Patch-clamp studies of the GABAA receptor using microfluidic methods
Ion channels are membrane proteins that passively transport ions. When activated, conformational changes within the protein lead to opening of a channel pore, and the flow of ions causes a change in the membrane potential. This is the fundamental process behind the generation and transduction of nerve impulses. The GABAA receptor is a ligand-gated ion channel which is activated by the inhibitory neurotransmitter γ-amino-n-butyric acid (GABA). This receptor is mainly situated in the post-synaptic membrane and conducts inhibitory synaptic currents. Understanding the kinetics of ion channels like the GABAA receptor is important in order to understand what output, in terms of the shape and length of the synaptic current, will be the result of different inputs, in terms of the concentration profile of the neurotransmitter in the synaptic cleft. The GABAA receptor exhibits binding, gating, and desensitization, i.e. transit into long-lived, non-conducting, states. Desensitization has been suggested to regulate the length and amplitude of the synaptic GABAA currents, and also to reduce GABAA currents during repetitive GABA application.
Using a combination of the patch-clamp technique and a microfluidic superfusion system, we have performed scanning experiments across gradients of the GABAA receptor agonists GABA and β-alanine. Results show that the order of application of different ligand concentrations, as well as the time and frequency of stimulations, affect the appearance of the dose-response curves and hence the EC50 values obtained by fitting data to the Hill equation. Also, repetitive applications cause slow desensitization which constitutes a memory function which lasts for minutes. By adding temperature control to the microfluidic chip, we have also obtained dose-response curves for temperatures between 25°C and 40°C. The EC50 value proved to be temperature dependent for GABA, but not for β-alanine. This could be due to differences in binding kinetics for GABA and β-alanine.