Oxidant/antioxidant effects of chronic exposure to

Authors: G E MejiaCarmona K L Gosselink G PérezIshiwara A MartínezMartínez

Publish Date: 2015/05/16

Volume: 406, Issue: 1-2, Pages: 121-129

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Abstract

The incidence of anxietyrelated diseases is increasing these days hence there is a need to understand the mechanisms that underlie its nature and consequences It is known that limbic structures mainly the prefrontal cortex and amygdala are involved in the processing of anxiety and that projections from prefrontal cortex and amygdala can induce activity of the hypothalamic–pituitary–adrenal axis with consequent cardiovascular changes increase in oxygen consumption and ROS production The compensatory reaction can include increased antioxidant enzymes activities overexpression of antioxidant enzymes and genetic shifts that could include the activation of antioxidant genes The main objective of this study was to evaluate the oxidant/antioxidant effect that chronic anxiogenic stress exposure can have in prefrontal cortex amygdala and hypothalamus by exposition to predator odor Results showed a sensitization of the HPA axis response b an enzymatic phase 1 and 2 antioxidant response to oxidative stress in amygdala c an antioxidant stability without elevation of oxidative markers in prefrontal cortex d an elevation in phase 1 antioxidant response in hypothalamus Chronic exposure to predator odor has an impact in the metabolic REDOX state in amygdala prefrontal cortex and hypothalamus with oxidative stress being prevalent in amygdala as this is the principal structure responsible for the management of anxietyStress induces a neurochemical cascade that can cause longlasting changes Activation of the hypothalamic–pituitary–adrenal HPA axis is the most characteristic response to stress in mammals In anxiety the prefrontal cortex PfC sends projections and dopamine inputs to the amygdala 1 initiating a glucocorticoid cascade through the HPA axis 2 Neuronal imaging studies in humans have shown that some anxiety and depressive disorder patients have increased amygdaline function 3 The increase in neuronal activity in the amygdaline zone during anxiety periods produces cardiovascular changes mediated by excitation of neurons in the hypothalamus 4 Predator odor induces a fear and anxietylike behavior in rodents similar to some anxiety disorders in humans in which risk assessment fight flight and exploration conducts are observed that is why in the past decade this model has been used to evaluate the effects of fear and anxietyrelated diseases such as general anxiety disorder GAD posttraumatic stress disorder PTSD and panic disorder and depression 5 6 7 8 Predator odor stress induces an elevation in dopamine turnover in the medial prefrontal cortex mPfC and amygdala 9 and changes in afferent and efferent transmissions of the amygdala 10 11 It selectively activates diverse nuclei of the amygdala PfC and hypothalamus 12 structures that are rarely activated in immobilization foot shock or forced swimming models of stress 13 Furthermore predator odor stress induces cardiovascular behavioral endocrine and autonomic longterm changes 4 12Cardiovascular modifications have been associated to an increase in metabolism with oxygen consumption and production of ROS by electron transport chain 14 which can lead to oxidative stress The compensatory response can include increased antioxidant enzymes activities overexpression of antioxidant mRNAs of enzymes and genetic shift that could include the activation of antioxidant genes 15 16 This effect has been observed by ArzateVázquez et al 17 when a modification in transcript expression occurred in amygdala hypothalamus hippocampus and PfC in rats exposed to a chronic live predator exposure paradigm later we observed modification in the activity of some antioxidant enzymes under an acute model of predator odor 18 Both models include an anxiolytic component and have studied structures responsible for the processing of emotions Nevertheless the effect of chronic predator exposure as an anxiogenic stimuli on the cellular antioxidant mechanisms of the amygdala PfC and hypothalamus has been poorly explored Thus the aim of this study was to determine the effect that chronic anxiogenic stress has on the oxidant/antioxidant capacity of amygdala and PfC centers for processing emotions and hypothalamus the primary structure involved in arising the stress responseAll experimental protocols were approved by the Bioethics Committee of the Universidad Autónoma de Ciudad Juárez following the guidance of Official Mexican Standard NOM062ZOO1999 Twelve singlehoused experimentally naïve adult male Sprague–Dawley rats weighing 250–280 g approximately 2 months of age were randomly assigned to control or stress group The model of chronic predator odor exposure was used according to Dielenberg et al 19 with some modifications Control group without