Animals
Male Sprague Dawley rats weighing 285–507 g were used in all experiments and were housed in a climate controlled environment and a dark/light cycle of 12 h of daylight and 12 h of darkness with ad libitum access to food and water. Electrophysiological and behavioral studies were performed in separate groups of rats. All procedures were conducted in compliance with the National Institutes of Health (NIH) guidelines for use of laboratory animals and institutional guidelines for animal care and use for research purposes were strictly followed and the study was approved by institutional review board.
Experimental groups
Hesperidin 100 mg/kg was used as SULT1A1 inhibitor found in citrus fruits, known migraine triggers and mefenamic acid (NSAID) 20 mg/kg, another SULT1A1 inhibitor, was used to induce medication overuse. In chronically treated rats, body weight was monitored weekly throughout the treatment period. Animals were randomly assigned to experimental groups.
Cortical spreading depression studies
CSD susceptibility was assessed by measuring the electrical threshold for CSD and the CSD frequency during continuous topical application of KCl in the experimental groups below. Other CSD attributes such as propagation speed, amplitude and duration at half-maximal amplitude were also measured.
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1)
Hesperidin 100 mg/kg (n = 16) or its vehicle-DMSO (n = 16) was administered intraperitoneally (ip) 30 min prior to CSD experiments.
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2)
Mefenamic acid 20 mg/kg/day (n = 12) or its vehicle-5% DMSO and sesame oil (n = 12) was administered (ip) chronically for 4 weeks and on day 28, CSD susceptibility was examined.
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3)
Mefenamic acid 20 mg/kg/day was administered (ip) for 4 weeks and on day 28, 30 min prior to CSD experiments, single dose hesperidin 100 mg/kg (n = 12) or its vehicle-DMSO (n = 12) was given intraperitoneally.
Behavioral studies
Behavioral tests were performed in a different set of experimental groups with intact skull.
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1)
Hesperidin 100 mg/kg (n = 12) or its vehicle-DMSO (n = 12) was administered intraperitoneally (ip) 30 min prior to behavioral studies.
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2)
Mefenamic acid 20 mg/kg/day (n = 12) or its vehicle-5% DMSO and sesame oil (n = 12) was administered (ip) chronically for 4 weeks and on day 28, behavioral tests were performed.
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3)
Mefenamic acid 20 mg/kg/day was administered (ip) for 4 weeks and on day 28, hesperidin 100 mg/kg (n = 12) or its vehicle-DMSO (n = 12) was given (ip) 30 min prior to behavioral studies.
General surgical preparation
Rats were anesthetized with isoflurane, the femoral artery was cannulated and a tracheostomy tube was placed for mechanical ventilation (SAR-830; CWE, Ardmore, PA). Rectal core temperature was maintained at 37 °C with a heating pad. Continuous blood pressure monitoring was established via femoral artery catheter (ADInstruments, Ardmore, PA) and intermittent arterial pH, pCO2 and O2 were measured (Rapidlab 248 blood gas/pH analyzer; Siemens HealthCare, Eschborn, Germany) and maintained within physiological range by adjusting ventilation parameter settings.
Cortical spreading depression susceptibility testing
Animals were placed on a stereotactic frame (Stoelting, Wood Dale, IL), craniotomies were drilled over the occipital (mm from bregma: 4.5 posterior, 2 lateral; diameter 2 mm), parietal (mm from bregma 1.5 posterior, 2 lateral,1 mm diameter), and frontal cortices (mm from bregma 1.5 anterior, 2 lateral; diameter 1 mm) under saline irrigation and dura was removed. Glass capillary microelectrodes (approximately 250 mm deep) were inserted and the cortex was allowed to rest for 15 min under saline. The electrocorticogram and direct current potentials were recorded and signals were amplified (EX1 differential amplifiers; Dagan Corporation, Minneapolis, MN) and recorded continuously (PowerLab; ADInstruments, Colorado Springs, CO).
Susceptibility to CSD was assessed by measuring the electrical threshold for CSD and the frequency of CSD during continuous topical KCl application [14, 15]. The electrical threshold for CSD was determined by direct stimulation of the cortex with a bipolar stimulation electrode (400 mm tip diameter, 1 mm tip separation; Frederick Haer Company, Bowdoin, ME) placed on the cortical pial surface and a stimulus isolator (WPI, Sarasota, FL). Single-square pulses of increasing duration and intensity (50–4000 mC) were applied at 5-min intervals until a CSD was triggered at the recording site. CSD frequency was analysed using a cotton ball (1.5–2 mm diameter) soaked with KCl (1 M) placed on the occipital cortex and the cotton ball was changed every 15 min and CSDs were continuously recorded for an hour. CSDs with an amplitude ≥5 mV were counted. Propagation speed, amplitude, and duration (at halfmaximal amplitude) of CSDs were also measured. Electrical thresholds and KCl induced CSD frequencies were measured in both hemispheres. Experimental set up is shown on Fig. 1.
