A Role for glutamate and Gaba in attention-deficit/hyperactivity disorder: a study of the spontaneously hypertensive rat
Doctoral Thesis
2014
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University of Cape Town
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Abstract
Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous, developmental disorder characterised by behavioural symptoms of inattention, hyperactivity, and impulsivity. The present thesis investigated how the main excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA respectively, play a role in the dysregulation of the norepinephrine system in a rat model of ADHD, the spontaneously hypertensive rat (SHR), compared to its control strain, the Wistar-Kyoto rat (WKY), and a third comparator strain, Sprague-Dawley rats (SD). Release of radio-actively labelled norepinephrine from hippocampal slices of SHR, WKY, and in some experiments SD, by glutamate, GABA, potassium, and nicotine, were measured, and the involvement of glutamate and GABA receptors and transporters in the stimulated release of hippocampal norepinephrine were investigated, using an in vitro superfusion technique. Since early life stress has been shown to increase the risk of developing ADHD, the present thesis also investigated how a model of early life stress (maternal separation) affects behavioural responses to novelty (behavioural assays), plasma corticosterone (ELISA kit), and neurochemical regulation of hippocampal norepinephrine release by glutamate and GABA (in vitro superfusion) in SHR, as well as in control strains. Effects of early life stress on hippocampal protein profile of SHR, WKY and SD were also investigated using proteomic analysis, followed by Western blot analysis to confirm main proteomic findings related to the glutamate transporter GLT-1, and the potassium-chloride co-transporter KCC2. The main findings of the present thesis were that glutamate-stimulated release of norepinephrine was elevated in SHR hippocampus compared to WKY and SD, and was via glutamate AMPA receptors. Elevating tonic levels of GABA in WKY and SD hippocampi increased glutamate-stimulated release of hippocampal norepinephrine to be equivalent to that of SHR, while elevating tonic levels of glutamate in SHR hippocampus reduced glutamate-stimulated release of hippocampal norepinephrine to be equivalent to that of control strains, suggesting that GABA tonic levels are reduced while glutamate tonic levels are elevated in SHR hippocampus in vivo compared to control strains. GABAA receptor inhibition of norepinephrine release, and GABA evoked release of norepinephrine, was reduced in SHR hippocampus compared to WKY. Nicotine stimulated release of norepinephrine was reduced in SHR, compared to WKY and SD, and lacked a GABAA receptor component that was evident in WKY hippocampus. Proteomic analysis showed that the expression of the glutamate transporter GLT1b splice variant and KCC2 was increased in SHR hippocampus compared to WKY, while GLT1 total was reduced in SHR hippocampus compared to WKY and SD. Experiencing early life stress did not influence behaviour of SHR, and had limited effect on the behaviour of WKY, suggesting that SHR may be resilient to the long -term behavioural effects of early life stress. Early life stress did, however, increase NMDA receptor- and GABAA receptor-mediated inhibition of locus coeruleus-norepinephrine varicosities in SHR hippocampus, while reducing the role of these receptors in WKY hippocampus. Early life stress increased GLT1 expression in all 3 strains and reduced the GLT1b splice variant in SHR hippocampus only. These neurochemical changes following early life stress possibly involved adaptations that prevented long-term changes in behaviours of these rats. In conclusion, the results of the present thesis provide evidence supporting a role for glutamate and GABA dysfunction in the hippocampus of an animal model of ADHD, SHR, and also provide evidence for altered glutamate and GABA neurotransmission in the hippocampus of SHR following early life stress.
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Includes bibliographical references.
Reference:
Sterley, T. 2014. A Role for glutamate and Gaba in attention-deficit/hyperactivity disorder: a study of the spontaneously hypertensive rat. University of Cape Town.