GABA (also known as gamma-aminobutyric acid) is the brain's
primary inhibitory neurotransmitter; it decreases the likelihood that a
particular neuron will fire. It works by opening ion
channels that allow chloride ions to flood into the neuron.
How do chloride ions make a neuron less likely to fire? Well, remember
that most neurons have a resting potential of about -70 mV--that
is, the voltage inside the neuron is 70 mV less that the voltage outside
the neuron. If that potential changes to about -60 mV or so, the neuron
will fire. Now, excitatory neurotransmitters (like
acetylcholine) make positive ions flow into the neuron, raising the
potential closer to that -60 mV threshold. If chloride ions (which are
negative) flow in, the resting potential will move further away from the
threshold (say, down to -90 mV). You'll therefore need lots more
excitatory neurotransmitter to get the neuron up to -60.
Benzodiazepines (drugs like Valium and Xanax) are GABA