NEUROMEDIATOR CONTROL OF ADAPTIVE BEHAVIOUR IN YOUNG RATS UNDER EMOTIONAL STRESS IN HYPOTHYROIDISM

Abstract

Background. Given that thyroid hormones are involved in the formation of cognitive functions and determine the favorable course of stress conditions, the study of the mechanisms of adaptive protective response formation under conditions of emotional stress in hypothyroidism in early ontogenesis becomes particularly relevant.

Aim. To investigate changes in the content of neurotransmitter amino acids and serotonin in the cortex and hippocampus of juvenile rats under condition of emotional stress in hypothyroidism during the formation of defensive behaviour.

Materials and methods. The study was carried out on 32 mongrel rats weighing 40-60 g, aged 30-40 days, which were divided into experimental (emotional stress in experimental hypothyroidism) and sham (emotional stress) groups. Defensive behavior was studied in an elevated plus maze and Morris water maze. An experimental model of hypothyroidism was created by administering Mercazolil (“Zdorov’je”, Ukraine) at a dose of 10 mg/kg with food. Chronic emotional stress was simulated by placing rats in an open water space for 5 minutes over 5 days, acute stress was simulated by placing them once for 5 minutes in a closed space measuring 1 m×1 m. The tests were conducted after creating a model of hypothyroidism, and after chronic and acute emotional stress. The content of neurotransmitter amino acids (GABA, glycine, glutamate) and serotonin in the cortex and hippocampus were determined by thin-layer chromatography followed by spectrophotometry. Statistical processing included calculations of arithmetic means, standard deviations, Student’s T-test, and Pearson’s correlation analysis.

Results. Emotional stress against the background of hypothyroidism was accompanied by an anxiolytic effect: the number of transitions into the light arms increased by 98% (p˂0.05) in chronic stress and by 274% (p˂0.05) in acute stress, and the duration of stay in them increased by 2.3 (p˂0.05) and 3.4 (p˂0.05) times, respectively; the duration of grooming decreased by 43% (p˂0.05) under chronic emotional stress and almost twice (p˂0.05) under acute stress; the number of transitions to the dark arms increased by 44% and 30% (p˂0.05), correspondingly. The latent period of searching for a saving platform in the Morris maze decreased by 25% (p˂0.05) in chronic and by 40% (p˂0.05) in acute emotional stress. Behavioural indicators correlated with serotonin and glycine levels in the cortex during chronic emotional stress and glutamate levels in the hippocampus during acute emotional stress.

Conclusions. Emotional stress against the background of hypothyroidism was accompanied by an anxiolytic effect and improvement in spatial memory, which correlated with an increase in serotonin (chronic emotional stress) and glutamate (acute emotional stress) content in the cortex and hippocampus (acute emotional stress) during the formation of protective behavior in juvenile rats.