Question

Draw a cell containing such a channel before and after application of GABA, indicating ionic movements through the channel. Next, draw a trace of membrane potential Vm over time for a cell before and after application of GABA. Make sure your axes are to scale. Explain your drawings.

Answer 1

There are two types of GABA receptors. GABA A receptors and GABA B receptors. Both have different effects on the

cell as well as the ion channels.

GABA A receptors: binding of GABA molecules to their binding sites in the extracellular part of the receptor triggers opening of a chloride ion-selective pore. Theincreased chloride conductance drives the membrane potential towards the reversal potential of the Cl¯ ion which is about –75 mV in neurons, inhibiting the firing of new action potentials. This mechanism is responsible for the sedative effects of GABAA allosteric agonists. In addition, activation of GABA receptors lead to the so-called shunting inhibition, which reduces the excitability of the cell independent of the changes in membrane potential.

GABA B receptors: GABA B receptors are coupled indirectly to K+ channels. When activated, these receptors can decrease Ca2+ conductance and inhibit cAMP production via intracellular mechanisms mediated by G proteins. GABA B receptors can mediate both postsynaptic and presynaptic inhibition. Presynaptic inhibition may occur as a result of GABA B receptors on nerve terminals causing a decrease in the influx of Ca2+, thereby reducing the release of neurotransmitters.