exposure to cat odor was exposed to a 25 × 25 cm clean fabric Stress group or cat odorexposed group was exposed to a 25 × 25 cm fabric in which a domestic cat slept overnight followed by vigorous rubbing against the fur of the cat Treatment cage for both groups consisted of two parts as described by Dielenberg et al 12 twothirds of the cage were made with clear transparent Plexiglas while onethird consisted of a black Plexiglas with a peaking hole meant to be a hiding box Treatment was applied for five consecutive days as follows First a habituation period of 20 min in the treatment cage followed by 20 min in their housing cage and finally 20 min in the treatment cage but with their respective fabrics without cat odor for control or cat odor for stress attached to the opposing wall from the black Plexiglas On the 6th day the rats were euthanized by decapitation for ROS evaluation or by intraperitoneal overdose of pentobarbital for enzyme activities and TBARSPlasma corticosterone CORT was determined before and after predator odor exposure Three days before treatment blood from the saphenous vein was drawn to measure basal levels of CORT and measured again on control and stressed rats on the day of euthanasia Day 6 After pentobarbital injection blood was drawn from cardiac puncture collected in 1 mg/mL EDTA containing microtubes centrifuged at 3000×g for 10 min then plasma was separated and stored at −80 °C CORT concentration was measured using the AssayMax Corticosterone ELISA kit Assaypro EC30011 following manufacturer’s instructions Results are expressed as ng of CORT per mL of plasmaRats euthanized by overdose injection of pentobarbital were perfused for 20 min with icecold KrebsHenseleit solution containing 148 mM NaCl 4 mM KCl 185 mM CaCl2 105 mM MgCl2 3 mM HEPES 135 μM EDTA pH 80 55 mM glucose and 00035  p/v ascorbic acid 20 Whole brain was washed in icecold 09  saline solution and frozen at −20 °C then Amygdala PfC and hypothalamus were dissected and immersed in solution containing 50 mM cold sodium phosphate buffer of pH 74 and 1  phenylmethylsulfonyl fluoride PMSF in a ratio of 110 w/v Tissues were homogenized and sonicated 3 times at 30 w 69 kHz Sonic Dismembrator Model 100 Fischer Scientific for 10 s at 1min intervals on ice Homogenates were centrifuged at 20800×g for 30 min at 0 °C 21 the supernatants were kept at −80 °C until use to evaluate superoxide dismutase SOD glutathione Stransferase GST and thiobarbituric reactive substances TBARS levelsSOD activity was determined by autooxidation of pyrogallol Sigma Cat No 254002 The pyrogallol method developed by Marklund and Marklund 22 is a convenient and reliable technique to measure the activity of SOD and it has been extensively used 23 24 25 26 27 Detection of SOD by pyrogallol depends on the inhibition of pyrogallol autooxidation by the enzyme which can reach up to 99  22 The specific detection of mitochondrial SOD which is dependent on manganese MnSOD relies on the addition of sodium cyanide NaCN to inhibit cytosolic SOD that is in turn dependent on copper and zinc Cu/ZnSOD thereby facilitating the detection of MnSOD 22 Thus a modified reaction of the method described by Marklund and Marklund 22 was used A microplate reaction mixture containing 270 μL of 50 mM Tris–HCl pH 82 containing 1 mM diethylenetriaminepentaacetic acid DTPA 27 μL of 100 mM NaCN and 14 μL of supernatant was prepared Reaction was started by the addition of 16 μL of 36 mM pyrogallol MnSOD activity was determined in a microplate reader inhibition of pyrogallol autooxidation was registered at 15s intervals for 12 min and 2 s of mixing at 420 nm and room temperature Only the firstorder reaction at initial maximal velocity Vo = Vmax Si  10 km Si ≥ 09Sf was taken into account at r = 099 One unit of MnSOD activity U was defined as the amount of enzyme that inhibits pyrogallol autoxidation rate by 50  per min It is expressed as U per mg of proteinThe activity of GST was determined according to the method of Habig et al 28 with some modifications On the day of the assay equimolar substrate solutions of 225 mM 24Dinitrochlorobenzene CDNB Sigma Cat No 237329 and 225 mM GSH Sigma Cat No G4251 were prepared separately in 01 M phosphate buffer of pH 65 Equal volumes of each substrate solution 225 mM CDNB and 225 mM GSH were mixed and heated to 37 °C Assay was prepared by adding 25 μL of supernatant to a 96well plate followed by 200 μL of the substrate solution mix to start the reaction Formation of the GSCDNB complex was registered for 22 min at 15s intervals for 5 s of mixing in a microplate reader at 340 nm and 37 °C Only the firstorder reaction at initial maximal velocity Vo = Vmax Si  10 km Sf ≥ 09Si was taken into account at r = 099 One unit of GST activity U was defined as the amount of enzyme that catalyzes the formation of one µmol of GSCDNB complex per min We expressed GST activity as mU per mg of protein

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