Proceedings of The Physiological Society
University of Bristol (2005) J Physiol 567P, PC65
The effects of stress on extracellular levels of GABA in the hippocampus: an in vivo microdialysis study in the rat
de Groote, Lotte; Linthorst, Astrid C.E.;
1. HW LINE, University of Bristol, Bristol, United Kingdom.
Gamma-aminobutyric acid (GABA) has been implicated in the central mechanisms controlling emotion and in the response to stress. An important brain structure involved in behavioural and neuroendocrine responses to stress is the hippocampus. There is, however, little known about the actual extracellular GABA concentrations in this brain structure under stressful conditions. The aim of the present study is therefore to characterise the effects of different types of stress on GABA levels in the hippocampus of the rat. In vivo microdialysis was performed to monitor extracellular GABA levels under baseline and stress conditions. Rats were equipped with a guide cannula (surgery under ketamine/xylazine anaesthesia, 100 and 5 mg/kg, respectively, i.p.) and after a 7 day recovery period, a microdialysis probe was inserted through the guide cannula into the hippocampus (under halothane anaesthesia). The microdialysis experiments started on the second day after implantation of the microdialysis probe. During the baseline period, six 10 min samples were collected, after which the animals were exposed to a stressful challenge. At the start of the stress paradigms the sampling time was 5 min for a 30 min period followed by the collection of 10 min samples for another 2.5 h. Rats were exposed to either novelty or forced swim stress (5-7 animals per group). For the novelty exposure rats were placed in a clean cage (similar to their home cage) for 30 min. For the forced swim stress rats were placed in a large beaker glass containing water at a temperature of 25°C for a period of 15 min. The behaviours displayed by the animals were scored throughout the microdialysis experiments, including the novelty and swim stress period. We have established a sensitive HPLC/electrochemical detection method (adapted from Smith & Sharp, 1994) for the analysis of GABA concentrations in dialysates of short sample duration. All animals were humanely killed at the end of the experiment. Exposure to a novel environment caused a moderate increase in hippocampal GABA to 120% basal levels that lasted throughout the exposure period and returned gradually back to baseline after return of the rat to the home cage. Forced swim stress resulted in a profound decrease of GABA levels with a minimum of 70% of baseline reached 15 min after the rats had been taken out of the water. Our preliminary data show that novelty and forced swimming, two different types of stressors that have primarily psychological (emotional) and combined psychological and physiological components, respectively, cause differential changes in hippocampal GABA levels. We conclude that in vivo microdialysis using short sampling times is a powerful method that can contribute to our understanding of the role of GABA-ergic neurotransmission under stressful challenges.
Where applicable, experiments conform with Society ethical requirements