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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Physiology and Pharmacology

Supervisor

Poulter, Michael O.

2nd Supervisor

Inoue, Wataru

Co-Supervisor

Abstract

γ-aminobutyric acid type A receptors (GABAA receptors) underlie the majority of inhibitory synaptic transmission in the brain. Modulation of GABAergic activity occurs in development and normal physiological functioning of the brain, and changes to GABAergic function has been implicated in numerous neurological disorders including epilepsy. Neurosteroids, metabolites of steroid hormones, and kinases are known to modulate GABAA receptor mediated currents in health and disease. This thesis aims to investigate the effects of kinases and neurosteroids on modulating GABAA receptor-mediated currents in cortical pyramidal cells and their effects on the piriform cortex (PCtx) circuit in naïve rats and in rats after kindling (animal model of epilepsy). Specifically, we investigated:

1) The effects of kinase activators on neurosteroid-induced modulation of GABAA receptor mediated currents, using inhibitory neurosteroid tetrahydrodeoxy-corticosterone (THDOC) and excitatory neurosteroid pregnenolone sulfate (PregS).

2) The effect of neurosteroid THDOC and protein kinase modulation on the PCtx circuit

3) The effect of THDOC and kinase modulation in the PCtx after kindling

We found that kinases differentially modulate the potentiating effect of THDOC on phasic and tonic inhibition, where they suppressed enhancement of tonic inhibition, but had no effect on phasic inhibitory postsynaptic currents (IPSCs). By contrast, kinases did not modulate the effect of excitatory neurosteroid PregS on phasic and tonic inhibition. In the PCtx circuit, THDOC suppressed the activity of the circuit, and this effect was blocked by prior kinase activation. After kindling, we found that THDOC no longer suppressed the circuit activity. Protein kinase C activation partially restored the effect of THDOC after kindling. Our findings show that protein kinase activities regulate neurosteroid-mediated modulation of GABAergic inhibition and that regulation of protein kinase activity may be able to restore normal neurosteroid functioning in epilepsy.

Summary for Lay Audience

The brain is a complex organ that controls our body and mental function. It consists of neurons that form intricate networks. These neurons communicate through excitatory and inhibitory electrochemical signals. The balance between excitation and inhibition in the brain is crucial for normal function of the brain, and disturbances to the balance is implicated in brain disorders such as epilepsy.

γ-aminobutyric acid type A receptors (GABAA receptors) are responsible for inhibition of the brain. Their activities can be altered by many molecules, including those made in the body. In this thesis, we investigated two classes of such molecules: neurosteroids and protein kinases. Neurosteroids are made from hormones in our body, and they can alter the function of GABAA receptors. The neurosteroid tetrahydrodeoxycorticosterone (THDOC) enhances GABAA receptor function and thus enhances inhibition of the brain. Protein kinases can also alter the function of GABAA receptors, but they can also alter the effect of neurosteroids on GABAA receptors. The main goal of this thesis was to investigate how neurosteroids and protein kinases alter GABAA receptor function at cellular level (in single neuron) and at network level (in network of neurons), specifically in the piriform cortex, a region of the brain known to be extremely susceptible to seizures, and how the effects of neurosteroids and protein kinases change in epilepsy, using an animal model.

We found that the effect of THDOC on GABAA receptors is decreased when protein kinases were applied at cellular level. Similarly, in the piriform cortex circuit, protein kinases blocked the inhibitory effect of THDOC. In the piriform cortex of epileptic brain, however, we found that THDOC no longer enhanced the inhibition by GABAA receptors. When the epileptic brain was treated with protein kinases, the effect of THDOC was restored, and THDOC was able to enhance GABAA receptor function and thus inhibition of the brain. Our findings show that changing protein kinase activities may be able to restore normal functioning of neurosteroids in epilepsy and could be a potential treatment to fight epilepsy.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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