Glutamic Acid Decarboxylase Concentration Changes in Response to Stress and Altered Availability of Glutamic Acid in Rabbit (Oryctolagus cuniculus) Brain Limbic Structures

SIMPLE SUMMARY: Glutamic acid decarboxylase (GAD) is an enzyme that catalyses the formation of γ-aminobutyric acid (GABA), the most important inhibitory neurotransmitter, from glutamic acid (Glu), which is the major neuromodulator in the central nervous system and is involved in most processes such as learning and memory, and in the mechanisms underlying aggressive animal behaviour. However, an excess of Glu in the neuronal space has a cytotoxic and neurodegenerative effect on neurons, and numerous studies have shown this negative effect on the proper functioning of the nervous system. GAD is therefore a key enzyme that ensures the balance between the concentration of Glu and GABA necessary for the proper functioning of brain mechanisms, including the stress response mechanism. The aim of this study was to examine if and how stress and Glu and its selected antagonists affect the level of the GAD enzyme in rabbit brain structures and, if so, in which structures these changes take place and whether GAD can be, next to adrenal hormones, an alternative marker to determine the level of stress in animals. In summary, the conducted study showed that selected rabbit brain structures showed variable GAD concentration in different ways under stressful conditions. The results presented in this paper improve our understanding of the rabbit’s limbic system and broaden our understanding of the stress response in this animal species under the influence of a stress factor. It is advisable that further studies assess precisely the Glu–GAD–GABA system under stressful conditions in other animal species, including farm animals, in particular those exposed to stress. ABSTRACT: Glutamic acid decarboxylase (GAD) is an enzyme that catalyses the formation of γ-aminobutyric acid (GABA), the most important inhibitory neurotransmitter, from glutamic acid (Glu), which is considered the most important excitatory transmitter in the central and peripheral nervous systems. GAD is a key enzyme that provides a balance between Glu and GABA concentration. Hence, it can be assumed that if the GAD executes the synthesis of GABA from Glu, it is important in the stress response, and thus also in triggering the emotional states of the body that accompany stress. The aim of the study was to investigate the concentration of the GAD in motivational structures in the brain of the rabbit (Oryctolagus cuniculus) under altered homeostatic conditions caused by stress and variable availability of Glu. Summarising, the experimental results clearly showed variable concentrations of GAD in the motivational structures of the rabbit brain. The highest concentration of GAD was found in the hypothalamus, which suggests a strong effect of Glu and GABA on the activity of this brain structure. The GAD concentrations in individual experimental groups depended to a greater extent on blocking the activity of glutamate receptors than on the effects of a single stress exposure. The results obtained clearly support the possibility that a rapid change in the concentration of GAD could shift bodily responses to quickly achieve homeostasis, especially in this species. Further studies are necessary to reveal the role of the Glu–GAD–GABA system in the modulation of stress situations as well as in body homeostasis.