Subjects
Fifty male Wistar rats aged 9–10 weeks with a weight range of 250 to 300 g were purchased from the animal laboratory of Hamadan University of Medical Sciences. The rats were kept in a standard condition (12 hours of light and 12 hours of darkness and a temperature of 22±1 ° C, relative humidity: 55% to 60%). Rats were fed with laboratory chow. The study protocol was approved by the Institutional Animal Ethics Committee.
Induction of Type 2 Diabetes Mellitus
The rats were deprived of food 12 h before the induction of type 2 diabetes. Induction of type 2 diabetes was done by nicotinamide (120 mg/Kg, soluble in normal saline, Sigma-Aldrich, St. Louis, USA) that was administrated by intraperitoneal (IP) injection following the fasting period. The IP injection of STZ (65 mg/kg, soluble in citrate buffer (0.05M, pH=4.5), Sigma-Aldrich, St. Louis, USA) was injected 15 min after nicotinamide injection (Nasri et al., 2020). The control group rats received only subcutaneous injections of citrate buffer. Seventy-two hours after injections, the FBS level in blood samples collected from the tail vein was measured and animals with the FBS of above 250 mg/dl were regarded as diabetic and used in the study (Fig. 1).
Experimental design
After a short-term acclimation of rats to the laboratory environment, healthy and diabetic rats were divided into the following groups:
Group 1: Control (Con)
Group 2: Untreated type 2 diabetic rats (D(
Group 3: Type 2 diabetic rats + P. oleracea and P. psyllium (D+PO+PP)
Group 4: Type 2 diabetic rats + exercise training (D+Tr)
Group 5: Type 2 diabetic rats + P. oleracea and P. psyllium + exercise training (D+PO+PP+Tr)
Exercise training
To alleviate stress without promoting adaptation to exercise, rats in the training groups were familiarized with water and swimming in the pool (70 × 80 × 100 cm, water temperature: 30-32° C) filled with water to a depth of 60 cm for 3 days (2 sessions per day, each session 10 min). The training protocol consisted of 12 weeks of progressive submaximal swimming exercise (5 days per week) (Chen et al., 2018, Lin et al., 2020). After the habituation, the rats swam for 15 min in the first two weeks, and then, gradually the swimming time increased to 50 minutes in the overload phase from the third to the tenth weeks, and in the last two weeks, the duration of swimming increased to 60 min. All rats swam while wearing a weight of 2% of their body weight attached to the tail. The body weight of all groups was monitored and recorded weekly.
Co-administration of P. oleracea and P. psyllium
P. oleracea and P. psyllium were collected from a local herb store of Hamedan and approved by the Department of Pharmacy of the Hamedan University of Medical Science. To prepare the food with P. oleracea and P. psyllium, 5 mg of P. psyllium, and 5 mg of P. oleracea (3 mg of P. oleracea seeds powder plus 2 mg of P. oleracea dried plant) were mixed with one liter of water to obtain a homogeneous solution. The solution was then mixed with 90% of normal food. P. psyllium and P. oleracea were mixed with standard pelleted food at a weight ratio of 10% and were received by the rats in the D+PO+PP and D+PO+PP+Tr groups for 12 weeks.
Shuttle box test
Passive avoidance memory was evaluated by the shuttle box test. The method of working with the device and process were fully mentioned in our previous papers (Zarrinkalam et al., 2016, Zarrinkalam et al., 2018, Karimi et al., 2020, Ahmadi et al., 2021). The device had two light and dark sections (20× 20 ×30 cm), with a grid stainless-steel rod floor attached to a shock generator and a guillotine door separated two compartments. At first, for acclimatization, the animals were placed in a lighted section and then, the guillotine door was opened and after 30 s of the entrance to the dark section, it was transferred to its home cage. Thirty minutes later, this test was repeated again. When the rat had its whole body in the dark section, the entrance latency to the dark chamber (step-through latency, STLa) was measured. The guillotine door between two sections was closed and then an electrical shock (0.8 mA) was applied to the rat for 2 s. Thirty seconds after an electrical shock, the rat was transferred to its home cage. The test was conducted again after 2 min. Each time the rat re-entered the dark section, it received an electric shock. When an animal stayed in the dark section for 120 s, the test was terminated and the number of trials was recorded (Zarrinkalam et al., 2016, Zarrinkalam et al., 2018, Ghaderi et al., 2020).
The retention test was executed 24 h after the PAL acquisition trial. In this phase, the rat was placed in the light section and the guillotine door was raised to the rat for 5 s and then, the step-through latency (STLr) and the time spent in the dark section (TDC) were measured for 600 (Zarrinkalam et al., 2016, Shiri et al., 2017).
Open field (OF) test
To determine the general locomotor activity and exploratory behavior of the subjects, the open field (OF) test was used. As described by our laboratory (Etaee et al., 2019), briefly, we carried out the test in a 100 × 100 × 40 cm hypethral box with the bottom divided into four identical squares on the floor of the arena. The rat was placed in the central square and had 10 min to explore. The total distance moved (locomotor activity) was recorded using a video camera and the data were analyzed through video track software (Etaee et al., 2019).
Novel object recognition (NOR) test
We used the novel object recognition (NOR) test to assess non-spatial memory in type 2 diabetic rats. As previously described (Lueptow, 2017, Kassab et al., 2019) with some minor modifications, the NOR test is a simple test that can be done over 3 days. During training, the rat could explore two similar objects. On the test day, one of the objects was replaced with a new one having a different shape and color. The rats prefer novelty, thus, when they recognize the familiar object, they prefer to spend most of their time with the new one (Lueptow, 2017, Kassab et al., 2019). The time spent exploring each object (sniffing or touching the object not standing, sitting on, or leaning against the object) was noted. The discrimination index (DI= (TNO – TFO)/(TNO + TFO) was also determined (TNO: the exploration time of the new object and TFO: the exploration time of the familiar object (Kassab et al., 2019, Shekarian et al., 2020). It should be noted that all sessions were video recorded and analyzed blindly.
Elevated plus-maze test (EPM)
The elevated plus-maze (EPM) was employed to evaluate anxiolytic activity. As previously described (Cavalcanti et al., 2020), the EPM apparatus consists of two opposing closed arms (10 × 50 cm) and two opposing open arms (50 × 10 × 50 cm) connected through a central square (10 × 10 cm), and the maze is 80 cm above the floor. Each rat was placed in the center of the device in front of one of the closed arms and could explore the maze for 10 min. The time spent in closed arms was video recorded and analyzed. The light intensity was 130 lux in the closed arms and 220 lux in open arms. The maze was cleaned using 10% ethanol after each test to get rid of any remaining odors.
Statistical analyses
Data were analyzed by SPSS version 20.0 (IBM SPSS Statistics). The Shapiro-Wilk test was used to assess the normal distribution of the data. The statistical difference between groups was estimated using one-way analysis of variance (ANOVA) with Tukey’s post-hoc test. Values were expressed as mean ± SD. Values with a p-value of ≤ 0.05 were considered significant.