The next neurotransmitter I will talk about is GABA (gamma-aminobutyric acid). GABA is the primary inhibitory neurotransmitter in the brain, but depending on the receptor type, it can be inhibitory or excitatory. We mainly talk about it’s actions as an inhibitory neurotransmitter, but it’s important to note that it can act as an excitatory one as well, depending on the receptor that it acts upon.
There are two main receptor subtypes for GABA, which are known as GABA-A and GABA-B receptors. GABA-A receptors are ligand-gated chloride channels. Hopefully, without further explanation, from my past entries, this makes sense. If you need a refresher, this means that when GABA interacts with these GABA-A receptors, they undergo a conformational change that opens their “pore” to allow chloride (Cl-) ions to flow through. Since Cl- is negative, it can hyperpolarize the cell (make it more negative) and make it less likely to fire, in the simplest explanation.
GABA-B receptors are more complex, as they are those G-protein coupled receptors. Downstream effects can be to open Cl- channels or K+ channels (since there is more potassium inside the cell, K+ might flow out or even if it does not move, shunt an excitatory signal if it arrives while the channels are open), amongst other things.
Inhibitory actions can be very complex on their own and really help to fine-tune the rest of the brain’s activity. The image above shows a presynaptic cell below and a postsynaptic neuron above it, as GABA is involved in the hypothalamus/feeding behavior- found in this paper (Richards & Berthoud, 2006).