Behavior
Behavioral tests were conducted at the same time period of the day. Spontaneous behavior such as grooming, freezing, immobility and head shakes was recorded for 10 min in plexiglass cages where rats were free to move. Elevated plus maze was used to assess anxiety like responses [16, 17]. The rats were placed at the center of the elevated plus maze set up facing the same closed arm and their spontaneous behavior was recorded for 5 min with the vide-camera sistem on the setup. Each animal was tested only once. The maze was cleaned with 70% alcohol after each animal. Duration spent in open and closed times and the number of open and closed arm entries are recorded.
Periorbital mechanical withdrawal thresholds were evaluated with von Frey filaments using ‘up and down method’ [18] starting with a force of 4 g. First, rats were allowed to accommodate to 40 cm high platforms and then von Frey filaments were applied to the periorbital region to assess mechanical withdrawal thresholds. Avoidance of rats from von Frey filament or ipsilateral head grooming was considered as a positive response.
Acetone evaporation test was performed to assess cold allodynia. One drop of acetone was applied to the periorbital region and the test was repeated 5 times with 5-min intervals. Grooming and avoidance within 1 min is considered as a positive response. Positive response was expressed as a percentage.
SULT 1A1 enzyme activity detection
At the end of the behavioral testing (2 h after the administration of hesperidin or its vehicle ip), the animals were sacrified by thiopental (50 mg/kg). Brains were harvested and rat brains were stored at − 80 °C. A spectrophotometric assay was used to study SULT1A1 enzyme activity where p-nitrophenyl sulfate was the sulfate donor and PAP was the sulfate acceptor molecule forming PAPS and p-nitrophenyl anion, which could be measured at 405 nm.
Homogenization
On the study day, the samples were taken from − 80 °C freezer and weighed one by one on a precision scale and the weights of the tissues were noted. For homogenization, tissue samples were diluted with homogenization buffer (%1.15 KCl, 1 mM EDTA ve 1 μg/mL protease inhibitor (aprotinin) containing 10 mM HEPES-NaOH, pH 7.4) in a ratio of 1/5. Brain tissues were homogenized in a mortar on ice. After homogenization, it was centrifuged at > 9000×g to obtain post-mitochondrial fraction in a refrigerated centrifuge. The supernatant obtained after centrifugation was placed in appropriate eppendorf tubes and stored at − 80 °C until the time of the analysis. On the analysis day, the eppendorf tubes were dissolved at + 4 °C.
Reagents
Assay buffer: 50 mM NaCl containing 25 MOPS buffer, pH 7.2, and 1 mM EDTA.
PAP reagent: 2 mM stock solution (8.6 mg/10 mL) in the assay buffer was prepared.
p-Nitrophenyl sulfate reagent: 2 mM (5.2 mg/10 mL) in the assay buffer was prepared.
Analysis and calculation
In a clean microplate or borosilicate tube, 50 μL of S10 (10,000×g supernatant for 20 min at 4 °C) or water, 25 μL of PAP reagent and 25 μL of p-nitrophenyl sulphate were pipetted and they were completed to 250 μL with assay buffer (150 μL). Reagents and samples were placed in 96 well plate. The formation of p-nitrophenyl anion was measured over 30 min at 405 nm at 10 min intervals with a plate reader. All absorbance values measured at 10 min intervals from t: 0 to t: 30 were recorded. According to Beer Lambert law, the activity of the solution with epsilon value was calculated according to the equation in Fig. 2. Epsilon value was taken as 15,000 M− 1 cm− 1 at 405 nm [19]. Obtained initial value was converted to IU (international unit = μmol/min) and other calculations were done based on it [20]. The specific enzyme activity was calculated with respect to the protein concentration (mg/mL).
The length of the solution in the equation was taken as the length of the well of the plate which was 0.5 cm. Calculation of enzyme activity was based on average absorbance values. Delta average absorbances and enzyme activities were calculated according to the equations below;
$$\mathrm{Delta}\ \mathrm{average}\ \mathrm{absorbance}=\left(\mathrm{A}\left(\mathrm{t}10-\mathrm{t}0\right)+\mathrm{A}\left(\mathrm{t}20-\mathrm{t}10\right)+\mathrm{A}\left(\mathrm{t}30-\mathrm{t}20\right)\right)/3\ \mathrm{where}\ \mathrm{A}\ \mathrm{is}\ \mathrm{absorbance}\ \mathrm{and}\ \mathrm{t}\ \mathrm{is}\ \mathrm{time}.$$
$$\mathrm{Enzyme}\ \mathrm{Activity}\left(\mathrm{IU}/\mathrm{L}\right)=\mathrm{Delta}\ \mathrm{Average}\ \mathrm{Absorbance}\ \left(10-0,20-10,30-20\ \min \right)\ \mathrm\times\ 2.1\times1{0}^4$$
Study design and statistics
All experiments were performed and analyzed in a blinded fashion. A priori exclusion criteria were surgical failure and poor systemic physiology. None of the animals died or experienced severe complications during the CSD recordings. Data were presented as mean ± standard deviation or as bar graphs. Analyses were made by Prism 7 (GraphPad Software, Inc., CA, USA). Continuous variables were analyzed with Student t-tests, or 2-way analysis of variance for repeated measures and post-hoc multiple comparisons were carried out using Šidák’s multiple comparisons test. A P value of 0.05 was considered statistically significant.