAD128 and ICD4157, served as non-oxime controls. Animals were injected subcutaneously with 1.0 x LD(50) of the nerve agents sarin, cyclosarin, VR or VX and treated intramuscularly 5 min later with one of these oximes. Toxic signs and lethality were monitored; tissue ChE activities were determined at 60 min after nerve agent. Some animals exposed to sarin or cyclosarin, with or without non-oxime treatment, died within 60 min; however, no animal treated with an oxime died. For VR or VX, all animals survived the 60 min after exposure, with or without non-oxime or oxime therapy. The four nerve agents caused differential degrees of inhibition in blood, brain regions and peripheral tissues. The tested oximes exhibited differential potency in reactivating nerve agent-inhibited ChE in various peripheral tissues, but did not affect ChE activity in the brain regions. There was no direct relation between blood and peripheral tissues in the reactivating efficacy of oxime treatments. ChE inhibited by sarin was the most susceptible to oxime reactivation while cyclosarin the least susceptible. There was no difference in the ChE reactivating potency between the dimethanesulfonate and dichloride salts of HI-6. MMB-4 significantly reactivated the ChE inhibited by these four nerve agents in blood and all three peripheral tissues of the guinea pig, and among all the oximes tested it was the most effective in vivo ChE reactivator against all four nerve agents.