Neurotransmitters can inhibit or excite neurons. _________ for example, is inhibitory whereas __________ is excitatory

The correct option is GABA; glutamate.

To understand why this is correct, we can examine the roles and mechanisms of these two primary neurotransmitters in the central nervous system:
1. GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the brain. When GABA binds to receptors on a postsynaptic neuron, it typically triggers the opening of ion channels that allow negatively charged chloride ions (Cl^-) to enter the cell or positively charged potassium ions (K⁺) to leave. This flow of ions hyperpolarizes the neuron's membrane, making it more negative and moving it further away from the threshold required to fire an action potential, thereby suppressing or inhibiting its activity.
2. Glutamate is the primary excitatory neurotransmitter in the brain. When glutamate binds to its receptors on a postsynaptic neuron, it opens channels that allow positively charged sodium (Na⁺) and calcium (Ca²⁺) ions to flow into the cell. This influx of positive charge depolarizes the cell membrane, bringing it closer to the threshold needed to generate an action potential, thereby promoting or exciting neural activity.

Consequently, GABA functions as an inhibitory neurotransmitter, whereas glutamate functions as an excitatory neurotransmitter, making "GABA; glutamate" the correct sequence to complete the sentence.