Role of basal stress hormones and amygdala dimensions in stress coping strategies of male rhesus monkeys in response to a hazard-reward conflict

Objective(s): In the present study the effect of stress on monkeys that had learned to retrieve food from a five-chamber receptacle, as well as the relationship between their behavior and the serum cortisol and epinephrine levels and relative size of the amygdala was evaluated. Materials and Methods: Six male rhesus monkeys were individually given access to the food reward orderly. They could easily retrieve the rewards from all chambers except for the chamber 4, which a brief, mild electric shock (3 V) was delivered to them upon touching the chamber’s interior. The coping behaviors were video-recorded and analyzed offline. Baseline serum cortisol and epinephrine levels were measured before the experiments using monkey enzyme-linked immunosorbent assay kit. One week after the behavioral experiment, the monkeys’ brains were scanned using magnetic resonance imaging under general anesthesia. The cross-sectional area of the left amygdala in sagittal plane relative to the area of the whole brain in the same slice was evaluated by the planimetric method using ImageJ software. Results: Exposure to the distressing condition caused different behavioral responses. Monkeys with higher baseline levels of serum cortisol and epinephrine and larger amygdala behaved more violently in the face of stress, indicating adopting emotion-focused stress-coping strategies. Conversely, those with low plasma epinephrine, moderate cortisol, and smaller amygdala showed perseverative behavior, indicating a problem-focused coping style. Conclusion: In dealing with the same stress, different responses might be observed from nonhuman primates according to their cortisol and epinephrine levels as well as their amygdala dimensions

The Effect of Extremely Low Frequency Electromagnetic Fields on Visual Learning & Memory and Anatomical Structures of the Brain in Male Rhesus Monkeys

Background: Humans in modern societies expose to substantially elevated levels of electromagnetic field (EMF) emissions with different frequencies.The neurobiological effects of EMF have been the subject of debate and intensive research over the past few decades. Therefore, we evaluated the effects of EMF on visual learning and anatomical dimensions of the hippocampus and the prefrontal area (PFA) in male Rhesus monkeys. Materials and Methods:In this study, four rhesus monkeys were irradiated by 0.7 microtesla ELF-EMF either at 5 or 30 Hz, 4 h a day, for 30 days. Alterations in visual learning and memory were assessed before and after irradiation phase by using a box designed that cchallenging animals for gaining rewards Also, the monkeys’ brains were scanned by using MRI technique one week before and one week after irradiation. The monkeys were anesthetized by intramuscular injection of ketamine hydrochloride (10–20 mg/kg) and xylazine (0.2–0.4 mg/kg), and scanned with a 3-Tesla Magnetom, in axial, sagittal, and coronal planes using T2 weight­ed protocol with a slice thickness of 3 mm. The anatomical changes of hippocampus and the prefrontal area (PFA) was measured by volumetric study. Results: Electromagnetic field exposure at a frequency of 30 Hz reduced the number of correct responses in the learning process and delayed memory formation in the two tested monkeys. While, ELF-EMF at 5 Hz had no effect on the visual learning and memory changes. No anatomical changes were found in the prefrontal area and the hippocampus at both frequencies. Conclusion: ELF-EMF irradiation at 30 Hz adversely affected visual learning and memory, pprobably through these changes apply through effects on other factors except changes in brain structure and anatomy